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California): Introduction and Implications

California): Introduction and Implications

Vol. 6, No. 2 February 1996 INSIDE • Penrose Conference Reports, p. 14, 16 GSA TODAY • Rocky Mountain Section Meeting, p. 24 A Publication of the Geological Society of America • North-Central Section Meeting, p. 30

Alternate Origins of the Coast Range (): Introduction and Implications

William R. Dickinson, Department of Geosciences, University of Arizona, Tucson, AZ 85721 Clifford A. Hopson, Department of Geological Sciences, University of California, Santa Barbara, CA 93106 Jason B. Saleeby, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125

ABSTRACT Correctly interpreting the tectonic evolution of the California requires understanding the ori- gin of the Coast Range Ophio- lite, which represents a fragment of mafic-to-ultramafic crust of oceanic character lying depositionally beneath the western flank of the Great Valley forearc basin in fault contact with the Franciscan complex of the . Three con- trasting hypotheses for genesis of the ophiolite as seafloor are each based on internally consistent logic within the framework of , but are mutually exclusive and lead to strikingly different interpretations of regional tectonic relations, even though each assumes that the batholith to the east represents the eroded roots of a magmatic arc linked to subduction along the continental margin. To encourage the further work or analy- Multiple basaltic sills of the sheeted and sill complex, Point Sal remnant of the Coast sis needed to develop a definitive inter- Range ophiolite. The ridge in background exposes sheeted sills and (to left of tree on the skyline) base of pretation, summary arguments for each the overlying pillow lavas. hypothesis of Coast Range Ophiolite genesis in mid- to time are presented in parallel: (1) backarc now incorporated within the continental larger than our area of disagreement. We spreading behind an east-facing intra- block (Bailey et al., 1970). The overall span each interpret the Coast Range Ophiolite oceanic island arc that then collided and of Middle to Late Jurassic radiometric ages layered assemblage as a profile of mafic amalgamated with the Sierran continen- for igneous components of ophiolite and crust and lithosphere of oceanic character, tal-margin arc; (2) paleoequatorial mid- postophiolite hypabyssal intrusions is and we infer that this profile was formed ocean spreading to form oceanic litho- ~170 to 155-150 Ma (Hopson et al., 1981, through magmatism induced by mantle sphere that was then drawn northward 1991; Saleeby et al., 1984; Mattinson and upwelling linked to lithospheric extension toward a subduction zone in front of Hopson, 1992). Understanding correctly or “spreading.” We each also argue for the Sierran continental-margin arc; and the origin and emplacement of the Coast emplacement of the ophiolite within the (3) forearc spreading within the forearc Range Ophiolite is essential for under- conceptual framework of plate tectonics, region of the Sierran continental-margin standing the Mesozoic evolution of the taking the Sierra Nevada composite batho- arc in response to transtensional defor- Cordilleran continental margin (Saleeby, lith to the east to be the deeply eroded mation during slab rollback. 1992). The time is long past when geosci- roots of Jurassic- magmatic arc entists could assume that all belts, and regarding Franciscan rocks of INTRODUCTION formed in the same way or have the same the California Coast Ranges farther west Widely distributed exposures of the tectonic significance. as part of the subduction complex Jurassic Coast Range Ophiolite in the Cali- With the help of co-authors, we out- accreted near the trench that was paired fornia Coast Ranges represent deformed line here three divergent views on the ori- with the Sierran-Klamath arc assemblage and structurally dismembered segments gin of the Coast Range Ophiolite. We of and uppermost mantle emphasize that our areas of agreement are Ophiolite continued on p. 2 IN THIS ISSUE GSA TODAY February Vol. 6, No. 2 1996 Alternate Origins of the Coast SAGE Remarks ...... 20 Range Ophiolite (California): GSA TODAY (ISSN 1052-5173) is published Introduction and Implications .... 1 Environment Matters ...... 22 monthly by The Geological Society of America, Inc., Calendar ...... 