cea f F A - 8 - y b - * : $ f k f # 4 & # : 4 - % 34 ko C R i & d i C ig h - u f * ile ¢ fume a 4 A . s + i $ \ + + & + % 1 «w; + & % & 4 i ~ 3 f R # 4 x A i * C 3% I § \ yisa muck f . fice R , s t @ . (%, - % | C Neg j t A P 4 ¢ } + P Melanges and Their Bearing on Late Mesozoic and Tertiary Subduction and Interplate Translation at the West Edge of the North American Plate By KENNETH F. FOX, JR. GEOLOGICAL SURVEY PROFESSIONAL PAPER: 1198 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1983 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data Fox, Kenneth F., 1933- Melanges and their bearing on late Mesozoic and Tertiary subduction and interplate translation at the western edge of the North American plate. (Geological Survey professional paper 1198) Bibliography: p. 36-40 Supt. of Does. no.: I 19.16:1198 1. Melanges (petrology). 2. Geology, stratigraphic-Mesozoic. 3. Geology, stratigraphic-Tertiary. 4. Plate tectonics. 5. Geology-North America. I. Title. II. Series: United States. Geological Survey. Professional Paper 1198. QE471.15.M44F69 1983 551.1'386 82-600355 For sale by the Distribution Branch, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 CONTENTS Page Page Abstract 1 Franciscan Complex 28 Introduction 2 Age of deformation resulting in the Franciscan Com- Late Cretaceous and Tertiary plate-tectonic setting ----- 2 plex 23 Definition of melange, broken formation, and petrotec- Cause of allochthonous deformation that created the tonic assemblage 3 Franti8CAN COMPI@X 24 Origin of melange 3 Comparison of Cretaceous through early Eocene tec- Objectives of present investigation ------------------------------ 4 tonic regimes as predicted by the triple-junction Geometry of Humboldt- and Mendocino-type triple junc- hypothesis with concepts suggested by other evi- tions 5 dence 25 Age and distribution of melanges and related rocks ----------- 10 Evidence from the Franciscan Complex contradictory San Onofre Breccia 10 to the triple-junction hypothesis ------------------------- "21 Olympic Peninsula and Oregon-Washington borderland - 10 Oregon-Washington borderland = ---------------------------------- 28 Franciscan Complex, Franciscan assemblage, Great Val- Age, distribution, and plate-tectonic setting of the ley sequence, and correlative rOCks -------------------------- 15 borderland 28 Semantic distinction between Franciscan Complex Origin of the borderland through plate collision ----- 29 and Franciscan assemblage and the relation of Reconciliation of the collision hypothesis with other these rocks to the Great Valley sequence ----------- 15 tectonic features of the Pacific Northwest --------- 32 Hsu's (1969) concept of allochthonous, mesoallochthonous, Melanges of the Olympic Peninsula ----------------------- 33 autochthonous, and neoautochthonous rocks -------------- 18 The Aja Fracture Zone and the formation of a Evidence bearing on the age of formation of the Francis- Humboldt-type triple junction ---------------------- PBJ can Complex 18 Origin of melanges and broken formations ------- 33 Morro Bay area 19 San Onofre Breccia 34 Clear Lake-COVeIO @rea --------------------......._........._.. 20 | Conclusions 36 Bandon area 21 | References cited 36 Main elements of melanges and their bearing on plate-tectonic history 28 ILLUSTRATIONS Page FIGURE 1. Sketch map showing geometry west of the North American plate from 60 to 80 Ma derived through extrapoliation of late Cenozoic plate motions 2 2. Schematic model of early and middle Tertiary evolution of the plate geometry of the northestern Pacific -------------------------- 2 3. Sketch map showing relation of oceanic features of the north eastern Pacific to melange and other features of the western part of the North American plate 4 4. Cross section showing hypothetical formation of melanges through tectonic churning and accretion of trench deposits to under- side of a subduction zone as postulated in subduction-complex theory 6 5-8. Diagrams showing: 5. Vector circuit representing the relative movement of three plates in mutual contact at a ridge-transform fault-subduction zone triple junction 7T 6. Evolution of plate geometry at a triple junction 8 7. Progressive advance of Humboldt-type triple junction along a continental plate and the volume of material that must be disposed of during this advance 9 8. Formation of fold belt, thrusts, gravity thrusts, and melanges at Humboldt-type triple junction ---------------------------- 10 9. Geologic map of western Oregon and Washington 11 10. Sketch map showing tectonic classification of the Franciscan Complex and associated rocks of western California and southwest- ern Oregon 14 11. Diagram showing correlation of boundary regimes and petrotectonic features along the west side of the North American plate - 17 12. Schematic view to northwest showing transit of Early Cretaceous Humboldt-type triple junction with accompanying formation of melange and a proto-Coast Range thrust to east 26 13. Paleomagnetic map of the floor of the northeastern Pacific Ocean 30 14. Diagram showing Late Cretaceous and early Tertiary plate geometry west of the North American plate ------------------------- 31 15. Sketch map showing successive positions of the Aja fracture zone relative to the North American plate ------------------------- 34 16. Schematic view to north showing deformation and generation of melanges and broken formations in early and middle Miocene time at the present site of the Olympic Peninsula 35 IH MELANGES AND THEIR BEARING ON LATE MESOZOIC AND TERTIARY SUBDUCTION AND INTERPLATE TRANSLATION AT THE WEST EDGE OF THE NORTH AMERICAN PLATE By KENNETH F. FOX, Jr. ABSTRACT Melanges are commonly considered to be material scraped off an oceanic plate descending at a subduction zone, tectonically churned, and accreted to the underside of the overriding plate. Yet the correla- tion of Late Cretaceous and Tertiary melanges of western North America with subduction zones of that age is poor. During much of the middle and late Tertiary, this area was continuously or discontinu- ously bordered by a subduction zone within which the Farallon plate and much of its successor, the Juan de Fuca plate, were consumed. Yet known melanges of this age that can reasonably be linked to this process are rare and limited to those of the Olympic Peninsula of Washington. Melanges are also present within the Franciscan Com- plex of western California and within the Otter Point Formation of southwestern Oregon, mostly Eocene or older. | An alternative to the subduction-complex theory is that melanges \ are material that was broken and sheared as it was plowed aside and | either coasted or was rammed inland at a triple junction migrating along the edge of the continental plate. The required triple junction is of a singular dynamic type, referred to as a Humboldt-type, formed where an oceanic plate obliquely underthrusts a continental plate and advances laterally along the edge of that plate while following a re- treating oceanic (or possibly continental) plate. The triple junction may be formed through the interection of either (1) a spreading ridge, transform fault, and subduction zone or (2) two transform faults and a subduction zone. The Franciscan Complex includes rocks that contain detritus eroded from preexisting melanges or detritus deposited by normal sedimentary processes on top of preexisting melange. These se- quences were subsequently sheared, fragmented, and intermixed to form new melanges or broken formations, strata similar to melanges but containing no exotic blocks. The Franciscan in places contains a re- cord of two or more distinct cycles of melange development. Evalua- tion of such constraints as are known on the ages of these cycles suggests three diachronous events, believed to represent the transit along the western margin of the continent of Humboldt-type triple junctions in Cretaceous and early Tertiary time. The youngest of these is fairly well bracketed by ages of nonpenetratively deformed rocks and penetratively deformed melange or broken formation near Morro Bay, Calif., and less satisfactorily in the Covelo-Clear Lake area of California. The ages suggest that the most recent period of for- mation of the Franciscan Complex and correlative rocks was during the Campanian at Morro Bay and early Eocene or perhaps later time near Covelo. Farther north, the age of the most recent overthrusting and imbrication of Franciscan-like rocks near Bandon, Oreg., also is bracketed within the early Eocene, but it is not certain that melange or broken formation formed contemporaneously with the thrusting. In California, the final episode of allochthonous deformation was probably a diachronous upheaval producing melange and broken for- mation that transited the continental margin at a rate of roughly 4 em/ yr, reaching northern California by the early Eocene. This timing nearly coincides with the transit of the Kula-Farallon-North American triple junction, as inferred by Tanya Atwater in her constant-motion model of Late Cretaceous and Tertiary plate geometry. In early Eocene time, however, this transit apparently evolved into an event in which coastal areas of southwestern Oregon and northwestern California were contemporaneously deformed and the allochthonous oceanic crust now underlying northwestern Oregon and western Washington was formed and accreted to the craton. The basement rock of this Oregon-Washington borderland con- sists of oceanic tholeiitic basalt of early and middle Eocene age, which, from published paleomagnetic data, is believed to have been rotated clockwise as much as about 70° by middle Tertiary time. The contact of the oceanic crust with the craton to the east is apparently defined by a zone of steep negative gravity gradients. The angular to jagged outline of this contact as inferred from published gravity maps suggests that the borderland is an aggregation of variably rotated blocks, rather than a single elongate and coherent crustal block. The reported attitude of source fissures of the tholeiite suggests derivation in part in a stress system with a tensional direction comparable to that of the Kula-Pacific ridge rather than of the supposedly nearby Kula- Farallon ridge. Prior to 56 Ma (million years before A.D. 1950), the paths of Pacific and North American plates may have been convergent rather than parallel to the trend of the Queen Charlotte and San Andreas faults, as they have been for the past 25-30 million years. If they were convergent, the allochthonous crust of the borderland could have been accreted to the North American plate during a collision between that plate and the Pacific plate. It has been proposed that the Kula-Pacific spreading ridge van- ished abruptly shortly after 56 Ma. The presence within the oceanic crust of the Oregon-Washington borderland of a tensional orientation comparable to that of this ridge suggests that the ridge, instead of vanishing, jumped northward, intersecting the North American plate northwest of the former Kula-Farallon-North American triple junc- tion. The Pacific plate, enlarged by the addition of part of the Kula plate, then sideswiped the North American plate, driving a widening wedge of recently formed oceanic crust inland while crunching and stacking adjacent rocks of the craton in Oregon and Washington. By the impact of this collision, the Pacific and North American plates were deflected into their present paths parallel to their bounding transforms as the allochthonous wedge of oceanic crust was sheared off, fragmented, and rotated clockwise to form the basement of the Oregon-Washington borderland. The core rocks of the Olympic Peninsula consist of melange and broken formation, infaulted or imbricated with blocks of intact strata. Rocks peripheral to the core consist of the oceanic tholeiitic basement 1 2 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE of the Oregon-Washington borderland with interfingering clastic de- posits, mainly overlain by shallow-water marine-shelf deposits. The core rocks are bathyal marine turbidite deposits. Melanges of the western core contain fossils whose reported ages are as young as early or middle Miocene. Published potassium-argon ages of the rocks of the eastern core suggest metamorphism after 29 Ma and cooling about 17 Ma. Magnetic lineations of the northeastern Pacific step right laterally across the Aja fracture zone. From the age of these anomalies, it ap- pears that north of the Aja, the spreading ridge system and coexisting subduction zone shrank, then vanished about 21% Ma. The Aja would then intersect the Queen Charlotte fault and the subduction zone to the south and, with continued right-lateral movement of the Pacific plate, would form a Humboldt-type triple junction. That triple junc- tion would then persist through about 5% m.y., finally dying 16 Ma as the ridge system south of the Aja stepped eastward and intersected the subduction zone. This timing nearly coincides with potassium- argon ages of cooling of the youngest melanges in the eastern core of the Olympic Peninsula. To account for the structural fabric and geo- graphic extent of the Olympic melanges of Miocene age through the tectonism associated with this triple junction, the junction must have been situated immediately west of the Olympic Peninsula. If this spa- tial and temporal relation is valid, northwestward movement of the Pacific plate relative to the North American plate has averaged about 6 em/yr at least since middle Miocene time, a rate comparable to ac- cepted estimates of the rate of movement of these plates averaged over the past 2 m.y. INTRODUCTION LATE CRETACEOUS AND TERTIARY PLATE-TECTONIC SETTING In Late Cretaceous and early Tertiary time, the North American plate was flanked to the west by the Kula plate (Grow and Atwater, 1970, p. 3717) and the Farallon plate (McKenzie and Morgan, 1969). Still farther west, the Kula and Farallon plates joined the Pacific plate (fig. 1) at spreading ridges from which all three plates grew. During this time, the oceanic plates, though moving in various directions with respect to each + NORTH L* Seanie AMEEICAN'-- -> Brate \\I Anchorage - . San: , ip Ay Aue. r <4» Francisco . ' -'. . Guaymas ;. 12 cm/yr i g ti ue .: "p ow A T k.. KULA PLATE «A* H FARALLON A t PLATE xz" "A Mendocino 7 1 I fracture zone PLATE * FIGURE 1.-Plate geometry west of North American plate from 60-80 Ma (million years before A.D. 1950) (derived by Atwater, 1970, p. 3531, through extrapolation of late Cenozoic plate motions). Ar- rows show directions of spreading and plate movements relative to the North American plate, arbitrarily held fixed. Pacific plate is assumed to be moving at a constant 6 em/yr parallel to trans- form faults that later (in late Cenozoic) developed between it and the North American plate. Model explained by Atwater (1970, p. 3531) as one of numerous alternatives permitted by her data. other, collectively moved right laterally past the North American plate (Atwater, 1970). Consequently, the sub- duction zone down which the Farallon plate plunged ob- liquely below the overriding North American plate gradually lengthened to the northwest. The spreading direction between Pacific and Faral- lon plates changed abruptly in early Tertiary time a KULA sas o PLATE _ = ~ - . AMERICAN ® .*.. * _PLATE'_.." PACIFIC PLATE JUAN / 7 Mendocino fracture zone 7e . sNORAH: '' :. . ._AMERICaAN : © __. :~ ~': PACIFIC / PLATE - Pioneer fracture *~ zone :! :'. I 7// op" / 7 Murray : fracture /zone / y cocos : PLATE B FIGURE 2.-Early to middle Tertiary evolution of plate geometry of northeastern Pacific (modified from Menard, 1978, p. 105). A, Early Tertiary (50 Ma). B, Middle Tertiary (36 Ma). INTRODUCTION 3 (Menard and Atwater, 1968). Magnetic anomalies formed after this change radiate slightly fanwise about a distant pole (Menard, 1978, p. 104), suggesting that the Farallon plate gradually pivoted counterclockwise with respect to the Pacific plate. Menard (1978, p. 104) has postulated that about the time anomaly 21 was formed (50 Ma) the Farallon plate broke along the east- ward projection of the Murray fracture zone into two parts, cutting the Juan de Fuca plate on the north away from the main part of the Farallon plate on the south, here referred to as the Cocos plate (fig. 2). At about the time of formation of anomaly 13 (36 Ma), the Pioneer fracture zone also broke eastward to the edge of the North American plate, forming a small plate between the Pioneer and Murray fracture zones (Menard, 1978, p. 104). Shortly after about 30 Ma, the eastward projection of the Pacific plate between the Pioneer and Murray fracture zones contacted the North American plate. After 27 Ma, some 3 to 4 m.y. later, the segment of Pacific plate between the Mendocino and Pioneer fracture zones also contacted the North Ameri- can plate. Subsequently, through northward advance of the Juan de Fuca- Pacific-North American triple junc- tion (now at Cape Mendocino) and southward retreat of the Cocos-Pacific-North American triple junction, the plate geometry evolved into its present configuration (fig. 37". The evolution of plate geometry outlined here is based on the history of spreading of the northeastern Pacific, deduced chiefly from the magnetic lineations and morphology of its basaltic floor, and extrapolation of the present rate (roughly 6 em/yr) and direction of rela- tive movement between the Pacific plate and North American plate back to Late Cretaceous time. Although the gross geometry is probably correct, the model be- comes progressively weaker proceeding back in time be- cause of the cumulative effect of errors in the assump- tions and because much of the pre-Tertiary spreading record has been eradicated by consumption of oceanic crust at converging plate margins. Acknowledgments.-I appreciate the penetrating yet constructive criticisms of the manuscript by J. C. Matti, R. W. Tabor, M. C. Blake, Jr., D. L. Jones, and P. D. Snavely, Jr. Continuing dialogs with R. J. McLaughlin and M. C. Blake, Jr., on problems of the Franciscan Complex were very helpful, as were discus- sions with R. W. Kopf on the distinction between rock- stratigraphic units and lithotectonic units, and distinc- 'Ages assigned to magnetic anomalies in this report are based on the time scale of La Breeque and others (1977). *The Juan de Fuca plate was called the Vancouver plate by Menard (1978). At the risk of some slight loss in precision, this plate and other fragments of the Farallon plate, such as the Cocos plate, are here referred to by the commonly accepted name of their major surviving remnant. tion between the terms "Franciscan Complex" and "Franciscan assemblage." f DEFINITION OF MELANGE, BROKEN FORMATION, AND PETROTECTONIC ASSEMBLAGE Geologic features indicative of particular plate-tec- tonic regimes of the past (petrotectonic assemblages of Dickinson, 1971) include ophiolites, paired metamorphic belts, old volcanic ares, thrust belts, and melanges. Fol- lowing Hsu (1968, p. 1065), "melange" refers to those enigmatic, though "mappable bodies of deformed rocks characterized by the inclusion of tectonically mixed frag- ments or blocks, which may range up to several miles long, in a pervasively sheared, fine-grained, and com- monly pelitic matrix." The melanges include both exotic and native blocks. Again quoting Hsu (1968, p. 1065), "Native blocks are disrupted brittle layers which were once interbedded with the ductilely deformed matrix. Exotic blocks are tectonic inclusions detached from some rock-stratigraphic units foreign to the main body of the melange." A body of broken strata containing no exotic blocks but otherwise similar to a melange is de- fined as a "broken formation" (Hsu, 1968, p. 1065-1066). These and analogous features are major guides to past plate geometries. Even where correctly identified, the precision with which they can be applied is reduced because (1) the features are inherently difficult to date; or (2) their genetic correlation with a particular plate- tectonic regime is tenuous; or (3) the spatial relation is diffused because the petrotectonic feature forms over a broad area at a considerable or indefinite distance away from the causal plate boundary regime; or (4) the spatial relation is confused by lateral translation of unknown magnitude before final accretion to the craton. Attempts to verify, calibrate, and rigorously extend Atwater's (1970) model by dating the petrotectonic features of the continental plate have generally been frustrated. ORIGIN OF MELANGE The recognition of the loose association of melanges with presumed consuming plate boundaries, the internal tectonic disruption of the melanges, and the incorpora- tion of trench deposits and ophiolitic bodies within them suggested to many workers that the melanges formed through scraping off and tectonic churning of the upper surface of the oceanic plate as it was being subducted (Hamilton, 1969, p. 2415-2416). By this concept, melanges were considered to be imbricated slices of ma- terial scraped off the descending oceanic plate and ac- creted to the underside of the overriding plate during the subduction process (fig. 4). Indeed, the presence of melanges within a geologic terrane is now the single most important indicator of the subduction complex. 4 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE One of the inadequacies of this subduction complex theory is that, in detail, the correlation between the lo- cation and age of melanges and the inferred location and duration of past subduction regimes is poor. For exam- ple, according to Atwater's (1970) model, during much of the middle and late Tertiary, the western side of the North American plate was continuously or discontinu- ously bordered by a subduction zone within which the Farallon plate and much of its successor, the Juan de Fuca plate, were consumed. Yet known melanges or melangelike rocks of this age that could possibly be re- lated to this subduction process are rare, being limited to those of the Olympic Peninsula (Stewart, 1971; Tabor and Cady, 19782). Perhaps the subduction process is in- herently occult, its products concealed from observation except when unusual cireumstances not now understood intervene. If it is, the subduction-complex hypothesis in its present form makes no verifiable predictions. An alternative hypothesis to explain the origin of the melanges of the Franciscan Complex was recently IQSOVI/ ~ 60 y advanced by Fox (1976). According to this hypothesis, melanges form as a series of coalescing and imbricated gravity slides and thrust slices at a triple junction as it migrates along the edge of the continental plate. Further, the required triple junction is of a singular dy- namic type referred to as a "Humboldt" triple junction. At a Humboldt-type triple junction, a transform fault bounding the continental plate is converted to a subduc- tion zone through lateral transport of the two adjacent oceanic plates. Melanges, then, represent a chronologi- cal and spatial tie between the point on the continental plate where it joins with two adjacent oceanic plates and one at which there is a transition from a strike-slip tec- tonic regime to a subduction regime. OBJECTIVES OF PRESENT INVESTIGATION One of the objectives of this paper is to test the sub- duction-complex and triple junction hypotheses by com- * ® # & 7 400 _ 500 KILOMETERS 2d L s FIGURE 3.-Relation of oceanic features of northeastern Pacific to melange and other features of western part of North American plate. Ob- lique Mercator projection about late Cenozoic pole of rotation between Pacific and North American plates at lat 52° N., long 73°W. (Chase, 1972). Melange terranes too small to show located in San Juan Islands (S.J.1.) and at Cedros Island (C.1.). Oceanic features after Atwater and Menard (1970), Naugler and Wageman (1973), Vine (1968), Raff and Mason (1961), Barr and Chase (1974), Moore (1973), and Larson (1972). INTRODUCTION 5 paring the timing and locus of melange formation with that suggested by the known or inferred evolution of plate geometry along the western margin of the North American plate. Except for the Neogene, however, neither the evolutionary history of plate geometry nor the age of formation of the melanges is known with suffi- cient exactitude to provide a very sensitive test. Mean- ingful comparisons can indeed be made for the Neogene, but only very crude comparisons for the early Tertiary and Cretaceous. For that part of the geologic record, the recognition of an association of melanges with specif- ic triple junctions could potentially improve both percep- tion of past tectonic regimes and precision in their corre- lation with particular plate configurations. This leads to the second objective of this paper; to summarize evi- dence now available concerning the ages of the melanges and, by interpreting them as records of the existence and passage of Humboldt-type triple junctions, to mod- ify and amplify the Late Cretaceous and early Tertiary plate-tectonic history of western North America. “If“ » -\/// San Onofre Breccia ncr \\\ CEDROS w/WLANDB >- & ~ \\ \ Sp L/ *, a % GEOMETRY OF HUMBOLDT- AND MENDOCINO-TYPE TRIPLE JUNCTIONS Triple junctions have been classified into 16 types according to the nature of the intersecting plate bound- aries, that is ridge-ridge-ridge, ridge- transform-sub- duction zone, transform-transform-subduction zone, and so on, and the conditions under which they are stable defined (McKenzie and Morgan, 1969). In this discus- sion, my interest is focused chiefly on understanding the tectonic processes at the margin of a continental plate. I therefore restrict my inquiry to the dynamics of those stable ridge- transform-subduction zone and transform- transform-subduction zone triple junctions in which the edge of a continental plate is a transform fault on one side of the triple junction and a subduction zone on the other side. The subduction zone is presumed to dip continent- ward at an angle within the range of dip of modern Be- nioff zones, approximately 20°%-65° (Turcotte and 7 ¢. 4 * EXPLANATION A_ A & A a_ Subduction zone-Sawteeth on upper plate; sawteeth in direction of dip Transform fault-Dotted where concealed 4 or inferred Fracture zone Magnetic anomaly-Queried where un- certain Boundary of Oregon-Washington bor- $ derland Spreading ridge-Dashed where inferred -A- FIGURE 3.-Continued 6 Schubert, 1973, p. 5880), whereas the transform fault is presumed to be nearly vertical, also in accordance with seismic studies of modern transform faults such as the San Andreas fault. The triple junction marks, then, the point at the surface of the Earth where the inclined plane of the subduction zone intersects the nearly verti- cal plane of the transform. The third leg of the triple junction is either a spreading ridge or transform separating two oceanic plates. The rate and direction of lateral migration of triple Junctions at which a continental plate contacts two adja- cent oceanic plates depends on the relative motion of the three plates with respect to each other. Although that fact will seem self-evident, the details of the geometric relation between the movement of the triple junction and that of the three plates may seem obscure except to those who closely followed the development of plate-tec- tonic theory. The geometric details may be clarified by considering the following hypothetical example of rela- tive movement between three plates, A, B, and C, as depicted in figures 5 and 6. I define the triple junction between plates A, B, and C to be stable in the sense of McKenzie and Morgan (1969); that is, the relative movement between these . plates sums to zero and can therefore be represented by the vector triangle shown in figure 5. In this example, spreading at the ridge between plates B and C is ortho- gonal to the ridge and is symmetrical; that is, equal in- Early Cretaceous turbidites Trench Late Cretaceous turbidites \ MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE crements of new oceanic crust are accreted to plates B and C during any given interval of time, and so plates B and C move away from the ridge at equal rates. That being the case, all points on the ridge and its imaginary prolongations at either end must lie on the perpendicu- lar bisector of line B-C (fig. 5). The triple junction must lie both on the perpendicu- lar bisector of line B-C and also on the line of which the vector between C and A is a segment, hence must be lo- cated at J in figure 5. The vector Ja thus represents the direction and velocity of movement of the triple junction J with respect to plate A, that is, 1 em/yr N. 40° W. if the spreading direction is N. 87° E. In this example, plate B moves N. 40° W. at 6 em/yr relative to plate A, and plate C moves N. 80° E. at 6 em/ yr relative to plate B. Were these rates the same, but the spreading direction N. 80° E. rather than N. 87° E. (fig. 5), the triple junction would plot at J, coincident with the apex of the triangle at A. In this situation, the triple junction would not move relative to plate A. Were the spreading direction N. 75° E. (fig. 5), the triple junc- tion would plot at J°® in figure 5, hence would move 1 em/ yr S. 40° E. relative to plate A. In this example, the di- rection and rate of movement of the triple junction rela- tive to plate A is very sensitive to minor differences in spreading direction. Differences in spreading rate, angle of intersection of ridge and the margin of the continental plate, or in rate of offset of the continental plate with re- Arc volcanism 20 - Late Cretaceous C A xs melange y Mas See "ree | made __M£h3. dfcgtfluiy hee tew ass 40 - > 4p}; I ~> Early Cretaceous wore \ b melange Toy | s 0 20 KILOMETERS w so - N R ¢ e E 0 CoE A No- Cc by \ I s y -e Anat Fi, NSSY ~ - 3—5 M A N T L E ~4 r € \A CONTINENTAL PLATE £ as ® # 100 - EXPLANATION is 3 ® % (-% L- * ¥ wy N CX: & Intrusive igneous rocks \ ® x ® ot AJ) - Sialic crust , \\\\ \\ (- AAH u \ A ~" Kx % ® x" X 149 - Early Cretaceous turbidites \ \ [-~ is e \\\ ho Late Cretaceous turbidites x » 3 i 160 ] \ Former location of Med Fault- Showing relative movement \ theisubduction zone =~" C, f x --- Base of oceanic plate \ FIGURE 4.-Cross section showing hypothetical formation of melanges through tectonic churning and accretion of trench deposits to underside of a subduction zone, as postulated in subduction-complex theory. INTRODUCTION T spect to the oceanic plates could similarly affect the movement of the triple junction. Relative to the continental plate, then, the triple junction could (1) move in the same direction as the transform-fault-bounded oceanic plate but at the same or a lesser rate, thereby extending the length of the subduction zone; or (2) not move; or (3) move in the di- rection opposite (1), thereby extending the length of the transform fault. Returning to our example (fig. 6) and its vectorial representation (fig. 5), it is convenient to consider the spreading ridge as simply a crack along which upwelling magma is plastered to plates B and C. From the vector diagram (fig. 5), we see that a point on this crack (P) must move 4 em/yr toward N. 3° W., that is, toward tri- ple junction J° (fig. 6). Hence the crack, or spreading ridge, is being shortened at a rate of 4 em/yr. The vector C;, which represents the direction and rate that plate C moves with respect to the triple junc- FIGURE 5.-Vector circuit representing the relative movement of three plates, A, B, and C, at a ridge-transform fault-subduction zone triple junction (J) that is stable in the sense of McKenzie and Morgan (1969) and at which the three plates are in mutual con- tact. A point on the spreading ridge is represented by P. The di- rection of spreading is orthogonal to the ridge, and the rate of spreading symmetrical with respect to the ridge. Three cases are represented; in all, plate B moves N. 40° W. at 6 em/yr with re- spect to plate A, and the spreading rate at the ridge is also 6 em/ yr. Case 1 (solid lines): plate C moves N. 87° E. with respect to plate B; case 2 (long-dashed lines): plate C moves N. 80° E. with respect to plate B; case 3 (short-dashed lines): plate C moves N. 75° E. with respect to plate B. tion J, trends N. 34° E. Hence the edge of the subducted slab (dotted line in fig. 6A) must also trend N. 34° E. (ignoring the effects that curvature, dip, partial melt- ing, deformation, and other processes might have on the outline of the subducted slab as projected to the sur- face). To clarify this picture, we define three points that initially are superimposed on triple junction J° in figure 6A. Point R is attached to plate B, point S to plate C, and point Q is fixed with respect to the crack (spreading ridge). At this instant in time, a brief magnetic polarity change is recorded in the cooling basalt along the ridge system, forming anomaly 5. After an interval of time, the plate geometry will have evolved to that portrayed in figure 6B. Relative to plate A, point R has moved 6 em/yr to N. 40° W., point S has been subducted and moved 5.4 em/yr to N. 24° E., where it lies on the N. 34° E.-trending edge of the sub- ducted part of plate C. The triple junction has moved from J° to J+, and point Q now lies on an imaginary pro- longation of the crack (spreading ridge), which has been shortened at the rate of 4 em/yr. Anomaly 5 is visible alongside the spreading ridges. The length of that ano- maly in plate C, including the part that has been sub- ducted, equals its length in plate B. The evolution of this plate geometry is followed through four more time periods, assuming no changes in spreading rates, in figures 6C, 6D, 6E, and 6F. Note that the triple junction and the leading edge of the sub- ducted slab move steadily northwest, until the ridge- ridge transform fault forming part of the boundary be- tween plates B and C intersects the margin of plate A. At that time, the velocity of the triple junction with re- spect to plate A abruptly accelerates from 1 em/yr to 6 cm/yr, and there is a concomitant acceleration and change in outline of the leading edge of the subducted slab. Consider now the space problems that arise if the triple junction and leading edge of the subducted slab are forced to migrate laterally along the margin of the continental plate. If, as in the example given, the sub- duction zone is extended at the expense of the transform fault bounding the continental plate, the descending plate must incrementally displace the wedge-shaped vol- ume of lithospheric material partly bounded by the transform, the base of the continental plate, and the lat- erally projected surface of the subduction zone (fig. 7). As shown in figures 6 and 7, the space problem may be exacerbated while a ridge-ridge transform fault is sub- ducted. Because of its buoyancy, the displacement of the volume of crustal material involved must be accom- plished by the plowing up and ramming back of the lip of the continental plate above and ahead of the advance- ing prow of the subducting slab (fig. 8). Thus elevated, 8 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE 6 cm/yr U > a ha C. j A_ EXPLANATION 4 Subduction zone-Sawteeth on upper plate; esesssseees Projection to surface of northwestern edge of arrow shows relative movement subducted extension of plate C __ ---.... Transform fault-Long: dashed where inactive:= O. \'}... ic) __ Projection to surface of former positions of edge short dashed where projected to surface. of subducted extension of plate C Arrows show relative movement Spreading ridge FIGURE 6.