23 with offices at 3300 Penrose Place, Boulder, Colorado. Washington Report ...... 10 Mailing address: P.O. Box 9140, Boulder, CO 80301- Rocky Mountain Section Meeting ...... 24 9140, U.S.A. Second-class postage paid at Boulder, Book Nook ...... 11 Colorado, and at additional mailing offices. Postmas- North-Central Section Meeting ...... 30 ter: Send address changes to GSA Today, Membership GSAF Update ...... 12 Services, P.O. Box 9140, Boulder, CO 80301-9140. 1995 Annual Meeting Revisited ...... 36 Copyright © 1996, The Geological Society of America, Penrose Conference Reports Inc. (GSA). All rights reserved. Copyright not claimed Fault-Related Folding ...... 14 GSA Meetings ...... 38 on content prepared wholly by U.S. Government Argentine Precordillera ...... 16 employees within the scope of their employment. Per- Classifieds ...... 39 mission is granted to individuals to photocopy freely all 1996 GeoVentures ...... 19 Northeastern Section Meeting Update . . . 39 items other than the science articles to further science and education. Individual scientists are hereby granted GSA on the Web ...... 19 permission, without royalties or further requests, to make unlimited photocopies of the science articles for use in classrooms to further education and science, and to make up to five copies for distribution to associates in the furtherance of science; permission is granted to Ophiolite continued from p. 1 We thank conveners R. G. Anderson, make more than five photocopies for other noncom- mercial, nonprofit purposes furthering science and edu- D. M. Miller, and R. M. Tosdal for arrang- cation upon payment of the appropriate fee ($0.25 per (Fig. 1). We concur that the east flank of ing the 1993 Penrose Conference on page) directly to the Copyright Clearance Center, 27 the Franciscan subduction complex was Jurassic Cordilleran magmatism at which Congress Street, Salem, Massachusetts 01970, phone (508) 744-3350 (include title and ISSN when paying). thrust beneath and otherwise faulted our opposing thoughts were pointedly Written permission is required from GSA for all other against the Coast Range Ophiolite , which juxtaposed, and we dedicate the following forms of capture, reproduction, and/or distribution of formed the westernmost segment of the discussions to the memory of E. H. Bailey any item in this journal by any means. GSA provides this and other forums for the presentation of diverse floor of the Great Valley forearc basin (who started it all). opinions and positions by scientists worldwide, regard- lying between and Franciscan less of their race, citizenship, gender, religion, or politi- trench. 1. COAST RANGE OPHIOLITE cal viewpoint. Opinions presented in this publication do not reflect official positions of the Society. We nevertheless ascribe generation of AS BACK-ARC–INTER-ARC the Coast Range Ophiolite to three differ- BASIN LITHOSPHERE ent tectonic settings: (1) Dickinson infers SUBSCRIPTIONS for 1996 calendar : William R. Dickinson, Department of Society Members: GSA Today is provided as part of “backarc” seafloor spreading behind a Geosciences, University of Arizona, membership dues. Contact Membership Services at migratory east-facing intraoceanic island (800) 472-1988 or (303) 447-2020 for membership Tucson, AZ 85721 arc, which collided with the west-facing information. Nonmembers & Institutions: Free with Richard A. Schweickert, Department of paid subscription to both GSA Bulletin and Geology, Sierran arc along the continental margin Geological Sciences, University of Nevada, otherwise $45 for U.S., Canada, and Mexico; $55 else- (as intervening oceanic lithosphere was where. Contact Subscription Services. Single copies Reno, NV 89557 consumed), to lodge the migratory arc and may be ordered from Publication Sales. Claims: For Raymond V. Ingersoll, Department of Earth nonreceipt or for damaged copies, members contact its backarc seafloor against the continental and Space Sciences, University of California, Membership Services; all others contact Subscription margin; (2) Hopson infers “midocean” Services. Claims are honored for one year; please allow , CA 90024-1567 sufficient delivery time for overseas copies. seafloor spreading along an intraoceanic ridge crest, followed by tectonic transport The concept that the Coast Range STAFF: Prepared from contributions from the of the resulting seafloor to the continental Ophiolite was formed by backarc-interarc GSA staff and membership. margin (as Sierran subduction drew it ever spreading behind an east-facing intra- Executive Director: Donald M. Davidson, Jr. Science Editor: Suzanne M. Kay closer), until the ophiolite docked against oceanic island arc that was accreted to the Department of Geological Sciences, Cornell University, the continental margin prior to the onset continent in Jurassic time by arc collision Ithaca, NY 14853 of Franciscan accretion; (3) Saleeby infers along a suture within the Sierra Nevada Forum Editor: Bruce F. Molnia U.S. Geological Survey, MS 917, National Center, “forearc” seafloor spreading induced by foothills has persisted for 25 (Moores, Reston, VA 22092 transtensional deformation within the 1970; Schweickert and Cowan, 1975; Managing Editor: Faith Rogers west-facing Sierran-Klamath arc system Moores and Day, 1984; Ingersoll and Production & Marketing Manager: James R. Clark Production Editor and Coordinator: Joan E. Manly (in response to rollback of the subducted Schweickert, 1986). Remnants of the Graphics Production: Joan E. Manly, Adam S. McNally slab during highly oblique convergence). intraoceanic arc complex are identified The three concepts have quite differ- as thick submarine successions of de- ADVERTISING ent implications for details of tectonic formed and disrupted Jurassic lavas and Classifieds and display: contact Ann Crawford history. For example, models 1 and 3 both pyroclastics, as much as 5000 m thick (303) 447-2020; fax 303-447-1133 involve varieties of so-called supra–sub- (Bogen, 1985), resting locally on shreds Issues of this publication are available electronically, in duction-zone ophiolite forming the floors of ophiolitic along the Sierran full color, from GSA as Acrobat “Portable Document of interarc basins, whereas model 2 envi- foothills belt. Eruptive activity in the Format” (PDF) files. These can be viewed and printed on personal computers using MSDOS or MSWindows, sions only “normal” seafloor spreading foothills arc was coeval with Jurassic on Macintoshes, or on Unix machines. You must use in an open ocean basin; models 2 and 3 phases of magmatism in the west-facing the appropriate Adobe Acrobat Reader, available for involve only a single west-facing Sierran Sierran continental-margin arc, whose axis free download from GSA and other online services. The more powerful Adobe Exchange program, available magmatic arc, whereas model 1 includes lay farther east along and beyond the Sier- from commercial software suppliers, may also be used. a separate east-facing arc that was accreted ran crest from mid- to mid-Jurassic Download the issues of GSA Today and/or the appropri- tectonically to the Cordilleran continental time (Schweickert, 1976; Busby-Spera, ate Readers using the Uniform Resource Locator (URL): http://www.geosociety.org. Issues of GSA Today are margin; models 1 and 2 both require 1988; Dilles and Wright, 1988). A strong posted about the first of the month of publication. tectonic transport of the ophiolite to case can be made that the Jurassic intrao- This publication is included on GSA’s annual CD-ROM the continental margin, whereas model ceanic and continental-margin arcs of the GSA Journals on Compact Disc. Call GSA Publication 3 envisions genesis of the ophiolite in Sales for details. place within an arc-trench system lying Printed with pure soy inks on recyclable paper in the U.S.A. along the continental margin. Ophiolite continued on p. 3

2 GSA TODAY, February 1996 Smartville complex, which is then inter- preted as the product of spreading in place within the west-facing Sierran forearc (Dilek, 1989a; Edelman et al., 1989), rely upon the presence in the northern foot- hills belt of Middle Jurassic (~165 Ma) granitoid plutons that cut thrusts placing Lower Jurassic elements of the foothills arc assemblage above metasedimentary melange. The resulting conclusion that accretion of the foothills arc complex was Figure 1. Geologic sketch map of part of complete by Middle Jurassic time is not California showing the robust, however, because the intruded regional relation of the melange unit is not tied firmly to the con- Coast Range Ophiolite tinent and underthrusting of the eastern to key lithotectonic belts; flank of an east-facing intraoceanic arc by SC—location of melange