-Evolution of plate geometry at a triple junction between three plates A, B, and C. Spreading rates, nomenclature, and assumptions are same as those in figure 5 (case 1). A, Initial configuration of the three plates. B, C, D, E, and F, Configuration after lapse of successive and equal increments of time. Movement and growth of plates B and C are shown with respect to plate A, arbitrarily held fixed. Dotted line, projection to surface of northwest edge of subducted extension of plate C. Former positions of this edge are shown in B through F by fine-dotted lines. Numbered lines 5, 4, 3, 2, and 1 represent magnetic anomalies formed at the time of frames A, B, C, D, and E, respec- tively, shown as solid lines at surface of plates B and C; dotted lines, where subducted beneath plate A. J°, J4, J°, J%, J', and J° represent location of triple junction at time of successive frames A, B, C, D, E, and F, respectively. R, Q, S, points mentioned in text. INTRODUCTION *] pue 'g 'y asoy; 03 juareamba sreaxojut aum arngng 12 qets pagonpans Jo agpa Sutpeoa1 Jo suontsod aatssa0ons moys ayefd requaurjuo Jo apts apun uo sauy poneg 'arefd requaumnuod yjmm Lrepunog urede adSpry ';) 'sopefd otureado ut souy poysep-3uo; 4q umoys y 12 soewoue anausep; 'uonsimnsguoa @uoz 03 saajoAa uoroun{ ardur se saqgaatooo®e arefd [equournuo Jo pred ;o uonsouniy, 'g 'oyerd requaunuoo 03 sapefd otusado Jo quawaaow Jo uotpatp Ssmoure yoyq Sutpeaids moys smoure uly, *;) pus g ut payoea1 gets paronpqns Jo agpa Butpeor yo uontsod moys arefd requaunmuoa Jo apts -1opun uo sour payseq 'sonuru09 Jutpeards se ystuea [[m unropsue.11 Jo Sutpeaids Jo quaudos Au, 'y 'woargoud aoeds saopeqaaoexa sajefd requaunuoa pus aruea00 usamoq j[ng; YIM adpu-a3pu Jo uonooasiojut are quawoAouw pug 'uononpqns 'Sutpeouds ;o saye ySnowyty 'soueape sty; Buump pooetdstp aq qsnu yey; reLojew ;o aumjoa pus (yoeq pay) arefd requaunuo) e Suore uonoun( ardry Jo aoueape aatssauSoud Sutmoys weadeg-} «NOL paranpans jo sabpa buipea7 10 the lip of crustal material will probably slough away as subhorizontal gravity slides, imbricated thrust fault slices, and intercalated melange and broken formation (Fox, 1976). A triple junction whose movement is associated with the lateral advance and impingement of the sub- ducted slab on space occupied by the continental plate, resulting in the displacement of parts of the continental plate by the subducted slab, has been defined as a Hum- boldt-type triple junction (Fox, 1976). Two other situa- tions can be visualized. In one, the triple junction does not perceptibly move relative to the continental plate. With time, the tectonic situation at the margin of that plate will stabilize, with a strike-slip regime to one side of the triple junction and a subduction regime to the other side. Alternatively, if the triple junction moves in such a way that the transform fault bounding the continental plate is extended at the expense of the subduction zone, then the wedge-shaped space formerly occupied by the obliquely descending lithospheric plate must be filled through upwelling of mantle or by accordion-folding of lithospheric material. This type of triple junction has been defined as a Mendocino-type (Fox, 1976). AGE AND DISTRIBUTION OF MELANGES AND RELATED ROCKS Melanges of late Mesozoic and possible early Ter- tiary age recognized along the western margin of the North American plate (fig. 3) have been cataloged by Jones and others (1978). Exclusive of melanges in Alaska, which are outside the scope of the present study, the melanges include those of the Franciscan Complex in California and its presumed correlatives in Oregon and the Baja Peninsula of Mexico and melanges on both the San Juan Islands of Washington and the west coast of Vancouver Island, British Columbia. To Continental plate Fold belt ___-~ nema ss Reverse Right-lateral fault em transform Gravity thrust Obliquely subducting oceanic plate Spreading ridge or transform FIGURE 8. -Diagram showing formation of fold belt, thrusts, gravity thrusts, and melanges at Humboldt-type triple junction. MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE these must be added the melanges of the Olympic Penin- sula of Washington, reported to be of Eocene (Snavely and Pearl, 1975; Snavely and others, 1977), and Miocene age (Rau, 1975). The melanges of the Olympic Penin- sula, because of their relative youthfulness, permit com- parison of the plate-tectonic situation that prevailed at the time of their formation, as deduced from the mag- netic lineations of the northeastern Pacific, with that predicted by the subduction-complex and triple-junc- tion theories. The Olympic melanges occupy part of the northern end of a 640-km-long coastal strip apparently underlain by oceanic crust composed chiefly of tholeiitic basalt and subordinate volcaniclastic sediments (Snavely and others, 1977, p. 9). The oceanic crust is early and middle Eocene and was probably accreted to the craton in mid- dle Eocene or later time (MacLeod and others, 1977, p. 226). Melanges are not found outside the Olympic Penin- sula within this voleanogenic borderland, but because its history forms an important link in the story of interac- tion between oceanic and continental plates, its geology is briefly outlined below. The San Onofre Breccia (Woodford, 1925), though not a melange, is briefly mentioned because its origin may be related to the plate interactions that are the main concern of this paper. Correlation of these rocks with a particular tectonic setting is deferred to the sec- tion below entitled "Main Elements of Melanges and ***." SAN ONOFRE BRECCIA The San Onofre Breccia consists of sandstone, con- glomerate, and breccia, together aggregating at least 795 m in thickness (Woodford, 1925, p. 185). The deposit is unusual in two respects, first because it contains an- gular blocks of blueschist and other crystalline rocks as much as 4.6 m long (Woodford, 1925, p. 186); second, be- cause, though interlayered with marine deposits derived entirely from the craton to the east, the breccia itself was derived through erosion of a briefly emergent sub- marine source area to the west (Woodford, 1925 p. 236- 239). The deposit formed as a subaerial alluvial fan and bordering shallow-water marine-fan and delta complex in late Saucesian, Relizian, and Luisian(?) stages of the Miocene (Stuart, 1976). OLYMPIC PENINSULA AND OREGON-WASHINGTON BORDERLAND Between Bandon, Oreg., and Vancouver Island, British Columbia (fig. 9), the continent is bordered by a 200- to 300-km-wide strip in which the apparent base- ment rock is chiefly tholeiitic basalt of early and middle Eocene age, overlain by younger volcanic and volcanic- lastic strata (Snavely and others, 1966). This area, here AGE AND DISTRIBUTION OF MELANGES AND RELATED ROCKS 11 EXPLANATION 23 Surficial deposits (late Miocene to Quaternary) MIOCENE TO EOCENE BEDROCK MARINE DEPOSITS M Shallow leep-w. sedm ry ock h calat d I ocks (Eoc CCCCCCCCCCCCCCCC cle an {h nuunmmw W” as: as; s Wig! i Q } ;:E.I:-. I 3. 'o Ap "NW Mum \ A4. ? 1; f l IMMIHIHIHW (mud FiGURE 9.-Geologic map of western Oregon and Washington. Chie efly from Huntting and others (1961), Wells and Peck (1961), Bro: romery and Snavely (1964), MacLeod and other rs (1977), and Tabor and Cady (1978b). 12 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE referred to as the Oregon-Washington borderland, un- derlies only the western part of an early and middle Ter- tiary eugeosyncline (Snavely and Wagner, 1963) that also included the area to the east underlain by fringing shelf deposits of eugeosynclinal aspect. As pointed out by Bromery and Snavely (1964, p. N-1), the contrast between the high density of the base- ment rock and the lower density of the superjacent sedimentary rocks makes feasible the mapping of the basement surface through gravity surveys. Gravity maps by Bromery and Snavely (1964), Bonini and others (1974) and MacLeod and others (1977) confirm the coin- cidence of positive gravity anomalies with areas in which the tholeiitic basement rocks crop out. These anomalies are bounded on the east by zones of steep negative gradients, here assumed to represent the con- tact in the subsurface of oceanic crust with less dense crustal material of the craton (fig. 9). The zones of high . gradient appear to be segmented, though linear within individual segments. Considering the smoothing effect of the gravity-measurement and map-contouring pro- cess, the actual contact is probably angular and in some areas jagged. The Eocene basalt includes, from south to north, the Siletz River Volcanics (Snavely and others, 1968), the Crescent Formation (Arnold, 1906), and the Metcho- sin Volcanics of Clapp (1910) (Snavely and others, 1966, p. 456). These volcanic rocks apparently are chiefly Ulatisian (late early and early middle Eocene) in age (Rau, 1964, p. G-4; Snavely and others, 1968, p. 467). The Siletz River Volcanics has been dated at 49.3 to 54.7 m.y. by potassium-argon methods (Duncan, 1977). The volcanic rocks generally include a thick lower member of marine pillow basalt and interbedded deep- marine sediments, locally overlain by an upper member of shallow-water to subaerial tholeiitic or alkalic basalt, the whole aggregating as much as 15 km in thickness (Cady, 1975, p. 575). According to Snavely and others (1968, p. 480), the lower part of the Siletz River Vol- canics rose from the mantle along north-trending fis- sures. Sedimentary rocks interlayered with basalt of the Crescent Formation contain clasts of continental prove- nance including boulders of quartz diorite as much as 3 m in diameter, suggesting proximity of the basalts to the North American continent at the time of their extru- sion (Cady, 1975, p. 579). In Washington, the Crescent basalt is conformably overlain by, or intertongues with, a sequence of deep-marine clastic sediments mapped as the Aldwell and overlying Lyre Formations (Olympic Peninsula) or McIntosh and overlying Northcraft For- - mations (southwestern Washington) of upper Ulatisian to middle Narizian (middle and early late Eocene) age (Rau, 1964, p. G-4; Snavely and others, 1958, p. 17-22). These units are unconformably overlain by shelf de- posits, the unconformity marking a significant orogenic episode in the opinion of Snavely and others (1977, p. 7, 20). In Oregon, the Siletz River Volcanics underlies the Tyee Formation, a 3,000-m-thick sequence of rhythmi- cally interbedded sandstone and siltstone of continental provenance deposited by northward-flowing turbidity currents (Snavely and Wagner, 1963, p. 7). The Tyee is in turn overlain by the late Eocene Yachats Basalt (Snavely and McLeod, 1974). The pre-Pliocene bedrock of the Olympic Peninsula (fig. 9) is broadly composed (Tabor and Cady, 19782) of peripheral rocks and core rocks. The peripheral rocks consist of a thick stratiform sequence of Eocene to Miocene age, which, though folded and faulted, is essen- tially stratigraphically intact (compare Glassley, 1974, p. 786). In contrast, the core rocks, though also of Eocene to Miocene age, consist of rocks that have been tectonically disrupted, forming a mass in which melanges and broken formations are imbricated and in- folded with fault-bounded blocks and slivers of intact strata. The peripheral rocks consist of tholeiitic basalt of the Crescent Formation and interfingering deep-water clastic deposits. Except for the northwestern part of the peninsula, these rocks are overlain by a succession of shallow-water marine shelf deposits. In contrast, the core rocks consist of bathyal marine turbidite deposits. Melanges of the northwestern core are "composed of sheared middle Eocene basalt, large infolded blocks of turbidite sandstone, and broken formation" according to Snavely and Pearl (1975). These rocks are depositionally overlain by deep-water marine siltstone and sandstone of latest Eocene to middle Miocene age, and the se- quence is itself strongly deformed and unconformably overlain by tilted strata of latest Miocene and Pliocene age (Snavely and Pearl, 1975). Melanges in the west-central part of the core zone are described by Rau (1973, p. 5) as part of the Hoh rock assemblage, which, in addition to melange, contains much-deformed turbidite deposits, chiefly rhythmically interbedded and graded sequences of siltstone, sandstone, graywacke, and conglomerate. Most of the foraminiferal assemblages contained within these de- posits indicate deposition at no less than upper bathyal depths (Rau, 1975). The melanges typically are com- posed of blocks or slabs of sandstone, graywacke, or conglomerate embedded in a matrix of much-sheared siltstone. The blocks commonly are as much as a meter or more in length; and Rau (1973, p. 8), in his excellent descriptions and photographs of the melanges, noted that the blocks include some as large as houses and some even larger ones that form sea stacks, promontories, AGE AND DISTRIBUTION OF MELANGES AND RELATED ROCKS 13 and small islands. The slabs range to several kilometers in length and appear to be "floating" in melange (Snavely and others, 1977, p. 21). At the seacliff exposures to which the I was di- rected by W. W. Rau, the blocks are angular to nearly equant and faceted. Outer surfaces are polished and commonly striated, and the blocks, though essentially intact, are crisscrossed by fractures. The melange loc- ally contains zones composed predominantly of sandstone or graywacke that is sheared ubiquitously but not penetratively at hand-specimen scale. Exotic clasts of greenstone are present in the melange, but blueschist and eclogite have not been reported. The foraminifers of the Hoh rock assemblage have been extensively collected and studied by W. W. Rau. The following paragraph summarizes his findings (in Rau, 1975): Foraminifers, though rare, have been collected from 50 localities within the Hoh rock assemblage. Most of these fossils suggest either an early or middle Miocene age (Saucesian or Relizian Stages of Kleinpell, 1938) of deposition; a middle Eocene assemblage was found at one locality and a late Eocene at two. Some of the youngest assemblages (five localities) suggest a Re- lizian age, and one very poorly preserved assemblage tentatively suggests an age as young as late Miocene. Collections from the melanges themselves suggest upper Saucesian to possibly Relizian (middle Miocene) age. The single significant megafossil assemblage found indicates an early or middle Miocene age, according to W. 0. Addicott (cited by Rau, 1975). The Hoh rock assemblage is beveled by an angular unconformity and on land is overlain by the late Miocene(?) and Pliocene Quinault Formation, flat-lying siltstone, sandstone, and conglomerate (Rau, 1975). Offshore, according to Rau (1975), late Miocene strata are at least in places present between the structurally complex rocks of early and middle Miocene age and the strata of Pliocene age. The eastern core rocks have recently been de- scribed by Tabor and Cady (1978a) as consisting of shale, siltstone, sandstone, and minor conglomerate, basalt, basaltic volcaniclastic rock, diabase, and gabbro, varyingly metamorphosed to slate, semischist, phyllite, greenstone, and greenschist. These rocks range from faulted or shear-zone bounded, but intact, bedded se- quences to completely disrupted broken formations com- posed of sandstone or semischist clasts embedded in a matrix of slate or phyllite. Exotic clasts such as bluesch- ist or eclogite have not been found. The rocks are multi- ply cleaved and lineated, and both bedding and cleavage have been folded and refolded. Fossils are very rare; those found range from early Eocene to early Oligocene. An intensive investigation of the potassium-argon geochronology of the northeastern core led Tabor (1972) to conclude that the age of regional metamorphism was about 29 Ma and the age of a later episode of faulting and quartz veining about 17 Ma. Measured ages (66 reported determinations) range from 16.2 to 227.4 m.y. (Tabor, 1972, table 1), with an appar- ent inverse correlation between metamorphic grade and age. Potassium-argon ages of graywacke and semischist range downward to about 29 m.y. This lower limit was defined by three samples from which separates of mat- rix material yielded ages of 31.2+0.6, 29.1+0.9, and 29.0+0.7 m.y.; separates of coexisting clast material yielded corresponding ages of 35.2 +0.7, 36.2 + 0.9, and 39.1+1.3 m.y. Considering the possibility of imperfect separation and consequent cross-contamination of mat- rix and clast material, noted by Tabor (1972, p. 1810), it seems probable that the age of pure metamorphic mat- rix material is significantly less than 29 Ma. The potassium-argon age of slate and phyllite ranged down to 27 Ma. The 10 determinations of the age of phyllite breccia, however, clustered at about 17 Ma and ranged from 16.2 to 19.9 Ma. The samples of breccia are from widely scattered localities, some adjacent to outcrops of rocks giving much older ages (Tabor, 1972, p. 1811, fig. 10). Core rocks are separated from peripheral rocks by a curving system of observed and inferred faults and shear zones following or splaying away to the inside of the Olympic horseshoe (fig. 9). On the north, the system includes the Calawah fault, which diverges westward from the inferred shear system between peripheral and core rocks, cutting through the core rocks as a zone loc- ally more than a kilometer and a half wide (Gower, 1960; Tabor and Cady, 19782). Gower (1960) inferred that the zone was a left-lateral strike-slip fault. MacLeod and others (1977, p. 227) concurred in this opinion, noting that major differences in lithology and provenance of Eocene deep-water marine sandstone north and south of the fault suggest strike-slip offset. They concluded that because of this lithologic contrast and the wide zone of shearing, offset was probably substantial. Tabor and Cady (19782), after analyzing the struc- tural fabric, concluded that the parental rocks of the core were isoclinally folded, then faulted, imbricated, overturned westward, and pressed by east-west com- pression into the basaltic horseshoe. The horseshoe could have formed by this deformation, or it could have been already in existence. Continued compression caused the core rocks to yield upward and outward by shear folding, forming a "mushroom-like dome" (Tabor and Cady, 19782). The age, or ages, of the deformation of the core, zone rocks is critical to any understanding of their ori- gin. In the eastern core, the age of some or all of the 14 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE broken formation and of the most recent tectonism must be late Oligocene or younger, on the basis of Tabor's (1972) potassium-argon age study. Tabor's work estab- lishes a strong presumption that these rocks were metamorphosed after 29 Ma and finally cooled through the blocking temperature of fine-grained mica about 17 Ma. In Tabor's opinion, the potassium-argon age data suggest two events, the ages of which are closely ap- proximated by the clusters of potassium-argon ages at 29 Ma (late Oligocene) and 17 Ma (early Miocene). The melanges of the Hoh rock assemblage of the western core must be entirely or in part at least as young as the youngest fossils found within them, that is, possibly Relizian. This age would imply a maximum age of about 16 m.y. (time scale of Van Eysinga, 1975). The presence of little-deformed late Miocene sediments un- conformably resting on the western core rocks similarly places a younger age limit of at least 5 m.y. on the age of the most recent deformation. That there was at least one previous episode of major deformation is indicated by the presence of latest Eocene sediments deposition- ally overlying melange containing middle and early Eocene rocks (Snavely and Pearl, 1975). In summary, the deformation of the core-zone rocks was apparently episodic, not continuous. Snavely and Pearl (1975) had suggested that the core rocks reflected two major orogenic events, the first of which was in late middle or early late Eocene, the last in middle Miocene. Their conclusion, insofar as it applies to the western core, seems amply sustained by the available evidence. Tabor's age data suggest that deformation of the east- ern core during the last orogenic event ended in latest early Miocene time and further may point to the exis- tence of a third major event of late Oligocene age. Con- firmation of the late Oligocene event should be sought in the stratigraphic record. < as a Ru lores. 4 ¥ iy ___ & § w $ By, 7 7 7 7 0 50 100 150 KILOMETERS | " y alley *- outlier ° (autoch.?) 196. e u n..." £3 Ce, + *y a : ef-s Sit» -. o Cape Sebastian Sandstone . yess - <% &.: fm s § # +# 5 fit . Mountain - and \p . cond ACA - o . We Felis t ue «Wm unit. : 49’s“? Hunters Cove Formation of 3 pez om .' ra eton iits iid: R. 4 § sao rr _s" Be 5 Dott (1971) Eureka "Coastal belt of Fra $$ $ gd. 3 EXPLANATION FRANCISCAN COMPLEX I Cape Mendocino Mesoallochthonous units Allochthonous slabs and plates ns Melanges and allochthonous and mesoallochthonous units, undifferentiated [ [ [ [ & 125» w & T > Ro h & ® & FIGURE 10.-Sketch map showing tectonic classification following Hsu (1969) (see also Hsu and Ohrbom, 1969) of the Franciscan Complex and associated rocks of western California and southwestern Oregon. Modified chiefly from Jennings (1977) with additions from Coleman (1972), Baldwin (1974), Dott (1971), and Beaulieu and Hughes (1976). AGE AND DISTRIBUTION OF MELANGES AND RELATED ROCKS 15 FRANCISCAN COMPLEX, FRANCISCAN ASSEMBLAGE, GREAT VALLEY SEQUENCE, AND CORRELATIVE ROCKS SEMANTIC DISTINCTION BETWEEN FRANCISCAN COMPLEX AND FRANCISCAN ASSEMBLAGE AND THE RELATION OF THESE ROCKS TO THE GREAT VALLEY SEQUENCE The late Mesozoic and early Tertiary rocks underly- ing much of the western seaboard of central and north- ern California are collectively referred to as the Francis- can assemblage (Bailey and others, 1964, p. 11) or Fran- ciscan Complex (Berkland and others, 1972). Although these names are applied to nearly the same rocks, the Franciscan assemblage and the Franciscan Complex em- body different stratigraphic concepts and are therefore not synonymous. The Great Valley sequence borders go w 38 0 y, the Franciscan Complex on the east, and similar rocks are found in isolated areas surrounded by the Francis- can Complex (fig. 10). The semantic distinction between the age of the as- semblage and that of the complex is like that between the age of the trees in a forest or several forests and a house built from those trees. The trees might be of various ages within a discrete range; after cutting, saw- ing, planing, assembling, and nailing, the result is a new entity, a house, whose age is less than that of any of its components. Thus when we refer to the Franciscan Complex, we refer to the "house"; when we refer to the Franciscan assemblage, we refer to the "trees" from which the house was built. The problem is more compli- cated than this, in that like a house to which various wings have been added at various times, the Franciscan Complex has been built in various stages, places, and times. 7 36° w y Lake Berryessa bo NINA ; ,,, $000.0000000002‘ e Atascadero Formation 0"; ? of Fairbanks (1904) (fig; SSF 5 heer oan XXX Asuncion Fxp CFovmanon ge r HAMA Las Tablas unit mesoallochthon ou *~San Luis Obispo 00 of Taliaferro , fam yd Broken y= Clear Lake formation ig ¢" > + a* 7° .+ ; outlier . 4 - # XS ere o 0 0 0.0.0.0 0 5, A w % entTia ecs ct n *s. 99 hes fl}. 0.334"? passa and Knoxville rock i> Santa Rosa °.~* > Fon FPDP J e rocks 25 p. ihe m RNR AAP 3 a itn * ei- .- CR NC toy ome ,~ OCEAN : o £ i-" = * paCIFIC F f > 5 is Salinian block Neo-autochthon Autochthon ........ M 722010 Contact-Dotted where inferred _ Fault-Dotted where inferred F4 F4 [ [ > lge F > ERES & w P A w Z ® & & & FIGURE 10.-Continued 16 The Franciscan Complex is a structural aggregation of rocks in which fault slices and blocks of relatively in- tact bedded deposits are imbricated and interwoven with shear zones, melanges, and broken formations. The structural fabric of the unit, dominated by subparallel to anastomosing fracture surfaces, closely spaced in the melanges and broken formations and widely spaced in the tectonic slabs and blocks, along with accompanying fragmentation and granulation, is the chief diagnostic feature of the complex. Berkland and others (1972, p. 2297) stipulated that the Franciscan Complex was the basement terrane of the California Coast Ranges, and their assignment of some rock units to the complex was partly based on an interpretation of whether or not they constituted part of the local basement (1972, p. 2299). As noted in an earlier paper (Fox, 1976, p. 740), the Franciscan Complex is probably not basement in some, and perhaps much, of its area of outcrop. Except for that element requiring it to be a basement terrane, Berkland and others' (1972, p. 2297) name "Franciscan Complex," with their definition of it, is here adopted in place of the name "Franciscan Formation." The parental rocks were chiefly massive graywacke interbedded or interlayered with siltstone, conglomer- ate, bedded chert, greenstone, and locally limestone. During their structural disruption, slabs of serpentine, greenstone, and ophiolitic rocks were tectonically in- troduced into the complex. These rocks were zeolitized and locally metamorphosed to low-grade blueschist, greenschist, or amphibolite. In addition, high-grade blueschist and eclogite of enigmatic origin is present in the melanges as ubiquitous, though commonly widely scattered, exotic clasts. The term "Franciscan assemblage" (Bailey and others, 1964, p. 11) is an informal designation collec- tively applied to various lithologies that make up the Franciscan Complex as defined above. Bailey and others (1964, p. 148) stressed that their so-called Franciscan rocks are sheared, disrupted, and deformed, but did not imply that the resulting structural fabric is a diagnostic part of their concept of the lithologic entity they desig- nated an assemblage. Though only sparsely fossilifer- ous, the assemblage does contain scattered fossils of Late Jurassic, Cretaceous, and early Tertiary age (Irwin, 1957; Bailey and others, 1964; Blake and Jones, 1974). It is therefore correct to say that the Franciscan assemblage, as indicated by the available fossil evi- dence, ranges in age from Late Jurassic to early Ter- tiary, since by "age" we mean depositional age of the various parts of the assemblage. The Franciscan Complex is defined as a structural complex, a rock-stratigraphic unit according to Article 6) of the "Code of Stratigraphic Nomenclature" (Ameri- can Commission on Stratigraphic Nomenclature, 1974) MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE though there seems to be a difference of opinion on this point (compare Berkland and others, 1972, p. 2299), and its age is that of its diagnostic structural fabric; its age at any given place is that of the deformation that pro- duced the melanges and tectonic disruption characteris- tic of the complex. The older limit of that age is the age of the youngest fossils locally present (R. W. Kopf, writ- ten commun., 1976). Contrary to the opinion of Berk- land and others (1972, p. 2300), the presence of fossils of Late Jurassic to early Tertiary age does not necessarily mean that the complex has a comparable range in age (melange rule three of Hsu, 1968, p. 1067). The defor- mation is penetrative with respect to the complex as a whole, though not necessarily at hand-specimen or even outcrop scale, a feature that could lead to difficulties in the practical application of this definition. More recent faulting and folding without this grossly penetrative as- pect may displace the Franciscan Complex but is not considered part of its distinguishing structural fabric. As defined by Berkland and others (1972, p. 2299) the Franciscan Complex geographically occupies three belts, an eastern belt, a central belt, and a coastal belt. In the eastern belt, the rocks are dominantly metaclas- tic and have a strong to barely perceptible schistosity. In the central belt, the dominant lithology is melange. The coastal belt, originally recognized and roughly de- lineated by Bailey and others (1964), contains graywacke, shale, and conglomerate but only a little greenstone, chert, serpentine, and blueschist and is structurally deformed in a manner analogous to other parts of the Franciscan Complex (see Bailey and others, 1964, p. 13). The ages given by Blake and Jones (1974, p. 351) for fossils and fossil assemblages from the Franciscan assemblage of northern California (fig. 11) range from Late Jurassic to Eocene. Except for those of Late Cre- taceous and early Tertiary age, most of which were found in the coastal belt, the fossils were found either in the matrix of the melange or within allochthonous blocks and slabs. Rocks included within the Great Valley sequence range in age from Late Jurassic (Tithonian) to Late Cre- taceous, making them equivalent in age to some of the lithologic elements incorporated within the Franciscan Complex (Irwin, 1957; Bailey and others, 1964, p. 123- 139; Berkland, 1973, p. 2396-2399). The Franciscan Complex and Great Valley sequence are not found in de- positional contact with each other. Rather, the two units are faulted together, or are faulted against folded sheet- like bodies of serpentine and variably serpentinized ul- tramafic rocks that separate the two units. In places, the basal part of the Great Valley sequence deposition- ally overlies these ultramafic rocks (Bezore, 1969; Bailey and others, 1970). 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These age and contact rela- tions hypothetically result from underthrusting of the Franciscan Complex below the Great Valley sequence (Bailey and others, 1964, p. 163-165; Irwin, 1964; Blake and others, 1967, p. 6-7; Bailey and Blake, 1969, p. 148). HSU'S (1969) CONCEPT OF ALLOCHTHONOUS, MESOALLOCHTHONOUS, AUTOCHTHONOUS, AND NEOAUTOCHTHONOUS ROCKS Rarely, the conglomerates within the Franciscan Complex contain scattered pebbles of blueschist similar to the blueschist found as exotic blocks within the melange, indicating that these deposits were derived through erosion of a preexisting part of the Franciscan Complex (Cowan and Page, 1975). Also rarely, shelf de- posits that appear locally to unconformably overlie the deformed rocks of the complex elsewhere have been broken, disrupted, and imbricated with melange and broken formation, thereby assuming the same struc- tural fabric as the complex as a whole (Hsu, 1969). The Franciscan Complex, in some places, then, contains a record of at least two episode(s) of deformation and melange formation. The earlier episode(s) was in places followed by cannibalization of the structural complex and deposition of detritus from it on a surface eroded across it. This cycle was followed by another cycle of de- formation and melange formation. The Franciscan Com- plex is in places the product of two and possibly more distinct cycles of sedimentation and deformation. The concept of tectonic cycles was first articulated by Hsu (1969), who classified the rocks in the Morro Bay area as allochthonous, mesoallochthonous, autochthon- ous, and neoautochthonous. According to Hsu (1969, p. 12-13): (1) Allochthonous rocks are those that have been deformed by overthrusting or by gravity sliding and have been transported for a considerable distance from their original site of deposition. (2) Mesoallochthonous rocks are those rocks that were deposited upon an allochthonous basement and transported for a considerable distance from their origi- nal site of deposition during a later episode of al- lochthonous deformation. A mesoallochthonous slab may include only mesoallochthonous sediments, or it may in- clude both the mesoallochthonous sediments and some of the basement that has been deformed by two or more episodes of allochthonous movements. (3) Autochthonous rocks are those rocks that have not been transported from their original site of deposi- tion. § (4) Neoautochthonous rocks are those rocks that were deposited upon allochthonous rocks after all al- lochthonous deformations had taken place. EVIDENCE BEARING ON THE AGE OF FORMATION OF THE FRANCISCAN COMPLEX In applying Hsu's (1969) concepts of allochthonous, mesoallochthonous, autochthonous, and neoautochthon- ous rocks to the Franciscan Complex (fig. 10), it is stipu- lated that: (1) the craton to the east be regarded as a fixed reference frame; (2) tectonic dislocations, lateral translation, and accretion as tectonostratigraphic ter- ranes (Beck and others, 1980) of the lithospheric crust underlying or including the Franciscan Complex and Great Valley sequence before the Cretaceous be ig- nored; and (3) the mode of deformation of the al- lochthonous rocks include overthrusting, gravity slid- ing, and (or) any other process capable of forming the penetrative fabric of the Franciscan Complex. The ages of the "allochthonous deformations" are bracketed by the ages of neoautochthonous and mesoal- lochthonous deposits and by the ages of mesoal- lochthonous and the parental autochthonous deposits. Hsu's classification appears to be relevant to the lithologic contrast between rocks of the coastal belt and the blueschist-bearing melange of the central belt. The coastal belt contains fossils as young as late early Eocene (Orchard, 1978), yet is imbricated, penetratively deformed, and includes broken formations, evidence of one or more episodes of allochthonous deformation (Blake and Jones, 1974, p. 347). The nature of the contact of coastal-belt rocks with the central belt is controversial. Kleist (1974) concluded that melange near Laytonville in the central belt was deposited on the coastal belt, but Kramer (1976, p. 4) found that the contact in the Fort Bragg-Willits area was a high-angle reverse fault. Kramer (1976, p. 4, 5) also found isolated remnants of the coastal belt deposi- tionally overlying central-belt rock and speculated that the reverse relations reported by Kleist (1974) might have resulted from Tertiary and Quaternary landslid- ing. Detailed mapping of the coastal belt is incomplete; it may be found that some of the Eocene rocks are neo- autochthonous. Nonetheless, exotic tectonic blocks of blueschist and eclogite are absent or very rare except at the margins of the belt. The coastal belt was probably deposited after conclusion of the tectonic event or events that stirred the exotic blocks into the melange of the central belt and, assuming that the coastal-belt rocks depositionally overlie central-belt rocks at least locally, must be chiefly mesoallochthonous. Blocks of foraminiferal limestone attributed to the coastal belt that contain Cenomanian fossils have been AGE AND DISTRIBUTION OF MELANGES AND RELATED ROCKS 19 tectonically mixed with central-belt melange (Blake and Jones, 1974, p. 350-351). Cenomanian fossils have been reported from "Franciscan" sandstone and questionable Cenomanian fossils from a metagraywacke unit (Hull Mountain belt) enveloped by melange (Blake and Jones, 1974). By these findings, fossils as young as Cenomanian are locally present along with blueschist blocks in the melanges of northern California. The episode of defor- mation during which these rocks were mixed together, then, occurred in Cenomanian or later time but probably before deposition of the bulk of the coastal belt. Farther south, near Jasper Ridge, 50 km southeast of San Francisco (fig. 10), rocks comparable in age to those in the coastal belt appear to be neoautochthonous. At this locality, greenstone and chert of the Franciscan Complex, along with serpentine that contains pods or blocks of blueschist, is owerlam by a thick sequence of sandstone and siltstone, containing fossils of middle and late Eocene age (Pampeyan, 1970; Page and Tabor, 1967). The Franciscan rocis form the core of an appres- sed west-northwest-trending anticline defined by steeply dipping and locally overturned Eocene rocks that form the limbs of the fold. The Eocene rocks of the south limb include a conglomerate as much as 60 m thick that depositionally overlies the Franciscan rocks (Pam- peyan, 1970). A conglomerate is also present at the base of the middle and upper Eocene sandstone on the north side, but there it is much thinner, being only about 3 to 9 m thick (Pampeyan, 1970), and measurably older, for it contains a fauna suggesting a late Paleocene or early Eocene age (Graham, 1967). This conglomerate deposi- tionally overlies the nearby serpentine, and is composed of packed granule-sized Franciscan detritus, mainly greenstone, in a calcium carbonate cement (Page and Tabor, 1967; Graham, 1967). Page and Tabor (1967, p. 5-8) observed that the superjacent middle and upper Eocene beds contain un- usual chaotic zones consisting of "disordered mudstone containing isolated sandstone bodies which have been detached from formerly continuous beds and have been more or less haphazardly distributed in the mudstone matrix." They concluded that the chaotic beds probably formed through submarine sliding in the late Eocene but that the steep folding of the Eocene beds and their close proximity to the San Andfieas fault might be suggestive of a tectonic origin. Because neither the granule con- © glomerate nor the overlying Eocene beds are penetra- tively sheared, these rocks are here tentatively consid- ered neoautochthonous (fig. 10). In this area, it appears that the Franciscan Complex had been tectonically formed, then exposed to erosion by early Eocene at the latest. Search of the literature reveals three areas where Hsu's (1969) classification either has been applied or could be applied on the basis of published descriptions: the Morro Bay area of central California, the Clear Lake-Covelo area of northern California, and the Ban- don area of southern Oregon. MORRO BAY AREA In the Morro Bay area, Hsu (1969, p. 17-18) map- ped a slab of broken formation ("broken formation A") composed in part of graywacke containing about 10-20 percent K-feldspar, a few granite clasts, and abundant Franciscan debris. Blocks of similar graywacke were found in a melange below this slab, justifying Hsu's de- signation of the unit as mesoallochthonous (Hsu, 1969, fig. 2, p. 18; Smith and Ingeroll, 1978). Two palynomorphs from two samples of shale interbedded with the graywacke in this slab were identified by W. R. Evitt (in Hsu, 1969, p. 18) and considered by him to be Late Cretaceous, most likely Campanian or older. Approximately 15 km inland, Cowan and Page (1975, p. 1089) found recycled Franciscan detritus, in- cluding glaucophane-lawsonite schist, as clasts in a mass of sandstone, the Las Tablas unit, about 2.0 by 0.75 km in area, tectonically enveloped in Franciscan melange. Three palynomorphs (including the two species found in broken formation A) from three samples of shale inter- calated with the sandstone were identified by W. R. Evitt (in Cowan and Page, 1975, p. 1093) and considered by him to be Late Cretaceous and, quite possibly but not necessarily, Campanian. Rocks about 25 km southeast of the Las Tablas unit, mapped as the Atascadero Formation by Hart (1976), are composed of massive sandstone and conglom- erate, along with bedded sandstone, siltstone, and mudstone. In this unit, according to Hart (1976, p. 15): Internal deformation is widespread and locally in- tense. This includes: (1) pervasive shears in sandstone subparallel to the bedding; (2) pinched-off sandstone beds, including occasional boudins; and (3) pinching and swelling of mudstone beds. Some of the deformation may be of "soft rock" type (for example, slumping and sliding contemporaneous with deposition), but much of it is "hard rock" (as indicated by common microscopic shears and deformed grains in sandstone) and probably is the result of large-scale overthrusting or gravity slid- ing. It appears, then, that these rocks have been af- fected by one or more episodes of allochthonous defor- mation. According to Hart (1976, p. 16), the Atascadero is probably Cenomanian or Turonian to late Campanian or Maestrichtian. The oldest rocks in this area that are neither dis- rupted by penetrative shearing nor found as tectonic in- clusions in the melanges make up the arkosic Asuncion 20 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE Formation of Taliaferro (1944), according to Hsu (1969, p. 25). Campanian and Maestrichtian fossils found in the upper part of this unit indicate the age of deposition (Taliaferro, 1944; Popenoe and others, 1960; Hsu, 1969, p. 25). The most recent period of melange formation in this part of California appears to have been in Campanian time. The presence of mesoallochthonous deposits im- plies that there was at least one earlier period of melange formation. The rocks of central California classified by Hsu (1969) as mesoallochthonous include rock units of Early Cretaceous age, mapped as part of the Marmolejo For- mation by Taliaferro (1944). Fossils reported by Taliaferro (1944, p. 469) from the Marmolejo were as- signed to the late