would be expected prior to final Smartville ophiolitic complex within foothills suturing to the continent. Arc plutons metamorphic belt; trend unrelated to the Sierran continental arc of Great Valley gravity- could thus cut arc-melange thrusts in the magnetic anomaly late phases of intraoceanic arc evolution (high) after Cady (1975). prior to accretion along the compound subduction complex of the foothills belt. Widespread Middle Jurassic deformation within the Sierran continental arc has been attributed in part to accretion (Edelman and Sharp, 1989; Edelman et al., 1989), but could as well reflect intra-arc contraction. Several workers (Shervais and Kim- brough, 1985; Shervais, 1990; Stern and Bloomer, 1992) have concluded that the Coast Range Ophiolite has geochemical affinities with supra–subduction-zone Ophiolite continued from p. 2 The arc assemblage of the Sierran (SSZ) ophiolites (Pearce et al., 1984), foothills metamorphic belt (Fig. 1) may implying the influence of a subducted Sierra Nevada are genetically unrelated represent a complex of related but dis- slab on its generation. These workers (Dilek et al., 1990). rupted arc segments and remnant arcs and others (Evarts, 1977; Lagabrielle et Deformed and variably metamor- juxtaposed across fault contacts (Paterson al., 1986; Robertson, 1989) have variously phosed Paleozoic-Mesozoic marine strata et al., 1987; Edelman and Sharp, 1989). inferred backarc, forearc, or intra-arc set- (mainly chert-argillite sequences and Volcanogenic successions locally overlie tings of either east-facing or west-facing ), exposed between and thrust ophiolitic sequences of both earliest arcs for its origin. Ophiolitic breccias beneath the two arc assemblages, are Jurassic (~210–200 Ma) and intra-Jurassic locally overlying the Coast Range Ophio- interpreted as a suture belt of compound (~165–160 Ma) age (Saleeby, 1982; Saleeby lite and resting concordantly beneath subduction complexes cut by multiple et al., 1989; Dilek, 1989b; Edelman et al., the Great Valley Group reflect local but fault zones and melange belts emplaced 1989). The best preserved remnant of widespread extensional deformation at during arc-arc collision (Schweickert and mafic crust occurs in the northwestern the sites of their formation (Robertson, Cowan, 1975). A modern example of a foothills within the Smartville ophiolitic 1990). Following initiation of Franciscan remnant ocean basin closing by face-to- complex (Fig. 1) formed by intra-arc rifting subduction to the west, Great Valley fore- face arc-arc collision is afforded by the and associated magmatism during Middle arc sedimentation was underway near the Molucca Sea (Ricci et al., 1985). In the to Late Jurassic time (Menzies et al., 1980; Kimmeridgian-Tithonian boundary Sierran foothills belt, metamorphosed Beard and Day, 1987); combined radio- (155–150 Ma). Upper Jurassic turbidites of the partly metric and fossil ages bracket the main Stern and Bloomer (1992) argued the volcaniclastic Mariposa Formation are interval of its formation as 165–155 Ma case for forearc spreading to produce the inferred to be an overlap assemblage (Day et al., 1985; Edelman and Sharp, Coast Range Ophiolite by drawing an deposited in part in a remnant ocean 1989). Widespread overlap of foothills analogy between the Jurassic Sierran arc basin but also onlapping the accreted volcanic units by the Mariposa Formation and early stages in the evolution of the intraoceanic arc complex (Ingersoll and near the Oxfordian-Kimmeridgian bound- modern Izu-Bonin-Mariana arc of the Schweickert, 1986). Once the foothills ary implies that the arc complex had western Pacific. As they note, however, arc-arc suture belt had fully closed, lodged along the foothills belt by ~155 Ma the analogy is not exact because the con- subduction stepped outboard to the in Late Jurassic time (Schweickert et al., cepts of “subduction-zone infancy” and California Coast Ranges, trapping 1984). Crosscutting plutons of the evolv- “infant-arc crust,” unquestionably applica- backarc-interarc Jurassic oceanic crust as ing Sierran arc were emplaced into foot- ble to the Izu-Bonin-Mariana arc, the Coast Range Ophiolite at the leading hills volcanogenic assemblages and cannot apply to the Sierran arc, for which edge of the overriding plate. With the melanges by latest Jurassic or earliest abundant radiometric ages for plutons onset of Franciscan subduction in the Cretaceous time (~150–140 Ma) indicate arc activity throughout the inter- California Coast Ranges, Sierran arc mag- (Saleeby et al., 1989). val 215–80 Ma (Stern et al., 1981; Chen matism also stepped westward to over- Recent interpretations that the foot- and Moore, 1982). Moreover, the Izu- print both the foothills suture belt and hills arc complex was accreted to the con- the accreted intraoceanic arc. tinental margin prior to formation of the Ophiolite continued on p. 4