Valanginian by Popenoe and others (1960, chart 10e, annotation no. 12, p. 1520), but accord- ing to Hsu (1969, p. 18), specimens of Buchia from the formation include both Late Jurassic and Early Cretace- ous species. Hsu (1969) described these rocks as being evenly bedded siltstone and shale characterized by the preservation of stratal continuity, occurring both as large mappable slabs and tectonic inclusions too small to map. Hsu (1969, p. 16) correlated broken formation B, . composed in part of graywacke commonly containing 2-5 percent detrital K-feldspar and conglomerate and sedimentary breccia containing clasts of glaucophane schist, with the rocks mapped as the Marmolejo by Taliaferro (1944) on the basis of their K-feldspar con- tent. The designation of the fossiliferous Marmolejo as mesoallochthonous, rather than simply allochthonous, hinges on this correlation. Fossils were not found in bro- ken formation B. If truly mesoallochthonous, an episode of tectonism in which blueschist exotics were incorpo- rated in melange occurred prior to the deposition of the Marmolejo. About 100 km southeast, the latest Jurassic and early Cretaceous (Valanginian) Toro Formation, as de- scribed by Hart (1976), seems to grade into melange. While evidence for a pre-Marmolejo and pre-Toro episode of melange formation is somewhat tenuous, there can be no doubt of one after deposition of these rocks. g CLEAR LAKE-COVELO AREA Near Clear Lake, Swe and Dickinson (1970) de- scribed a succession of clastic sedimentary deposits, ranging in age from Late Jurassic to Late Cretaceous, that they correlated with the Great Valley sequence to the east. They postulated that these deposits, aggregat- ing 10,700 m (35,000 ft) in thickness, along with lower Tertiary beds, represented outliers of imbricated sheets that were overthrust to the west onto the Franciscan assemblage, or that the Franciscan was underthrust to the east below Great Valley rocks, and that these imbri- cated sheets were subsequently infolded with the Fran- ciscan. The rocks correlated with the Great Valley se- quence form four fault-bounded segments within which the strata appear to be conformable. The lower Tertiary strata consist of a lower unit of sandstone and shale aggregating 1,300 m (4,230 ft) in thickness and an upper unit of conglomeratic sandstone aggregating 335-365 m (1,100-1,200 ft) in thickness (Brice, 1953, p. 28-30). These two units had originally been assigned to the Martinez (early Paleocene) and Tejon (late Eocene) Stages, respectively, by Dickerson (1914, 1916), but as Berkland (1973, p. 2391) reminds us, the "Tejon" beds were subsequently reassigned to the Meganos (late Paleocene) Stage by Clark and Vokes (1936, p. 856, fig. 2). The basal contact of the Paleocene beds was given careful attention by Swe and Dickinson (1970, p. 183). They found that the Paleocene strata are in thrust contact with underlying rocks correlative with the lower part of the Great Valley sequence. On this basis, they inferred that the Great Valley and Tertiary rocks were tectonically emplaced against Franciscan rocks after deposition of the lower Tertiary beds. The thrusting may have produced a certain degree of mixing of Franciscan and Great Valley rocks, for Swe and Dickinson (1970, p. 171) observed such mixing in several areas. In one area, small fossiliferous thrust slices of the Great Valley sequence were caught up in the underlying Franciscan tectonic breccia. In another (1970, p. 169) the Great Valley is extensively sheared and intricately mingled with serpentinite breccia. In a third, highly deformed massive graywacke and pebble conglomerate mapped as the Franciscan assemblage lie on the strike of a 6/-km (4 mi)-long belt of graywacke and conglomerate in the adjacent Great Valley sequence (1970, p. 168-169); these Franciscan and Great Valley rocks had previously been mapped as a continuous belt of the Knoxville Formation by Brice (1953, p. 13-14, pl. 1). The preservation of the Cretaceous and Paleocene depositional sequence suggests that the Great Valley se- quence found within the Clear Lake outlier has not been grossly dismembered and therefore probably has not been transported great distances from its original de- positional site. These rocks are probably autochthonous. However, their deformation and local incorporation in Franciscan tectonic breccia imply that here the Francis- can Complex includes rocks that were tectonically de- tached from the Great valley sequence. The episode of allochthonous deformation during which these rocks were mixed necessarily occurred in Paleocene or later time. At Rice Valley, Cretaceous strata correlated with the Great Valley sequence and overlying Paleocene and Eocene(?) strata form an elongate slab 0.7 km by 3 km AGE AND DISTRIBUTION OF MELANGES AND RELATED ROCKS 21 in plan that is tectonically enclosed by ultramafic rock and chaotic rocks of the Franciscan Complex (Berkland, 1973). The Cretaceous rocks consist of greenish-gray shale with thin-bedded K-feldspar-bearing sandstone in- terlayers aggregating 915 m (3,000 ft) in thickness, overlain by biotitic arkosic K-feldspar-rich sandstone about 150 m (500 ft) thick. The Paleocene strata overlie the Cretaceous strata with slight angular discordance (Berkland, 1973, p. 2396) and in ascending order consist of an unfossiliferous massive sandstone at the base, overlain by polished pebble conglomerate, also unfos- siliferous but containing abundant reworked Franciscan detritus; quartz grit; fossiliferous concretionary sandstone containing a rich fauna of Meganos (upper Paleocene) age; and an uppermost unit of glauconitic sandstone containing a sparse fauna that could repre- sent a horizon "possibly as young as Eocene" according to W. 0. Addicott (in Berkland, 1973, p. 2398). According to Berkland (1973), the Cretaceous part of the Rice Valley outlier is a conformable sequence, 965 m (3,500 ft) thick, that ranges in age from Hauterivian to Cenomanian on the basis of fossils from two horizons. As Berkland (1973) suggested, the difference in thick- ness between this section and the 12,000-m (40,000 ft) section of the Great Valley sequence at the western edge of the Sacramento Valley is in part due to erosion prior to deposition of the overlying Paleocene rocks. The Hauterivian to Albian part of the autochthonous Great Valley sequence in the Wilber Springs area (fig. 10), 50 km southeast of the Rice Valley outlier, is itself 5,100 m (17,000 ft) thick (Bailey and others, 1964, p. 132). Hence the thinness of the Rice Valley sequence implies ex- treme primary depositional thinning of the Great Valley sequence toward the west. Berkland's (1973, fig. 2) map shows that the slab is folded into an appressed syncline with the dip of both limbs approaching the vertical near the bounding faults. Berkland presumed that the Rice Valley sequence is a remnant of an upper-plate underthrust by an oceanic plate of Franciscan materials (Berkland, 1973, p. 2400- 2402). He suggested that the melange characteristic of the Franciscan Complex in this area may represent a tectonic mixture of adjacent thrust plates. In support of this concept, Berkland mentioned the presence along the east side of Rice Valley of a slab of unmetamorph- osed K-feldspar-bearing graywacke and minor chert- pebble conglomerate similar to the Lower Cretaceous part of the Great Valley sequence. The structural relations of the Rice Valley outlier to surrounding ultramafic rock and melange of the Francis- can Complex are equivocal. The actual contacts are chiefly high-angle faults, according to Berkland's (1973) map and cross sections. There is little direct evidence to support the conventional view that the slab is a down- faulted block or an erosional remnant of a once-continu- ous sheet of Great Valley sequence thrust over the Franciscan. On the basis of structural relations de- scribed by Berkland (1973), the possibility cannot be ruled out that the slab is (1) an upfaulted block of the au- tochthonous basement on which the Franciscan rests, or (2) a large rip-up of that basement engulfed in melange, or (3) a mesoallochthonous deposit. In the Covelo area, irregular "islands" formed by intact sequences of Upper Cretaceous, Paleocene, and Eocene strata are surrounded by a "sea" of broken and sheared rocks of the Franciscan Complex (Clark, 1940). The Franciscan Complex here consists of layers of melange and coherent sequences of mudstone and feldspathic arenite or graywacke; the melange contains fossils as young as Late Cretaceous (Albian through Cenomanian) according to Guewa (1975, p. 107). Guewa (1975) noted that one of the coherent units-the White Rock unit of Late Cretaceous age-is depositionally overlain by melange. He attributes this contact relation- ship to emplacement of the melange as a gravity mass flow during Late Cretaceous time or later. The Upper Cretaceous strata forming the islands consist of sandstone, shale, and minor conglomerate that is in places conformably or disconformably overlain by Martinez (lower - Paleocene), - Meganos (upper Paleocene), and Capay (lower Eocene) sandstone (Clark, 1940). Locally this sequence of sediments deposi- tionally overlies the Franciscan Complex (Guewa, 1974, p. 39, 66). We ask, then, are these Cretaceous and early Ter- tiary sequences mesoallochthonous or neoautochthon- ous? The field evidence bearing on this question seems to be somewhat equivocal. Guewa (1974, p. 39) noted that one small outlier consisting of strata similar to the fossiliferous Upper Cretaceous rocks is overturned. He suggested that this section was deposited on melange and then partly incorporated into the melange. How- ever, other sections of the Upper Cretaceous and Ter- tiary rocks do not give any indication of substantial postdepositional displacement, hence in Guewa's opinion were probably deposited after formation of the subja- cent melange. Probably additional field evidence will be needed to resolve this question. However, on the basis of the map and contact relations depicted and described by Clark (1940) and Guewa (1974, 1975), it seems likely that the Late Cretaceous strata were deposited on melange, then dismembered and engulfed within melange in early Tertiary, hence are probably mesoal- lochthonous. BANDON AREA In southern Oregon, the closest lithologic correla- tive of the Franciscan Complex is the Otter Point For- 22 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE mation. The Otter Point consists of interstratified mudstone, graded sandstone (arkosic to lithic wacke), pebbly mudstone, volcanic breccia, bedded chert, con- glomerate, and pillowed lava flows (Koch, 1966, p. 36- 43). The presence of numerous detached garnetiferous blueschist blocks, universal shearing, and the absence of preserved bedding (Beaulieu, 1971, p. 30) imply that the Otter Point is in fact a melange (Dott, 1971, p. 27; Baldwin and Beaulieu, 1973, p. 13), hence should be con- sidered a northern continuation of the Franciscan Com- plex. The Otter Point Formation is regarded as Late Jurassic by the presence within it of a sparse fauna that includes Buchia piochii (Gabb) (Koch, 1966, p. 42-43). Cretaceous fossils have been found in other formations in the area, for example, the Upper Cretaceous Cape Sebastian Sandstone and Hunters Cove Formation of Dott (1971, p. 31-42) and the Lower Cretaceous Rocky Point Formation and underlying Humbug Mountain Conglomerate (Koch, 1966, p. 43-48). The Cape Sebas- tian Sandstone unconformably overlies the sheared Otter Point Formation (M. C. Blake, Jr., written com- mun., 1979). But as the contact between the Humbug Mountain and Rocky Point Formations and Otter Point melange is apparently tectonic (Koch, 1966, p. 43; Dott, 1971, p. 57), the episode or episodes of deformation that produced the Otter Point melange must be pre-Cape Sebastian Sandstone and could be post-Rocky Point Formation in age. Along the coast, the type Humbug Mountain Con- glomerate is gradationally overlain by the Rocky Point Formation; here the two formations are composed of in- terbedded conglomerate, mudstone, and sandstone, and aggregate about 2,750 m (9,000 ft) in thickness (Koch, 1966, p. 43-50). The two formations appear to be Valan- ginian in age (Koch, 1966, p. 44-45). The Humbug Mountain unconformably overlies the Upper Jurassic Galice Formation, composed of black argillite, slaty or phyllitic mudstone, and gray sandstone (Dott, 1971, p. 11, 21-28). Overthrusting of the Humbug Mountain and Rocky Point Formations by the Colebrooke Schist (Col- eman, 1972, pl. 1) shows that their deposition was fol- lowed by one and possibly more than one episode of al- lochthonous deformation. The youngest rocks in the Bandon area that have been penetratively deformed (at least locally) and tec- tonically juxtaposed against the Otter Point melange are those of the Roseburg Formation of Baldwin and Beaulieu (1973) and Baldwin (1974, p. 60), of early Ter- tiary age. Their Roseburg includes strata classified by Turner (1938, p. 5) as the lower member of Diller's (1898) Umpqua Formation. As defined, their Roseburg consists of a lower member of basaltic flows, pillow lavas, volcanic breccias and interflow sediments, chiefly conglomerate and tuffaceous sandstone, and an upper member of tuffaceous siltstone and rhythmically bedded sandstone. The formation could aggregate 3,700 to 4,600 m (12,000 to 15,000 ft) in thickness (Baldwin, 1974, p. 6- 8). According to Baldwin (1974, p. 8-10), the Roseburg includes strata that at various localities have yielded a fauna of Paleocene, early Eocene, and rarely, Late Cre- taceous age. In his opinion, rocks containing the Late Cretaceous fossils may have been tectonically in- troduced into the Roseburg. The Roseburg is isoclinally folded and thrust over severely deformed strata in part considered to be the sedimentary upper member of the formation (Baldwin and Beaulieu, 1973, p. 20), and in places, as at Coos Bay, the Roseburg (nee lower Umpqua) "displays sev- eral melange features" (Beaulieu, 1971, p. 30). Steeply folded Roseburg strata also appear to be overthrust by the Colebrooke Schist (Baldwin and Lent, 1972, p. 125; Beaulieu and Hughes, 1976, p. 13). The Roseburg is he- rein considered to be autochthonous (fig. 9), although it is apparent from the descriptions given that parts of the formation might be allochthonous. The Colebrooke Schist is composed of thin-bedded shale, sandstone, and associated minor pillow lavas, tuff, and chert metamorphosed at temperatures and pressures intermediate between those of the blueschist facies and greenschist facies (Coleman, 1972, p. 29, 43). This formation occupies an area of about 260 km" (100 mi) in southwestern Oregon, much of which, according to Coleman (1972, p. 56), represents an allochthonous sheet thrust eastward over the Otter Point, Dothan, and other pre-Tertiary formations in the area. The folded and beveled Roseburg and pre-Tertiary rocks are unconformably overlain by a mildly deformed conglomerate, - sandstone, - and siltstone, - the Lookingglass Formation of Baldwin (1974), which con- tains a Penutian (late early Eocene) fauna (Baldwin, 1974, p. 12-16). Strata assigned to the Lookingglass by Baldwin (1974, p. 16, and geologic map) were considered by Turner (1938, p. 5) to be the upper member of the Umpqua. They also include the beds at one locality (Boulder Creek) mapped as Umpqua by Coleman (1972, p. 19, 28, pl. 1) that Baldwin (1974, geologic map) shows depositionally overlying the Colebrooke Schist, its basal decollement, and part of the subjacent imbricated ser- pentinite sheet. The structural fabric of the Colebrooke Schist suggests multiple episodes of penetrative deformation, the most recent of which Coleman believed to be Late Cretaceous (1972, p. 19, 29, 56). However, the field rela- tions of the Colebrooke to the Roseburg and Lookingglass appear to bracket the age of final tectonic emplacement of the Colebrooke Schist as early Eocene, as suggested by Baldwin and Lent (1972, p. 125) and Beaulieu and Hughes (1976, p. 13). MAIN ELEMENTS OF MELANGES AND THEIR BEARING ON PLATE-TECTONIC HISTORY 283 MAIN ELEMENTS OF MELANGES AND THEIR BEARING ON PLATE-TECTONIC HISTORY The origin of the melanges of the western part of the North American plate and their bearing on plate- tectonic history from Cretaceous through Tertiary time involves the formation of three main elements: (1) Cre- taceous through early Tertiary melanges of the Francis- can Complex, (2) the Eocene Oregon-Washington bor- derland, and (3) middle(?) Tertiary melanges of the Olympic Peninsula of Washington. Here, attention is fo- cused on whether the age of the most recent episode of allochthonous deformation during which part of the Franciscan Complex was formed coincides with the con- version of a transform boundary to a subduction zone at a northwestward-migrating triple junction, or spans the entire time during which the subduction zone was ac- tive. The topic of the Oregon-Washington borderland represents a departure from the main subject of this paper, that is, the tectonic implications of the melanges. Yet an understanding of the origin of the borderland is necessary in order to link the Cretaceous through early Tertiary history of the Franciscan Complex with the middle(?) Tertiary history of the Olympic Peninsula. In the third section of the discussion, attention is focused on whether the age of deformation of the Olym- pic Peninsula coincides with the age or ages of momen- tary formation and transit of a Humboldt-type triple junction between the Pacific, Juan de Fuca, and North American plates, as deduced from the magnetic record of the ocean floor. FRANCISCAN COMPLEX AGE OF DEFORMATION RESULTING IN THE FRANCISCAN COMPLEX The internal disruption and deformation character- istic of the Franciscan Complex has been attributed to (1) processes occurring at or above an active subduction zone (Hamilton, 1969) and, alternatively, to (2) proces- ses occurring at or near a triple junction whose migra- tion is associated with conversion of a transform plate boundary to a subduction zone (Fox, 1976). According to the subduction-zone hypothesis, deformation continues along the entire extent of the subduction zone for its life. According to the triple-junction hypothesis, defor- mation is episodic, occurring only in the immediate vic- inity of the triple junction. Further, because of the mig- ration of this triple junction, the age of formation is progressively younger along its course. We ask, then, was the formation of the Franciscan Complex a continu- ous process or was it an episodic process? If episodic, was all or most of the complex synchronously formed, or was the deformation that produced the complex diac- hronous, becoming younger northwestward? The fact that units within the Franciscan Complex can be classified as allochthonous, mesoallochthonous, autochthonous, and neoautochthonous strata, a division made by Hsu (1969), implies that deformation was an episodic process, interrupted at least locally by periods of erosion and sedimentation. The age of the terminal episode of deformation is closely bracketed within Late Cretaceous (Campanian) in the Morro Bay area and early Eocene in the Bandon area. The youngest deposits presumed to be mesoal- lochthonous in the Covelo-Clear Lake area are late Paleocene, and possibly early Eocene at Rice Valley, and late Paleocene and early Eocene at Covelo. The coastal-belt rocks immediately to the west also are at least locally as young as Early Eocene. The age of this terminal episode of allochthonous deformation varies from place to place and apparently becomes progressively younger to the northwest (fig. 11). The simplest history consistent with the data (fig. 11) is as follows: beginning in Late Cretaceous, a diac- hronous upheaval producing melange or broken forma- tion and the younger part of the Franciscan Complex progressed northwest along the margin of the continen- tal plate at about 4 em/yr, reaching central California in early Eocene time. The age of the deformation of the Eocene coastal belt and rocks in the Covelo area and Eocene(?) rocks in Rice Valley is not narrowly limited by the ages of younger neoautochthonous deposits. Conceivably, in these areas this event could be younger than early Eocene, implying that its rate of northwestward migra- tion was less. On the other hand, the velocity could be greater if the Franciscan Complex in the Morro Bay area has been telescoped by right-lateral movement ex- ceeding the 315 km of Neogene and later offset recog- nized by Matthews (1973) on the San Andreas fault. The Campanian and early Tertiary event probably did not extend southeastward to the Los Angeles low- land. Crystalline rocks forming the basement of the Los Angeles lowland were beveled in middle Cretaceous time, forming the "Los Angeles erosion surface" (fig. 11), and were unconformably overlain, beginning in the Cenomanian, by a sequence of strata that, though folded and faulted, have apparently not been affected by a re- gional episode of penetrative allochthonous deformation (Woodford and Gander, 1977). The continental borderland west of the Los Angeles lowland is underlain in places by the Catalina Schist (Bailey and others, 1964, p. 93), in other places by Cre- taceous (Maestrichtian?, Campanian, Coniacian, and Cenomanian) and younger strata (Paul and others, 1976, p. 16). Neither the contact relation of the schist to the Cretaceous sediments nor the relation of the basement 24 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE rocks of the borderland to those of the Los Angeles low- land has been established with certainty (Howell and Vedder, 1981). According to Howell and Vedder (1981), the border- land is composed of four terranes, probably represent- ing large structural blocks. By northwest-directed lat- eral movement, the blocks have been individually dislo- cated with respect to each other and collectively dislo- cated with respect to the Los Angeles lowland. Hence, deformational episodes affecting the borderland may not be synchronous with those affecting the Los Angeles lowland. Howell and Vedder (1979, 1982) noted that "middle Cretaceous and late Paleocene hiatuses in sedimentation and concurrent regional lapses in mag- matism may represent times of transform faulting that interrupted subduction." Until the movement history of the borderland is accurately reconstructed, the transi- tions from transform faulting to subduction implied by this statement cannot be placed in context with episodes of allochthonous deformation sensed in the Morro Bay area to the north. That there were episodes of allochthonous deforma- tion preceding the Campanian and early Tertiary event is implicit in the characterization of the Las Tablas unit of Cowan and Page (1975), the "broken formation A" of Hsu (1969), the Atascadero Formation of Hart (1976), and similar units as mesoallochthonous. Although accu- rate definition of these earlier deformational episodes is not yet feasible, available information is sufficient to at least suggest their existence and place rough limits on their age and extent. In northern California, the latest Cretaceous and early Tertiary coastal belt is presumed to be chiefly me- soallochthonous and deposited after an episode of pene- trative deformation that occurred in Cenomanian and Maestrichtian time. This Late Cretaceous episode (fig. 11) probably postdates the Hauterivian to Cenomanian sequence at Rice Valley and the Late Jurassic to Early Cretaceous (Valanginian) Marmolejo Formation of Taliaferro (1944) and the Toro Formation of the Morro Bay area. It must predate the early Tertiary Roseburg Formation and Covelo outlier, the Paleocene rocks at Rice Valley, the "broken formation A" of Hsu (1969), the Las Tablas unit, and the Atascadero Formation. At Rice Valley, this episode of allochthonous deformation probably occurred during the time represented by the unconformity between Cenomanian and Paleocene rocks. A middle Early Cretaceous episode of melange for- ~) _ mation in northern California also seems required, for / most of the melanges there contain only a comparatively abundant Tithonian through Valanginian fauna without admixture of younger fossils (Blake and Jones, 1974, p. 350). The middle Early Cretaceous event could also ac- count for pre-Marmolejo development of melange in cen- tral California and perhaps at Baja, Calif. (fig. 11). The age of melange at Baja is not known with any certainty, however. At one locality on Cedros Island in Baja, Calif., the latest Jurassic through Early Cretaceous Eugenia Formation reportedly includes blueschist de- tritus believed to have been derived from the nearby melangelike Cedros Formation of probable Late Juras- sic age (Kilmer, 1977). Potassium-argon ages of white mica and blue amphibole from blueschist at Cedros Is- land, however, range from 94.4+4 to 110+2 m.y. (Suppe and Armstrong, 1972). If not thermally retrog- raded, these ages indicate metamorphism and possibly formation of melange in or after Turonian time, that is, after deposition of the Eugenia Formation. This middle Early Cretaceous episode could also be responsible for some of the structural disruption of the allochthonous terranes of the San Juan Islands. The age or ages of these rocks has not been firmly established. Some parts contain fossils as young as Cenomanian and Valanginian age (J. T. Whetten, written commun., 1977). In summary, the development of the Franciscan Complex and allied rocks was apparently an episodic process. In some areas, such as the Morro Bay area, the evidence suggests that there were several episodes of deformation. The final episode was apparently diachron- ous; earlier events may have also been diachronous, but the data for them is less compelling. The age of much of the complex probably decreases systematically to the northwest, ranging from Late Cretaceous (Campanian) - at Morro Bay, California, to early Tertiary (Eocene) in northern California. Within this geographic span, how- ever, parts of the complex formed prior to Campanian time and were unaffected by more recent episodes of penetrative deformation may represent discrete ter- ranes. CAUSE OF ALLOCHTHONOUS DEFORMATION THAT CREATED THE FRANCISCAN COMPLEX The structural fabric of the Franciscan Complex may have been produced through gravity thrusting and slid- ing away from one or more triple junctions that migrat- ed northwestward along the interface between the North American plate and the oceanic plates to the west. The chief bases for this hypothesis, as originally proposed (Fox, 1976), were (1) the existence of triple Junctions of the required dynamic type is implicit in the plate-tectonic theory; and (2) the structural fabric of the Franciscan Complex is more compatible with shearing at low confining pressure inherent in this process than the high confining pressures seemingly required by the MAIN ELEMENTS OF MELANGES AND THEIR BEARING ON PLATE-TECTONIC HISTORY 25 subduction theory; and (3) this process provides a means of recovering and stirring exotic clasts of eclogite and blueschist into the melange. To these reasons a fourth may now be added, namely, that the deformation that produced the Franciscan Complex was apparently episodic and, where best dated, was diachronous. If the premise of episodic deformation is accepted, the age data schematically projected in figure 11 provide a means of identifying and tracking the causal triple junctions. The Late Cretaceous and early Eocene defor- mational event is the track of a Humboldt-type triple junction that migrated northwestward along the plate margin at a rate of roughly 4 em/yr. At any given time, the margin of the North American plate was bordered by a transform fault to the northwest and by a subduc- tion zone southeast of this triple junction. Hence the northwest transit of the triple junction marks the end of a strike-slip-dominated tectonic regime and the begin- ning of a subduction-dominated tectonic regime. Both the middle Early Cretaceous and middle Late Cretaceous events are presumed to represent north- westward transits of Humboldt-type triple junctions (fig. 11), as yet too obscure to justify any firm conclusion as to their precise age or rate of northwestwardly mig- ration. Tentatively, they appear to have moved some- what faster than the Late Cretaceous (Campanian) and early Tertiary event of central and northern California (fig. 11). The Campanian and early Tertiary event marks a transition from what was previously strike-slip-domi- nated to subduction-dominated tectonism along the con- tinental plate margin. Presumably, each of the preced- ing episodes of allochthonous deformation do also. Inter- vening episodes in which the subduction zone was con- verted to a strike-slip transform fault could be episodes during which a Mendocino-type triple junction (Fox, 1976) transited the margin of the continental plate, or episodes during which a change in plate motion was coupled with a ridge jump. Cooper and others (1976) showed that the lithos- phere underlying the eastern Bering Sea basin is a frag- ment of oceanic plate that broke away and was trapped as the subduction zone at the northern edge of the Kula plate jumped south to its present location at the Aleu- tian trench. They estimated the date of this event to be approximately 70 Ma (Cooper and others, 1976, p. 1125). Presumably this event was part of a general reorganiza- tion of the boundaries of the Kula plate that roughly coincided with the middle Late Cretaceous (approxi- mately 75 Ma) transition along the California coast from subduction to strike-slip faulting. If so, that transition was probably precipitated by a ridge jump (fig. 11), rather than by transit of a Mendocino-type triple junc- tion. On the basis of the spreading history of the Pacific plate, Larson and Chase (1972) postulated a 2,000-km southeastward jump of the Kula-Farallon ridge about 100 Ma. This jump probably entailed a jump of the Kula- Farallon-North American triple junction, hence coin- cides with a transition from subduction to strike-slip faulting along a segment of the western edge of the North American plate (fig. 11). ; The following model (fig. 12) is proposed: Beginning in latest Jurassic or Early Cretaceous time, the spread- ing ridge between two oceanic plates first intersected the margin of the North American plate, forming a tri- ple junction with the transform fault to the northwest and the subduction zone to the southeast. Subsequently, this triple junction migrated northwest along the margin of the continental plate at a rate comparable to that suggested in figure 11, that is, about 5% em/yr (or more, if, as is likely, pre-Late Cretaceous elements of the Franciscan Complex have been telescoped by faulting an amount greater than the 315 km restored in fig. 11). The space problem at the triple junction caused by the encroachment of the subducting oceanic slab on the transform-bounded edge of the continental plate was re- lieved by the plowing up of the lip of the continental plate and the consequent detachment of slabs and sheets of the Late Jurassic and Early Cretaceous turbidite de- posits underlying the continental slope. These slabs cas- caded or were rammed inland, stacking melanges, bro- ken formations, and allochthonous imbricated masses of rocks on the floor of the foreare basin to the east, which was locally underlain by Late Jurassic (Knoxville For- mation) and Early Cretaceous rocks deposited over oceanic basement. Rip-ups from this autochthonous floor were locally incorporated in the melanges. The subjacent autochthon was depressed beneath the weight of the melange and, being under compression from the west, broke along the crustal flexure that formed near the eastern extremity of the allochthonous mass, and was then partially overridden by the Knoxville Forma- tion and the Great Valley sequence of the craton on a zone of thrust and high-angle reverse faults (the first phase of thrusting on the Coast Range thrust of Bailey and others, 1970). This scenario was apparently re- peated with varying degrees of severity during middle Late Cretaceous and again in Late Cretaceous (Campa- nian) and early Tertiary time. In these later episodes, strata laid down on the melange were themselves dis- rupted and imbricated. COMPARISON OF CRETACEOUS THROUGH EARLY EOCENE TECTONIC REGIMES AS PREDICTED BY THE TRIPLE-JUNCTION HYPOTHESIS WITH CONCEPTS SUGGESTED BY OTHER EVIDENCE The plate configuration from which the Early Cre- taceous Humboldt-type triple junction evolved is un- 26 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE known. Ernst (1965, p. 905) suggested that the Late Jurassic elements of the Franciscan "Group" represent trench deposits, a suggestion adopted as a working hypothesis by Hamilton (1969). This implies that in Late Jurassic and Early Cretaceous time, central western North America was bounded by a subduction zone or zones. Plutonic rocks (fig. 11) that could be related to this subduction zone include both Late Jurassic and Early Cretaceous plutonic rocks with representatives in northern California (Lanphere and others, 1968) and central California (Evernden and Kistler, 1970, p. 623). Further definition of the Jurassic setting is required be- fore the nature of the transition, which could involve as yet undefined plates, from Late Jurassic subduction to the Cretaceous melange-forming transit of the Kula- Farallon spreading ridge can be specified. The timing and geographic extent of the Late Cre- taceous subduction zone required by the scenario given in figure 11 correlates roughly with the plutonic episode Evernden and Kistler named the Cathedral Range in- trusive epoch (1970, p. 623). The Early Cretaceous sub- duction zone correlates less satisfactorily with the Hun- tington Lake intrusive epoch. The Yosemite intrusive epoch and the age of plutonic rocks of the Trinity Moun- tains (Lanphere and others, 1968) do not correlate with any subduction episode represented in the boundary re- gimes (fig. 11). This may imply that marginal parts of the North American plate have been dislocated by Wrangellia strike-slip faulting from the Early Cretaceous volcanic ares of the craton and subsequently rejoined elsewhere to the craton as tectonostratigraphic terranes. The relations depicted in the schematic chart (fig. 11) imply that a strike-slip regime prevailed along the western margin of the North American plate for about 20 m.y. in Late Cretaceous and early Tertiary time. This conclusion is vital to the interpretation of plate geometry developed in later sections, hence corrobora- tion independent of the triple-junction hypothesis is par- ticularly desirable. Snyder and others (1976) postulated development of a strike-slip regime in latest Cretaceous and early Tertiary time, on the basis of timing of offset on the proto-San Andreas fault, which seemingly re- quires a period of strike-slip faulting roughly 50-70 Ma (1976, p. 103). Development of the transform through a rise-trench encounter was followed by a cessation of are magmatism to the east. Their compilation of ages of igneous rocks indicates that there was a null period in magmatism about 65-40 Ma. They attributed the appar- ent 5- to 10-m.y. timelag in cessation and renewal of magmatism to the time required for the trailing edge of the subducted slab to pass and the leading edge of the succeeding slab to reach appropriate depth for melting to begin (Snyder and others, 1976, p. 92, 103). The Late Cretaceous to early Eocene transit of a Humboldt-type triple junction inferred from the ages of melanges is roughly synchronous with the transit of the Source of Great /Vlll|y sequence > a (6 % <.co « €, A ® a av 8 ax voor oy a w NTIN‘ENTALV PLATE: a 6s £1.* £ h4>< a a a * s F3 cov FIGURE 12.-View to northwest showing transit of Early Cretaceous Humboldt-type triple junction with accompanying formation of melange and a proto-Coast Range thrust (CRT) to east. Western source of detritus in the Franciscan Complex is a tectonstratigraphic terrane (Beck and others, 1980), such as Wrangellia (Jones and others, 1977), locked in an oceanic plate and drifting with it to northwest relative to craton and source area of the Great Valley sequence. MAIN ELEMENTS OF MELANGES AND THEIR BEARING ON PLATE-TECTONIC HISTORY 27 Kula-Farallon-North American triple junction originally proposed by Atwater (1970, p. 3531) through extrapola- tion of present rates of plate movement back to that time. The rate of right-lateral offset between the Pacific and North American plates of 6 em/yr adopted by Atwa- ter (1970) in her constant-motion model was based prin- cipally on Larson and others' (1968) estimate of the av- erage spreading rate for the past 2 m.y. across the ex- tension of the East Pacific Rise into the Gulf of Califor- nia. The slip rate was amended slightly by Minster and others (1974) to 5% em/yr on the basis of simultaneous solution of rotation vectors between 11 major plates. Atwater and Molnar (1973) deduced greatly differ- ent slip rates between the North American and Pacific plates throughout most of Tertiary time by chaining spreading rates between the North American, African, Indian, East and West Antarctic, and Pacific plates. On this basis, they suggested that the rate of motion be- tween the Pacific and North American plates averaged about 5 em/yr between 29 and 21 Ma, 1.3 em/yr between 21 and 10 Ma, 4 em/yr between 10 and 4.5 Ma, and 5.5 cm/yr since 4.5 Ma. While cautioning that the details should not be taken literally, in particular noting weak- nesses in the 21-m.y. and 29-m.y. reconstructions, they concluded that "an average relative Tate of about 2 ecm/ yr between 38 and 10 Ma is almost certainly realistic" (Atwater and Molnar, 1973, p. 142). If the rates postulated by Atwater and Molnar (1973) are approximately correct, the transition from strike-slip faulting to subduction at any given place along the plate margin would be expected to have oc- curred much more recently than is implied by the latest Cretaceous through middle Eocene age of the younger melanges of the Franciscan Complex. If the triple-junc- tion hypothesis for the origin of the melanges is correct, then the deformational record of those melanges suggests that the average rate of movement from Cre- taceous to the present approaches the present rates of - plate movement and in general validates Atwater's (1970) direct extrapolation of those rates back to the Cretaceous ("constant-motion model"). Conversely, if the rates of offset proposed by Atwater and Molnar (1973) are approximately correct, the triple-junction hypothesis must be wrong. EVIDENCE FROM THE FRANCISCAN COMPLEX CONTRADICTORY TO THE TRIPLE-JUNCTION HYPOTHESIS Resistance to the hypothesis that melanges of the Franciscan Complex originated through tectonism at migrating triple junctions as advanced by the author (Fox, 1976) commonly focuses on two questions: (1) Do any of the ophiolite-Knoxville Formation-Cretaceous se- quences found as numerous discrete terranes structur- ally enclosed by Franciscan Complex actually represent exposed elements of the autochthonous pre-melange basement, or are they all part of an allochthonous upper plate? (2) Does the triple-junction hypothesis provide a structural setting in which the exotic blocks of high- grade blueschist and the more extensive terranes of low-grade blueschist could form? The contact relations between internally deformed rocks of the Franciscan Complex and the stratally intact ophiolite-Knoxville-Cretaceous sequences that are com- monly referred to the Great Valley sequence have been and remain a vexing problem. In part because of inher- ent stratigraphic and structural complexity and in part because of poor exposure, geologic opinion on this prob- lem has been dominated by successive reigns of various ruling theories which may or may not have a sound ob- servational basis. Three such theories can be readily dis- tinguished: (1) the Knoxville Formation is younger than, and grades downward into, the Franciscan "Formation" of former usage (Taliaferro, 1943); (2) the Great Valley sequence is broadly coeval with the Franciscan as- semblage, and the two groups of rocks have been jux- taposed by faulting (Bailey and others, 1964; Irwin, 1957); (3) the various isolated remnants of the Great Valley sequence are parts of a formerly continuous sheet underthrust by the Franciscan assemblage (Bailey and others, 1970). Contributing to the credibility of the third hypothesis is the fact that in broad areas of central California, Upper Cretaceous rocks contiguous with, and commonly correlated with, the Great Valley se- quence do indeed structurally overlie Franciscan rocks. This relation is perhaps best illustrated in the area of the Diablo antiform (fig. 10) (Bailey and others, 1964, p. 154). Detailed maps (Dibblee, 1973, 1975) show that a conformable sequence of stratally intact Upper Cretace- ous to Eocene rocks wrap around Franciscan rocks ex- posed in the core of the southeast-plunging nose of this fold. And as previously noted, north of the Diablo anti- form, both Lower and Upper Cretaceous rocks of the Great Valley sequence, sensu stricto (that is, as exposed along the western flank of the Great Valley), have struc- turally overridden the Franciscan Complex along a re- verse fault or thrust fault. Proponents of the subduction-complex theory, ex- trapolating from these relations, argue not unreasona- bly that all the isolated patches of Great Valley-like rocks to the west structurally overlie the Franciscan Complex; if the local contact is in fact a high-angle fault or if it can be demonstrated that the Franciscan struc- turally overlies the Great Valley-like rocks, then the contact is interpreted as a folded or overturned thrust fault with Franciscan in the lower plate. In some recent compilations, all patches of Great Valley-like rocks are 28 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE shown as klippen, regardless of the present attitude of the contact and of whether the original mapper believed the contact to be depositional or structural. This dogma is contradicted by both the conception of the Franciscan Complex as being part mesoal- lochthonous and partly allochthonous (Hsu, 1969) and by the implication of the triple-junction hypothesis that the Franciscan could overlie a basement composed in part of Great Valley-like rocks. What is in question, then, is the structural and depositional relations of the isolated areas of Great Valley-like rocks to the surrounding Franciscan Complex. Field evidence is commonly am- biguous because the contacts between these rocks are typically poorly exposed and either are, or have been modified by, high-angle faults. The presence of Francis- can structurally overlying Great Valley rocks locally has been reported, as at the St. John Mountains thrust fault, on which Franciscan rocks have been thrust over the Knoxville Formation a distance of at least 8 km (5 mi) (Weaver, 1949, p. 137; Bailey and others, 1964, p. 157). Stratigraphic differences between the rocks in these "outliers" and the Great Valley sequence, sensu stricto, have been noted, leading some workers to post-. ulate that the outliers were deposited in local basins of deposition separate from the main body of the Great Valley sequence (Maxwell, 1974). Moore and Karig (1976) postulated that the outliers might represent sedi- ments accumulated in local basins on the trench slope adjacent to, or within the surface trace of, the subduc- tion zone. In their view, these sediments may have been deposited on a part of the accretionary wedge previ- ously converted to melange, and subsequently or con- currently deformed by distributed thrusting as subduc- tion continued. This concept, which was echoed by How- ell and others (1977) and by Underwood (1977), has merit in that it does recognize and, to a degree, accounts for the contact and structural relations peculiar to the mesoallochthonous deposits. In summary, observational and theoretical basis is now sufficient to reopen the question of the relations, both structural and stratigraphic, of the outliers of Great Valley-like rocks to the Franciscan Complex. These outliers could include: (1) klippen of the Coast Ranges thrust, (2) an autochthonous basement that is structurally overlain by the Franciscan Complex, (3) mesoallochthonous slabs that depositionally overlie older melange, and are structurally overlain by younger melange; and (4) neoautochthonous strata that deposi- tionally overlie the Franciscan Complex. The origins of (1) the regionally metamorphosed generally low grade blueschist formations, such as the South Fork Mountain Schist (Blake and others, 1967) and the Colebrooke Schist (Coleman, 1972), and (2) the high-grade blueschist and eclogitic exotic blocks are two of the very puzzling riddles of the Franciscan Complex. It may be significant that the potassium-argon ages of the exotic blocks reported by Coleman and Lanphere (1971) are older at any given place than the admittedly speculative middle Early Cretaceous event (fig. 11). This could imply that these blocks formed during a period of strike-slip-dominated tectonism preceding that event. Possibly they originated as blocks of material that were scraped off the wall of the oceanic plate at its transform boundary, metamorphosed at local sites of high contact pressure, and left as horses in the fault zone, then elevated and incorporated in melange during the ensuing passage of the Humboldt-type triple junc- tion. The rubidium-strontium isochron age of 125 +18 m.y. for the Colebrooke Schist (reported as 128 +18 m.y. by Coleman, 1972, p. 54, and recalculated by Lan- phere and others, 1978, using refined decay constants) compares with the probable metamorphic age of 115-120 m.y. of the South Fork Mountain Schist (Lanphere and others, 1978). These ages straddle the time of passage of the middle Early Cretaceous triple junction as conjec- tured in figure 11, suggesting that metamorphism of these rocks was related to this episode of allochthonous deformation. The questions raised here bear directly on the val- idity of the triple-junction concept. Even assuming that this concept is correct in principle, the tectonic scenario given in figure 11 for the Early and early Late Cretace- ous must be regarded as tentative because of the ex- treme fuzziness of the dating of the melanges of those ages, and because of the possibility of undetected lateral translation of the data points in part, perhaps, as dis- crete tectonostratigraphic terranes. The speculative nature of that scenario can perhaps be accentuated by framing some of the more trouble- some questions that occur to the author: (1) Were there in fact two diachronous episodes of allochthonous defor- mation in Cretaceous time prior to the Campanian, or only one, possibly with a more complex path? (2) What is the significance of the fact that exotic clasts of blueschist and eclogite were mainly introduced into the melange during the deformational events prior to the Campanian? OREGON-WASHINGTON BORDERLAND AGE, DISTRIBUTION, AND PLATE-TECTONIC SETTING OF THE BORDERLAND The early and middle Eocene age of the allochtho- nous oceanic crust represented by basalts of the Metcho- sin, Crescent, and Siletz River Volcanics, the basement of the Oregon-Washington borderland, roughly equates MAIN ELEMENTS OF MELANGES AND THEIR BEARING ON PLATE-TECTONIC HISTORY 29 with the expected time of transit past northwestern California and western Oregon of the Kula-Farallon spreading ridge (fig. 11). Presuming that the basalts are indeed oceanic crust, their restricted age indicates that they represent a block sliced away from the near-flank and perhaps the axial zone of a spreading ridge. That the spreading ridge was near the continental plate is confirmed by the 3-m-diameter quartz diorite boulders incorporated in sedimentary beds interlayered with Crescent Formation as reported by Cady (1975). Paleomagnetic data from the western Cordillera of North America suggest that the region consists of large crustal blocks which were rotated clockwise and trans- lated northwestward relative to the North American craton, as if caught and rotated like ball bearings in a wide zone of right-lateral shear (Beck, 1976). In this context, rocks of the Oregon-Washington borderland are remarkable and to a degree atypical, in that some, though relatively youthful, show very substantial rota- tions (Cox, 1957; Simpson and Cox, 1977) but no detect- able northward translation (Plumely and Beck, 1977; Beck and Burr, 1979, p. 178). Paleomagnetic data reported by Simpson and Cox (1977) indicate that after deposition the Siletz River Volcanics was rotated clockwise en bloc about 68° + 12°, the overlying middle Eocene Tyee Formation and re- lated rocks about 64°+16°, the late Eocene Yachats Basalt about 51°+33°, an Oligocene sill about 28°, and nearby Miocene basalt 0 +44°. Simpson and Cox propose two alternative tectonic models: (1) a 225-km segment of coastal Oregon originated as a small plate related to northward migration of the Kula-Farallon-North Ameri- can triple junction, then rotated clockwise about a pivot near its southern end; (2) a much longer block encom- passing what is now the entire Oregon-California bor- derland rotated clockwise about a pivot near its north- ern end. A variation of the first model better accords with the plate geometry required if the Campanian and early Tertiary episode of allochthonous deformation also rep- resents the transit of this triple junction. To document this opinion requires a slight digression from the main thread of this discussion but one necessary to put the melange-forming processes that occurred before and after formation of the borderland into better perspec- tive. The angular and, in places, even jagged outline of basement as inferred in figure 9 suggests that the bor- derland is an aggregation of numerous crustal fragments wedged into a reentrant in the continental plate, rather than a single coherent crustal block. If it is, the blocks must have been individually rotated clockwise after their formation, some as much as about 70°, to account for the paleomagnetic data. The north-trending fissures that were the source of part of the Siletz River Vol- canics (Snavely and others, 1968, p. 480) therefore must have trended about N. 70° W. when formed. These fis- sures thus reflect a tensional axis oriented parallel to that of the Kula-Pacific spreading ridge rather than the supposedly nearby but northeast-trending Kula-Faral- lon ridge. On the basis of Duncan's (1977) potassium-argon ages, which show that the Siletz River Volcanics ranges in age from 49.3 to 54.7 Ma, it appears that anomaly 22 (53 Ma) and possibly anomaly 23 (55 Ma) are rep- resented within the rotated blocks of oceanic crust mak- ing up the basement of the borderland. But study of the paleomagnetic map of the northeastern Pacific (fig. 13) shows that at the time of anomaly 24 (56 Ma), the eas- ternmost part of the Kula-Pacific spreading ridge termi- nated at a ridge-ridge-ridge triple junction some 1,300 km west of the North American plate. Yet it also appears from this map (fig. 13) that the Kula-Pacific spreading ridge either vanished (Byrne, 1978; Geotimes, 1978, p. 24) or jumped some undefined but considerable distance northward shortly after for- mation of the Y-shaped anomaly labeled "24?" located at about 54° N., 158° W. The ridge jump is the preferred hypothesis because it better rationalizes the presence of the Kula-Pacific spreading orientation with the Siletz River Volcanics and, as will be discussed later, exten- sional faulting and rift-related volcanism in British Col- umbia and northern Washington. If there was such a jump, the Y-shaped anomaly 24? could be the fossilized triple junction between the Kula, Pacific, and Farallon plates formed immediately before the jump. ORIGIN OF THE BORDERLAND THROUGH PLATE COLLISION The fragmentation and differential rotation of the allochthonous blocks forming the borderland indicates that their emplacement was forcible. Could the border- land have formed through a collision of oceanic and con- tinental plates? Atwater (1970, p. 3531), in her constant- motion model (fig. 1), assumed that in the Late Cretace- ous and early Cenozoic, the motion of the Pacific plate was parallel to the edge of the North American plate and that the motion of the Kula and North American plates had a significant component of strike slip. Indeed, the Kula-North American motion may have been essen- tially strike-slip, as judged by the evidence summarized of a time-transgressive change in tectonism associated with northwestward advance of the subducting Farallon plate. But the paths of the Pacific and North American plates, not then in mutual contact, could have been slightly or even markedly convergent. 30 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE If the paths of the North American and Pacific plates were convergent, a collision would inevitably fol- low, and the borderland could be a product of that colli- sion. This concept accommodates the paleomagnetic evi- dence of a ridge jump, the evidence of tectonic fragmen- tation and rotation of the borderland, and the deforma- tional features east of the borderland. The following scenario is suggested (fig. 14). As the Pacific plate ap- proached the North American plate through plate growth and path convergence, the shrinking Kula plate broke apart, causing the Kula-Pacific spreading ridge to jump northward (fig. 14C) and intersect the continental margin. Enlarged by the addition of part of the Kula plate, the Pacific plate sideswiped the North American plate, driving a widening wedge of recently formed oceanic crust inland (fig. 14D) and crunching adjacent rocks of the craton in northern California, Oregon, and Washington. The Pacific and North American plates were deflected by the impact of this collision, ultimately adopting paths parallel to their bounding transform 160° w | 1 faults. As they did, the allochthonous wedge was sheared off and fragmented. Caught between two major plates moving right laterally past each other, the frag- ments rotated clockwise and lodged against the North American plate. The change in spreading directions between the Pacific and Farallon plates recorded by the difference in trend of anomalies 22 and 21 (53-50 Ma) (fig. 13) may represent adaptation of the movement of the Farallon plate to the changing paths of Pacific and North Ameri- can plates. As a consequence of this change in spreading direction, several right laterally stepping fracture zones, including the Aja, formed along the new segment of Pacific-Farallon spreading ridge (the northeast-trend- ing segment that had been the Kula-Farallon spreading ridge). Relative movement between the Pacific and North American plates has assuredly been parallel to the transform between them since contact of the two plates in the area between the Pioneer and Murray fracture 140° W 130° W | | 60° N ALASKA 100 200 _ 300 KILOMETERS §§5.N . - || [[{HIT f | 22 20 16 15 13 50° N - - 14 \ mA \\ I % 't ( * j ® \ \ \ 45°N - \ ‘\ 20 E - 99 in I Z 3133” 40 N \ A é | 3 22 20 \ \#¥ 28 \\ l o 1 40° N | 1T FIGURE 13.-Paleomagnetic map of floor of northeastern Pacific Ocean (after Hilde and others, 1976, p. 223, except detailed inset, which is after Atwater and Menard, 1970, p. 448). Heavy lines indicate fracture zones, dashed where uncertain; light numbered lines indicate magnetic anomalies, dashed where uncertain. MAIN ELEMENTS OF MELANGES AND THEIR BEARING ON PLATE-TECTONIC HISTORY 31 zones after about 30 Ma. The adjustment of the mechanism which drives the plates that was required to bring the direction of their relative movement into con- «Seattle © x sa sae" NORTH AMERICAN PLATE + 3 ancouver Cp v Msn NMNpovre ond. . _. __ *_ » Portland G £ ay Cape . ._ _ ban. °. § fin as £ nik '' .* Mendocino . Francisco . * Guaymas - 47—5? - - ,A.nchorage ............ A 75 Ma "> Vancouver, *Seattlé . + NORTH AMERICAN PLATE «<. soo ome -ePortland /: !. .._. : Ch 5 Cape San - > tual =+ a *, + Mendocino: Francisco _ ' [Sif eee. * ermine og 6-64 nAnghorage PLATE B 56 Ma y T <4 « : «Seatle ." : - NORTH AMERICAN PLATE® 26 f -»..'..Pon.1lar)d""' 7 e. j j § .4Cape'_~u San 4.7; ¥ :_:?,; Mendocino . "' Ke Guaymafi "nAfichomge CS5 Ms PACIFIC PL‘éTE »Seattle . ... - ~ NORTH AMERICAN PLATE: s ancouver - - : A 69, 5 4 7G 9 - . «Portland . _. (e ie st e n e '~ Cape: . :'.' San _ fig. at fee ee * -:Mer?docino, © Francisco + * >* +-Guaymas. fy: PACIFIC PLATE D 51 Ma «". Vangoucet . »Seatile -* - NORTH AMERICAN PLATE] i - -+ »Portland / F '~ Cape:: ' San! : . '> ..>. ; Agi" | ~ Mefldfmin‘i. ©, Francisco ". ©_ * . Guaymas® . E 42 Ma formity with the bounding transforms by about 30 Ma probably began contemporaneously with the jump of the Kula-Pacific ridge about 55 Ma, continued through the change in spreading directions between the Pacific and Farallon plates at 53 to 50 Ma (anomalies 22 and 21), and was essentially complete before 30 Ma. The Earth's hot spots appear to constitute a fixed frame of reference by which absolute motion of the lithospheric plates may be defined (Minster and others, 1974). The only significant shift in direction of move- ment of the Pacific plate relative to the Hawaiian hot spot during early and middle Tertiary time occurred about 42 Ma, when the Pacific plate veered from a northward to a northwestward path (Dalrymple and Clague, 1976). During the earlier part of the Tertiary, the Pacific plate was apparently holding steady on course. The initial contact between Pacific and North American plates, if, as suspected, it occurred simultane- ously with the jump of the Kula-Pacific ridge about 55 Ma, is detectable in the magnetic record of the ocean floor some 13 m.y. before the Pacific plate finally veered away to the northwest. The path change at 42 Ma proba- EXPLANATION -&_&A.&A- Subduction zone-Sawteeth on upper plate; dashed where approximately located m Transform fault-Queried where uncertain. Arrows show relative movement =?==--- Spreading ridge-Queried where approximately lo- cated FIGURE 14.-Late Cretaceous and early Tertiary plate geometry west of North American plate. Spreading directions shown by thin ar- rows, plate movements relative to a fixed North American plate by broad arrows. Projection is oblique Mercator as in figure 3. A, About 75 Ma, Kula plate was moving right laterally past North American plate, Farallon plate was being slowly subducted, and Pacific plate was drifting toward North American plate. B, By 56 Ma, with movement continuing as in A, triple junction between Kula, Farallon, and North American plates had moved northwest- ward past "San Francisco." C, About 55 Ma, ridge between Kula and Pacific plates jumped northward, adding a large block of Kula plate to Pacific plate. D, About 51 Ma, spreading directions be- tween Pacific and Farallon plates changed, causing reorientation of ridge system. Aja (A.F.Z.) and Sila (S.F.Z.) Fracture Zones originated as ridge-ridge transform faults at this time. Motion of Pacific and North American plates was convergent, causing a sa- lient of more rigid oceanic crust to impinge on North American plate. E, As direction of movement between North American and Pacific plates shifted to right-lateral strike-slip about 42 Ma, wedge of Pacific plate that had impaled North American plate sheared off, and Pacific-Farallon ridge jumped northwestward to position shown by dashed lines. 32 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE bly marks the end of the period of convergence and colli- sion between the North American and Pacific plates. RECONCILIATION OF THE COLLISION HYPOTHESIS WITH OTHER TECTONIC FEATURES OF THE PACIFIC NORTHWEST The collision hypothesized here provides an expla- nation for several otherwise enigmatic structural fea- tures of probable Eocene age in Washington and adja- cent areas. The Devils Mountain fault (fig. 9), for which Whetten (1978) has postulated 60 km of left-lateral dis- placement, may represent a tear fault formed during the early phase of the collision when plate paths were most - convergent. The Straight Creek fault (fig. 9) and its probable continuation in Canada, the Hope fault, appear to be right-lateral strike-slip faults with displacement estimated at 200 km (120 mi) by Misch (1977). The trace *of the fault is interrupted by younger plutonic rocks but appears to truncate the Devils Mountain fault zone and to truncate or merge with the Olympic-Wallowa linea- ment. The Straight Creek fault may have been an active strike-slip fault during the later phases of the collision as the paths of the North American and Pacific plates approached parallelism and the salient of oceanic crust forming the borderland was torn away from the Pacific plate. The eastern continuation of the Devils Mountain fault could be marked by the southeastern part of the Olympic-Wallowa lineament of Raisz (1945) (fig. 9), a zone of structural weakness marked chiefly by en eche- lon anticlines and associated reverse faults. According to Bentley (1977), these features formed by draping of Miocene and younger rocks over rotated basement blocks. Alternatively, the lineament may represent a pre-Miocene fault zone separating basement rocks of somewhat contrasting rheomorphic properties. By focusing regional stresses along this discontinuity, the fault has printed up through the cover of Miocene and younger rocks that are present along most of its extent. In British Columbia, the long period of volcanic quiescence that had begun in the Late Jurassic ended . abruptly in the early Tertiary with widespread explo- sive eruption of lavas, mainly acidic to intermediate (Souther, 1970, p. 559). According to Souther, their eruption was accompanied by block foundering, north- south block faulting, formation of large cauldron subsi- dence features, and emplacement of north-south dike swarms. Correlative volcanic rocks were erupted in northern Washington, particularly thick sequences ac- cumulating in north-northeast-trending volcanotectonic depressions such as the Republic graben (Muessig, 1967, p. 95-96) and, Toroda Creek graben (Pearson and Ob- radovich, 1977, pl. 1). Potassium-argon ages show that this episode of volcanism began about 53 Ma, climaxed about 51-50 Ma, and continued with interruptions through 45 Ma (Mathews, 1964; Pearson and Ob- radovich, 1977). These volcanic rocks and the tensional features associated with them formed contemporane- ously with the collision of the Pacific and North Ameri- can plates postulated above. Because the formation of north-south tensional features is incompatible with east- west compression, the impact area was probably en- tirely south of this area of volcanism. If it was, the oceanic plate immediately west of the volcanic field was probably decoupled from the Pacific plate during the col- lision. This suggests that rather than vanishing, the Kula-Pacific ridge jumped northward 56-55 Ma, rees- tablishing itself southwest of the area of volcanism. Ten- sional features and the related volcanic rocks probably formed in response to the tearing effect of the right-lat- eral component of motion of the impinging Pacific plate. In Washington, the grabens and allied tensional fea- tures along with the associated volcanic and hypabyssal intrusive rocks have a north to north-northeast trend. The en echelon alinement of these volcanic features de- fines a diffuse, east-trending volcanic field that forms the western section of the so-called "Challis are." The association of these volcanic rocks, at least those in the segment of the are in Washington, with extensional fea- tures suggests that the are originated through rifting, not through melting of a subducted slab. This sugges- tion is supported by the fact that in southern British Columbia and northeastern Washington, lavas and as- sociated intrusive rocks near the base of the middle Eocene sequences are alkalic, including analeitic lava, extrusive rhomb prophyry, trachyte, phonolite, and trachyandesite (Daly, 1912, p. 98; Monger, 1968; Church, 1971; Fox, 1973). Though overlain by andesite, rhyodacite, and rhyolite, the alkalic rocks at the base give the sequence a compositional flavor more commonly associated with tension or rift-related volcanism than with the calc-alkaline andesitic volcanism of conven- tional island ares. In southern Oregon, the early Eocene deformation of the Roseburg Formation associated with, or perhaps culminating in, the emplacement of an allochthon of the Colebrooke Schist cannot be readily explained by defor- mation related to the postulated transit in Campanian to Eocene time of the Humboldt-triple junction along the continental margin to the south. But the age of thrust- ing and related deformation (fig. 11) does roughly coin- cide with the change in spreading direction of the Pacific-Farallon ridge (53-50 Ma), and the rift-related volcanism to the north (53-45 Ma), may thus date ex- treme compression that occurred at the initial site of Pacific-North American plate contact and convergence. MAIN ELEMENTS OF MELANGES AND THEIR BEARING ON PLATE-TECTONIC HISTORY 33 MELANGES OF THE OLYMPIC PENINSULA THE AJA FRACTURE ZONE AND THE FORMATION OF A HUMBOLDT-TYPE TRIPLE JUNCTION The magnetic lineations in the northeastern Pacific, as mapped by Naugler and Wageman (1973) and incor- porated in figure 13, appear to step right laterally across the Aja fracture zone. The fracture zone and nearshore anomalies trend eastward into a disturbed zone within which the magnetic anomalies have appar- ently been obliterated by later heating (Naugler and Wageman, 1973). Judged by the mapping, it is likely that north of the Aja fracture zone, the spreading ridge system and coexisting subduction zone shrank, then vanished about 21% Ma (fig. 13). A right-lateral transform, the ancestor of the Queen Charlotte fault, would at that time have intersected the Aja fracture zone and the still-active subduction zone along the continental shelf southeast of the Aja, forming a transform-transform-subduction zone triple junction. With northwest movement of the Pacific plate relative to the North American plate, this triple junction would necessarily be of the Humboldt-type. That triple junc- tion would then persist through part of early Miocene time, finally dying as the ridge system south of the Aja fracture zone stepped eastward, intersecting the sub- duction zone. Judged by spreading rates and amount of right-lat- eral offset of the spreading ridge at the Aja fracture zone indicated by offset of the magnetic lineations, the Humboldt type triple junction would have persisted through about 5% m.y. (until about 16 Ma) before being succeeded by the stationary to slow-moving (Rid- dihough, 1977, p. 392-393) transform-spreading ridge- subduction zone triple junction that now forms the northern terminus of the Juan de Fuca ridge. The al- lochthonous deformation related to transit of the triple Junction might then diminish gradually as the position of the leading edge of the subducted plate stabilized (fig. 7C). ORIGIN OF MELANGES AND BROKEN FORMATIONS Melanges and other deformational features of the Olympic Peninsula were formed during at least two, and possibly three, episodes of deformation, the earliest in middle or late Eocene time (Snavely and Pearl, 1975) and the most recent in latest early and middle Miocene time. Potassium-argon ages from the rocks of the east- ern core may record a third event at about 29 Ma (Tabor, 1972), somewhat later if those ages represent older material incompletely retrograded during metamorphism. The middle or late Eocene event evidently coin- cided with the initial impact of the Pacific and North American plates, which, as postulated in the preceding section, culminated in suturing of the block of oceanic crust now forming the basement of the Oregon- Washington borderland to the North American plate. As allochthonous deformation and formation of melanges or broken formation within and near the su- ture zone, particularly at the leading edge of the block, might be expected, no further explanation of the middle or late Eocene melanges seems required. The timing of the Humboldt-type triple junction formed by the intersection of the Aja transform with the Queen Charlotte fault and the subduction zone to the south coincides at least approximately with the time of formation of the latest early and middle Miocene melanges. In order to account for the structural fabric and geographic extent of these melanges through tec- tonism associated with this Humboldt-type triple junc- tion, that triple junction at its inception about 21% Ma would probably have been near the southern part of the Olympic melanges (fig. 15) and would have existed until it reached the approximate latitude of the northern part about 16 Ma. If this spatial relation is valid, the Olympic melanges and broken formations of Miocene age proba- bly originated as follows. At about 21% Ma, the Aja fracture zone intersected the edge of the continental plate, forming a northwestward-migrating Humboldt- type triple junction (figs. 15, 16). At this time the under- thrusting part of the Juan de Fuca plate began incre- mentally extending itself northwestward along the con- tinental plate margin at a rate commensurate with the northwestward slip of the Pacific plate. As it did, the underthrusting slab rammed, then plowed aside, the Queen Charlotte transform-fault-bounded lip of the con- tinental plate, sending the contents of one or more deep marginal basins cascading toward the east in a catas- trophic series of gravity slides and thrusts. These dis- placed masses bent the Crescent Volcanics and superja- cent beds into the Olympic horseshoe, where, arrested by the restraining bulk of this barrier, they piled up at the present site of the Olympic Mountains. At about 16 Ma, the spreading Juan de Fuca ridge again intersected the subduction zone (figs. 15, 16), and shortly thereafter the creation of melanges and broken formations of the Olympic Peninsula ceased. In examining the map of the northeastern Pacific (fig. 13), one observes other right-stepping fracture zones north of Aja; the northernmost of these persisted at least through the time of anomaly 10(?). Other such zones may once have existed farther north, but if they did, they have now been subducted beneath Alaska. As each of the active ridge-ridge transform faults whose 34 existence the fracture zones record in turn intersected the Queen Charlotte fault, a Humboldt-type triple junc- tion would form. The Olympic Peninsula or other areas immediately to the south could have been affected by tectonism associated with these triple junctions. As re- constructed by me, a Humboldt-type triple junction formed at the eastern end of the northernmost fracture zone shown on the map (fig. 13) about 29 Ma and was in existence for about 2 m.y. (fig. 11). Correlation of the Miocene allochthonous deforma- tion of the Olympic Peninsula with transit of the Aja fracture zone, as postulated above, requires an average slip rate of about 6 em/yr for the past 20 m.y. or so, com- parable to that measured by Larson and others (1968) for the past 2 m.y. Were the average rate substantially MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE less, as postulated for example by Atwater and Molnar (1973), tectonism associated with transit of the Aja frac- ture zone between 21% Ma and 16 Ma would be focused off the coast of British Columbia rather than the Olym- pic Peninsula. SAN ONOFRE BRECCIA The San Onofre Breccia of southern California may have originated through tectonism associated with the transit of an identifiable feature of the Pacific plate. Magnetic anomalies of the northeastern Pacific floor ap- pear to be offset left laterally about 275 km on the Pioneer fracture zone (fig. 3). This relation implies that the segment of spreading ridge north of the Pioneer J 3 & 1 > & 725°W is § oaw C I / / VANCOUVER 0 100 200 300 400 500 KILOMETERS 139 w Iona 735W "aps ® PORTLAND OLYMPIC PENINSULA Pacific-Juan de Fuca ridge t ~~ : & 1 to F $ ~ "i R \ ¥ wi o x_ i $i ~23%2 M *% A £ S § p ¥ % sf p e \ 4 2 ' 7. /x * g» < "49> * g» * 0 $ my $ w R $ a $ &" EXPLANATION -A_A._-&A- Subduction zone-Sawteeth on --6-- Magnetic anomaly upper plate r: Transform fault-Dotted where sub- - a merged. Arrows show relative horizontal movement < Fracture zone-Showing relative horizontal movement; dashed where inactive Spreading ridge FIGURE 15.-Successive positions of Aja fracture zone relative to North American plate (arbitrarily considered fixed). About 23% Ma, a short segment of Pacific-Juan de Fuca ridge is still present north of Aja Fracture Zone but will vanish about 21% Ma through continued spread- ing. About 16 Ma, ridge south of the Aja Fracture Zone first intersects plate margin. Humboldt-type triple junction that existed between 21% and 16 Ma probably created the Olympic melanges of early and middle Miocene age. Assuming that to be true, spatial relations be- tween oceanic and continental features at 23% and 16 Ma shown above were adjusted to associate melanges with Humboldt-type triple junc- tion. MAIN ELEMENTS OF MELANGES AND THEIR BEARING ON PLATE-TECTONIC HISTORY 35 fracture zone intersected the North American plate sig- nificantly later than did the segment of ridge to the south. Assuming that the half-spreading rate of about 4% ecm/yr measured between anomalies 13 and 8 was main- tained, the northern end of the spreading ridge between the Mendocino and Pioneer fracture zones probably in- tersected the North American plate after 27 Ma and was active at its southern end until after 25 Ma, some 5 to 6 m.y. after initial contact of the Pacific and North Ameri- can plates to the south. The-spatial and temporal associ- ation of the San Onofre Breccia with the jump of the tri- ple junction from the Pioneer to the Mendocino fracture zone (fig. 11) suggests that this jump might have in- itiated the catastrophic uplift and erosion of the source _ AdA_ EBACTuURE T> - _ JUAN DE FUCA -- \Gr|vity slides area of the San Onofre Breccia. That spatial and tem- poral association is admittedly less perfect than ex- pected, however. In figure 11, the position of the San Onofre has been adjusted to compensate for the 315-km offset on the San Andreas fault, and the northwestern part of the deposit has been moved 160 km southeast to restore movement postulated by Stuart (1976) on the East Santa Cruz Basin fault system. With these adjust- ments, and given a rate of offset between the Pacific and North American plates of 6 em/yr (as in fig. 11), the San Onofre appears to be displaced about 200 km farther northwest than expected. Were the rate only 5% em/yr as calculated by Minster and others (1974), the discre- pancy would be about 100 km. This amount can probably be accounted for by the displacement between the FIGURE 16.-Views to north showing deformation and generation of melanges and broken formations at present site of Olympic Peninsula in early and middle Miocene time. A, In early Miocene, Aja transform fault has intersected margin of North American plate, forming a Hum- boldt-type triple junction. North side of the Juan de Fuca plate, defined by Aja transform at surface and by its subducted extension in sub- surface, follows northwestward movement of Pacific plate, causing north side of subducted part of Juan de Fuca plate to impinge on North American plate at their interface in subsurface (toward direction indicated by broad arrow). North American plate is compressed and buck- les upward near triple junction and downward along a curving axis to north and east. Speculatively, a curving reverse fault forms at inflec- tion in North American plate north and east of this curving downfold. B, In latest early Miocene, continuing impingement of Juan de Fuca plate drives a segment of North American plate landward and beneath main part of this plate. At triple junction, lip of North American plate buckles upward, and upper part of this uplifted mass sloughs away to north and east in a series of catastrophic gravity slides and thrust plates, forming melange and broken formation. These sheets of displaced material pile up within curving downfold where they are further deformed through compression against overriding and upfaulted edge of North American plate to north and east (Olympic "horse- shoe" of Cady, 1975). Lower parts of this pile are sufficiently heated through reestablishment of normal geothermal gradient to be weakly metamorphosed. 