GSA TODAY, February 1996 3 Ophiolite continued from p. 3

Bonin-Mariana arc is indisputably an intraoceanic arc, and geochemical anal- ogies between the Coast Range Ophiolite and igneous rocks of the Izu-Bonin-Mari- Pillow basalt with interpillow ana system can be interpreted as strong pelagic limestone. Volcanic evidence for origin of the former in close member of the Middle Juras- relation to an intraoceanic arc, rather than sic Coast Range ophiolite, Llanada remnant, southern to the Sierran arc along the continental , California. margin. Recent work near intraoceanic island arcs in the southwest Pacific has shown the difficulty of distinguishing geo- chemically among arc-related magmas erupted in backarc, intra-arc, and forearc settings (Hawkins, 1994). Accordingly, origin of the Coast Range Ophiolite by backarc spreading behind an precluding east-west transport behind an 2. COAST RANGE OPHIOLITE AS intraoceanic island arc that lodged in the arriving island arc. PALEOEQUATORIAL MID-OCEAN Sierran foothills late in Jurassic time Two residual questions remain. The LITHOSPHERE remains a viable hypothesis. Scraps of first pertains to relations between the Sier- remnant arc structures within the ophio- Clifford A. Hopson, Department of ran foothills belt and the Klamath Moun- lite are to be expected in this case, along Geological Sciences, University of California, tains, where the Josephine Ophiolite is with overall SSZ geochemistry. If forearc Santa Barbara, CA 93106-9630 inferred to have formed by interarc rifting of the Sierran arc (model 3) were Emile A. Pessagno, Jr., Programs in spreading along the continental margin the correct interpretation, one would Geosciences, University of Texas at Dallas, within the interval 165–155 Ma (Saleeby expect to find rifted fragments of prerift Richardson, TX 75083-0688 et al., 1982; Harper and Wright, 1984; Sierran foothills melange units within the James M. Mattinson, Department of Wyld and Wright, 1988). This interval Coast Ranges, but such has never been Geological Sciences, University of California, overlaps the time span inferred above for reported. Moreover, the Coast Range Santa Barbara, CA 93106-9630 arc rifting within an offshore intraoceanic Ophiolite is capped locally, as at Llanada, Bruce P. Luyendyk, Department of arc complex to form the Smartville com- by ~1500 m of intermediate volcaniclastic Geological Sciences, University of California, plex and Coast Range Ophiolite. In the rocks (Robertson, 1989; Hull et al., 1993), Santa Barbara, CA 93106-9630 Klamaths, however, the Rogue Volcanics which could readily be derived from a rift- Ward Beebe, Department of Geological form a frontal arc coeval with the interarc ing intraoceanic arc but are unlike chert- Sciences, University of California, basement of the Josephine Ophiolite, rich quartzolithic Upper Jurassic to Lower Santa Barbara, CA 93106-9630 whereas no analogous assemblage has Cretaceous derived from a Sier- Donna M. Hull, Programs in Geosciences, been discovered within the Coast Ranges. ran provenance and deposited in both the University of Texas at Dallas, Spreading to form the Josephine Ophiolite Mariposa Formation of the foothills belt Richardson, TX 75083-0688 may have been a response to arc-arc colli- and at lower horizons of the Great Valley Ivette M. Muñoz, Programs in Geosciences, sion in the Sierran region farther south forearc basin (Ingersoll, 1983; Short and University of Texas at Dallas, Richardson, (Ingersoll and Schweickert, 1986). Ingersoll, 1990). Upward transitions from TX 75083-0688 The second issue pertains to the time distal to proximal volcaniclastic strata Charles D. Blome, U.S. Geological Survey, of initiation of Franciscan subduction west above the Coast Range Ophiolite are inter- MS 919, Box 25046, Denver, CO of the Coast Range Ophiolite. Ages (K-Ar, preted here as the result of progradation 80255-0046 U-Pb, Ar-Ar) of high-grade blueschist from arc sources, rather than the record blocks within the Franciscan assemblage The igneous pseudostratigraphy, of tectonic transport toward the arc (as range from 140–145 to ~160 Ma (Wak- structure, seismic velocity profile, petro- in model 2). abayashi, 1992). The oldest ages appear to logy, and geochemistry of the mid-Jurassic If the Coast Range Ophiolite, as overlap with the final phases of formation (~170–165 Ma) Coast Range Ophiolite argued here, is an accreted fragment of of the Smartville complex and Coast seem consistent with tectonically thinned, backarc-interarc crust, then its formation Range Ophiolite, whereas the tectonic multiply altered oceanic crust, but provide at essentially the same time as the intra- model favored here holds that Franciscan no clear-cut guide to original tectonic set- arc Smartville complex of the Sierran foot- subduction should postdate accretion of ting. Lacking decisive evidence from the hills reflects the same general interval of those intra-arc and backarc features by igneous rocks, we turn to the associated extensional tectonism within an intra- arc-arc collision in the Sierran foothills. Jurassic sedimentary rocks. The succession oceanic arc-trench system. The rather Perhaps resolution of this paradox lies in of sediments entrapped within and accu- mafic crustal profile of the intervening a better understanding of the mechanisms mulating on top of the igneous crust of a Great Valley (Cady, 1975; Holbrook and by which subduction was arrested in the mobile oceanic plate make up its plate Mooney, 1987) can be understood as Sierran foothills and initiated in the Coast stratigraphy (Berger and Winterer, 1974), representing similar ophiolitic materials, Ranges to the west. Some overlap in the which is applicable to ophiolites and can perhaps telescoped by deformation during timing of those two events is not difficult provide a travel history for an ancient accretion and certainly overprinted by to envision as an intraoceanic arc system oceanic plate. For example, the plate subsequent Sierran plutonism. Recent gradually lodged firmly against the stratigraphy of ophiolites formed at a interpretations of paleomagnetic data continental margin. divergent plate margin (mid-ocean ridge) for several remnants of the Coast Range will reflect transport toward a convergent Ophiolite suggest paleolatitudinal concor- margin, marked by progressive increase dance with North America (Butler et al., in the sedimentary products of arc 1991; Mankinen et al., 1991; Hagstrum and Murchey, 1993), requiring no major north-south transport (model 2) but not Ophiolite continued on p. 5

4 GSA TODAY, February 1996 Ophiolite continued from p. 4 Figure 2. Tectonostratigraphic diagram compar- ing Coast Range Ophiolite (CRO) –VP– basal Great Valley Group (GVG) succession at Cuesta Ridge, volcanism. Arc-related ophiolites, born Point Sal, Stanley Mountain, and Llanada with behind convergent plate margins (i.e., the Josephine Ophiolite (JO)–Galice succession. above subduction zones), lie adjacent to Time scale from Gradstein et al. (1994); top of the arc volcanism from birth. Their travel his- Jurassic from Bralower et al. (1990); radiolarian tories might keep them close to the active zonation from Pessagno et al. (1993). Minimum arc (e.g., arc-parallel strike-slip transport) and estimated maximum possible ages of Coast Range Ophiolite remnants (only tops shown) are or take them farther away in the case of based on U/Pb and Pb/Pb isotopic ages, respec- prolonged back-arc spreading, but will tively (Mattinson and Hopson, 1992). The not carry them toward the arc from a Josephine Ophiolite age is from Harper et al. distant birthplace. (1994); Devils Elbow outlier (JODE) age from Jurassic plate stratigraphy at the Point Wyld and Wright (1988). Black intervals span the Sal, Stanley Mountain, Cuesta Ridge, and depositional hiatus between Coast Range Ophio- lite remnants and the overlying VP succession; Llanada Coast Range Ophiolite remnants also a hiatus within VP. VP spans time of vol- (Fig. 1) shows that the igneous oceanic canopelagic sedimentation including distal tuffa- crustal rocks originated beyond reach of ceous (VPt) and proximal sandy-fragmental (VPs) terrigenous or sedimentation facies, respectively. Terrigenous