36 MELANGES-SUBDUCTION AND INTERPLATE TRANSLATION, NORTH AMERICAN PLATE Pacific and North American plates that was distributed across faults outside the San Andreas fault system. For example, Thompson and Burke (1973) estimated that N. 55° W. extension across the entire Basin and Range pro- vince is about 100 km. Hence it is likely that the total displacement between the San Onofre Breccia and the North American plate east of the Basin and Range pro- vince substantially exceeds the 315 km allowed for offset on the San Andreas fault, and that the outcrop of the San Onofre correlates more perfectly with the Pioneer- Mendocino triple-junction jump than is shown on figure 11. CONCLUSIONS Latest Cretaceous and Tertiary melanges of the North American plate apparently can be correlated in both time and location with triple junctions that migrat- ed along the western edge of this plate. The existence of triple junctions of the required dynamic type, the Hum- boldt-type triple junctions of Fox (1976), is implied by the magnetic record of crustal formation preserved in the ocean floor. The temporal and also the spatial corre- lation is required if the rate of movement of the North American plate with respect to the Pacific plate has av- eraged roughly 6 em/yr for the past 21% m.y. and ap- proached that figure for the entire Tertiary. Atwater and Molnar (1973) have calculated that this rate averaged only 2 em/yr between 38 Ma and 10 Ma. The discrepancy between this rate and the 6- em/yr rate invoked to correlate the Miocene melanges of the Olym- pic Peninsula with the transit of the causal triple junc- tion must be resolved before a genetic link between melanges and triple junctions can be accepted with com- plete confidence. Direct correlation of the melanges of the Olympic Peninsula with what are presumed to be the causal plate-tectonic features suggests that plate movements have been continuous and the rate of rota- tion roughly constant from at least earliest Miocene time to the present. If the melanges of the Franciscan Complex and Olympic Peninsula did form at migrating triple june- tions, the ages of these melanges, together with the paleomagnetic record preserved in the ocean floor, suggest the following scenario: (1) In latest Cretaceous to early Eocene time, a tri- ple junction between the Kula, Farallon, and North American plates migrated northwestward along the edge of the North American plate at about 4 em/yr. (2) In early Eocene time, the southern part of the Kula plate broke away and was added to the Pacific plate through a northward jump of the Kula-Pacific spreading ridge. (3) During middle through late Eocene time, the convergent motion of the obliquely colliding Pacific and North American plates was arrested as fragments of the Pacific plate were sheared off, rotated clockwise, and sutured to the North American plate, forming the al- lochthonous Oregon-Washington borderland. (4) In latest Oligocene to middle Miocene time, right-stepping ridge-ridge transform faults, including the Aja, intersected the Queen Charlotte fault, forming Humboldt-type triple junctions west of the present site of the Olympic Peninsula. At these triple junctions, the convergence of the subducted part of the oceanic plate on the south and the transform-bounded part of the North American plate on the north culminated in the formation of the melanges and broken formations of the Olympic Peninsula. Latest Cretaceous to early Tertiary transit of the Kula-Farallon-North American triple junction appar- ently introduced a subduction-dominated tectonic re- gime that was to persist for at least 30 m.y. along the coast of California and to the present time along the coast of Oregon and Washington. Were melanges pro- duced as part of the subduction process, melanges rep- resenting that age and age span should be widespread. Their general absence, except for the Olympic Penin- sula, and the prevalence instead of melanges dating from the time of the postulated transition from strike- slip to subduction-dominated tectonism suggest that these melanges originated near the migrating Kula- Farallon-North American triple junction. Corroboration or refutation of this hypothesis should be sought through refinement of the age(s) of the melanges of the Franciscan Complex, through reevaluation of the con- tact relations of the Franciscan to the patches of Great Valley-like rocks, and through further consideration of the origin of blueschist. The allochthonous Oregon-Washington borderland with its varyingly rotated blocks of oceanic crust seems to have originated through a glancing collision of the Pacific and North American plates. 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PETERMAN Radioclement Distribution in a 3.06-Kilometer Drill Hole in Precambrian Crystalline Rocks, Wind River Mountains, Wyoming By CARL M. BUNKER and CHARLES A. BUSH Ages of Igneous Rocks in the South Park-Breckenridge Region, Colorado, and their Relation to the Tectonic History of the Front Range Uplift By BRUCE BRYANT, RICHARD F. MARVIN, CHARLES W. NAESER, and HARALD H. MEHNERT Potassium-Argon and Fission-Track Zircon Ages of Cerro Toledo Rhyolite Tephra in the Jemez Mountains, New Mexico By G. A. IZETT, J. D. OBRADOVICH, C. W. NAESER, and G. T. CEBULA Fission-Track Dating of the Climax and Gold Meadows Stocks, Nye County, Nevada By C. W. NAESER and FLORIAN MALDONADO GEOLOGICAL SURVEY PROFESSIONAL PAPER 1199 A-£E UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1981 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary GEOLOGICAL SURVEY Doyle G. Frederick, Acting Director Catalog No. 81-600048 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 (A) (B) (C) (D) (E) CONTENTS [Letters designate the chapters] Archean gneisses in the Little Rocky Mountains, Montana, by Zell E. Peterman Radioelement distribution in a 3.06-kilometer drill hole in Precambrian crystalline rocks, Wind River Mountains, Wyoming, by Carl M. Bunker and Charles A. Bush Ages of igneous rocks in the South Park-Breckenridge region, Colorado, and their relation to the tectonic history of the Front Range uplift, by Bruce Bryant, Richard F. Marvin, Charles W. Naeser, and Harald H. Mehnert............. Potassium-argon and fission-track zircon ages of Cerro Toledo Rhyolite tephra in the Jemez Mountains, New Mexico, by G. A. Izett, J. D. Obradovich, C. W. Naeser, and G. T. Cebula Fission-track dating of the Climax and Gold Meadows stocks, Nye County, Nevada, by C. W. Naeser and Florian Maldonado ...................................._.____.. SYMBOLS AND ABBREVIATIONS °C CIPW USED IN THIS VOLUME Degrees Celsius Cross, Iddings, Pirsson, and Washington (igneous rock classification system) Square centimeters Grams International Union of Geological Sciences Kilometers Meters Millimeters Million years Neutrons Parts per million Radium-equivalent uranium Years Decay constant for spontaneous fission of **U Decay constant based on beta-particle emission Decay constant based on orbital electron capture Microcalories per gram-year Micrometers (10 ° m) Induced fission-track density Spontaneous (fossil) fission-track density Standard deviation Neutron flux Page 1 15 37 45 Archean Gneisses in the Little Rocky Mountains, Montana By ZELL E. PETERMAN SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES GEOLOGICAL SURVEY-PROFESSIONAL PAPER 1199 -A CONTENTS Page Abstract 1 Introduction 1 Acknowledgments .._. 1 Samples and analytical methods 2 Results 3 Regional considerations 3 References cited 6 ILLUSTRATIONS Page FIGURE 1. Generalized geologic map of the Little Rocky Mountains, Montana ...... 2 2. Rb-Sr isochron plot for samples of gneiss and amphibolite from the Little Rocky Mountains 4 3. Map showing regional Precambrian framework of north-central United States and adjacent Canada 5 TABLES Page TABLE - 1. Rb-Sr analyses of Precambrian rocks from the Little Rocky Mountains .... 3 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES ARCHEAN GNEISSES IN THE LITTLE ROCKY MOUNTAINS, MONTANA By ZELL E. PETERMAN ABSTRACT Rb-Sr dating of a layered complex of tonalitic and granitic gneisses and amphibolite in the Little Rocky Mountains of north- central Montana defines their age as late Archean. Previous K-Ar ages of 1,710 and 1,750 m.y. on hornblende and biotite, respectively, reflect a widespread resetting of K-Ar and Rb-Sr mineral ages in the basement of the Williston basin and of the western Canada sedimentary basin. Although exhibiting some scatter, the Rb-Sr data are consistent with an age of at least 2,550 m.y., and consideration of reasonable initial *"Sr/**Sr ratios indicates that the gneisses may be substantially older. Even though the Rb-Sr data do not provide an exact age, an Archean age is firmly established. This age, coupled with determinations on basement cores from southeastern Montana and northeastern Wyoming, extends the Archean terrane of the Wyoming age province well into the buried basement towards the Williston basin. Continuity of the Archean of the Superior province and the Wyoming age province in the subsurface of the Dakotas and eastern Montana remains to be demonstrated, but the gap is narrowed considerably by the data reported here. INTRODUCTION The Little Rocky Mountains of north-central Montana are well known for excellent sections of Paleozoic and Mesozoic sedimentary rocks exposed in a domical uplift associated with several Tertiary syenite porphyry stocks (fig. 1). Precambrian rocks are also exposed in the area but as blocks within and marginal to the intrusions. These blocks consist of biotite schist and gneiss, quartzite, and amphibolite that are thought to represent a sequence of sedimentary and volcanic rocks metamorphosed to amphibolite grade (Knechtel, 1959). Hearn and others (1977) report K-Ar ages of 58 to 66 m.y. for the syenite and assign it to the Paleocene. The syenite was locally fractured, silicified, and altered during an episode of mineralization that produced gold-silver deposits in the area. Burwash, Baadsgaard, and Peterman (1962) determined K-Ar ages of 1,710 and 1,750 m.y. on hornblende and biotite respectively for Precambrian rocks just north of Zortman (fig. 1). These ages are in the common range of K-Ar ages of basement rocks of the western Canada sedimentary basin and of the Williston basin of western north Dakota and eastern Montana. The concordance of the ages from the Little Rocky Mountains suggests that a specific thermal or metamorphic event is being recorded by the K-Ar systems. That these systems were apparently little affected by the Tertiary intrusion is surprising in view of the association of the syenite and the Precambrian rocks. The syenite intrudes Precambrian and possibly Cambrian rocks but is in fault contact with younger rocks (Knechtel, 1959). Considering the relatively small areas of Precambrian rocks, the thermal effects must have been significant but not sufficient to reset the K-Ar systems. The K-Ar determinations provide a minimum age for the Precambrian rocks of the Little Rocky Mountains and identify them as Precambrian X or older. The nearest Archean rocks crop out in the Little Belt Mountains approximately 200 km southwest of the Little Rocky Mountains; they have been dated by Catanzaro and Kulp (1964), and are part of the major Archean terrane of the Wyoming age province. The extent of Archean rocks in the buried basement of northern and eastern Montana is poorly defined. Gneisses exposed in the Little Rocky Mountains provide indirect access to this otherwise buried basement, and the results obtained from Rb- Sr dating of these gneisses are reported here. ACKNOWLEDGMENTS The technical support of G. T. Cebula and J. W. Groen in the careful preparation of the rock samples and of Kiyoto Futa in the analytical work is gratefully acknowledged. 1 2 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES 108°30' 48°00' o Lodgepole I 5 110 KILOMETERS | 1 1 i i EXPLANATION Tertiary syenite Mesozoic sedimentary rocks Paleozoic sedimentary rocks Archean metamorphic rocks & R Mine Contact Fault Sample locality FIGURE 1.-Generalized geology of the Little Rocky Mountains, Montana (adapted from Knechtel, 1959). SAMPLES AND ANALYTICAL METHODS Rb and Sr analyses were completed following procedures slightly modified from those described earlier (Peterman and others, 1967). Ages are calcu- lated using the isotopic and decay constants recom- mended by the International Union of Geological Sciences Subcommission on Geochronology (Steiger , and Jager, 1977). | Seven samples were collected from an exposure of Precambrian rocks in a blasted cut along the old haulage road from Zortman to the Ruby Gulch gold mine (fig. 1). A layered complex of gneiss and amphibolite has nearly horizontal banding and foliation at this locality. Fine-grained biotite gneiss is in bands from several centimeters to several tens of centimeters thick. Biotitehornblende gneiss occurs in layers as much as a meter thick. Amphibolite layers range from a few centimeters to a meter or more, and thinner layers are commonly boudined and tightly folded. Samples LRM-1, -5, and -6 are light- to medium- gray, fine-grained biotite granite gneiss. A well developed foliation is defined by biotite alinement and by thin (up to 2 mm) lenses of composite quartz. ARCHEAN GNEISSES IN THE LITTLE ROCKY MOUNTAINS, MONTANA 8 Quartz, plagioclase, and microcline are present in nearly equal amounts, and biotite forms about 5 percent of the gneiss. Samples LRM-3 and -4 are biotite-hornblende gneiss of tonalitic to granodioritic composition. Poikiloblastic hornblende grains (as much as 2 mm long) are present in fine-grained, , well-foliated biotite gneiss with abundant quartz and plagioclase but only sparse microcline. Both samples contain composite augen of quartz, plagioclase, and microcline. LRM-4 contains single plagioclase grains (as much as 3 mm in diameter) that are commonly surrounded by healed mortar structure. Sample LRM-7 is a biotite granite gneiss similar to LRM-1 but with composite augen of quartz and feldspar. LRM-2 is a dark-gray, fine-grained amphibolite with abundant and nearly equal amounts: of hornblende and plagioclase, and minor quartz. Accessory minerals in the amphibolite are sphene, apatite, and altered allanite. The felsic gneisses contain accessory sphene, apatite, zircon, pyrite,. magnetite-ilmenite, and altered allanite. Plagioclase is slightly to moderately sericitized. Biotite is generally fresh with only local chloritization. Hornblende is commonly altered along cleavages. Minor carbonate occurs in most of the samples. Sphene is partially altered to leucoxene on grain margins and along fractures. The primary mineral assemblages indicate metamorphism in the amphibolite facies. Some of the alteration may be related to the intrusion of the syenite, but the iron staining is probably due to weathering. RESULTS The felsic gneisses have similar Rb and Sr contents (table 1) and, therefore, a limited range in Rb/Sr ratios that precludes clear definition of a Rb-Sr isochron by these data alone. The amphibolite has a much lower Rb/Sr ratio, and a reference isochron corresponding to an age of 2,550 m.y. is drawn through this point and the points for the felsic gneisses (fig. 2). The points scatter well outside of analytical error, and the data do not warrant a more sophisticated regression. The reference isochron has to be drawn through a relatively high initial §78r/8$Sr ratio of 0.706 in order to accomodate the data point for the amphibolite. Model ages can be calculated for the felsic gneisses using an initial-Sr ratio of 0.071, a value more common for rocks of late Archean age. These model ages range from 2,920 to 2,620 m.y., and the average value of 2,770 m.y. would correspond approximately to an average "wholerock" composite age for the felsic gneisses. TABLE 1.-Rb-Sr analyses of samples of Precambrian rocks from the Little Rocky Mountains, Montana [Analyst: Kiyoto Futa) i Sample Rb (ppm) Sr (ppm) 8 rp/865p *875p/865p LRM-1+ 124 219 1.655 0.7636 LRM-lf 123 219 1.630 7635 LRM-2 10.7 269 116 7105 LRM-3 114 285 1.159 . 7486 LRM-4 98.7 220 1.306 7563 LRM-5 124 232 1.555 21617 LRM-6 110 258 1.241 .7516 LRM-7 103 181 1.649 . 7660 *Normalized to 86sr/88sr = 0.1194. TtDuplicate analyses. Although the Rb-Sr systematics do not provide an especially accurate determination of the age, the data clearly establish the gneisses as Archean. The scatter of data points is not surprising in view of the metamorphic event at 1,700 to 1,800 m.y., the contact effects of the Paleocene syenite, and recent weathering. If the whole-rock Rb-Sr systems have been modified by any or all of these events, the effect would most likely have been to lower the age through loss of radiogenic Sr, gain of Rb, or internal redistribution of both elements. Consequently, the Rb-Sr systematics provide a firm minimum age for the gneisses. REGIONAL CONSIDERATIONS Most of the Precambrian of the United States is concealed beneath Phanerozoic cover (see King, 1976, fig. 1, p. 4-5) and not readily accessible for geologic studies. Significant advances in understanding the Precambrian history of the buried basement have been made through rediometric dating of basement samples obtained in drilling, largely in petroleum exploration. A major dating study of the basement of the U.S. culminated in a series of four reports (Goldich, Lidiak, and others, 1966; Goldich, Muehlberger, and others, 1966; Muehlberger and others, 1966; and Lidiak and others, 1966) followed by a regional synthesis for North America (Muehlberger and others, 1967). Very few ages of basement rocks have been reported since 1966. A goal of the dating studies is the delineation of age provinces in the basement. Many of these are subsurface extensions of the geologic and age provinces of the Canadian shield; others appear to be unique to the subsurface. The extent of these provinces, their age and lithologic character, and their mutual relationships are fundamental 4 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES 0.780 T T T T u u T 0.770 0.760 0.750 o ~ a - 875r/865p 0.730 0.720 0.710 i il il il i i i 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 a 0.700 87Rb/865r FigurRE 2.-Rb-Sr isochron plot for samples of gneiss and amphi- bolite from the Little Rocky Mountains, Montana. The slope of the isochron corresponds to an age of 2,550 m.y., and the initial *"Sr/*Sr intercept is 0.706. problems that have not been fully resolved in the buried basement nor in many exposed Precambrian terranes for that matter. The basement of eastern Montana, eastern Wyoming, North Dakota, and South Dakota is of particular interest because it lies between two major exposed Archean terranes: the Wyoming age province on the west and the Superior province on the east (fig. 3). Three major problems are (1) whether or not Archean crust is continuous in the subsurface between the exposed Archean terranes, (2) the extent of lower Proterozoic (Precambrian X) supracrustals in the subsurface, and (3) the nature of the event that produced the 1,600- to 1,800-m.y. K-Ar and Rb-Sr mineral ages and the amount of intrusive rock emplaced during this event. Only a few dated samples from the basement provide clear evidence of Archean rocks (fig. 3). In the eastern Dakotas, proximity to the exposed Archean rocks of Minnesota and Manitoba and geophysical data coupled with basement lithologies provide additional evidence for an Archean basement in this region (Muehlberger and others, 1967; Lidiak, 1971). In the Black Hills, a key area in the western part of this region (fig. 3), folded and metamorphosed lower Proterozoic supracrustals lie on Archean basement. The deformation and intrusive event in this area between 1,600 and 1,800 m.y. was named the Black Hills orogeny (Goldich, Lidiak, and others, 1966). The Harney Peak Granite associated with this event is dated at 1,710 m.y. (Riley, 1970). Zartman and Stern (1967) established the Little Elk Granite in the northeastern Black Hills as Archean basement. Rb Sr systems in whole rocks and minerals were reset by cataclasis and recrystallization during the Black Hills orogeny, but the U-Pb systems in zircon re tained the Archean age. Archean rocks were also discovered in the western Black Hills, where Ratté and Zartman (1970) obtained 2,500m.y. ages on granite and pegmatite from the core of a gneiss dome that penetrates supracrustals at Bear Mountain. The extent of lower Proterozoic supracrustals in the basement to the west and north of the Black Hills is largely conjectural. However, Lidiak (1971), in his construction of a geologic map of the basement of South Dakota, interprets gravity and aeromagnetic anomalies in the central and western part of the state as reflecting 1,600- to 1,800-m.y.-old structural ele ments that trend north to northwest. On a more regional scale, Camfield and Gough (1977) have delineated a conductive zone in the basement that skirts the east side of the Black Hills and extends north to the Canadian shield and south to the Laramie Range of southeastern Wyoming. The ori- gin of the anomaly is problematic, but Camfield and Gough (1977) note a correspondence or alinement with highly sheared and deformed rocks in both Saskatchewan and southeastern Wyoming, and they suggest that the zone may be a suture related to plate collision in the Proterozoic. The southernmost part of the anomaly is more or less coincident with part of the Colorado lineament described by Warner (1978). Warner presents abundant evidence for a Precam- brian wrench-fault system extending from northern Arizona northeasterly to the southwestern tip of Lake Superior and postulates that the fault was approximately marginal to a continental plate in the early Proterozoic. The present study of the Precambrian of the Little Rocky Mountains does not bear on the extent of the lower Proterozoic in the basement or on the nature of the orogenic event, but it does document an exten- sion of the Archean into north-central Montana. Further evidence for this extension is provided by data for two basement cores obtained in U.S. Geo- logical Survey drill holes that have been dated by the Rb-Sr method (Peterman, unpub. data, 1978). A hole in northeastern Wyoming (fig. 3) penetrated a band- ed gneiss complex that gives an isochron age of 2,640 m.y. A hole to the north in Montana encountered a porphyritic biotite granite that gives a model Rb-Sr age of approximately 2,800 m.y. These data, coupled with data for the Little Rocky Mountains, extend the Archean basement well into the subsurface towards the Wiliston basin. Whether or not the Archean was continuous between the Wyoming age province and ARCHEAN GNEISSES IN THE LITTLE ROCKY MOUNTAINS, MONTANA -__ - _J 4 % CANA -z t—\/:,/:, 218-1 bz 1871 me /x * maas, Tx m mades > sees, i SASKATCHEWAN MANITOBA 1 ® TTT seme meee »CAWADA UNITED l STATES 0 Little Rocky Mountains NORTH DAKOTA 1 | 1 | 9 l 1 Little Belt Mountains I, \ | 1 MONTANA l . LJ M Tt A mee & sores it Stor Smo oo it mae ~ F= J & 1 Y §" "=" 1 ! SOUTH DAKOTA "Ga | && Black Hills 5| - =/ % A «& g wyominc | I y 9 | 1 5 100 0 100 200 KILOMETERS Ll vfs 002 L2 _i EXPLANATION PRECAMBRIAN Y ARCHEAN 7 3 Belt Supergroup of Montana, Keweenawan Supergroup A £ Superior Province of Canada and Archean rocks of of Minnesota, and Sioux Quartzite of Minnesota and Minnesota and of the Wyoming age province South Dakota € Subsurface samples dated as Archean, and two Archean outcrop areas in the Black Hills (the Little Elk Granite and PRECAMBRIAN X Penokean granites of Minnesota, and Churchill Province of Canada (in part Archean) Animikie Group of Minnesota, and metasedimentary and metavolcanic rocks of the Black Hills (including the Harney Peak Granite) the granite gneiss at Bear Mountain) FIGURE 3. -Regional Precambrian framework of the north-central United States and adjacent Canada. Adapted from the "Basement Map of North America'" (Flawn, 1967). Age data for basement samples are from Burwash and others (1962); Peterman and Hedge (1964); Goldich, Lidiak, and others (1966); Peterman (unpub. data, 1978). Archean ages in the Black Hills are from Zartman and Stern (1967) 'and Ratté and Zartman (1970). 6 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES the Superior province will have to be determined by additional dating of basement samples by methods that are capable of penetrating the effects of the 1,600- to 1,800-m.y. event. REFERENCES CITED Burwash, R. A., Baadsgaard, H., and Peterman, Z. E., 1962, Precambrian K-Ar dates from the western Canada sedimen- tary basin: Journal of Geophysical Research, v. 67, p. 1617- 1625. Camfield, P. A., and Gough, D. I., 1977, A possible Proterozoic plate boundary in North America: Canadian Journal of Earth Sciences., v. 14, p. 1229-1238. Catanzaro, E. J., and Kulp, J. L., 1964, Discordant zircons from the Little Belt (Montana), Beartooth (Montana) and Santa Catalina (Arizona) Mountains: Geochimica et Cosmochimica Acta, v. 28, p. 87-124. Flawn, P. T., 1967, Basement map of North America: U.S. Geo- logical Survey. Goldich, S. S., Lidiak, E. G., Hedge, C. E., and Walthall, F. G., 1966, Geochronology of the midcontinent region, United States, 2., Northern area: Journal of Geophysical Research, v. 71, p. 5389-5408. Goldich, S. S., Muehlberger, W. R., Lidiak, E. G., and Hedge, C. E., 1966, Geochronology of the midcontinent region, United States, 1., Scope, methods, and principles: Journal of Geo- physical Research, v. 71, p. 5375-5388. Hearn, B. C., Jr.. Marvin, R. F., Zartman, R. E., and Naeser, C. W., 1977, Geochronology of igneous activity in the north- central alkalic province (abs.): Geological Society of America Abstracts with Programs, v. 9, no. 6, p. 732. King, P. B., 1976, Precambrian geology of the United States; an explanatory text to accompany the geologic map of the United States: U.S. Geological Survey Professional Paper 902, 85 p. Knechtel, M. W., 1959, Stratigraphy of the Little Rocky Mountains and encircling foothills, Montana: U.S. Geological Survey Bulletin 1072-N, p. 723-752. Lidiak, E. G., 1971, Buried Precambrian rocks of South Dakota: Geological Society of America Bulletin v. 82, p. 1411-1420. Lidiak, E. G., Marvin, R. F., Thomas, H. H., and Bass, M. N., 1966, Geochronology of the midcontinent region, United States, 4., Eastern region: Journal of Geophysical Research, v. 71, p. 5427-5488. I Muehlberger, W. R., Denison, R. E., and Lidiak, E. G., 1967, Base- ment rocks in continental interior of United States: American Association of Petroleum Geologists Bulletin, v. 51, p. 2351- 2380. Muehlberger, W. R., Hedge, C. E., Denison, R. E., and Marvin, R. F., 1966, Geochronology of the midcontinent region, United States, 3., Southern area: Journal of Geophysical Research, v. 71, p. 5409-5426. Peterman, Z. E., and Hedge, C. E., 1964, Age of basement rocks from the Williston basin of North Dakota and adjacent areas, in Geological Survey research 1963: U.S. Geological Survey Professional Paper 475-D, p. D100-D104. Peterman, Z. E., Doe, B. R., and Bartel, Ardith, 1967, Data on the rock GSP -1 (granodiorite) and the isotope-dilution method of analysis for Rb and Sr, in Geological Survey research 1967: U.S. Geological Survey Professional Paper 575-B, p. B181- B186. Ratte , J. C., and Zartman, R. E., 1970, Bear Mountain gneiss dome, Black Hills, South Dakota-Age and structure (abs.): Geological Society of America Abstracts with Programs, v. 2, no. 5, p. 345. Riley, G. H., 1970, Isotopic discrepancies in zoned pegmatites, Black Hills, South Dakota: Geochimica et Cosmochimica Acta, v. 34, p. 713-725. Steiger, R. H., and Jager, E., 1977, Subcommission on geochro- nology-Convention and use of decay constants in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, p. 359-362. Warner, L. A., 1978, The Colorado lineament-A middle Pre- cambrian wrench fault system: Geological Society of America Bulletin, v. 89, p. 161-171. Zartman, R. E., and Stern, T. W., 1967, Isotopic age and geologic relationships of the Little Elk Granite, northern Black Hills, South Dakota: U.S. Geological Survey Professional Paper 575-D, p. D157-D163. Radioelement Distribution in a 3.06-Kilometer Drill Hole in Precambrian Crystalline Rocks, Wind River Mountains, Wyoming By CARL M. BUNKER and CHARLES A. BUSH SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES GEOLOGICAL SU ERVEY PROFESSIONAL PAPER 1199-B CONTENTS Abstract Introduction Analytical method Results Conclusions References cited ILLUSTRATIONS FicurE 4. Map showing location of 3.06-kilometer-deep drill hole in the Wind River Mountains, Wyoming 5. Plots of radioelement concentrations and ratios and radiogenic heat in 3.06-km-deep drill hole 6.) Histograms showing frequency distribution of radioelement concentrations and ratios and radiogenic heat = TABLES TABLE 2. Summary of radioelement and radiogenic heat analyses on 299 core samples 3. Averages of radioelement contents and ratios and radiogenic heat for samples from drill hole, and summary of published averages for igneous rocks Page wa im Q0 00 <1 1 Lader Page 11 13 Page 12 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES RADIOELEMENT DISTRIBUTION IN A 3.06-KILOMETER DRILL HOLE IN PRECAMBRIAN CRYSTALLINE ROCKS, WIND RIVER MOUNTAINS, WYOMING By C. M. BUNKER and C. A. BUSH ABSTRACT Uranium, thorium, and potassium contents were measured in 229 samples of cuttings from a drill hole that penetrates 3.06 kilometers of Precambrian crystalline rock on the southwestern flank of the Wind River Mountains, Wyoming. Thorium and potassium contents are similar to reported averages for inter- mediate to silicic igneous rock, and uranium content is in the range of averages for mafic to intermediate rock; these data indicate that the rock is deficient in uranium. The radioelement data support geologic and geophysical data which indicate that the drill hole penetrates a thick section of layered, fractured, heterogeneous rock. INTRODUCTION A 3.06- kilometer exploratory borehole was drilled into Precambrian crystalline rocks on the south- western flank of the Wind River Mountains, Wyo- ming, to obtain geologic and geophysical data. The hole is in sec. 2, T. 32 N., R. 197 W. (lat 42°45'30" N., long 109°31'30" W.) about 23 km southeast of Pine dale, Wyo.; the collar elevation is 2219 m above mean sea level (fig. 4). The Wind River Mountains are a broad asym- metrical anticline exposing Precambrian crystalline rocks in the core. Several kilometers of rock may have been eroded above the existing surface at the drill-hole site. The flank of the Wind River Moun- tains overthrusts the Green River Basin to the southwest; the thrust dips gently to the northeast. Rocks exposed at the surface near the drill hole and all core samples show nearly horizontal foliation. Petrographic descriptions of cores taken from depths 50 to 300 m apart have been reported by Ebens and Smithson (1966). The rock cores are granitic (quartzo-feldspathic) in general character, but they are very heterogeneous. Composition rang- es from quartz-dioritic to granodioritic to quartz- monzonitic gneiss; texture ranges from fine grained to porphyroblastic. Neither composition nor texture shows any relationship to depth, although the miner- alogy suggests an increase in grade of metamor- phism with depth. Ebens and Smithson (1966) con- cluded that the hole penetrates a heterogeneous layered sequence of metamorphic rocks. A seismic velocity survey and sonic, caliper, and neutron logs of the drill hole indicate that the rock is fractured throughout the depth of the hole (Smithson and Ebens, 1971). The fracturing is very likely related to the proximity of the drill hole to the thrust fault. A major fracture zone with cracks exists to a depth of 460 m; a sharp increase in velocity at that depth is interpreted as being related to the absence of cracks below that depth. Other highly fractured zones are indicated at 1,642 m and from 2,665 to at least 2,700 m. Relatively unfractured zones occur from about 2,000 to 2,650 m, which is also a zone of relatively high rock density, and from 2,865 to 2,890 m. A gamma-ray log of the hole shows general changes in radiation intensity which are probably 112 110° 108° 106° 104° I __ 1M0N_TAM________—-—~\l‘8 fe C af E I | 44° |- ' ‘g > o WYOMING 13 3) ' . 3 a: Pingdsl Wind River Mountains F.-— 7 . | 2 II Drill hole H; 42°_--~1 ‘33 E (A -q l Fal --- FIGURE 4.-Location of 3.06-kilometer-deep drill hole in the Wind River Mountains, Wyoming. 7T 8 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES related to changes in rock type. The gamma-ray log also shows many zones ranging in thickness from about 0.3 to 15 m in which the radioelement content is greater than that of the host rock. These relatively thin zones of anomalous radioactivity are not ap- parently related to anomalies on the neutron log; however, the radioactivity anomalies can be inter- preted as indications of either mineralogic differ- ences in the host rock or enrichment of radioelement contents in at least some of the fracture zones. These geophysical data confirm that the rock is heterogen- eous both in composition and physical properties, as was previously determined from a few core samples. In addition to the 19 cores that were obtained, drill cuttings were collected from virtually the entire depth of the hole. Ninety-nine of these samples were used for measurements of radiogenic heat produc- tion (Lachenbruch and Bunker, 1971); those plus an additional 130 samples were used in the present investigation. Most of the samples represent about 3 m of rock penetrated by the drill hole; a few represent 1.5-m intervals. The distance between samples range from 0 to 36 m, but most of the sampled intervals are 3.3 to 9.8 m apart. The principal objective of the radioelement ana- lyses was to determine the distribution of uranium, thorium, potassium, and the associated radiogenic heat through a thick vertical section of Precambrian crystalline rocks. We also plan to compare these data with radioelement analyses of Precambrian granites in central Wyoming. ANALYTICAL METHOD Radioelement (K, RaeU, and Th) contents of the samples were measured by gamma-ray spectrome- try. Approximately 600 g of the material were sealed in 15-cm-diameter plastic containers. The containers were placed on a sodium iodide crystal 12.5 cm in diameter and 10 cm thick. The gamma radiation penetrating the crystal was sorted according to energy by the associated electronic devices, and the resulting spectra were stored in a 512-channel me mory. The spectra were interpreted with the aid of a linear-least-squares computer method which match- es the spectrum from a sample to a library of radioelement standards; the computer method for determining concentrations is a modification of a program written by Schonfeld (1966). Standards used to reduce the data include the USGS standard rocks, New Brunswick Laboratories standards, and several samples for which uranium and thorium concentrations have been determined by isotope dilution and mass or alpha spectrometry. Uranium contents were measured indirectly by measuring the daughters (?"4Bi and *"**Pb) to obtain radium-equivalent uranium (RaeU) values. Radium-equivalent uranium is the amount of ura- nium required for secular isotopic equilibrium with the Ra and its daughters measured in a sample. Isotopic equilibrium between these daughters and was accomplished by allowing the sealed sample containers to sit for at least 21 days prior to the analyses. All uranium concentrations measured in the drill-hole samples are radium-equivalent values. Although thorium is also measured from daughter products ("Bi, *"°Pb, and **TI), isotopic dis- equilibrium is improbable because the: daughter products measured have such short half-lives that their concentration directly reflects that of thorium. Potassium is determined from the 4°K isotope, which is radioactive and directly proportional to the total potassium. All the radioelement data (table 2) reported in this paper are based on replicate analyses. The coeffi- cient of variation for the accuracy of these data, when compared to isotope-dilution and flame-photo- metry analyses, is about +2 percent for RaeU and Th and about +l percent for K. These percentages are in addition to minimum standard deviations of about 0.05 ppm for RaeU and Th and 0.03 percent for K. RESULTS The highly variable character of the distribution of the radioelement contents and ratios in the rock penetrated by the drill hole (fig. 5) reflects the layering, fracturing, and heterogeneity in mineral content and rock types that were observed by exa- mination of core samples or interpreted from geo- physical logs. An exception to the highly variable radioelement distribution is the depth interval from about 2,040 to 2,650 m, in which the uranium and potassium contents are nearly constant, thereby indicating a fairly homogenous layer of rock. This is a relatively unfractured zone in which the transport and redistribution of radioelements is less likely than in the highly fractured zones. We believe that the measured radioelement contents in the 2,040- to 2,650-m interval are the intrinsic values of the rock, which has been described as foliated porphyroblas- tic quartz monzonite (Ebens and Smithson, 1966). RADIOELEMENT DISTRIBUTION, PRECAMBRIAN CRYSTALLINE ROCKS, WIND RIVER MOUNTAINS, WYOMING 9 TABLE 2.-Summary of radioelement and radiogenic heat anal- yses on 229 core samples from a drill hole in Wind River Mountains, Wyoming Sample Rael Th F Heat Th RaeU Th depth (ppm) (ppm) - (per- (ucal/ (meters) cent) _ g-yr) Rael! - Kx10"% 6.1- 9.1 0.42 5.95 4.94 2.83 14.17 0.09 1.20 18.3- 21.3 0. 84 15.60 3.45 4.66 18. 57 0.24 4.52 36.6- 39.6 1.19 28.53 3.92 7.63 23.97 0.30 7.28 45.7- 48.8 1.15 18.04 4.09 5.55 15.69 0. 28 4.41 51.8- 54.9 1.07 21.32 3.77 6.06 19.93 0.28 5.66 61.0- 64.0 2.23 24.09 3.62 7.42 10. 80 0.62 6.65 67.1- 70.1 1.15 15.78 4.31 5.16 13.72 0.27 3.66 76.2- 79.2 1.16 78.27 5.23 17.91 6 .47 0.22 14.97 8B5.3- 88.4 1.74 72.18 4.53 16.93 41.48 0.38 15.93 97.5- 100.6 1.78 52.11 4.76 13.01 29.28 0.37 10.95 106.7- 109.7 1.50 43.32 4.35 10.93 28.88 0.34 9.96 112.8-. 115.8 1.25 32.91 4.56 8.73 26.33 0.27 7.22 121.9- 125.0 1.40 30.98 4.11 8.33 22.13 0.34 7.54 131.1- 134.1 0.90 28.69 3.81 7.42 31.88 0.24 7.53 140.2- 143.3 1.43 34.90 3.64 9.01 24.41 0.39 9.59 152.4- 155.4 1.06 8.10 3.55 3.35 7.64 0. 30 2.28 161.5- 164.6 1.84 9.29 2.38 3.84 5.05 0.77 3.90 167.6- 170.7 2.06 9 . 68 1.67 3.89 4.70 1.23 5.80 176.8- 179.8 1.31 9.38 3.40 3.75 7.16 0.39 2.76 185.9- 189.0 1.48 15.95 2.52 4.95 10.78 0.59 6.33 204.2- 207.3 2.70 19.60 3.43 6.82 7.26 0.79 5.71 213.4- 216.4 2.56 17.85 3.26 6.32 6.97 0.79 5.48 222.5- 225.6 2.13 12.74 2.75 4.85 5.98 0.77 4.63 231.6- 234.7 2.02 21.81 3.58 6.80 10. 80 0.56 6.09 240.8- 243.8 1.76 27.26 3.42 7.66 15. 49 0.51 7.97 249.9- 253.0 5.04 21.58 3.16 R. 85 4.28 1.59 6.83 259.1- 262.1 12.94 33.24 2.79 16. 85 2.57 4.64 11.91 268.2- 271.3 3.17 21.84 2.64 7.39 6.89 1.20 8.27 277.4- 280.4 2.88 26.78 3.55 8.42 9.30 0. 81 7.54 283.5- 286.5 1.55 12.62 4.82 4.96 8.14 0. 32 2.62 289.6- 292.6 3.29 31.25 3.60 9.62 9.50 0.91 8. 68 295.7- 298.7 2.74 21.48 3.29 7.18 7. 84 0.83 6.53 304.8- 307.8 3.87 11.96 2.78 5.97 3, no 1.39 4,30 313.9- 317.0 2. 48 10.41 2.19 4. 48 4.20 1.13 4.75 320.0- 323.1 3.81 14.20 2.66 6.34 3.73 1.43 5.34 329.2- 332.2 2.09 10.76 2.03 4.23 5.15 1.03 5.30 338.3- 341.4 2.71 20.28 3.60 7.01 7.48 0.75 5.63 344.4- 347.5 .62 21.79 3.73 7.28 8.32 0.70 5. 84 350.5- 353.6 2.25 19.87 3.11 6.46 8. 83 0.72 6.39 359.7- 362.7 2.83 19.03 3.07 6.70 6.72 0.92 6.2 368.8- 371.9 3.64 21.72 3.27 7.88 5.97 1.11 6.64 381.0- 384.0 2.96 15. 82 2.81 6.08 5.34 1.05 5.63 387.1- 390.1 2.29 13.86 2.13 5.02 6.05 1.08 6.51 396.2- 399.3 2.62 16.82 2.85 6.05 6.42 0.92 5.90 405.4- 408.4 2.27 16.12 3.32 5.78 7.10 0. 68 4.86 414.5- 417.6 2.35 16.57 2.98 5.83 7.05 0.79 5.56 423.7- 426.7 2.57 18.00 2.86 6.25 7.00 0.90 6.29 432.8- 435.9 2.22 16.00 3.05 5.64 7.21 0.73 5.25 442.0- 445.0 3.73 20.54 3.69 7.83 5.51 1.01 5.57 448.1- 451.1 2.37 16.04 3.55 5.90 6.77 0.67 4.52 454.2- 457.2 2.19 16.74 3.69 5.94 7.64 0.59 4.54 460.2- 463.3 1.95 14.99 3.26 5.30 7.69 0. 60 4.60 493.8- 496.8 2.15 16.06 3.31 5.68 7.47 0.65 4. R5 521.2- 524.3 1.81 14.04 3.31 5.02 7.76 0.55 4.24 545.6- 548.6 1.30 7.47 2.78 3.19 5.75 0.47 2.69 554.7- 557.8 1.72 11.88 3.10 4.47 6.91 0.55 3.83 579.1- 582.2 «23 9.32 1.99 4.03 4. 18 1.12 4 . 68 591.3- 594.4 1.52 8.94 2.73 3.45 5.29 0.56 2.95 597.4- 600.5 2.45 21.73 3.29 7.02 R. R7 0.74 h. 60 609.6- 612.6 1.38 10.19 2. 8h 3.82 7.38 0. 48 3.56 618.7- 621.8 2.18 16.90 2.71 5.70 7.75 0. A0 6.24 624.8- 627.9 2.48 26.14 3.95 8.10 10. 54 N. 63 6.62 634.0- 637.0 3.00 20.06 3.42 713 6.69 0. AA 5. A7 646.2- 649.2 1.80 10. 59 3.41 4.35 5. 88 0.53 3.11 658.4- 661.4 1.41 11.00 3.00 4.04 7.80 0.47 3.67 TABLE 2.-Summary of radioelement and radiogenic heat anal- yses on 229 core samples from a drill hole in Wind River Mountains, Wyoming-Continued Sample RaeU Th K Heat Th RaeU Th depth (ppm) (ppm) (per- (ucal/ -- (meters) cent) g-yr) RaeU - Fx10~* Kkx10% 664.5- 667.5 1.98 - 13.87 2.96 5.02 7.01 0.67 4.69 679.7- 682.8 _ 2.02 17.96 3.64 6.05 8.89 0.55 4.93 685.8- 688.8 2.04 24.75 3.58 7.41 12.13 0.57 6.91 694.9- 698.0 2.39 24.37 3.82 7.65 10.20 0.63 6.38 701.0- 704.1 2.09 21.62 3.61 6.82 10. 34 0.58 5.99 713.2- 716.3 1.96 10.92 3.12 4. 46 5.57 0.63 3.50 728.5- 731.5 1.86 20.57 3.59 6.44 11.06 0.52 5.73 740.7- 743.7 1.57 11.05 2.62 4.06 7.04 0.60 4.22 752.9- 755.9 1.59 20.86 3.45 6.26 13.12 0.46 6.05 759.0- 762.0 1.75 15.67 3.10 5.25 8.95 0.56 5.05 762.0- 765.0 1.58 13.64 3.11 4.72 8.63 0.51 4.39 795.5- 798.6 1.80 12.59 4.14 4.95 6.99 0.43 3.04 823.0- 826.0 2.09 12.53 3.47 4.97 6.00 0.60 3.61 847.3- 850.4 1.90 10.14 3.01 4.23 5.34 0.63 3.37 B83.9- 887.0 3.19 14.61 3.57 6.21 4.58 0.89 4.09 914.4- 917.4 1.94 _ 10.15 2.72 4.18 5.23 0.71 3.73 938.8- 941.8 1.79 11.56 3.19 4. 48 6.46 0.56 3.62 975.4- 978.4 2.37 13.74 3.91 5.53 5.80 0. 61 3.51 984.5- 987.6 2.05 14.99 3.58 5.46 7.31 0.57 4.19 993.6- 996.7 2.19 14.11 2.85 5.19 6.44 0.77 4.95 1005. 1008.9 3.44 16.42 3.37 6.71 4.77 1.02 4.87 1015.0-1018.0 2.19 14.46 3.58 5.46 6.60 0.61 4.04 1033.3-1036.3 1.97 6.71 4.35 3.95 3.41 0.45 1.54 1048.5-1051.6 2.55 10.31 3.48 4.86 4.04 0.73 2.96 1063.8-1066.8 _ 2.81 11.96 3.23 5.32 4.26 0. 87 3.70 1094.2-1097.3 2.40 11.54 3.00 4.87 4.81 0. 80 3.85 1130.8-1133.9 2.65 17.36 3.18 6.27 6.55 0.83 5.46 1155.2-1158.2 2.79 20.26 3.35 6.99 7.26 0.83 6.05 1164.3-1167.4 1.51 12.38 3.43 4. 50 8.20 0.44 3.61 1170.4-1173.5 1.84 14.16 3.51 5.12 7.70 0.52 4.03 1182.6-1185.7 2.34 18.51 3.44 6.34 7.91 0.68 5.38 1191.8-1194.8 3.30 14.93 3.14 6.24 4.52 1.05 4.75 1213.1-1216.2 2.82 16.03 3.38 6.18 5.68 0.83 4.74 1225.3-1228.3 1.93 8.56 2.10 3.69 4. 44 0.92 4.08 1240.5-1243.6 2.27 9.92 2.67 4.36 4.37 0.85 3.72 1249.7-1252.7 2.66 14.30 3.10 5.64 5.38 0.86 4.61 1283.2-1286.3 2.42 11.12 2.85 4.76 4.60 0.85 3.90 1313.7-1316.7 2.01 14.75 2.85 5.19 7.34 0.71 5.18 1325.9-1328.9 _ 2.82 12.83 3.07 5.45 4.55 0.92 4.18 1338. 1-1341.1 4.06 20.74 3.50 8.06 5.11 1.16 5.93 1344.2-1347.2 3.68 27.49 3.86 9.23 7.47 0.95 7.12 1353.3-1356.4 3.15 25.37 3.96 8.44 8.05 0. &n 6.41 1356.4-1359.4 3.01 20.29 3.68 7.25 6.74 0. 82 5.51 1368.6-1371.6 3.15 12.20 3.35 5.64 3.87 0.94 3.64 1380.7-1383.8 1.66 11.90 2.70 4. 32 7.17 0.61 4.41 1402.1-1405. 1 2.07 12.57 2.91 4.81 6.07 0.71 4.32 1432.6-1435.6 1.61 12.19 2.82 4.37 7.57 0.57 4.32 1463.0-1466.1 2.37 11.91 2.28 4.73 5.03 1.04 5.22 1499.6-1502.7 2.77 10.26 2.49 4.75 3.70 1.11 4.12 1527.0-1530.1 1.92 9.75 2.88 4.13 5.08 0.67 3.39 1557.5-1560.6 2.16 14.67 2.67 5.23 6.79 0. 81 5.49 1566.7-1569.7 3.43 12.20 2.65 5.66 3.56 1.29 4.60 1572.8-1575.8 1.88 16.51 2.51 5.35 8.78 0.75 6.58 1581.9-1585.0 4.09 44.90 2.56 12.66 10.98 1.60 17.54 1588.0-1591.1 2.75 30.91 2.60 8.89 11.24 1.06 11.89 1597.2-1600. 2 1.76 18.73 3.29 5.92 10.64 0.53 5.69 1615.4-1618.5 1,90 12. 89 2.60 4.67 6.78 0.73 4.96 1645.9-1649.0 1.64 16.13 3.15 5.27 9. 84 0.52 5.12 1682.5-1685.5 1.46 17.15 2.92 5.28 11.75 0.50 5.87 1703.8-1706.9 1.87 15.08 2.46 5.05 8.06 0.76 6.13 1713.0-1716.0 1.62 11.82 2.65 4.26 7.30 0.61 4.46 1743. 5-1746.5 2.74 23.65 2.55 7.42 R. 63 1.07 9.27 1764.8-1767.8 1.85 13.56 2.90 4.85 7.33 0.64 4.68 1801.4-1804.4 1.98 13.52 2.70 4.88 6.83 0.73 5.01 1831.8-1834.9 2.44 12.69 2.62 5.03 5.20 0.93 4.84 10 TABLE 2.-Summary of radioelement and radiogenic heat anal- yses on 229 core samples from a drill hole in Wind River Mountains, Wyoming-Continued SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES TABLE 2.-Summary of radioelement and radiogenic heat anal- yses on 229 core samples from a drill hole in Wind River Mountains, Wyoming-Continued Sample RaeU Th K Heat Th RaeU Th depth (ppm) _ (ppm) (per- (ucal/ (meters) cent) g-yr) RaeU - Kx10®¢ Ex10~4 1859.3-1862.3 1.47 12.58 2.91 4.37 8.56 0. 51 4.32 1868.4-1871.5 1.40 12.99 3.05 4.44 9.28 0.46 4.26 1874.5-1877.6 1.60 17.12 2.60 5.29 10.70 0.62 6.58 1883.7-1886.7 1.80 13.75 2.43 4.72 7.64 -_ 0.74 5.66 1889.8-1892.8 Pill 14.25 2.43 5.05 6.75 0.87 5.86 1895.9-1898.9 1.38 9.23 2.70 3.58 6.69 0.51 3.42 1902.0-1905.0 0.77 4.57 2.51 2.15 5.94 0.31 1.82 1914.1-1917.2 1.51 8.57 2.34 3.45 5.68 0.65 3.66 1923.3-1926.3 1.05 11.67 2.30 3.72 11.11 0.46 5.07 1932. 4-1935.5 1.68 22.02 2.11 6.20 13.11 0.80 10.44 1941.6-1944.6 1.20 15.98 2.07 4.63 13.32 0.58 7.72 1947.7-1950.7 1.33 20.35 1.87 5.55 15.30 0.71 10. 88 1950.7-1953.8 0.91 12.08 1.82 3.57 13.27 0.50 6.64 1959.9-1962.9 1.06 35.42 2.92 8.65 33.42 0.36 12.13 1969.0-1972.1 2.21 38.03 2.87 9.99 17.21 0.77 13.25 1978.2-1981.2 1.83 21.12 2.90 6.34 11.54 0.63 7.28 1984.2-1987.3 1.39 16.64 2.71 5.07 11.97 0.51 6.14 1993. 4-1994.9 1.81 21.89 2.58 6.40 12.09 0.70 8.48 2002.5-2004.1 2.08 25.65 2.47 7.32 12.33 0. 84 10.38 2011.7-2013.2 1.22 10.34 2.17 3.54 8.48 0.56 4.76 2020.8-2022.3 1.36 15.71 2.47 4.80 11.55 0.55 h. 36 2026.9-2030.0 1.31 12.12 2.74 4.12 9.25 0.48 4.42 2037.6-2039.1 1.23 13.78 3.23 4.53 11.20 0.38 4.27 2045.2-2046.7 1.15 15.85 3.22 4.88 13.78 0.36 4.92 2062.0-2063.5 1.27 16.92 3.16 5.16 13.32 0.40 5.35 2072.6-2075.7 1.20 17.84 3.30 5.33 14.87 0.36 5.41 2094.0-2097.0 1.01 12.50 3.29 4.13 12.38 0.31 3.80 2106.2-2107.7 1.16 21.23 3.26 5.97 18.30 0.36 6.51 2110.7-2112.3 17.47 3.30 5.21 15.46 0.34 5.29 2121.4-2124.5 1.05 19.61 3.10 5.53 18.68 0 . 34 6.33 2130.6-2133.6 1.56 18.31 3.20 5.66 11.74 0.49 5.72 2139.7-2142.7 1.15 18.49 3.41 5.46 16.08 0.34 5.42 2148.8-2151.9 1.06 13.24 3.53 4.37 12.49 0.30 3.75 2158.0-2161.0 2.88 30.30 3.44 9.09 10.52 0.84 8.81 2167.1-2170.2 1.62 20.49 3.09 6.11 12.65 0.52 6.63 2176.3-2179.3 1.07 14.11 3.06 4. 43 13.19 0.35 4.61 2188.5-2191.5 1.28 15.76 2.98 4.89 12.31 0.43 5.29 2200.7-2203.7 1.03 16.44 3.39 4.96 15.96 0.30 4.85 2206 . 8-2 209.8 0.83 11.79 1.58 3.39 14.20 0.53 7.46 2218.9-2222.0 0. 88 15.67 3.23 4.65 17.81 0.27 4.85 2225.0-2228.1 0.91 12.91 3.28 4.13 14.19 0. 28 3.94 2237.2-2240.3 1.06 14.74 3.46 4.66 13.91 0.31 4.26 2249.4-2252.5 1.05 15.16 3.52 4.75 14.44 0.30 4.31 2258.6-2261.6 1,20 15.08 3.43 4.82 12.57 0.35 4.40 2267.7-2270.8 0.99 14.64 3.38 4.56 14.79 0.29 4.33 2279.9-2283.0 0.99 15.75 3.48 4.81 15.91 0.28 4.53 2289.0-2292.1 1.09 14.22 3.12 4. 48 13.05 0.35 4.56 2319.5-2322.6 1.05 19.93 3.43 5.68 18.98 0.31 5.81 2350.0-2353.1 1.11 16.52 3.51 5.06 14.88 0.32 4.71 2383.5-2385.1 1.16 17.18 2.65 5.00 14.81 0.44 6.48 2411.0-2414.0 0.93 14.35 3.25 4.43 15.43 0.29 4.42 2435.4-2438.4 1.10 13.65 3.37 4. 44 12.41 0.33 4.05 2441.4-2444.5 0.84 11.75 3.38 3.88 13.99 0.25 3. 48 2465.8-2468.9 1.00 11.62 3.54 4.01 11.62 0.28 3.28 2505.5-2508. 5 1.07 15.55 3.36 4.80 14.53 0.32 4.63 2535.9-2539.0 1.10 15.76 3.20 4.82 14.33 0.34 4.93 2560 .3-2563.4 1.11 15.55 3.30 4.81 14.01 0.34 4.71 2584.7-2587.8 1.09 14.14 3.00 4.43 12.97 0.36 4.71 2593. 8-2596.9 1.19 15.13 3.38 4.81 12.71 0.35 4.48 2621.3-2624.3 1.06 14.76 3.38 4.64 13.92 0.31 4.37 2654.8-2657.9 1.18 17.76 1.35 4.78 15.05 0.87 13.16 2664.0-2667.0 1.81 26.43 2.53 7.29 14.60 0.72 10.45 2673.1-2676.1 1.71 26.33 2.41 7.16 15.40 0.71 10.93 2682.2-2685.3 1.62 26.55 2. 85 7.26 16.39 0.57 9.32 2694. 4-2697.5 2.04 27.75 2.97 7.84 13.60 0.69 9.34 ! Sample RaeU Th K Heat Th RaeU Th depth (ppm) _ (ppm) (per- - (ucal/ 1 (meters) cent) g-yr) RaeU - Ex10~¢ Kx104 I |2700.5-2703.6 2.45 27.96 2.72 8.11 11.41 0.90 10.28 , 2715.8-2718.8 2.41 25.82 2.72 7.66 10.71 0.89 9.49 2724.9-2728.0 2.14 28.33 2.76 7.97 13.24 0.78 10.26 2734.1-2737.1 2.30 20.48 2.36 6.41 8.90 0.97 8.68 2746.2-2749.3 1.64 18.53 2.64 5.62 11.30 0.62 7.02 2767.6-2770.6 1.99 24.57 2.67 7.09 12.35 0.75 9.20 2776.7-2779.8 1.87 23.24 2.68 6.74 12.43 0.70 8.67 2788.9-2792.0 2.37 18.78 2.69 6.21 7.92 0.88 6.98 2798.1-2801.1 2.91 17.17 2.86 6.33 5.90 1.02 6.00 2807 .2-2810.3 1.58 8.14 3.11 3.62 5.15 0.51 2.62 2816.4-2819.4 2.33 12.79 2.78 5.01 5.49 0. 84 4.60 2828.5-2831.6 1.12 7.87 2.81 3.15 7.03 0.40 2.80 2840.7-2843.8 1.18 8.02 3.09 3.30 6.80 0.38 2.60 2846.8-2849.9 1.22 9.60 3.05 3.63 7.87 0.40 3.15 2862.1-2865.1 0.91 6.48 3.04 2.78 7.12 0.30 2.13 2874.3-2877.3 1.19 13.13 2.87 4.27 11.03 0.41 4.57 2886.5-2889.5 1.15 16.06 3.06 4.88 13.97 0.38 5.25 2898.6-2901.7 1.22 16.16 3.05 4.95 13.25 0.40 5.30 2907.8-2910.8 13.86 23.05 3.74 15.74 1.66 3.71 6.16 2920.0-2923.0 6.16 21.71 3.21 9.71 3.52 1.92 6.76 2932.2-2935.2 2.50 22.21 2.96 7.07 8.88 0. 84 7.50 2941.3-2944.4 1.90 21.40 3.06 6.49 11.26 0.62 6.99 2950.5-2953.5 1.72 16.76 2.88 5.39 9.74 0.60 5.82 2962.7-2965.7 2.08 18.22 2.86 5.93 8.76 0.73 6.37 2974.8-2977.9 1.71 15.54 2.63 5.07 9.09 0.65 5.91 2984.0-2987.0 2.31 13.90 2.43 5.12 6.02 0.95 5.72 2993.1-2996.2 1.87 14.14 2.86 4.97 7.56 0.65 4.94 3002.3-3005.3 1.56 18.48 2.83 5.60 11.85 0.55 6.53 3014.5-3017.5 1.52 17.77 2.87 5.44 11.69 0.53 6.19 3023.6-3026.7 2.56 23.03 2.54 7.16 9.00 1.01 9.07 3035.8-3038.9 2.16 13.02 2.46 4.84 6.03 0.88 5.29 3045.0-3048.0 1.73 10.52 2.42 4.02 6.08 0.71 4.35 3051.0-3054.1 1.71 9.89 2.65 3.94 5.78 0.65 3.73 3060.2-3063.2 2.66 16. 89 2.57 6.01 6.35 1.04 6.57 The thorium content for all samples ranges from 4.57 to 78.3 ppm; the mean and standard deviation is 17.6+8.76 ppm, which is very near the average for silicic igneous rocks (table 3). Most of the thorium analyses are within the range of averages reported for intermediate and silicic igneous rocks. The tho- rium content in the depth interval from 2,040 to 2,650 m is limited to a narrower range than that of all samples, but the mean content (16.1+3.38 ppm) is not significantly different. Virtually all the samples in which the thorium content is greater than three standard deviations from the mean are from a highly fractured zone near the top of the drill hole. The potassium content ranges from 1.35 to 5.23 percent; the mean and standard deviation is 3.09+ 0.58 percent. The potassium content in most of the samples is within the range commonly measured in rock types ranging from granodiorite to quartz mon- zonite. The mean and standard deviation of the potassium analyses in the interval from 2,040 to 2,650 m is 3.24+0.34 percent. 11 pardures ;o qurodptuw 1e are syjdap ardureg areas puo42q squrod sy1ew yor; Su07 'ajoy [[lup daop-wur-90°g ut jeay otuaSotpe1 pue some pue quowafo0IpPEY-'*C aeaA-B/jeo r 43d SLHVd RADIOELEMENT DISTRIBUTION, PRECAMBRIAN CRYSTALLINE ROCKS, WIND RIVER MOUNTAINS, WYOMING as to ho - on £ & ho as N co A o n o m o a n o n - o. 0 o o o a B to ho - 0 06 0 0 c o p a w n os o ____.~. T ;.T__._.M.___ T L_ ie 25 flfi__.”_._ Freep faye s r of ( Te / (7 unl G (7 tare 7 's' G5. | a - - 0€ a+ . f P .+ 6 ¢ |_ - 0 ide (s > fs toss [>< - > % : s L- eal & +3 - sel fin. & x/ 2 i & (- & * *. & ¥ * 1+ i] » | +2 No , , M t F z o s A & [~ ... i P ... +,* .. « 7 a i %* « i+ RS & ? A 4. t= : +, +2 **4 * 4 ** - t A& % * Mad th® |- *. » * 4, * *s r° * at. m fl n.- bo * kd d L6 at &+ £0. 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[x 1 1 LT | --L It po a t ) IVIH v-OL X (X/uL) v+-0L x ngey/uL UL noey SH3LIWO1TX NI'3DVIHNS WOH4 HL430 I1IWVS 12 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES TABLE 3.-Averages of radioelement contents and ratios and radiogenic heat for samples from a 3.06-kilometer drill hole, Wind River Mountains, Wyoming, and summary of published averages for igneous rocks [Drill hole data reported as average +1 standard deviation] All samples 2040- to 2650-m interval Average in igneous rocks Constituent Average Range Average Range Continental Mafic Inter- Silicic crust mediate a b e é a RaeU (ppm) ------- 2.02%1.33 0.42-13.86 1.15%0.33 0.83- 2.88 2.8 0.9 2.0-2.6 94 4.1 ©4.75 c e e a Th (ppm)------~-- 17.6 +8.8 4.57-78.3 16.1 #3.4 11.6 -30.3 6-10 2:7 8.5-9.3 20 18 a d a a K (percent) ------ 3.09%0.58 1.35- 5.23 3.24%0.34 1.58- 3.54 1.6-2.6 0.6-0.75 2.7-3.0 3.6 b b 2.6 3.179 a ¢ £ £ Th/RaeU---------- 10.3 16.7 1.66-67.5 14.3 £+2.0 10.5 -19.0 b3.5-4 £3 41.1 84.5 3.6 4.8 4.0 -4 a b g b RaeU/Kyx10 _------ 0.09- 4.64 0.36+0.10 0.25- 0.84 1 0.6 0.7-1 1.29 Th/leO—4 ———————— 5.76%12.43 1.20-15.9 5.01%1.12 3.28- 8.81 "313 b2 8 43.8-3.4 25.0 4.9 Heat (ucal/g-yr)- 5.84%2.20 1.93-17.9 4.93%0.90 3.39- 9.09 93.7-4.6 "1.4 "3.9-4.6 aRogers and Adams (1969b) . iClark, Peterman, and Heier (1966). Heier and Rogers (1963). Z. E. Peterman (written commun., 1963). Rogers and Adams (1969a) . Calculated from published values. Cocco and others (1970). The uranium (RaeU) content of all samples from the exploratory hole ranges from 0.42 to 13.86 ppm. Most of the uranium analyses are within the range of averages for mafic to intermediate rocks. The mean and standard deviation is 2.02+1.33 ppm, which is near the average for diorite and quartz diorite (Clark and others, 1966). The mean and standard deviation of the uranium content for the depth interval from 2,040 to 2,650 m is 1.15+0.33 ppm, which is close to the average for mafic igneous rocks (table 3). The uranium content in this 610-m interval is less than the content usually measured in quartz monzonite. A histogram of the data (fig. 6) shows that low uranium content is not unique to this interval, but most of the samples have higher content. The Th/RaeU ratio ranges from 1.66 to 67.5. A histogram of the data (fig. 6) indicates a bimodal distribution with the two groups separated at a ratio of about 10.0. The data from the 2,040- to 2,650-m depth interval are limited to and contribute greatly to the group of higher ratios. The average Th/U ratio in most rocks ranges from 3 to 5; virtually all samples from the drill hole have ratios greater than normal. These abnormally high ratios are indicative of either uranium depletion or excess thorium. Average U/KX10- ratios range from 0.6 in mafic [ rocks to 1.3 in silicic igneous rocks. The mean and standard deviation for all samples from the explora- tory hole is 0.68+0.44, and for the 2,040- to 2,650-m depth interval they are 0.36+0.10. The ratios from the drill hole samples indicate either that the rock types are in the mafic to intermediate classification, that uranium is depleted, or that potassium is in excess. Average Th/KX10- ratios range from 2.8 in mafic rocks to 5.0 in silicic igneous rocks. Mean and 'standard deviations for the drill hole data are 5.76++ 2.43 for all samples, and 5.01+1.12 for the 2,040- to 2,650-m depth interval; a histogram of the data indicates no significant difference in the two sets of data. The means are not greatly different from the average for silicic igneous rocks, although the ratios in the small percentage of the samples that contain anomalously large amounts of thorium are signifi- cantly greater than normal. A histogram of the radiogenic heat data (fig. 6) indicates no major difference between the distribu- tion for all samples and that for the 2,040- to 2,650-m depth interval. The means and standard deviations for these two groups of samples are 5.84+2.20 and 4.93+0.90, respectively. Most of the radiogenic heat values are similar to those calculated in intermediate to silicic igneous rocks. RADIOELEMENT DISTRIBUTION, PRECAMBRIAN CRYSTALLINE ROCKS, WIND RIVER MOUNTAINS, WYOMING 13 20 - 20 15 10 Th/RaeU 1.4 1.6 1.8 6 R =- (RaeU/K) X 10-4 >1.8 D as &n PERCENT OF SAMPLES 10 (Th/K) X 10-4 HEAT, IN ucal/g-YEAR FiGurE 6. -Frequency distribution of radicelement concentrations and ratios and radiogenic heat. Open bars are all 229 samples; shaded bars are 37 samples at depth interval 2,040-2,650 m plotted as a percent of the total. 14 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES CONCLUSIONS The variable distributions of the uranium, thorium, and potassium contents throughout the depth of the drill hole substantiate observations of similar variations in rock type and composition determined from petrographic examinations of a few core samples from the hole. The radioelement data and geophysical logs of the hole indicate that the drill hole penetrates a thick section of layered, fractured, heterogeneous rock. Thorium and potassium analyses indicate that the rock types range from intermediate to silicic and that virtually all the samples are more silicic than granodiorite, based on published average radioelement contents for those rock classifications. Radium equivalent uranium analyses indicate that the uranium content in most of the samples is less than the average reported for granodiorite. The amount of uranium measured may be less than the amount present because of a deficiency of daughter products in the uranium decay series. If the uranium is not in equilibrium, the disequilibrium is probably long lived and probably occurs among the isotopes 28U, U, and **°Th. If the uranium series is in equilibrium, abnormally low uranium content in the original material from which the rock was formed is indicated. We favor the possibility that the uranium was mobilized and removed, probably at the time of metamorphism. Uranium content is lower in relatively unfractured sections, for example, in the depth interval from 2,040 to 2,650 m, than it is in highly fractured sections of the hole. The uranium content in the unfractured sections may represent the content of that rock type prior to the occurrence of the over- thrust and its associated fracturing; if so, the higher | uranium content in the fractured zones may repre- sent enrichment through migration of uranium from other areas in the Wind River Mountains. If the uranium loss in the unfractured rock occurred during or after the thrusting, the uranium may have mi- grated vertically and redeposited in the fractures. REFERENCES CITED Clark, S. P., Jr., Peterman, Z. E., and Heier, K. S., 1966, Abun- dances of uranium, thorium, and potassium, sec. 24 of Hand- book of physical constants (revised ed.): Geological Society of America Memoir 97, p. 521-541. Cocco, G., Fanfani, L., Zanazzi, P. F., Heier, K. S., and Billings, G. K., 1970, Potassium, chapter 19 of Wedepohl, K. H., ed., Handbook of geochemistry, v. 2, no. 2; New York, Springer- Verlag, p. A1-N1. Ebens, R. J., and Smithson, S. B., 1966, Petrography of Pre cambrian rocks from a 3.05-kilometer-deep borehole, Wind River Mountains, Wyoming: University of Wyoming Contri- butions to Geology, v. 5, p. 31-38. Heier, K. S., and Rogers, J. J. W., 1963, Radiometric determination of thorium, uranium, and potassium in basalts and in two magmatic differentiation series: Geochimica et Cosmochi- mica Acta, v. 27, no. 2, p. 137-154. Lachenbruch, A. H., and Bunker, C. M., 1971, Vertical gradients of heat production in the continental crust, 2. Some estimates from borehole data: Journal of Geophysical Research, v. 76, no. 17, p. 3852-3860. Rogers, J. J. W., and Adams, J. A. S., 1969a, Thorium, chapter 90 of Wedepohl, K. H., ed., Handbook of geochemistry, v. 2, no. 4: New York, Springer-Verlag, p. B1-Ol. ____1969b, Uranium, chapter 92 of Wedepohl, K. H., ed., Hand- book of geochemistry, v. 2, no. 4: New York, Springer-Verlag, p. B1-O1. j Schonfeld, Ernest, 1966, Alpha M-An improved computer pro- gram for determining radioisotopes by least-squares resolu- tion of the gamma-ray spectra: Oak Ridge, Tenn., U.S. Nation- al Laboratory (Publications), ORNL-3975, 43 p. Smithson, S. B., and Ebens, R. J., 1971, Interpretation of data from a 3.05-kilometer borehole in Precambrian crystalline rocks, Wind River mountains, Wyoming: Journal of Geo- physical Research, v. 76, p. 7079-7087. Ages of Igneous Rocks in the South Park- Breckenridge Region, Colorado, and their Relation to the Tectonic History of the Front Range Uplift By BRUCE BRYANT, RICHARD F. MARVIN, CHARLES W. NAESER, and HARALD H. MEHNERT SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES GEOLOGICAL SURVEY PROFESSIONAL PAPER 1199-C CONTENTS Page Abstract 15 Introduction 15 Outline of the geology of the South Park-Breckenridge region 17 Fission-track dating procedure 17 Age of the South Park Formation 17 Intrusive rocks 19 Sample localities 21 Age of the intrusive rocks 21 Extrusive rocks .. 0 23 Age of the main deformation in South Park and along the margin of the Front Range uplift ...... 24 Implications of ages for the extent of the Eocene erO8i0N SUFfACG 24 References cited.. 200 25 ILLUSTRATIONS Page FIGURE 7. Index map of central and western COIOFAGO 16 8. Geologic map of the South Park-Breckenridge TORIONM isin 18 9. Graphical representation of mineral ages 19 10. Normative quartz-plagioclase-orthoclase diagram for intrusive rocks of the South Park-Breckenridge region 21 TABLES Page TABLE - 4. K-Ar and fission-track ages of igneous rocks in the South Park- Breckenridge region 20 5. Comparative classifications of intrusive rocks in the South Park- Breckenridge region 21 6. Location and description of dated and analyzed samples ...... 28 7. Analytical data for K-Ar ages of igneous rocks in the South Park- Breckenridge region . 32 8. Analytical data for fission-track ages of igneous rocks in the South Park-Breckenridge region 33 9. New chemical analyses and norms of Tertiary igneous rocks from the South Park-Breckenridge region 34 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES AGES OF IGNEOUS ROCKS IN THE SOUTH PARK-BRECKENRIDGE REGION, COLORADO, AND THEIR RELATION TO THE TECTONIC HISTORY OF THE FRONT RANGE UPLIFT By BRUCE BRYANT, RICHARD F. MARVIN, CHARLES W. NAESER, and HARALD H. MEHNERT ABSTRACT Potassium-argon ages of biotite and fission-track ages of zircon, sphene, and apatite show that porphyries of the Colorado mineral belt between the Breckenridge and Tarryall districts are predominantly of late Eocene and early Oligocene age, ranging from 35 to 42 m.y. (million years) old. A discrepancy between biotite ages of 44 to 50 m.y. and zircon, sphene, and apatite ages of 35-42 m.y. in some rocks may be due to excess argon derived from Precambrian basement rocks rather than to later regional heating or slow cooling. Concordant ages of biotite and zircon were found in one sample of porphyry at 41 m.y. The existence of coeval extrusive rocks is indicated by a 40-m.y.-old andesite in a paleovalley fill in southwestern South Park. Radiometric ages show that sedimentary and volcanic rocks were deposited in the South Park basin throughout the Paleocene. The main phase of the Laramide orogeny in South Park took place in the early Eocene, when the basin fill was folded, faulted, and overridden by the west margin of the Front Range uplift and before intrusion of the porphyries. INTRODUCTION The South Park-Breckenridge region lies at the intersection of the northeast-trending Colorado mineral belt and the north-northwest-trending wes- tern margin of the Front Range uplift (fig. 7). The Colorado mineral belt, characterized by many Late Cretaceous and Tertiary intrusive rocks and asso- ciated mineral deposits, was recognized by Spurr and Garrey (1908) and described in some detail by Lovering and Goddard (1950). Lovering and God- dard had no way of determining the ages of the intrusive rocks, but determined relative ages in part on the assumption that petrographically similar rocks were of similar age. They concluded that intrusion of the igneous rocks of the belt occurred over a long period of time and progressed from southwest to northeast. More recently, isotopic dating has furnished ages of the intrusive porphyries of the mineral belt. K-Ar ages of minerals from the porphyries in the north- eastern part of the belt and in the Leadville district adjacent to the Sawatch Range are 72 to 64 m.y., or Late Cretaceous to earliest Paleocene (Hart, 1960; 1964; Pearson and others, 1962). Further isotopic dating revealed that a few of the intrusives are 26 to 38 m.y. old (Oligocene), principally the ones asso- ciated with molybdenum deposits (McDowell, 1971; Taylor and others, 1968; Schassberger, 1972). Far- ther southwest along the mineral belt, isotopic da- ting of rocks in the Aspen region showed that small intrusives in the mining district are 67 to 72 m.y. old (Late Cretaceous) but that the larger igneous com- plexes of the Elk Mountains are 29 to 34 m.y. old (Oligocene) (Obradovich and others, 1969). This latter age range is similar to that of the bulk of the volcanic field of the San Juan Mountains in south- western Colorado (Lipman and others, 1970). Rela- tively few ages in the range 45-55 m.y. have been obtained from intrusive rocks in the mineral belt, but some have been reported. Intrusive rocks along a 20-km segment of the mineral belt in the South Park-Breckenridge region have been determined by us to be late Eocene to early Oligocene in age, 41 to 35 m.y. old. The relation of these rocks to the fault at the west margin of the Front Range uplift indicates that these intrusions. occurred after Laramide uplift of the Front Range block. 15 16 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES a* 108° f ( i I m i 0 o C North Park Mi Park © s 40° |- 9“ s % oGolden p Gore Range o * Yy Tenmile Range: DENVER (£1 FA Aspen «__/ e Q’O - # .. /. 38° - Elk Mountains o \ @ ~ eColorado Springs 30° - 0 50 100 KILOMETERS EXPLANATION Tertiary volcanic rocks Tertiary and Upper Cretaceous intrusive rocks Phanerozoic sedimentary rocks Area of figure Precambrian rocks __a __a Thrust fault, teeth on upper plate Fault —————— Outline of Colorado mineral belt FIGURE 7.-Index map Pf central and western Colorado showing the location of the South Park-Breckenridge area outlined and major tectonic features associated with the Front Range uplift. Simplified from King and Beikman (1974). AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO 17 OUTLINE OF THE GEOLOGY OF THE sOUTH PARK-BRECKENRIDGE REGION A continuous downfolded and faulted belt of Cre taceous and older sedimentary rocks, 250 km long, borders the Front Range on the west and separates it from the Park, Gore, Tenmile, and Mosquito Ranges farther west (fig. 7). Mountains formed by Tertiary igneous rock divide this belt into three broad basins: North, Middle, and South Parks. Along 150 km of the west margin of the Front Range uplift, Precambrian rocks are thrust westward over sedimentary rocks of the basins along the Williams Range fault in Middle Park and the Elkhorn fault in South Park (fig. 8). In the South Park-Breckenridge region, Tertiary intru- sives of the Colorado mineral belt interrupt the belt of sedimentary rock west of the Front Range uplift and separate Middle Park from South Park. In South Park, Tertiary sedimentary and volcanic rocks as much as 3,000 m thick form the South Park Formation (Sawatzky, 1967; Wyant and Barker, 1976). The South Park Formation overlies the Upper Cretaceous Laramie Formation, Fox Hills Sandstone, and Pierre Shale. At the base of the South Park Formation are andesitic flows and breccias and tuffaceous sandstone and conglomerate of the Reinecker Ridge Volcanic Member (Wyant and Barker, 1976). These volcanic and volcaniclastic rocks are overlain by a conglomerate rich in igneous clasts in the basal part. Stratigraphically higher, the conglomerate also contains sedimentary clasts from Paleozoic and Mesozoic rocks that crop out on the west side of the South Park basin. Beds of sandstone, mudstone, and tuff interlayered with the conglomerate make up a large proportion of the upper part of the unit. The Link Spring Tuff Member, about 2,000 m above the base of the South Park Formation, consists of tuff, tuffaceous conglomerate, and andesite as much as 200 m thick. The upper part of the South Park Formation is composed of arkose, con- glomerate, mudstone, and tuff and contains beds of boulder conglomerate in its highest part. Boulders in the conglomerate are as much as 3 m in diameter and are composed mainly of Precambrian granitic rock like that ex- posed along the margin of the Front Range uplift east of the Elkhorn fault. Although these rocks rest unconfromably on Cretaceous rocks, the principal deformation in the South Park basin occurred after deposition of the South Park For- mation, because the main folds and faults deform both the Cretaceous and Tertiary rocks. Volcanic rocks in the South Park Formation are thicker in the western part of South Park (fig. 8) and must have been derived from the area southwest of the intrusive rocks we have dated in this study, probably from near Leadville or east of Leadville, - where igneous intrusive rocks of Paleocene age are exposed (Pearson and others, 1962; Young, 1972). FISSION-TRACK DATING PROCEDURE Sphene and zircon separates were dated by the external detection method, using muscovite as the detector. A geometry factor of 0.5 was used to determine the induced track density for the age equation. The apatite ages were determined using the population method. The defect density in most of the apatite concentrates was too high to permit the use of the external detection method. The neutron dose was determined by counting tracks in a musco- vite detector, which covered National Bureau of Standards glass SRM 962 during its irradiation. This muscovite-glass pair has been calibrated using the NBS copper flux values. The errors on the apatite, sphene, and zircon (less than 4 grains) were calculated by combining the standard deviations (number of counts) for the induced and fossil counts. The standard deviations for samples in which five or more sphene or zircon grains were counted were calculated using the procedure outlined by Naeser and others (1978). AGE OF THE SOUTH PARK FORMATION The South Park Formation (Denver Formation of Stark and others, 1949) contains leaves indicative of a Paleocene age (Stark and others, 1949; Brown, 1962). The localities specified in these publications are all in the lower part of the South Park Formation, below the Link Springs Tuff Member. Sawatsky (1967) obtained a late Paleocene or early Eocene K/Ar whole-rock age of 56+2.6 m.y. from the lowest exposed flow in the Reinecker Ridge Volcanic Mem- ber at Devils Gap (fig. 8), the type locality of the Reinecker Ridge Volcanic Member (Wyant and Bar- ker, 1976). That date suggested that much of the South Park Formation might be significantly youn- ger than the comparable lower Tertiary deposits of the Denver Formation on the east side of the Front Range uplift. Our studies of the South Park Formation, however, show that the epoch of sedimentation in the South Park basin was roughly comparable to that of the Denver Formation in the Denver basin, as earlier - co SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES to yuya 37100IWN EXPLANATION intrusive rocks Tertiary volcanic rocks: Twm, Wall Mountain Tuff; Tan, andesite Tsp, Paleocene South Park Formation: Tsl, Link Springs Tuff Member; Tsr, Reineker Ridge Volcanic Member Other Tertiary and Mesozoic and Paleozoic sedimentary rocks Precambrian rocks __a__a__. Thrust fault, teeth on upper plate, dashed where inferred ___L____ Fault, bar and ball on downthrown side, dashed where inferred -—*— Axis of syncline e4 Locality of dated sample, listed in tables 4 and 6 eS Locality of sample dated by Sawatzky (1967) Devils Gap Fairplay sOUTH PARK 15 KILOMETERS FIGURE 8. -Geologic map of the South Park-Breckenridge region showing location of samples from which minerals were dated (numbers indicate locality). Simplified from Bryant and others (1980); Tweto and others (1978), and Scott and others (1978). AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO 19 workers believed. The age of samples from the base of the South Park Formation (localities 12 and 13, table 4 and fig. 8) is early Paleocene. The sample at locality 13 was taken about 200 m above the base of the Reinecker Ridge Volcanic Member and thus is from a horizon higher than that corresponding to Sawat- zky's sample. According to Sawatzky's description of his dated sample and our observations at his sample locality, the rock there is altered. (See field no. 323, table 9.) Consequently, his whole-rock age may be too young. The sample from locality 12 is a crystal tuff from about 3 m above the base of the South Park Formation in an area where rocks youn- ger than the Reinecker Ridge Volcanic Member form the base of the formation. The rock at this locality probably is stratigraphically equivalent to rocks directly above that member. The biotite ages from localities 12, 11, and 10 are progressively younger and fit the stratigraphy. Fission-track ages of co- existing zircons from these localities are in good agreement with the biotite ages-in great contrast to some of the biotite and zircon ages for the younger intrusives (fig. 9, table 4). These data show that deposition of the South Park Formation occupied the entire Paleocene. Some of the lowest beds may be of latest Cretaceous age, and the uppermost beds, which were not dated, might be as young as earliest Eocene. INTRUSIVE ROCKS In intrusive bodies of the Breckenridge area, Ran- some (1911) and Lovering (1934) recognized three main rock types: a quartz monzonite porphyry dis- tinguished by phenocrysts of potassic feldspar as much as several centimeters long; a monzonite por- phyry characterized by smaller phenocrysts of horn- blende, biotite, and plagioclase and by a lack of quartz phenocrysts; and an intermediate type they called quartz monzonite. Ransome (1911) found no contacts between the rock types and believed that they were closely related in age and origin. Lovering (1934) found a few exposures where the quartz monzonite porphyry cuts monzonite and quartz mon- zonite, and he inferred, assuming a normal sequence of magmatic differentiation, that the porphyritic quartz monzonite was younger than the other two rock types. In some intrusives, both Lovering and we have noted gradations between the monzonite and quartz monzonite. The mapping of those two rock , types may be rather arbitrary in many places. Pride and Robinson (1978) determined a sequence of intrusion in the area of the Wirepatch Mine, in the > (- E GEOLOGIC AGE Z T D Q CRETACEOUS PALEO- OLIGO- 9 CENE ene CENE f T T T *- .~! T -o- o az. 2 -e- wen -o- -t- -i- e -A- $ 3 hse w 3 "* k cC o ---(}... & § | =, E macys. § s __O__ -e- 6 -A- 2 -a- _- C * p 8 3% © f, -A- 5 8 9 gpo § -k ui 10 -o- C -i- 2 ere f_ t € 11 -e- el --a& -- LL ¥ o 12 +s- a. |-- £ f. ol} 3 y 13 O | | | 70 60 50 40 30 AGE IN MILLIONS OF YEARS EXPLANATION ® K-Ar biotite age Fission-track ages aA Zircon - A Sphene - O Apatite FiGurE 9.-Graphical representation of mineral ages from rocks in the South Park-Breckenridge region. Horizontal bars show range of uncertainty (2 sigma) for each determination. Two lines between epochs delineate range of uncertainity of absolute age of boundary (J. D. Obradovich and G.A. Izett, written commun, 1979). eastern part of the Breckenridge district, in which quartz monzonite porphyry is younger than mon- zonite poyphyry and also includes a younger rhyo- dacite porphyry and intrusive breccia. They cite an age of 41.4 m.y. for the rhyodacite porphyry but do not say how that age was determined. In the Tarryall district, south of Breckenridge, Muilenburg (1925) mapped the south end of a large sill (the Bald Mountain sill, fig. 8) as quartz mon- zonite porphyry, because quartz phenocrysts were 20 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES TABLE 4.-K-Ar and fission-track ages of igneous rocks in the South Park-Breckenridge region [Localities and sample descriptions in table 6; analytical data in tables 7 and 8 in appendix. Leaders (......) indicate no determination made.] Loc. Field K-Ar Fission, Mineral No. No. biotite age track age dated Intrusive rocks 1 223 44 .8+1.5 35.1% 3.6 Zircon. 43.1%1.0 1 a4.0%1.0 2 222 50.641.7 38.3% 1.8 Do. 45.5%1.1 36.64 2.0 Do. 2 49.1%1.2 36.2% 1.6 Do. 466 ___ _ -------- 38.2% 1.8 Do. 3 74 44,.0*1.5 40.4% 1.7 Sphene. 37.9% 1.6 Zircon. 42.74 8.2 Apatite. 4 233 == === _ -------- 39.9t 2.0 Sphene. 35.5% 1.4 Zircon. 41.1*10 Apatite. 5 503 * 40.7%1.4: .: 1.8 Zircon. 40.6+ 5.8 Apatite. 503a 43.2%1,5 __ <--------< __ 6 502 ___ _ -------- 41.5% 1.8 Zircon. 7 328 ___ _ -------- 37.64% 1.5 Do. * 56.4 7.4 _ Apatite. Extrusive rocks 8 430a 37.8%0.9 41,5% 1.8 Zircon. 42.8% 5.1 Apatite. 9 420 ___ _ -------- 36.4% 1.9 Zircon. South Park Formation 10 W-1-75 56.3%1.3 55.5% 2.7 Zircon. 