sedimentation on and were then carried progressively closer Coast Range Ophiolite–VP began with basal strata of the Great Valley Group in the latest Jurassic. to the coeval Jurassic arc that fringed west- The GALICE interval spans the terrigenous ern North America. The lithostratigraphic graywacke-mudstone sequence above Josephine succession begins with the sedimentary Ophiolite and thin VP strata. Nevadan rocks entrapped as small scraps within the (Klamath phase) from Harper et al. (1994). CT ophiolite volcanic member. These are interval spans sedimentation in Central Tethyan mainly basaltic rubble, red jasper (silicified Province, NT in Northern Tethyan Province, and SB in Southern Boreal Province; question-mark intervals lack diagnostic radiolarians. Asterisk wedges ferruginous hydrothermal sediment), and mark Late Jurassic magmatic (intrusive) and hydrothermal events. pelagic limestone. Limestone is the only externally derived , indi- cating an open-ocean setting. Claims of arc-derived volcaniclastic strata interbed- ded with Coast Range Ophiolite lavas tephra) mixed in varying proportions facies) downwind of an active Jurassic are incorrect; those strata belong to the with radiolarian ooze. A thick upper sandy- arc, then partly into the proximal unconformably overlying Late Jurassic fragmental facies composed of up to 300 m volcaniclastic submarine apron (VP (volcanopelagic) (VP) succession (see of bedded pumiceous and lithic lapilli tuff, sandy-fragmental facies). below), locally isolated between subvol- volcaniclastic (including tur- The Coast Range Ophiolite–VP con- canic intrusive sheets (postophiolite sills, bidites) and conglomerate, with interbeds tact is unconformable: pillow lavas below mistaken for ophiolite lava flows) that of radiolarian tuffaceous mudstone, over- this contact carry interpillow limestone, commonly concentrate along and just lies the tuff- facies at some cen- whereas VP strata immediately above are above the Coast Range Ophiolite–VP tral Coast Range localities (Llanada–Del mixtures of radiolarian ooze and volcanic contact in some Coast Range Ophiolite Puerto–Hospital Creek), and locally ash. The unconformity marks a deposi- remnants. (Llanada) grades up into an additional 500 tional hiatus (Fig. 2) that began when Resting depositionally on the ophio- m of cobbly to bouldery andesitic subma- spreading carried Coast Range Ophiolite lite lava is an Upper Jurassic (Oxfordian- rine debris-flow deposits capped by tuffa- oceanic crust below the calcite compensa- Tithonian) VP succession (Hull et al., 1993) ceous radiolarian chert. The tuff-radiolar- tion depth (CCD), ending carbonate depo- composed mainly of two original compo- ite facies represents the submarine distal sition, and lasted until its entry into the nents: radiolarian ooze and rhyolitic to tephra fringe of an active, emergent vol- tephra fringe of an arc. The Upper Jurassic andesitic volcaniclastic marine sediment. canic arc; the upper sandy-fragmental and Cretaceous Great Valley Group of ter- Most VP remnants consist of a thin facies is the corresponding coarser proxi- rigenous clastic marine strata overlies the (50–130 m) tuff-radiolarite facies of tuffa- mal submarine apron. This succession VP succession conformably. The upper- ceous radiolarian mudstone and chert, reflects transport of the oceanic plate most Jurassic lower portion of the Great and altered tuffs representing submarine (Coast Range Ophiolite) through the Valley Group, composed of mudstone deposits of pyroclastic fallout (airborne distal tephra fringe (VP tuff-radiolarite with interbeds of turbiditic siltstone and sandstone, plus local lenticular (channel- fill) pebble conglomerate well above the base of the succession (Bailey et al., 1964; Page, 1972; Suchecki, 1984), correspond to submarine slope deposits prograding over basin-plain deposits (Suchecki, 1984). These basal Great Valley Group strata, derived from Klamath-Sierran tectonic Interpillow pelagic (cocco- lithic) limestone near the top highlands at the North American accre- of the upper lava, Point Sal tionary margin (Dickinson and Rich, 1972; remnant of the Coast Range Ingersoll, 1983), represent a terrigenous ophiolite, Santa Barbara clastic apron that prograded over the deep County, California. ocean floor (Coast Range Ophiolite–VP succession) following onset of the (Pessagno et al., 1996).