66.5+11.6 Apatite. 11 150 59.7+2.0 54.7% 4.8 Zircon. 66.4*12 Apatite. 12 LAL 65.5+1.6 64.1%. 3.7 Zircon. 59.34 8.1 Apatite. 13 325 ___ _ -------- 60.94 3.2 Zircon. 63.3% 8.5 Apatite. Contains minor hornblende. Contains some hornblende. Contains minor hornblende and chlorite. Has many defects; age only approximate. F w N visible in the rock there. The north end of this sill was called monzonite porphyry by Ransome (1911). Muil- enburg called an intermediate rock, containing less quartz, Silverheels Quartz Monzonite Porphyry, al- though he pointed out that it was impossible to consistently differentiate the two rock types. A third, more mafic rock type, he designated diorite por- phyry. In the same region, Singewald (1942; 1951) mapped a monzonite porphyry containing little or no visible quartz, a quartz monzonite porphyry, and Lincoln Porphyry, characterized by large pheno- crysts of potassic feldspar and resembling the quartz monzonite porphyry of Ransome (1911) in the Breck- enridge district. Singewald (1942) also pointed out the difficulty of distinguishing between the first two map units. On his maps he designated the rock in the sill on Bald Mountain southwest of Breckenridge as quartz monzonite and diorite (1942) and as monzo- nite porphyry (1951). Stark and others (1949) gave a formal name, Esche Porphyry, to the igneous rocks, including the south end of the Bald Mountain sill, on the northwest side 'of South Park near Como, and they described the Esche Porphyry as quartz monzonite porphyry and quartz diorite porphyry. They thought that the igne ous rocks were emplaced contemporaneously with deposition of the South Park Formation, although they added that "at least some intrusions are later than the large north-trending faults of South Park." This review of the rock names applied to the intrusive rocks of the South Park-Breckenridge re- gion illustrates the lack of precision of field and petrographic determinations of the rock types. Chem- ical analyses (table 9; Ransome 1911; Muilenburg, 1925; Phair and Jenkins, 1975) show that these rocks range from rather potassic diorite to quartz mon- zonite (fig. 10). Most of the rocks are granodiorite to quartz monzonite, even though the term monzonite has been applied to some of the larger bodies, such as the Bald Mountain sill (localities 4 and 5, fig. 8). Bryant and others (1980) generally followed the classification of Ransome (1911), because it seemed the most practical for purposes of field mapping (table 5). The intrusive rocks occur as sills, plugs, and dikes, and they form the high peaks separating Middle Park from South Park. For example, Mt. Guyot (4,075 m) is a plug of the quartz monzonite (fig. 8, locality 3). Heat from this intrusion has metamorphosed Pierre Shale and a slice of Dakota Sandstone in the hang- ing wall of the Williams Range fault (fig. 8). Bald Mountain (4,170 m) is a sill of monzonite porphyry as much as 1 km thick; this sill cuts branches of the AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO 21 TaBLx 5.-Comparative classifications of intrusive rocks in the South Park-Breckenridge region Ransome (1911) Bryant and others (1980) Rock types based on Localities chemical analysis dated Quartz monzonite porphyry - Porphyritic quartz monzonite - Quartz monzonite------------ 1 Quartz monzonite--------- Quartz monzonite------------ Granodiorite---------------- 3 Monzonite porphyry------- MonzOMi tee------------------ Quartz monzonite (7), grano- - 2, 4, 5, diorite (2, 4, 5), and very 6, 7 minor potassic diorite. Boreas Pass-South Park fault zone (fig. 8, localities 4 and 5). A small plug of monzonite porphyry located at the junction of the Middle and South Forks of the Swan River is intrusive into and has metamor- phosed brecciated Precambrian rock along the Wil- liams Range fault zone (fig. 8, locality 2). The intrusive bodies predate some fault move ments. A sill in the Benton Group west of Kenosha Pass is cut by faults that, if they are vertical, have displacements as much as 200 m (fig. 8). The west margin of the Bald Mountain sill is offset by faults. Lovering (1934) shows numerous faults, with dis- placements as great as 200 m, that cut the intrusive rocks near Breckenridge. a & GD_ee (a) EIOM 4,5 3 @7 PI Or {=] M FIGURE 10.-Normative quartz-plagioclase-orthoclase diagram for intrusive rocks of the South Park-Breckenridge area. Based on chemical analyses from Ransome (1911), Muilenburg (1925), Phair and Jenkins (1975), and table 8, this paper. Circled points are analyses of rocks dated in this study; numbers indicate locality (fig. 8). Squares are averages for rock types from Nockolds (1954); D, diorite; M, monzonite; GD, granodiorite; QM, quartz monzonite. SAMPLE LOCALITIES We sampled each of the three major rock types for age determinations (table 5; figs. 8 and 9). One sample each of monzonite and quartz monzonite are from intrusives at or near the Williams Range fault. Locality 1 is in a sill of porphyritic quartz monzonite west of the Williams Range fault. Locality 2 (fig. 8) is in a small monzonite intrusive in the Pierre Shale along the fault zone. Dikes from the intrusive cut the fault zone, and brecciated Precambrian rock in the fault zone has been contact metamorphosed. Local- ity 3 is on the northeast margin of the Mt. Guyot plug near metamorphosed Pierre Shale and Dakota Sand- stone west of the Williams Range fault. Localities 4 and 5 are in monzonite porphyry from the southeast margin of the Bald Mountain sill in the area where it cuts branches of the Boreas Pass-South Park fault zone. Locality 6 is in a thin sill of monzonite in the Pierre Shale. Thin section study indicates that the rock at locality 6 is similar to that at localities 4 and 5 but is much more altered. AGE OF THE INTRUSIVE ROCKS K-Ar and fission-track ages of minerals are shown in fig. 9 and table 4; analytical data are listed in tables 7 and 8 in the appendix. Biotite ages from localities 1 through 3 are 44 to 55 m.y., but fission-track ages of zircon from these same localities are 35-38 m.y. Thus there are discrepancies of 9 m.y., 8 to 13 m.y., and 6 m.y. between the K-Ar ages of the biotite and the fission-track ages of the zircon in samples from localities 1, 2, and 3 respec- tively. Ages were determined for biotite from three different separates of the samples from localities 1 and 2 in an attempt to resolve these discrepencies. To confirm the zircon ages, a second determination was made from the original separate from the sample of locality 2, and a third was made on a new separate from the same sample. To further test the validity of the discordant ages for samples from locality 2, we dated a zircon from 22 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES contact-metamorphosed brecciated Precambrian rock (locality 2, sample 466) adjacent to the intrusive. The age of 38.2 m.y. is the same as that obtained from zircons from the intrusive rock. Fission-track ages of apatite (Bryant and Naeser, 1980) from Precambrian rocks 1-2 km from the exposed plutons are older than the zircon ages from the intrusive rocks; thus the discrepancy between zircon and biotite ages cannot be due to regionai heating or cooling 35-38 m.y. ago. Excellent agreement of ages of about 41 m.y. was obtained from biotite, zircon, and apatite from the Bald Mountain sill at locality 5 (table 4). A second biotite determination from locality 5 was made on an inclusion, and it agrees well with those from the sill, within the respective limits of analytical uncertain- ty. The sample from locality 4 is from the same sill, about 2 km from locality 5. The two samples are petrographically similar (table 6), except that the rock at locality 4 is somewhat more altered than that at 5. Ages of sphene and apatite from locality 4 agree with the ages from locality 5, but the zircon is about 5 m.y. younger at 4 than at 5. We conclude that the Bald Mountain sill was emplaced about 41 m.y. ago in the late Eocene. A small sill at locality 6, 6 km to the southeast, has the same age. The general agreement of ages of apatite, sphene, and zircon from the intrusive rocks indicates that the rock cooled quickly, for the annealing temperature of apatite is much lower than that of sphene and zircon (Naeser and Faul, 1969). A notable exception is from locality 7, where the apatite age of 56.4 m.y. is older than the zircon age of 37.6 m.y. In that sample, however, the apatite has many crystal defects that may have been counted as fission tracks. The authors have found, in many previous inves- tigations where both fission-track and K-Ar ages were determined, that agreement of K-Ar and fission- track ages is usually good, except where a thermal event has caused loss of radiogenic argon and (or) annealing of fission tracks (Cunningham and others, 1977; Naeser and others, 1977). Following such an event, the fission-track ages of sphene are usually similar to the K-Ar age of biotite, which, in turn, is usually greater than the fission-track age of zircon, which is greater than the fission-track age of apatite (Harrison and others, 1979). In the present study, however, the K-Ar and fission-track ages for min- erals from localities 1 through 3 indicate a rather unusual situation. The K-Ar biotite ages are signi- ficantly older than the fission-track zircon ages of sphene, zircon, and apatite. The fission-track ages of these three minerals are concordant. It seems il- -/ logical that a thermal event, after emplacement, could anneal fission tracks in the zircons without resetting the apatite ages in nearby Precambrian country rock. Furthermore, it seems unreasonable that 5 to 10 m.y. could have elapsed between the time biotite became a closed system, and the time that zircon fission tracks were annealed, without pro- ducing marked discrepancies in fission-track ages of sphene, zircon, and apatite. Based on the information given above, the most logical explanation is that there is excess radiogenic argon in the biotite. It appears that the biotite from locality 1 has a nearly constant amount of excess argon distributed throughout the biotite grains, re- sulting in good agreement for triplicate argon deter- minations (table 7). Even the third biotite separate, which has a discernible hornblende impurity (in- dicated by the K, O content, table 7, appendix), has a K-Ar age concordant with those of the two clean biotite separates. The ages of biotite from locality 2, however, indicate that excess argon varies in a- mount among individual biotite grains. Again, a significant amount of impurity in the third biotite sample run from locality 2 did not produce a biotite age greater than that determined for one of the clean samples. Therefore, excess argon does not appear to be concentrated in the impurities, but must be partitioned among the mafic minerals in an approximately even manner. In line with this reasoning, the fission-track ages are accepted as the ages of emplacement of the plutons at localities 1 through 3, which are 35, 37, and 38 m.y., respectively, or early Oligocene. The Montezuma stock, northeast of Breckenridge (fig. 5), has a biotite K-Ar age, a Rb-Sr biotite whole- rock age, and zircon and apatite fission-track ages that are all concordant at 39 m.y. (McDowell, 1971; Simmons and Hedge, 1978; A. A. Bookstrom and C. W. Naeser, written communication, 1979). A zircon fission-track age of 35 m.y. has been obtained from a satellitic intrusion just to the south (Cunningham and others, 1977). These ages are similar to those obtained from rocks in the South Park-Breckenridge region. K-Ar ages of 43-45 m.y. cannot be dismissed, however, for some intrusive rocks in this region. Simmons and Hedge (1978) obtained a Rb-Sr biotite whole-rock age of 45 m.y. from a sill of porphyritic quartz monzonite at Swan Mountain, north of Breck- enridge. V. E. Surface of Climax Molybdenum Co. obtained biotite K-Ar ages of 35-47 m.y. from quartz monzonite porphyry of the Humbug stock in the Tenmile Range, about 10-km west of Breckenridge (Marvin and others, 1974). In the Humbug stock, fission-track ages of zircon also are younger than the F bo AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO 28 biotite K-Ar ages (M. A. Kuntz and C. W. Naeser, oral communication, 1978). Fission-track ages of zircon and sphene are con- cordant with K-Ar and Rb-Sr ages of biotite in the Twin Lakes stock in the Sawatch Range (fig. 7) (Moorbath and others, 1967; Obradovich and others, 1969; Marvin and others, 1974; and Marvin and Dobson, 1979). The main part of the Twin Lakes stock was intruded about 45 m.y. ago. A younger granitic intrusion that cuts the main Twin Lakes stock (Wilshire, 1969) yields zircon and sphene ages of about 35 m.y., about 10 m.y. younger than the main part of the Twin Lakes stock. This age is similar to that of a stock intruding the Grizzly Peak cauldron in the Sawatch Range about 10 km farther west (Obradovich and others, 1969). Apatite fission- track ages from the Twin Lakes stock, the younger intrusion that cuts the Twin Lakes stock, and the Precambrian rocks on the west margin of the Sa- watch Range all are less than 29 m.y. old and are all related to later uplift and cooling of the mountain block (Bryant and Naeser, 1980). Relatively few ages in the range 45-55 m.y. have been obtained on rocks in the Colorado mineral belt, but a few are reported from the northeast end of the mineral belt (Marvin and others, 1974). The number of intrusive rocks in the mineral belt that have been dated as Eocene suggests that the former idea of distinct Laramide and mid-Tertiary intrusive events is no longer acceptable. It also is now clear that neither proximity nor similarity in chemistry and (or) mineralogy implies con- temporaneity of intrusions. This point is well illustrated by the difference in the ages of the Lincoln Porphyry (64 my.) near Leadville (Pearson and others, 1962; McDowell, 1971) and the petrographically and tex- turally similar coarse-grained porphyritic quartz monzonite of the Breckenridge district at locality 1, which has an age of about 35 m.y. The latter was correlated with the Lincoln Porphyry by Ransome (1911) and Lovering (1934). In some areas of the mineral belt in the Front and Sawatch Ranges, rhyolites in small bodies and granites in larger bodies have been dated as being as young as late Oligocene or early Miocene, (Taylor and others, 1968; Naeser and others, 1973; Schass- berger, 1972; Van Alstine, 1969; Limbach, 1975). Since dikes and plugs of rhyolite commonly are altered and, consequently, not datable by some methods, the dis- tribution of these younger, more silicic rocks is imperfectly known. However, they are volumetrically less significant than the early Oligocene and late Eocene intrusives. j Traditional concepts of the igneous history of the mineral belt, involving either an orderly pro- gression of intrusive events from southwest to north- east or two well-defined intrusive events, seem to be yielding to the idea of a more continuous period of intrusion spanning the period from 70 to 25 m.y. ago. Geophysical studies suggest that the mineral belt is underlain by a batholith (Tweto and Case, 1972; Isaacson and Smithson, 1976), which may be a 'composite batholith with a long and complex his- tory. In some areas, however, such as the San Juan Mountains, most of the magma that formed the top of the batholith apparently was emplaced during Oligocene time. EXTRUSIVE ROCKS In the Antero syncline, in the southwestern part of South Park, andesite flow breccia apparently occurs in a northwest-trending paleovalley (fig. 8). The Wall Mountain Tuff, about 36 m.y. old, occurs in shallow channels carved into the andesite. These volcanic rocks are overlain by the Antero Formation of Oligocene age and the Wagontongue Formation of Miocene age. Much of the folding of the syncline is younger than the Wagontongue Formation. The age of the andesite flow breccia (locality 8, table 4 and fig. 8) is about 40 m.y. (average of biotite K-Ar age of 38 m.y. and zircon and apatite fission-track ages of 42 m.y.). This age indicates that the flow is contempor- aneous with some of the intrusive rocks discussed above but is older than the Wall Mountain Tuff and the stratigraphically higher Buffalo Peaks Andesite and Badger Creek Tuff (Epis and Chapin, 1974; Sanders and others, 1976). The intrusive bodies nearest to locality 8 are 10 km to the northwest, but they have not been dated. Stark and others (1949) classified these intrusives as quartz monzonite. Chem- ical analysis of the andesite flow breccia (field no. 430a, table 9) shows that it is much more siliceous than is apparent in outcrop or thin section and is a quartz latite. The nearby intrusives therefore are a reasonable source for the flow breccia. Southeast of Kenosha Pass (fig. 8), an andesitic ash flow (field No. 430a, mapped as andesite but shown to be a rhyodacite by chemical analysis (table 9)) occupies a southeast-trending paleovalley, the bottom of which is 50 to 150 m below a widespread erosion surface on Precambrian rocks east of South Park. This surface, known as the Elkhorn surface, has been correlated with a widespread erosion sur- face of Eocene age in the region to the south (Scott, 1975; Epis and Chapin, 1975). A fission-track age of 36 m.y. on zircon from the volcanic rock in the paleovalley is similar to ages obtained from the 24 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES intrusive rocks to the west. The trend of the paleo- valley, however, suggests that the ash flow in it is an outflow from a vent in the area near the Montezuma stock, where the intrusives are also of that age. AGE OF MAIN DEFORMATION IN SOUTH PARK AND ALONG THE MARGIN OF THE FRONT RANGE UPLIFT Our data on the age of rocks in the South Park-Breckenridge region put some constraints on the timing of deformation in the region. The Boreas Pass-South Park fault zone, northwest of Como, is cut by the Bald Mountain sill (localities 4 and 5, fig 8). Latest major movements on this fault zone occur- red after deposition of the South Park Formation in the Paleocene and before emplacement of that sill about 41 m.y. ago. This fault zone also had large pre- Laramide movements and formed the front of a late Paleozoic range. Boulder conglomerates in the up- permost part of the South Park Formation, adjacent to the Elkhorn fault, were derived from source areas east of the Elkhorn fault. They indicate that uplift and probably faulting along the west margin of the Front Range occurred during the earliest Eocene. Ages of rocks from localities 2 and 3, farther north, put a limit of middle Eocene on the minimum age of movement of the Williams Range fault. A connection between the Williams Range and Elk- horn faults in the northern part of South Park is not exposed and is inferred on the basis of geophysical data (Barker and Wyant, 1976). In the absence of a known connection with the Elkhorn fault, no infer- ence can be made about the earliest possible time of movement on the Williams Range fault except at its northernmost exposures 90 km to the northwest (fig. 7). There, earliest movement was very Late Creta- ceous, before deposition of the Middle Park For- mation, a basin fill analogous to the South Park Formation. (Izett and Barclay, 1973). However, to the east, the Middle Park Formation is folded with the Cretaceous rocks and cut by faults (Izett 1968; Tay- lor, 1975), indicating a time relation between early Tertiary deposition and deformation similar to that in South Park. In Paleocene igneous rocks along the east side of the Front Range uplift between Golden and Lyons, Hoblitt and Larson (1975) found paleomagnetic evi- dence that Laramide deformation there was older to the north than to the south. Perhaps this relation- ship is also true along the west side of the Front Range. Between Denver and Colorado Springs, several faults cut the Upper Cretaceous and Paleocene Den- ver Formation and the Paleocene and Eocene Daw- son Arkose at the eastern margin of the Front Range (fig. 7). Because the Denver and the Dawson are mostly structurally concordant with the underlying Cretaceous rocks, the structural relief at the present mountain front is due to post-Paleocene faulting. Soister (1978) and Soister and Tschudy (1978) have recently found that some of the Dawson Arkose above a well-developed soil horizon is of Eocene age, but as yet it has not been possible to map this soil zone and determine relations between rocks above and below it along the mountain front. However, north of Colorado Springs, lower Oligocene rocks (Wall Mountain Tuff; Castle Rock Conglomerate) that occur on the highest hills on the margin of the Great Plains can be projected westward to the range front. Then, if we assume that a well-developed surface on the crest of the Rampart Range, just west of the margin of the Front Range, is approximately at the level of an erosion surface of Eocene age (Scott, 1975) and about on grade with deposits laid down in the early Oligocene, we can infer about 600 m of offset by post-Oligocene movement along the range front fault. Cross sections and stratigraphic relations along the mountain front in that area suggest post- Paleocene but pre-Oligocene displacement of as much as 3 km along the same fault. Much of this displace ment may have been contemporaneous with the Eocene deformation of the South Park Formation on the west side of the Front Range uplift. IMPLICATIONS OF AGES FOR THE EXTENT OF THE EOCENE EROSION SURFACE The highest peaks between South Park and Middle Park are composed of intrusive igneous rocks of latest Eocene and earliest Oligocene age, which crop out at altitudes as great as 4,170 m, but no extru- sive volcanic rocks are known from that area. Extrusive rocks of this age were deposited in shallow valleys in the Elkhorn surface on the east side of South Park, and these are preserved at 2,900-m altitude. The Elkhorn surface correlates with the Eocene erosion surface that is so widespread to the south and east (Scott, 1975; Epis and Chapin, 1975). If we assume a minimum cover of 500 to 1,000 m on the latest Eocene and earliest Oligocene intrusive rocks at the time of intrusion, a postulated Eocene erosion surface west of South Park would presently be at an altitude of about 5,000 m. If such an erosional surface did exist west of South Park in the area underlain by intrusive rocks, substantial vertical displacement of that area since the middle Tertiary is required. Bryant and A- AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO 25 others (1980), Stark and others (1949), and Barker and Wyant (1976) have not detected any major fault bounding the west side of the park; the mapped faults trend north-northwesterly and cross the high coun- try of the continental divide underlain by the in- trusive rocks. 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W., 1967, Evidence for the origin and age of some mineralized Laramide intru- sives in the southwestern United States from strontium isotope and rubidium-strontium measurements: Economic Geology, v. 62, p. 226-236. Muilenburg, G. A., 1925, Geology of the Tarryall district, Park County, Colorado: Colorado Geological Survey Bulletin 31, 64 p. Naeser, C. W., and Faul, Henry, 1969, Fission-track annealing in apatite and sphene: Journal of Geophysical Research, v. 74, p. 705-710. Naeser, C. W., Hurford, A. J., and Gleadow, A. O. W., 1977, Reply to comment by G. A. Wagner on Fission-track dating of pumice from the KBS tuff, East Rudolf, Kenya: Nature, no. 5612, p. 649. Naeser, C. W., Izett, G. A., and White, W. H., 1973, Zircon fission-track ages from some middle Tertiary igneous rocks in northwestern Colorado [abs.]: Geological Society of America Abstracts with Programs, v. 5, no. 6, p. 498. Naeser, C. W., Johnson, N. M., and McGee, V. E., 1978, A practical method of estimating the standard error of age in the fission-track dating method, in Zartman, R. E., ed., Short papers of the fourth international conference of geo- chronology, cosmochronology, and isotope geology: U.S. Geo- logical Survey Open-File Report 78-701, p. 303-304. Nockolds, S. R., 1954, Average chemical composition of some igneous rocks: Geological Society of America Bulletin, v. 65, no. 10, p. 1007-1032. Obradovich, J. D., Mutschler, F. E., and Bryant, Bruce, 1969, Potassium-argon ages bearing on the igneous and tectonic history of the Elk Mountains and vicinity-A preliminary report: Geological Society of America Bulletin, v. 80, no. 9, p. 1749-1756. 26 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES Pearson, R. C., Tweto, Ogden, Stern, T. W., and Thomas, H. H., 1962, Age of Laramide porphyries near Leadville, Colorado in Short papers in geology and hydrology: U.S. Geological Survey Professional Paper 450-C, p. C78-C80. Phair, George, and Jenkins, L. B., 1975, Tabulation of uranium and thorium data on the Mesozoic-Cenozoic intrusive rocks of known chemical composition in Colorado: U.S. Geological Survey Open-File Report 75-501, 57 p. Pride, D. E., and Robinson, G. S., 1978, Multiple intrusion and hydrothermal activity, eastern Breckenridge mining district, Summit County, Colorado: Geological Society of America Bulletin, v. 89, p. 866-874. Ransome, F. L., 1911, Geology and ore deposits of the Brecken- ridge district, Colorado: U.S. Geological Survey Professional Paper 75, 187 p. Sanders, G. F., Jr., Scott, G. R., and Naeser, C. W., 1976, The Buffalo Peaks Andesite of central Colorado: U.S. Geological Survey Bulletin 1405-F, 8 p. Sawatzky, D. L., 1967, Tectonic style along the Elkhorn thrust, eastern South Park and western Front Range, Colorado: Colorado School of Mines D. Sc. thesis, 206 p. Schassberger, H. T., 1972, K-Ar dates on intrusive rocks and alteration associated with molybdenum mineralization at Climax and Urad,; Colorado, and Questa, New Mexico: Iso- chron/West, no. 3, p. 29. Scott, G. R., 1975, Cenozoic surfaces and deposits in the southern Rocky Mountains, in Curtis, B. F., ed. Cenozoic history of the southern Rocky Mountains: Geological Society of America Memoir 144, p. 227-248. Scott, G. R., Taylor, R. B., Epis, R. C., and Wobus, R. A., 1978, Geologic map of the Pueblo 1°X2° quadrangle, south- central Colorado: U.S. Geological Survey Miscellaneous Geo- logic Investigations Map I-1022. Shapiro, Leonard, 1975, Rapid analysis of silicate, carbonate, and phosphate rocks-Revised edition: U.S. Geological Sur- vey Bulletin 1401, 76 p. Simmons, E. C., and Hedge, C. E., 1978, Minor element geo- chemistry and Sr-isotopes of Tertiary stocks, Colorado Min- eral Belt: Contributions to Mineralogy and Petrology, v. 67, p. 379-396. Singewald, Q. D., 1942, Stratigraphy, structure, and minerali- zation in the Beaver-Tarryall area, Park County, Colorado: U.S. Geological Survey Bulletin 928-A, 44 p. _____1951, Geology and ore deposits of the upper Blue River area, Summit County, Colorado: U.S. Geological Survey Bulletin 970, 74 p. Soister, P. E., 1978, Stratigraphy of uppermost Cretaceous and lower Tertiary rocks of the Denver basin, in Pruit, J. D., and Coffin, P. E., eds., Energy resources of the Denver basin: Denver, Rocky Mountain Association of Geologists, p. 223-229. Soister, P. E., and Tschudy, R. H., 1978, Eocene rocks in the Denver basin, in Pruitt J. D., and Coffin, P. E., eds., Energy resources of the Denver basin: Denver, Rocky Mountain Association of Geologists, p. 231-235. Spurr, J. E., and Garrey, G. H., 1908, Economic geology of the Georgetown quadrangle, Colorado with General geology by Sidney H. Ball: U.S. Geological Survey Professional Paper 63, 422 p. Stark, J. T., Johnson, J. H., Behre, C. H., Jr., Powers, W. E., Howland, A. L., Gould, D. B., and others, 1949, Geology and origin of South Park, Colorado: Geological Society of America Memoir 33, 188 p. Taylor, R. B., 1975, Geologic map of the Bottle Pass quadrangle, Grand County, Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ-1224. Taylor, R. B., Theobald, P. K., and Izett, G. A., 1968, Mid- Tertiary volcanism in the central Front Range, Colorado, in Epis, R. C., ed., Cenozoic volcanism in the southern Rocky Mountains: Colorado School of Mines Quarterly, v. 63, no. 3, p. 39-50. Tweto, Ogden, and Case, J. E., 1972, Gravity and magnetic features as related to geology in the Leadville 30-minute quadrangle, Colorado: U.S. Geological Survey Professional Paper 726-C, 31 p. Tweto, Ogden, Moench, R. H., and Reed, J. C., Jr., 1978, Geo- logic map of the Leadville 1° X 2° quadrangle, northwestern Colorado: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-999. Van Alstine, R. E., 1969, Geology and mineral deposits of the Poncha Springs NE quadrangle, Chaffee County, Colorado: U.S. Geological Survey Professional Paper 626, 52 p. Wilshire, H. G., 1969, Mineral layering in the Twin Lakes Grano- diorite, Colorado, in Igneous and metamorphic geology -A volume in honor of Arie Poldervaart: Geological Society America Memoir 115, p. 235-261. Wyant, D. G., and Barker, Fred, 1976, Geologic map of the Milligan Lakes quadrangle: Park County, Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ-1343. Young, E. J., 1972, Laramide-Tertiary intrusive rocks of Colo- rado: U.S. Geological Survey Open-File Report, 206 p. TABLES 6-9 28 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES TABLE 6.-Location and description of dated and analyzed Loc. Field North West Locality description Ouadrangle Rock type No. No. Latitude Longitude Intrusive rocks 1 223 39°30'19" _ 105°57'47" _ Knob with spot elevation 11,212, Keystone--- _ Porphyritic hornblende, 500 m NE. of Brewery Hill. biotite quartz Sill(?) in Pierre Shale. monzonite. 2 222 39°29'33" _ 105°56'31" Small cliff 300 m S. 22° E. of spot Boreas Pass - Pyroxene-bearing elevation 9977 on sharp corner on hornblende-bioti te Middle Fork Swan River road. granodiorite. Small plug in Pierre Shale and Williams Range fault. 466 39°29 "31" 105°56'14" _ 3291-m altitude on ridge between -__- ----- do---- _ Brecciated and contact Middle and South Forks Swan River. metamorphosed gneiss. Williams Range fault zone. 3 74 39°27'34" _ 105°55"44" _ Small- cliff at 3672-m altitude on E. - ----- do---- _ Biotite quartz shoulder of Mt. Guyot 1 km N. 80° monzonite. W. of Georgia Pass. Stock in Pierre Shale 120 m from Williams Range fault. 4 233 39°21 "37" 10595612" Cut in abandoned railroad grade 500 Como------- Porphyritic biotite m N. 81° W. of BM 10,576 at Half- granodiorite. way Gulch. Large irregular sill in Morrison Formation to lower Pierre Shale. 5 503 39°20" 40" 10595537" Cut in old railroad grade and road __ ----- do---- _ Porphyritic hornblende- to Boreas Pass. 300 m S. 24° E. biotite quartz of Peabodys site. Same intrusive monzonite. as sample 4. 503a Biotite-rich xenolith-- 6 502 39°17'54" _ 105©53'50" _ Roadcut on U.S. Highway 285, 2 km -----do---- _ Altered porphyritic S. 15° W. of roundhouse in Como. quartz monzonite. Probable sill in Pierre Shale. 7 328 39°25'19" _ 105°50'04" _ Jefferson Hill, 3100-m altitude on Jefferson-- _ Hornblende-biotite southwest side. Sill in the andesite porphyry. Benton Group. Extrusive rocks 8 430a s9°00'48" - 1085953°55" - Sec. 9, T. 12 S., R. 75 W., 550 m Garo------- OQuartz-bearing andesite E. of SW. corner and 60 m N. flow breccia. of section line. Andesite underlying Wall Mountain Tuff. 9 420 39°22'44" - sec. 1, T. 8 S., R. 75 W., on Mt. Logan-- _ Andesitic ash flow tuff centerline of SW 1/4 and 300 m N. of S. boundary. Fills paleovalley in Elkhorn surface. ed £2 AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO samples fror:: the South Park-Breckenridge region 29 Loc. - Field Sample description Accessory Chemical analysis No. , No. minerals of similar rock Intrusive rocks i 223 Light-gray rock containing quartz phenocrysts to 0.5 mm, Sphene, apatite, - Ransome (1911, biotite to 2 mm, plagioclase to 0.5 mm, and widely zircon, and p. 45, No. 1). scattered potassic feldspar to 1 cm. Euhedral sodic opaques. andesine and calcic oligoclase with normal zoning, somewhat resorbed euhedral quartz, euhedral biotite, and small, euhedral, light-green, locally chloritized amphibole 0.2 to 0.3 mm. A matrix of quartz and potassic feldspar with a grain size of 0.05 to 0.1 mm. 2 222 Euhedral to subhedral andesine to labradorite to 1.5 mm Opaque mineral, Table 9. long; anhedral, partly interstitial quartz to 0.2 mm apatite, and in diameter; interstitial potassic feldspar; very light zircon. green anhedral monoclinic pyroxene intergrown with biotite; light-green euhedral to subhedral amphibole to 1 mm long partly intergrown with biotite and pyroxene; and anhedral to subhedral biotite to 1 mm long. 466 Fragments of quartz as large as 4 mm in diameter, fractured, Epidote, seri- Not available. partly altered garnet and feldspar in a fine-grained matrix cite, zircon, of quartz and feldspar, and postkinematic brown biotite. and opaque minerals. 3 74 Subhedral to euhedral sodic andesine to 1.5 mm long with Opaque minerals, Ransome (1911, normal and oscillatory zoning, anhedral cryptoperthitic apatite, and p. 58). potassic feldspar to 3 mm, interstitial quartz 0.3 to 1 mm, zircon. subhedral brown biotite 0.5 mm (average) to 1.2 mm (maximum), and a few grains of olive-green hornblende 0.1 to 0.6 mm long, partly altered to biotite. 4 233 Euhedral phenocrysts of sodic andesine to 2 mm long, partly Muilenburg (1925, altered to sericite and carbonate; euhedral to anhedral $. 36). resorbed phenocrysts of quartz to 1.5 mm; euhedral, partly chloritized phenocrysts of brown biotite to 1 mm in a matrix of quartz and feldspar with a grain size less than 0.05 mm. 5 503 Partly resorbed phenocrysts of andesine as long as 2 mm, Apatite, alla- Not available. somewhat resorbed euhedral quartz to 1 mm, euhedral nite, zircon, biotite to 1.4 mm, euhedral light-green hornblende to and opaque 0.6 mm, and sanidine to 3 mm in a matrix of quartz minerals. Some and feldspar with a grain size of 0.02 mm. carbonate from alteration of plagioclase. 6 502 Phenocrysts of partly resorbed quartz to 3 mm; plagioclase as Apatite, Do. much as 1.3 mm in diameter altered to albite and carbonate; allanite, and chlorite after biotite and hornblende in a fine-grained and opaque matrix of quartz, altered feldspar, chlorite, and carbonate. minerals. 7 328 Euhedral phenocrysts of biotite to 2 mm, phenocrysts and Opaque minerals, Table 9. glomerophenocrysts of andesine with oscillatory zoning apatite, and in a sodic trend to 5 mm in diameter, and hornblende zircon. and monoclinic pyroxene in a very fine grained matrix of feldspar and quartz. Extrusive rocks 8 430a _ Phenocrysts and glomerophenocrysts of andesine to 5 mm Opaque minerals, Table 9. in diameter, of biotite to 2 mm, hornblende to 1 mm, and apatite, and quartz as fragments as much as 2.5 mm in diameter, all zircon. set in a fine-grained matrix. 9 420 Euhedral to anhedral cyrstals and crystal fragments of ----do----------- See sample No. andesine, quartz, biotite, and green hornblende in a fine-grained matrix of devitrified glass. 421, table 9. 30 Loc. Field North SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES TABLE 6.-Location and description of dated and analyzed West Locality description Quadrangle Rock type No. No. Latitude Longitude South Park Formation 10 W-1-75 3991349" - Iins%an'31" - T..9 8., .R. 75 w., 120 m Elkhorn---- - Hornblende-biotite- S. of NE. corner sec. 31. quartz-plagioclase About 1600 m above base of crystal tuff. South Park Formation. 11 150 39°15'27" - 105°%49'32" _ Borrow pit on Elkhorn road 1020 m Milligan Hornblende-bioti te- SW. of BM 9405 on Elkhorn Road. Lakes. plagioclase-quartz Link Springs Tuff Member of the crystal tuff. South Park Formation. 12 Lak _ 10s%48'55" t. 10 S., R. 75 W., 200 m Elkhormn---- _ Felsic crystal tuff---- S. 51° E. from center sec. 30. Base of South Park Formation. 13 325 s0°17'51"~ i08s°51"'10" Tt. 9 s., R. 76 w., 120 m 8. 60° &. Milligan Biotite-hornblende of center of sec. 2. Unaltered Lakes. andesite flow. lens in andesite about 160 m above base of formation. Analyzed samples closely related to rocks dated in the study 14 323 39°15'17" _ 105°55'01" - Stratigraphically lowest outcrop at Como------- Andesite-------------- 2993-m altitude 122 m N. 37° E. of spring. - Sawatzky (1967) gives K-Ar whole-rock age from this outcrop. 15 421 3992211" _ 105°%42'07" - 2993-m altitude 167 m N. 70° E. of Observatory - Andesitic ash flow----- junction on Lost Park Road marked Rock. 9733 on map. Same unit as dated rock from locality 9. AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO samples from the South Park-Breckenridge region 31 Loc. Field Sample description Accessory Chemical analysis No. No. minerals of similar rock South Park Formation 10 - W-1-75 Fragments of quartz 0.1 to 1.5 mm, andesine 0.1 to 1.5 mm, Apatite, zircon, - Table 9. biotite to 1 mm, and hornblende to 0.4 mm in a matrix of and opaque devitrified glass containing some pumice fragments. One minerals. fragment of feldspathic sandstone was seen. X-ray study indicates the glass has been converted to clinoptilolite. 11 150 Biotite to 0.5 mm in diameter, dark-green to brownish-green -=---dg----------- Do. hornblende to 0.3 mm, andesine 0.2 to 0.5 mm, quartz, and partly devitirified glass. X-ray study shows the glass has been replaced by clinoptilolite. 12 Lak Crystal fragments of quartz 0.1 to 1 mm, sodic andesine ----do----------- Do. 0.1 to 1 mm, biotite to 1 mm, and green to brown hornblende in a matrix of pumice and devitrified glass. X-ray study indicates much of the devitrified glass is clinoptilotite. 13 325 Euhedral, dark-greenish-brown hornblende 0.3 to 3 mm, Epidote, opaque, Do andesine 0.2 to 2 mm with delicate oscillatory zoning, and apatite. biotite to 1 mm, anhedral quartz to 0.5 mm, and light-green monoclinic pyroxene in a matrix of devitrified glass. Quartz in amygdules. Analyzed samples closely related to rocks dated in this study 14 323 Euhedral phenocrysts of reddish-brown hornblende 0.15 to Opaque minerals Table 9. 1.5 mm long, partly altered to greenish-brown hornblende, and apatite. and of altered plagioclase 0.15 to 2 mm. Matrix of altered glass contains a few aggregates of quartz. 15 421 Fragments of andesine to 2 mm, quartz to 1.5 mm, euhedral Zircon, apatite, Do to subhedral biotite to 1.5 mm, and greenish-brown hornblende with a partly zeolitized glassy matrix. opaque minerals and carbonate. 32 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES TABLE 7.-Analytical data for K-Ar ages of igneous rocks in the South Park-Breckenridge region «[All determinations made on biotite; some additional minerals intermixed 28 noted below. "K Ae=0.581X10"%/yr. Ajg=4.962X10/yr, Atomic abundance atom/atom K. Potassium determinations made with an Li internal standard. Analysts: R. F. Marvin, H. H. Mehnert, and Violet Merritt] Loc. - Field K,0 Radiogenic "" Ar Age (m.y.) No. No. (percent) - Moles/g Percent of £20 (x10 ~!9) total Ar 1 223 8.71, 8.69 5.689 85 44 .8%+1,.5 9.01, 8.93 5.634 74 43. 1%1.5 ' 8.17, 8.20 5.252 72 44 .0*1.5 2 222 8.42, 8.42 6.214 87 50.641. 7 8.70, 8.65 5.754 87 45 2 6.07, 6.10 4.361 87 49.1+1.6 3 74 8.57, 8.61 5.511 89 44.0*1.5 5 503 3 2.57, 2.57 1.524 50 40, 741.4 503a 7.79, 7.76 4.896 78 43.2%1,.5 8 430a 7.66, 7.64 4.211 77 37.8%1.3 9 W-1-75 2.90, 3.95 3.238 45 56.3+1.9 3.99, 3.90 10 150 8.11, 8.07 7.075 80 11 Lba4 8.26, 8.30 7.952 92 65,.512.2 ! Biotite mixed with minor hornblende. Biotite mixed with hornblende. 3 Biotite mixed with minor chlorite and hornblende. AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO f 33 TABLE 8. -Analytical data for fission-track ages of igneous rocks in the South Park-Breckenridge region [Constants: A=7.03X10 yr". Analyst C. W. Naeser. Number of tracks counted is given in parentheses] Loc. Field - Lab Fossil-track Induced-track Neutron Age (129) Number - Uranium No. No. No. Mineral density! (P4) density (p ;) flux (¢) (m.y.) of content DF- (108 tracks/cm*) (10° tracks/cm) (10'° n/cm*) grains (ppm) 1 223 592 Zircon 10.4 (723) 20.4 (700) 1.09 35.1% 3.6 3 570 2 222 817 ---do-- 6.08 (1188) 9.00 (943) 1.02 38.3+ 1.8 19 290 817 ---do-- 3.39 (392) 5.32 (308) .965 36.6% 2.0 6 290 841 ---do-- 5.22 (628) 8.44 (508) .980 36.2% 1.6 6 275 466 1504 ---do-- 5.55 (719) 8.76 (568) 1.01 38.2% 1.8 6 250 3 714 597 Sphene 5.93 (1456) 11.12 (1364) 1.27 40.4% 1.7 6 280 588 Zircon 3.61 (852) 6.87 (795) 1.18 37.9% 1.6 6 190 509 Apatite .243 (113) .387 (118) 1.13 42.7% 8.2 50 11 4 233 598 Sphene 2.10 (670) 3.92 (626) 1.25 39.9% 2.0 6 100 594 Zircon 5.56 (1055) 10.68 (1014) 1.14 35.5% 1.4 6 300 595 Apatite .144 (134) .238 (220) 1.13 41.1+10. 50 6.8 5 503 1802 Zircon 7.176 (1149) 12.30 (911) 1.07 40.3% 1.8 6 330 1803 Apatite 159 (331) .239 (497) 997 40.6% 5.8 50 6.9 6 502 1800 Zircon 4.24 (610) 6.59 (474) 1.08 41.5% 1.8 6 180 7 328 1345 ---do-- 4.09 (910) 8.32 (924) 1.28 37.6% 1.5 6 190 1341 Apatitez .108 (450) .116 (485) 1.02 56.4% 7.4 100 3:3 8 430a 1322 Zircon 3.62 (787) 5.41 (589) 1.04 41.5% 1.8 6 150 1306 Apatite 115 (479) .160 (667) 1.00 42.8% 5.1 100 4.6 9 420 2337 Zircon 4.75 (1122) 8.80 (1039) 1.13 36.4+ 1.9 6 220 10 W-1-75 1320 =---do-- 6.35 (912) 7.16 (514) 1.05 55.5% 2.7 6 200 1304 Apatite .149 (275) 136 (251) 1.02 66.5%+11.6 100 3.8 11 150 590 Zircon 8.12 (1240) 10.38 (793) 1.17 54.7+ 4.8 6 280 591 Apatite .162 (337) .164 (342) 1.13 66.4+12. 50 4.7 12 Lad 1323 Zircon 10.30 (1045) 9.82 (500) 1.03 64.1% 3.7 4 270 1307 Apatite .102 (427) .102 (425) 990 59.3% 8.1 100 3.0 13 325 1321 Zircon 5.12 (1208) 5.25 (620) 1.05 60.9% 3.2 6 140 1305 Apatite .110 (457) .104 (434) 1.01 63.34 8.5 100 3.0 1 Numbers in parentheses show total number of tracks counted in each determination. Sample has many defects. 34 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES TABLE 9.-New chemical analyses and norms of Tertiary igneous rocks from South Park-Breckenridge region [Major oxides determined by Hezekiah Smith, U.S. Geological Survey, 1976, using rapid methods described by Shapiro (1975, p. 43-45). Minor elements determined by semiquantitative spectrographic methods by Leung Mei, U.S. Geological Survey, 1976. CIPW norms calculated on the basis of major-oxide analyses recalculated to 100 percent after deduction of volatiles. The standard deviation of any single answer should be taken as plus 50 percent and minus 33 percent. Looked for but not found: Ag, As, Au, Bi, Cd, Dy, Er, Gd, Ge, Hf, Ho, In, Ir, Li, Lu, Mo, Os, Pd, Pr, Pt, Re, Rh, Ru, Sb, Sm, Sn, Ta, Tb, Th, Tl, Tm, U, WJ Paleocene volcanic rocks Eocene and Oligocene Intrusive rocks Volcanic rocks Field No.--- 323 325 150 W-1-75 222 328 421 430A Lab No.----- -191534 W-191538 W-191539 W-191533 W-191540 wW-191535 W-191536 W-191541 W-191542 Major oxides (percent) $10,=-----~~ 55.9 62.9 66.3 67.0 67.8 58.1 63.3 66.3 69.6 Al,Qg=-~---- 16.7 15.6 15.5 14.6 16.1 16.4 16.7 15.5 15.1 3.9 All 3.4 1,7 1,7 3.6 3.1 1.9 2.0 FeQ--------- 3.2 3.2 1.6 1.4 .88 4.0 1.2 2.3 1.1 MgO--------- 3.4 1.9 1.5 1.6 .96 3.3 1.4 1.0 +712 5.4 4.6 3.8 4.5 4.5 5.8 3.5 3.4 3.1 NazO-~------ 3.5 4.1 2.8 2.4 2.8 3.2 3,5 3.4 "3.1 K2“: ———————— 2.1 2.8 1.3 1.9 1.7 3.2 4.7 3.9 3.4 H20_ -------- 3.1 80 2.0 2.8 2.1 . 94 717 80 .79 85,0 -------- 1.9 .40 2 AA 1.5 1.1 12 . 78 .49 .28 Ti0a--~----- 74 59 .39 47 35 .99 72 79 . 48 p;0f-------= 40 .34 18 .33 .16 .48 . 32 .36 . 20 MnQO--------- . 08 14 .06 . 08 .05 12 .07 .05 . 04 . N8 .01 .02 .O1 .03 .05 .03 .01 .02 R----------- . 04 . 04 .02 . 04 .01 .07 . 08 «10 .05 Sum 100 100 101 100 100 100 100 100 100 Major oxides recalculated to 100 percent after deduction of volatiles (percent) Si0j=-=-~~--- 58.61 64.01 68.47 69.61 69.90 58.57 64.26 66.97 70.27 A1gO3~--->--~- 17.51 15.87 16.01 15.21 16.60 16.53 16.95 15.76 15.35 Feso;-~------ 4.09 2.14 3.51 1.77 1.75 3.63 3.15 1.92 2.03 FeQ--------- 3.36 3.26 1.65 1.46 .91 4.03 1,22 2.32 1.12 MgO--------- 3.56 1.93 1.55 1.67 .99 3.33 1.42 1.01 73 5.66 4.68 3.92 4.69 4.64 5.85 3.55 3.43 3.15 Na,j0-------- 3.67 4.17 2.89 2.50 2.89 9.23 3.55 3.43 3.15 K20 ————————— 2.20 2.85 1.34 1.98 1.75 3.23 4.77 3.94 3.46 Ti0j---~---- I8 .60 .40 . 49 . 36 1.00 +13 . 80 . 49 P,0§-=------- 42 35 19 34 .16 .48 132 . 36 . 28 15 14 .06 . 08 .05 #12 .07 05 . 04 AGE OF IGNEOUS ROCKS, SOUTH PARK-BRECKENRIDGE REGION, COLORADO 85 TABLE 9.-New chemical analyses and norms of Tertiary igneous rocks from South Park-Breckenridge region-Continued Paleocene volcanic rocks Eocene and Oligocene Intrusive rocks Volcanic rocks Field No.--- 323 325 150 W-1-75 222 328 421 430A Lab No.-----W-191534 W-191538 W-191539 W-191533 w-191540 W-191525 W-191536 - W-191541 W-191542 CIPW Norms Q----------- 11.89 16.35 36.32 35.87 35.44 11.01 16.52 73.42 31.43 G----------- --- --- 3.11 1.25 1.91 --- .26 47 1,18 Opr---------- 13.01 16.84 7.93 11.70 10.36 19.06 28.19 23.28 20.43 Ab---------- 31.05 35.30 24.47 21.16 24.43 27.30 30.06 29.06 26.67 An---------- 24.80 16.17 18.25 21.01 21.94 21.11 15.50 14.66 14.31 Hy-en------- 8.71 3.70 3.86 4.15 2.46 6.94 3.54 2,52 1.82 Hy- fs------- 1.74 2.68 --- .56 --- 2.50 --- 1.46 --- Di-Wo------- 23 2.00 --- ke e 1.98 R- --- l Di-en------- A7 1.11 --- --- --- 1.34 e --- --- Di-fs------- .03 . 81 --- --- --- .48 --- --- --- Mt---------- 5.93 3.10 4.36 2.957 2.05 5.26 2.04 72.718 2.32 Hm---------- --- --- &50 --- .34 --- 1.74 --- .43 Il---------- 1.47 1.14 .76 . 93 . 69 1.98 1,39 1,52 . 93 Ap-e--------- .99 .82 .44 .81 .39 1,15 My i d .86 .48 Minor elements (parts per million) -me rea mn senare ss 6.5 pA 8.4 10 <6.5 Pb---------- <14 14 15 <14 18 22 30 17 18 Soe---------- 14 18 12 9.9 7.9 21 15 15 9.1 ooo 1200 940 1200 1100 1500 980 770 870 750 SJ na lan ne soe a s ae uus ui an 100 110 40 37 Sl 120 70 120 53 Mere rem ave mene nese 15 25 13 26 12 20 25 29 16 Ybe--------- 2:5 3.3 2.3 3.1 1.3 3.4 4.7 3.8 2.1 Zn---------- 87 50 39 49 36 98 38 56 A4 93 220 120 200 120 110 490 220 170 Potassium-Argon and Fission-Track Zircon Ages of Cerro Toledo Rhyolite Tephra in the Jemez Mountains, New Mexico By G. A. IZETT, J. D. OBRADOVICH, C. W. NAESER, and G. T. CEBULA SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES GEOLOGICAL SURVEY PROFESSIONAL PAPER 1199-D CONTENTS Page Abstract 37 Introduction 37 Methodology 38 Discussion 41 References cited 48 ILLUSTRATIONS Page FIGURE 11... Map of western United States showing distribution of lower Pleistocene ash beds derived from pyroclastic eruptions from the Jemez Mountains area 38 12. Diagram showing nomenclature of lower Pleistocene pyroclastic units of the Jemez Mountains area 40 13. Photograph showing sampled interval of Cerro Toledo Rhyolite between the Otowi and Tshirege Members of the Bandelier Tuff in Pueblo Canyon, Los Alamos County, New MeXiCO 40 14. K-Ar isochron plot for three minerals from sample 77G55 of a pumice unit in Cerro Toledo Rhyolite 42 TABLES Page TABLE 10. Description of localities of Jemez Mountains-derived tephra deposits shown in figure 11 39 11. Reported and recalculated K-Ar ages of the Otowi and Tshirege Members of the Bandelier Tuff of north-central New Mexico ...................... 40 12. K-Ar ages of minerals from pumice units of the Cerro Toledo Rhyolite ...... 42 13. Zircon fission-track age determinations for sample 77G55, from an 42 airfall pumice unit of the Cerro Toledo Rhyolite .................................... SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES POTASSIUM-ARGON AND FISSION-TRACK ZIRCON AGES OF CERRO TOLEDO RHYOLITE TEPHRA IN THE JEMEZ MOUNTAINS, NEW MEXICO By G. A. IZETT, J. D. OBRADOVICH, C. W. NAESER, and G. T. CEBULA ABSTRACT Pumice units of the Cerro Toledo Rhyolite of early Pleistocene age in Pueblo Canyon in the eastern part of the Jemez Mountains of north-central New Mexico lie stratigraphically between the Otowi (lower) and Tshirege (upper) Members of the Bandelier Tuff. The K-Ar ages of sanidine, plagioclase, and hornblende from a lower unit of air-fall pumice of the Cerro Toledo are 1.46+0.03 m.y., 1.50+0.03 m.y., and 1.58+0.11 m.y., respectively, based on the newly recommended decay constants for "K, A, K-Ar isochron age for the three minerals is 1.47+0.04 m.y. The K-Ar age of sanidine from the uppermost pumice unit of the Cerro Toledo is 1.23 +£0.02 m.y. These K-Ar ages are stratigraphically compatible with K-Ar ages of the lower and upper members of the Bandelier Tuff determined by G. B. Dalrymple in 1968. Zircon fission-track ages of the lower unit of the Cerro Toledo are 1.39 +0.11 m.y. and 1.46 +0.12 m.y. The isotopic ages here reported for the Cerro Toledo Rhyolite coupled with those determined by Dalrymple for the Bandelier Tuff provide the basis for dating their downwind tephra correlatives in the southern High Plains of Kansas, New Mexico, and Texas. INTRODUCTION At several localities in the southern High Plains (fig. 11, table 10), such as the Borchers Ranch locality (Hibbard, 1941) in Meade County, Kans., deposits of volcanic ash occur (9 m stratigraphically above the type B Pearlette ash) that have mineralogical and chemical affinities with tephra of early Pleistocene age in the Jemez Mountains, north-central New Mexico. Elsewhere in the southern High Plains of New Mexico and Texas (fig. 11), volcanic ash beds tentatively correlate with two large pyroclastic units, the Guaje and Tsankawi Pumice beds (of the Otowi and Tshirege Members, respectively, of the Bande lier Tuff) (Izett and others, 1972; G. A. Izett, unpub. data, 1968-1979); but until the present, downwind equivalents of the Cerro Toledo Rhyolite, which lies between these two units, have not been recognized. If, in the future, these deposits of volcanic ash can be correlated with certainty with their suspected source- area tephra units, then they will have the potential of increasing the stratigraphic resolving power of the established sequence of ash beds and increasing the stratigraphic usefulness of associated fossil land mammals. The usefulness of these potential marker volcanic-ash beds may be further enhanced if reli- able K-Ar ages can be assigned to their source-area equivalents. It is the purpose of this report to present K-Ar ages of the Cerro Toledo Rhyolite tephra units, which previously have not been dated but whose stratigraphic position between the Ctowi and Tshi- rege Members of the Bandelier Tuff is well esta- blished. The samples chosen for K-Ar age determinations were collected from the eastern Jemez Mountains (fig. 12) in north-central New Mexico. Large samples (77G55 and 77G56) were taken from two air-fall pumice units (fig. 13) of the Cerro Toledo Rhyolite of early pleistocene age in Pueblo Canyon east of Los Alamos, N. Mex. Smith, Bailey, and Ross (1970) did not map the deposits of Cerro Toledo Rhyolite at the sample locality in Pueblo Canyon, owing to their small areal extent (R. L. Smith, oral commun., 1977). According to Smith (oral commun., 1977) and Smith, Bailey, and Ross (1970), the pumice units sampled are pyroclastics associated with the emplacement of rhyolite domes. These domes were emplaced fol- lowing the eruption of the lower Bandelier Tuff (Otowi Member) of early Pleistocene age and the subsequent collapse that formed the Toledo caldera. At the sample locality, the two units sampled are the uppermost of several discrete air-fall pumice beds interlayered with tuffaceous sediments that lie in 37 38 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES 120° 110° 100° 90° COLORADO ( VALLES AND TOLEDO CALDERAS 35° A 9 ®, 104 8 s* A 15 | NEW MEXICO | 3 o TEXAS Borchers Ranch | 11 FIGURE 11.-Map of Western United States showing the distribution of lower Pleistocene volcanic-ash beds derived from pyroclastic eruptions from the Jemez Mountains area, New Mexico. Volcanic-ash-bed localities: Guaje ash, open circle; Cerro Toledo ash, filled triangle; Jemez Mountains-derived ash suspected to be Cerro Toledo ash, open triangle; and Tsankawi ash, filled circle. Correlation of volcanic-ash beds here provisionally assigned to the Cerro Toledo and Tsankawi ashes with their suspected source-data tephra is tentative until more mineralogical and chemical data are available. Localities are described in table 10. stratigraphic succession above the Guaje Pumice bed and overlying ash flows of the Otowi Member and below the Tsankawi Pumice Bed and overlying ash flows of the Tshirege Member. Figure 12 shows the nomenclature for lower Pleistocene pyroclastic units of the Jemez Mountains, N. Mex. K-Ar sanidine ages of samples of the Bandelier Tuff were deter- mined by G. B. Dalrymple and reported in a paper by Doell and others (1968, p. 238). Dalrymple's K-Ar sanidine ages are given in table 11, as well as those ages recalculated using the new decay constants recently recommended by the IUGS Subcommission on Geochronology (Steiger and Jager, 1977). METHODOLOGY Mineral separations were made on the two sam- ples from two of the air-fall pumice units of the Cerro Toledo Rhyolite by G. T. Cebula, M. G. Sawlan, and J. W. Groen of the U.S. Geological Survey. About 28 kg of sample 77G55 was used to obtain 39 g of calcic albite, 10 g of sanidine, and 0.04 g of zircon. About 2 kg of sample 77G55 was used to obtain about 10 g of POTASSIUM-ARGON, FISSION-TRACK ZIRCON AGES OF CERRO TOLEDO RHYOLITE TEPHRA, NEW MEXICO TABLE 10.-Description of localities of Jemez Mountains-derived tephra deposits shown in figure 11. [Leaders (---) indicate data unavailable] 39 Tephra bed Approx. thickness (m) Locality description Collector Sample Nos. 6. 7. 10. 11. 12. 13. 14. 15. Tsankawi-----~-- t =--dg=---------- 1.5 Cerro Toledo(?)- 1 Tsankawi (?) 3 1. 2 Cerro Toledo---- 1. 4 Guaje----------- 1.5 Cerro Toledo(?)- =--do----------- 1 Guaje----------- «4 =--do----------- .6 =--dg----------- 2 ===gg=----<<---- 1 In Pleistocene deposits in roadcut along U.S. Highway 64 about 0.98 km southwest of junction with New Mexico State Highway 96, about 1.46 km southwest of the church at Ranchos de Taos in the Taos Southwest 7 1/2-minute minute quad., Taos County, N. Mex. N. Mex. In Pleistocene deposits near center of sec. 26, T. 7 N., R. 1 E., in Dalies 7 1/2-minute quad., Valencia County, About 2 m above base of 35-m-thick Pleistocene sequence (Tule Formation) along Rock Creek about 3 km north of Rock Creek Store in Cope Creek 7 1/2-minute quad., Briscoe County, Tex. NE 1/4 sec. West of Channing, Tex., east of Rita Blanca Creek in 18, block 49, above windmill marked with altitude 3,555 feet in Channing NW 7 1/2-minute quad., Hartley County, Tex. Sample 77628 is from second tephra bed about 3 m higher, above intervening sand- stone. Pumice lapilli bed in abandoned pumice pit in SE 1/4 NE 1/4 sec. 24, T. 16 N., R. 8 E., Turquoise Hill 7 1/2-minute quad., Santa Fe County, N. Mex. Abandoned tephra pit about 13 km south of U.S. Highway 66 about 12.8 km west of junction with U.S. Highway 84, Guadalupe County, N. Mex. Pumice gravel bed about 4.8 km southeast of San Felipe Pueblo in NE 1/4 NE 1/4 sec. San Felipe 7 1/2 minute quad., Santa Fe County, N. Mex. 33, T. 14 N., R. 5 E., Pumice gravel at southwest corner of sec. 26, T. 4 S., R. 1 E., San Antonio l5-minute quad., Socorro County, N. Mex. Ash and pumice lapilli bed in roadcut along Edith Blvd. near Albuquerque, N. Mex., NW 1/4 SE 1/4 SW 1/4 sec. 22, T. 11 N., R. E., Alameda 7 1/2-minute quad., Bernalillo County, N. Mex. Pleistocene channel deposit at north edge of New Mexico Institute of Mining Technology at Socorro, N. Mex. Tephra deposits now destroyed. About 9.1 m above Pearlette type B ash (Pliocene) in Pleistocene sediments in Borchers Ranch area, SE 1/4 sec. 16, T. 31 S., R. 28 W., Irish Flats NE 7 1/2- minute quad., Meade County, Kans. called type S Pearlette by some. sec. Ash bed incorrectly Roadcut along Texas State Highway 193 in NW 1/4 NE 1/4 1, block 3, of Eastland County school lands in Floydada SE 7 1/2-minute quad., Crosby County, Tex. Roadcut along U.S. Highway 60 about 3.85 km west of Pecos River bridge in SE 1/4 NE 1/4 sec. 32, T. 3 N., R. 25 E., Fort Sumner 7 1/2-minute quad., DeBaca County, N. Mex. Pumice lapilli bed in Arroyo Hondo at south edge of Santa Fe 7 1/2-minute quad., Santa Fe County, N. Mex. Roadcut of Farm Road 835 along south wall of Yellow, house Canyon just north of junction with Farm Road 3020 in Buffalo Springs Lake 7 1/2-minute quad., Lubbock County, Tex. G. R. Scott----- 77429 R. E. Wilcox---- 66469 G. A. Izett----- 68C63, 75632, 776104, ==--dg---------- 71621, R. E. Wilcox---- 65W123 R. H. Weber----- 66W162 R. E. Wilcox---- 65W121 R. H. Weber----- 664156 R. E. Wilcox, 65W116 P. W. Lambert. R,. H. Weber----- 66W 160 C. W. G. A. Hibbard--- Izett----- 73428 74G42, 74G51, G. A. Izett----- 70617 -=--do----------- 74630 R. E. G. A. 65W 124 75G3 Wilcox---- lzett----- G. A. Izett----- 75628 75G11 776103, 76620 77628 74G49, 786140 40 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES TABLE 11.-Reported and recalculated K-Ar ages of the Otowi and Tshirege Members of the Bandelier Tuff of north-central New Mexico K-Ar age (m.y.) Unit Subunit Reported by Recalculated Doell and using new decay others (1968)l constants* r é Ash flows 1. 0240. 04 1. 0540. 04 § 1.06%0.03 1.09£0.03 = o hel sl bel 0 3 y C4 A ¥ | £ Tsankawi Pumice Bed 1.09£0.03 1. 1240.03 3 $ § x Ash flows 1. 480. 09 1, 52%0. 09 m 2 1. 44%0.04 1. 4840.04 G "l > 0 5 Guaje Pumice Bed 1. 3740.04 1. 40£0.04 * K/Koga1=1- 191074 ; Ag=0, 585x107 yr ; Ag=4.72x107 0yr~! 2 " ; Ag +, -=0.581x107!0yr Ag=4. 962x107 -1 (Steiger and Jager, 1977). 1. > JEMEZ MOUNTAINS AREA, NEW MEXICO u ASH FLOW t UNITS g | TSHIREGE - memBenr | TSANKAW (~ PUMICE i BED CERRO TOLEDO RHYOLITE g ASH FLOW iz NITS © otTowl C m | MEMBER GUAJE e PUMICE Fi BED FiGgurE 12.-Diagram showing the no- menclature of lower Pleistocene pyro- clastic units of the Jemez Mountains area, north-central New Mexico. Modi- fied from Smith, Bailey, and Ross (1970). mgr hase hue ta FrgurRE 13.-View in Pueblo Canyon, NW 4WNW% sec. 13, T. 6 E., R. 19 N., Guaje Mountain 7%-minute quadrangle, Los Alamos County, N. Mex., showing the stratigraphic succession where samples 77G55 and 77G56, used for K-Ar age determinations, were collected from the Cerro Toledo Rhyolite of early Pleisto- cene age. Topmost part (covered) of the lower Bandelier Tuff (Otowi Member) is in lower part of photograph; the Otowi Member is overlain by a succession of air-fall pumice units of the Cerro Toledo Rhyolite and the Tshirege Member of the Bande lier Tuff. sanidine. The separation procedure for sample 77G55 consisted of screening the raw sample with a 4.75- mm (4-mesh) sieve and collecting only those 4.75-mm or larger pumice lumps that floated in water. Only the water-floatable pumice lumps were used for the mineral separations to insure that non-pumiceous accidental lithic fragments would not contribute POTASSIUM-ARGON, FISSION-TRACK ZIRCON AGES OF CERRO TOLEDO RHYOLITE TEPHRA, NEW MEXICO - 41 material to the mineral separations. The pumice lumps were ultrasonically scrubbed, dried at about 50°C, and crushed and pulverized so that the sample would pass through a 300-um (50-mesh) sieve. About 2 kg of sample was lost during sample preparation. Plagioclase was recovered using a bromoform and acetone mixture cut to appropriate specific gravity "from the 300-t0 100-um size range (-50 to +150 mesh), whereas the hornblende, sanidine, and zircon were recovered from the less-than-300-um (-50 mesh) size range owing to their small size and scarcity. The purity of the mineral separates was improved by using a magnetic separator set at appropriate cur- rent, forward tilt, and side tilt. Sanidine was re covered from sample 77G56 using the same proce- dures as for 77G55. Because many of the mineral grains had glass welded to their edges, the mineral grains were etched with hydrofluoric acid (24 per- cent) to remove the glass, using the following etch times: 77G56 sanidine ...... 2 minutes 77G55 sanidine .................. 3 minutes 77G55 plagioclase.............. 3 minutes 77G55 hornblende.............. 6 minutes Following the hydrofluoric acid etch, the minerals were ultrasonically scrubbed for about 5 minutes and sized as follows: sanidine (77G56) ...... 300 to 106m (-50 to +140 mesh) sanidine (77G55) ..... 300 to 100 um (-50 to +150 mesh) plagioclase (77G55) ..... 300 to 106 um (-50 to +140 mesh) hornblende (77GSS) neenee =300 um (-50 mesh) After the samples were sized, they were split, using a Jones-type microsplitter', into fractions for argon and potassium analysis. Argon was collected and argon isotopic ratio determined from melted splits of the samples. The potassium content of splits of the minerals used for argon analysis was determined by three methods-isotope dilution, flame photometry, and electron microprobe (table 12), although only the potassium contents as determined by isotope dilution were used to calculate the ages. Isotope dilution analysis of the samples was done by J. D. Obradovich and Kiyoto Futa on all samples. Flame 'Use of trade names in this report is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey. photometry was done by G. A. Izett and Wayne Montjoy to compare the results (table 12) with those done by isotope dilution. The potassium content of the samples was 'determined by G. A. Izett using an Applied Research - Laboratories EMX - model - electron microprobe to compare the results by this method with the isotope dilution results. Splits of the sam} les were mounted in epoxy in holes drilled in an aluminum wafer. The potassium content was determined by analyzing the specimens using 15 kilovolts operating voltage and 15 nanoamperes sample current measured on benitoite. A fixed count of beam current was used with a counting period of about 15 seconds. The analytical procedure consisted of analyzing 10 grains of each of four standard feldspar samples. A computer program linked to the electron microprobe (1) calculated a mean value of the number of counts and its associated standard deviation for potassium in the standard feldspars and (2) determined a least-squares curve relating the number of counts to weight percent potassium. Ten grains of each of the samples of unknown potassium content were analyzed, and their mean counts were compared by the computer with the curve relating counts to weight percent of the standard feldspars. The potassium contents determined in this way for the different runs were averaged to give the potassium content of each sample, and the results . are in fairly good agreement with the results from isotope diluton (table 12). DISCUSSION The K-Ar age determinations (table 12) made on pumice units of the Cerro Toledo Rhyolite of Pleisto- cene age are compatible, within their analytical uncertainty, with K-Ar ages of underlying and over- lying units of the Bandelier Tuff determined by G. B. Dalrymple (reported in Doell and others, 1968; and table 10). The results suggest that the lower pumice unit of the Cerro Toledo, herein dated 1.47+0.04 m.y. (77G55), is very close in age to the Otowi Member of the Bandelier Tuff as determined by G. B. Dalrymple (1.45 m.y.; average of 3 determinations (weighted mean)) and cannot be separated from it based on the K-Ar age determinations at hand. The uppermost pumice unit of the Cerro Toledo Rhyolite (77G56) has a K-Ar age (1.23+0.02 my.) significantly younger 42 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES TABLE 12.-K-Ar ages of minerals from pumice units of the Cerro Toledo Rhyolite, Jemez Mountains, New Mexico Sample Mineral Lab No. K,0 (percent) Sample Moles Percent K-Ar ageLi DKA- weight radiogenic radiogenic (m.y.) C): (3 0) (g) "Sar 77656 Sanidine--~=, 3502 7.21 7.33 7.22 3.9867 - 5.l11ix10~'!! 56.8 1. 2340.02 77G55 ---do------- 3500 . 8:36 8.62 --- 1.0727 - 1.89%10 !! 24.5 1. 46£0. 03 77G55 Plagioclase- ' 3409 - 1.98 3.0L 1.98 12.5853 5.36x10~'!' 28.3 1.50+0.03 776553 Hornblende-- - 3497 B6 --- 4.2865 - 5.50x107'2 6.1 1. 584%0. 11 ' Potassium determined by J. D. @Potassium determined by G. A. 3Potassium determined by G. A. “Decay constants: AB = "°K abundance: *°K=1.167x10~* atom/atom K. l-sigma level. dilution. Obradovich and K. Futa by the isotope dilution technique. Izett using the electron microprobe. Izett and W. Mountjoy by flame photometry technique. 4.962210740 yr"; yr~'. Precision of age determinations given at the K-Ar ages calculated using the potassium content as determined by isotope TABLE 18. fission-track age determination for sample 77G55, from an airfall pumice unit of the Cerro Toledo Rhyolite, Jemez Mountains, New Mexico [8 / yr~! 1 Lab No. Spontaneous-t rack densityl Induced-t rack densitg1 Neutron flux Fission-track U DF- (ps) (106 tracks/cmz) (pi) (106 tracks/cm* ) (¢)(1015n/cm2) age2 (m.y.) (ppm) 1646 0.423 (45) 19.14 (1019) 1.05 1. 39£0. 11 520 1646A &374 (71) 7.55 (717) «493 1. 46%0. 12 440 'Total number of tracks counted shown in parentheses. Error is #2 sigma. than the K-Ar age of the underlying pumice unit (77G55) of the Cerro Toledo (1.47+0.04 m.y.), and significantly older than K-Ar ages of the overlying Tshirege Member of the Bandelier Tuff as dated by G. B. Dalrymple (about 1.09 m.y.; average of 3 determinations (weighted mean)). The K-Ar ages of sanidine (1.46+0.03 m.y.), plagio- clase (1.50+0.03 m.y.), and hornblende (1.58+0.11 m.y.) of sample 77G55 are in agreement with each other within their analytical uncertainty at the 2-0 level. Because of the small amount of hornblende recovered through mineral separations from the phenocryst-poor rhyolite (77G55), and because of the low potassium content of the hornblende, the K-Ar age of the hornblende has a fairly high uncertainty. A K-Ar isochron plot of the analytical data for the three minerals from sample 77G55 is shown in figure 14, and the age calculated from the data is 1.47+0.04 m.y. The analytical precision estimate for this iso- chron age was calculated using a modified York linear regression. The minerals contain no analy- tically significant amount of inherited argon, inas- much as the +°Ar/3$Ar intercept is about 297. Two fission-track age determinations (table 13) were made by C. W. Naeser on zircon separated from sample 77G55 of the Cerro Toledo. The calculated ages are 1.39 +0.11m.y. and 1.46+0.12 m.y. (¥), and these ages are concordant with the K-Ar ages within their analytical uncertainty. The fission-track age of DKA 3499 PLAGIOCLASE DKA 3500 SANIDINE DKA 3497 HORNBLENDE gee Ar"%/Ar®® intercept= 297.1 al i i 1 I i 1 1 i | 1 1 i 1 0 5 10 18 K*ar® (x10) FIGURE 14.-K-Ar isochron plot for three minerals from sample 77G55 of a pumice unit in Cerro Toledo Rhyolite in the eastern Jemez Mountains, N. Mex. POTASSIUM-ARGON, FISSION-TRACK ZIRCON AGES OF CERRO TOLEDO RHYOLITE TEPHRA, NEW MEXICO 1.46 m.y. is perhaps the better of the two, inasmuch as it was made following re-irradiation of the ori- ginal sample, which had too high an induced-track density for optimum counting conditions. REFERENCES CITED Doell, R. R., Dalrymple, G. B., Smith, R. L., and Bailey, R. A., 1968, Paleomagnetism, potassium-argon ages, and geology of rhyolites and associated rocks of the Valles Caldera, New Mexico, in Studies in volcanology-A memoir in honor of Howel Williams: Geological Society of America Memoir 116, p. 211-248. 43 Hibbard, C. W., 1941, The Borchers fauna, a new Pleistocene interglacial fauna from Meade County, Kansas: University of Kansas Science Bulletin 38, pt. 7, p. 197-220. Izett, G. A., Wilcox, R. E., and Borchardt, G. A., 1972, Correlation of a volcanic ash bed in Pleistocene deposits near Mount Blanco, Texas, with the Guaje Pumice bed of the Jemez Mountains, New Mexico: Quaternary Research, v. 2, no. 4, p. 554-578. Smith, R. L., Bailey, R. A., and Ross, C. S., 1970, Geologic map of the Jemez Mountains, New Mexico: U.S. Geological Survey Miscellaneous Investigations Map I-571, scale 1:250,000. Steiger, R. H., and Jiger, E., 1977, Subcommission on Geochro- nology: Earth and Planetary Science Letters, v. 36, p. 359-362. Fission-Track Dating of the Climax and Gold Meadows Stocks, Nye County, Nevada By C. W. NAESER ard FLORIAN MALDONADO SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES GEOLOGICAL SURVEY PROFESSIONAL PAPER 1199-E CONTENTS Abstract Results and interpretation Other ages Analytical methods Acknowledgments References cited TABLES TABLE 14. Fission-track data for samples from the Climax and Gold Meadows stocks, Nye County, Nevada 15. Sample localities in the Climax and Gold Meadows stocks, Nye County, Nevada Page 45 45 46 46 46 Page 46 46 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES FISSION-TRACK DATING OF THE CLIMAX AND GOLD MEADOWS STOCKS, NYE COUNTY, NEVADA C. W. NAESER and FLORIAN MALDONADO ABSTRACT Fission-track ages indicate an age of 101 million years for the Climax stock and a minimum age of 93.6 million years for the Gold Meadows stock, both at the Nevada Test Site, Nye County, Nevada. Younger fission-track ages for some of the apatite concentrates suggest that the stocks have been within 4 kilo- meters of the surface since late Paleocene time. RESULTS AND INTERPRETATION The Climax stock, Nye County, Nevada (Maldona- do, 1977), has been sampled in five locations for fission-track dating. Apatite from five of the samples was dated, as were zircon from two and sphene from one. A sample of the Gold Meadows stock, Nye County, Nevada, was also dated using apatite and zircon. The purpose of this study was to determine the thermal history of the respective stocks. Because apatite records the rock's most recent cooling regime (Naeser and Forbes, 1976), it is a very useful indi- cator of tectonic history. Four of the five samples collected from the Climax stock (Maldonado, 1977) are outcrop samples (tables 14 and 15, Nos. 2-5). The fifth sample (table 14, No.1) is a drill core from a depth of 536 m in drill hole U15BGZ. Apatite from the four surface samples gave concordant fission-track ages, with an average of 101 +3.2 m.y. (+ one standard error (0) of the mean). This age is in excellent agreement with the stock's sphene and zircon ages, which give an average of 101+1.2 m.y. However, the apatite from the drill core gave a discordant age, 78.6+3.4 m.y. The concordance of the fission-track ages of the surface samples means that the rock now exposed at the surface has never been above 100°C in the last 100 m.y.; these concordant ages also indicate the probable time of emplacement of the stock. As- suming that the geothermal gradient for this area was the same during the late Cretaceous as it presently is, about 25°C/km (Roy and others, 1968), we can state that this stock was intruded at a depth of less than 4 km. The discordant age of the drill-core apatite points to a somewhat different thermal his- tory for the buried parts of the stock. There are two possible interpretations: The first is that the stock was emplaced at such a depth in the crust that the 100°C isotherm was between the rock presently at the surface and the rock at a depth of 500 m. This placement would have permitted the "surface" sam- ples to cool to less than 100°C while the lower samples remained above 100°C for an extended time. The second interpretation is that the accumulation of sedimentary or volcanic materials buried the stock to such a depth that partial annealing of the apatite took place at some time after intrusion. It is not possible to distinguish between these two possibi- lities using the present data. If it is assumed that sample 1 (core) cooled through the 100°C isotherm 79 m.y. ago, an average uplift rate of 0.05 mm/yr can be calculated. Most of that uplift would have taken place prior to the deposition of the Belted Range Tuff on the Climax stock 13 m.y. ago. The Gold Meadows stock, Nye County, Nevada (Snyder, 1977), was sampled at one surface site (tables 1 and 2, no. 6). The apatite and zircon from this sample have discordant fission-track ages: 55.6+2.8 m.y. and 93.6+4.3 m.y., respectively. These ages suggest a slightly more complex history for this stock than for the Climax stock. Apparently the Gold Meadows stock either was emplaced at a deeper level in the crust or was, at some later time, buried to a 45 46 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES TABLE 14. -Fission-track data for samples from the Climax and Gold Meadows stocks, Nye County, Nevada [Decay constant A=7.03X10 yr] f Spontaneous -track Induced-track Neutron Fission- Uranium Sample _ Analysis density! (og) density! (r;) flux (0) track age content No. No. Mineral (10° tracks/cm*) (10° tracks/cm2) (10'5 n/cm2) (m.y.) +20 (ppm) Climax stock 1 DF- 1637 Apatite 0.287 (1195) 0.243 (1013) 1.12 78.61 6.7 6.3 1 DF- 1638 Sphene 19.79 (733) 43.84 (812) 3.87 104 +11 330 1 DF- 1639 Zircon 10.99 (1628) 7.21 (534) 1.12 101 + 6.4 190 Z DF- 1640 Apatite 431 (1794) .307 (1280) 1.12 93.3% 6.8 7.9 3 DF-1641 Apatite sse7 (1362) 203 (845) 1:11 106 +9 5:3 4 DF-1642 Apatite +372 (1552) .248 (1032) 1.10 98.3+ 7.9 6.5 5 DF - 1645 Apatite . 349 (1454) .212 (885) 1.10 107: x 9 5.6 5 DF - 1644 Zircon 17.36. (1366) 11.44 (450) 1:11 100 + 6.4 300 Gold Meadows stock 6 DF-1646 Apatite 0.163 (681) 0.193 (803) 1.10 55.6% 5.6 5.0 7 DF-1647 Zircon 13.71 (1904) 9.58 (665) 1.10 93.61 8.5 250 1Numbers in parentheses show total number of tracks counted in each determination. TABLE 15.-Sample localities in the Climax and Gold meadows stocks, Nye County, Nevada Sample North West Site No . latitude longitude description Quadrangle Granodiorite from Climax stock 1 37°14 '04" 116°03*27" From drill Oak Spring hole U15bG2; 536m below collar. 2 37°14 23" 116°03'16" Surface out- Do.. crop 3 37°14 23" 116°03'50" ----do.----- Do. 4 37°14 18" 116°03'54" ----do . ----- Do. 5 3791332" 116°03'40" ----do.----- Do. Quartz monzonite from Gold Meadows stock 6 37°13'52" 16°12'28" Surface out- - Rainier Mesa crop deeper level. It has a calculated uplift rate of about 0.07 mm/yr for the last 56 m.y. The zircon from the Gold meadows stock is apparently younger than the sphene and zircon from the Climax stock. This younger age could be the result of a deeper level of intrusion and (or) later burial of the Gold Meadows stock, or of the emplacement of the stock about 7 m.y. after the emplacement of the Climax stock. OTHER AGES Marvin and others (1970) reported six biotite K-Ar ages from the Climax stock and one from the Gold Meadows stock. The six biotite ages from the Climax stock range from 89 to 97 m.y. Using the IUGS constants for °K, we recalculated the ages, Obtain- ing 91 to 100 m.y. The oldest age is very close to the average fission-track age for the Climax stock. The 91.8-m.y. K-Ar biotite age for the Gold Meadows stock recalculates to 94.3 m.y., which is in very good agreement with the zircon age of 93.6 m.y. ANALYTICAL METHODS The apatites were dated by the population method (Naeser, 1976); they were etched in 7 percent HNOs for 25 seconds at room temperature. The zircon and sphene were dated by the external-detector method (Naeser, 1976). The zircon was etched in a KOH- NaOH eutectic melt (Gleadow and others, 1976), and the sphene, in 50 M NaOH at 140°C (Naeser, 1976). ACKNOWLEDGMENTS We wish to thank G. T. Cebula of the U.S. Geologi- cal Survey for his excellent mineral separations on these rocks. FISSION-TRACK DATING, CLIMAX AND GOLD MEADOWS STOCKS, NYE COUNTY, NEVADA 47 REFERENCES CITED Barnes, Harley, Houser, F. N. and Poole, F. G., 1963, Geologic map of the Oak Spring quadrangle, Nye County, Nevada: U.S. Geclogical Survey Geologic Quadrangle Map GQ-214. Gibbons, A. B. Hinrichs, E. N. Hansen, W. R., and Lemke, R. W., 1963, Geology of the Rainier Mesa quadrangle, Nye county, Nevada: U.S. Geological Survey Geologic Quadrangle Map GQ-215. Gleadow, A. J. W., Hurford, A. J., and Quaife, R. D., 1976, Fission-track dating of zircon-Improved etching techniques: Earth and Planetary Science Letters, v. 33, p. 273-276. Maldonado, Florian, 1977, Summary of the geology and physical properties of the Climax stock, Nevada Test Site: U.S. Geo- logical Survey Open-File Report 77-356, 27 p. Marvin, R. F., Byers, F. M., Jr., Mehnert, H. H., Orkild, P. P., and Stern, T. W., 1970, Radiometric ages and stratigraphic. sequence of volcanic and plutonic rocks, southern Nye and western Lincoln Counties, Nevada: Geological Survey of America Bulletin, v. 81, p. 2657-2676. Naeser, C. W., 1976, Fission-track dating: U.S. Geological Survey Open-File Report 76-190, 68 p. Naeser, C. W. and Forbes, R. B., 1976, Variation of fission-track ages with depth in two deep drill holes: EOS, American Geo- physical Unior Transactions, v. 57, p. 353. Roy, R. F., Decker, E. K., Blackwell, D. D., and Birch, Francis, 1968, Heat flow in the United States: Journal of Geophysical Research, v. 73, p. 5207-5222. Snyder, R. P., 1977, Geology of the Gold Meadows stock, Nevada Test Site: Available only from U.S. Department of Commerce, National Technical Information Service, Springfield, VA 22161, as U.S. Geological Survey report USGS-474-179, 13 p. # U.S. GOVERNMENT PRINTING OFFICE: 1980-777-034/39