Ophiolite continued on p. 6

GSA TODAY, February 1996 5 Ophiolite continued from p. 5 show that the mid-Jurassic Coast Range Late Jurassic subduction zone lies buried Ophiolite oceanic crust formed near the beneath California’s Great Valley and We accordingly infer that the Coast paleoequator and was transported north- thrust sheets of the ; Range Ophiolite formed at a spreading ward, passing progressively through (7) the 162–164 Ma Josephine ophiolite center in an open-ocean region of pelagic Central Tethyan, Northern Tethyan, (Figs. 1 and 2), formed in the backarc carbonate sedimentation. Seafloor spread- and Southern Boreal provinces during VP region behind the Middle to Late Jurassic ing carried Coast Range Ophiolite crust to sedimentation in the Late Jurassic (Fig. 2). Rogue-Chetco arc of the Klamath region sub-CCD abyssal depths, ending pelagic Coast Range Ophiolite–VP remnants (Harper, 1984; Harper et al., 1994), and is carbonate deposition for up to ~12 m.y. at Cuesta Ridge and Llanada (also Del not related to the Coast Range Ophiolite; (Fig. 2), then into a realm of oceanic Puerto) host swarms of Upper Jurassic and (8) the diachronous Late Jurassic sub- upwelling where radiolaria flourished basaltic-diabasic, keratophyric-microdi- volcanic igneous and hydrothermal events and radiolarian ooze deposition began. oritic and quartz keratophyric–grano- took place beneath deep sea floor during This coincided approximately with entry phyric sills and dikes, and are overprinted VP sedimentation, and may represent into the distal tephra fringe of an active by hydrothermal . The rift-tip propagation of a new, Late Jurassic volcanic arc, where airborne ash mixed widespread assumption that ophiolite oceanic rift system through the older with radiolarian remains in the water genesis (mid-Jurassic) and pyroclastic arc (mid-Jurassic) Coast Range Ophiolite column. Most parts of the mobile Coast volcanism were contemporaneous and plate (Hopson et al., 1991). Range Ophiolite plate moved through closely adjacent, and consequently that only the tephra fringe of the arc, accumu- the ophiolite formed near or within an 3. COAST RANGE OPHIOLITE AS lating radiolarian ooze and mainly fine active arc (Evarts, 1977; Evarts and PARAUTOCHTHONOUS FORE- ash (Fig. 2, VP tuff-radiolarite facies). But Schiffman, 1982; Robertson, 1989), ARC LITHOSPHERE part of the Coast Range Ophiolite plate comes from the occurrence of sills (mis- Jason B. Saleeby, Geological and Planetary approached the volcanic arc more closely, taken for ophiolite lava flows), dikes, and Sciences, 170-25, California Institute of passing first through its deep-sea tephra hydrothermal alteration within the VP Technology, Pasadena, CA 91125 fringe and then into its proximal apron of succession. This interpretation is now volcaniclastic turbidites and debris flows rendered untenable by (1) recognition The forearc generation model for the (Fig. 2; Llanada remnant). Following VP of the Coast Range Ophiolite–VP uncon- Coast Range Ophiolite is based on petro- arc sedimentation, which ended in the formity with a long depositional hiatus, chemical and stratigraphic features of the late Tithonian, latest Jurassic terrigenous (2) identification of supposed “ophiolite ophiolite, relations of coeval ophiolitic turbidites and muds from Klamath-Sierran lava flows interbedded with arc volcani- and arc rocks of the western Klamath accreted advanced out over the clastics” as sills invading Upper Jurassic Mountains and Sierra Nevada, and consid- deep ocean floor. This Jurassic Coast strata and yielding Late Jurassic radiomet- eration of relations in west Pacific fringing Range Ophiolite–VP–basal Great Valley ric ages, and (3) evidence that the Late arc systems. A corollary of the forearc Group oceanic succession, uplifted when Jurassic “sill event” took place at more generation model is that at relatively short Franciscan subduction began farther out- northerly paleolatitudes than creation time scales (~5 m.y.) juvenile forearc crust board, then floored the new Cretaceous of Coast Range Ophiolite oceanic crust may find itself residing either within an forearc basin. (Fig. 2). interarc basin or within the locus of arc A mobile interpretation of Coast The lithostratigraphic columns of construction. Such changes in tectonic Range Ophiolite–VP oceanic crust also Figure 2 can be used as map tracklines setting may arise from evolving loci of stems from paleomagnetic and faunal for individual segments of moving Coast arc construction working in series with evidence of large-scale Jurassic paleolatitu- Range Ophiolite oceanic lithosphere. the production of juvenile ophiolitic crust, dinal displacement indicated by (1) paleo- The assemblage of tracklines, positioned and in the case of oblique subduction the magnetic measurements on pillow lavas geographically according to constraints tangential migration of active and inactive at three Coast Range Ophiolite remnants, imposed by the lithofacies succession, arc segments and basinal tracts into (and and (2) provinciality of radiolarian and paleolatitude–faunal province, and age out of) ephemeral juxtapositions. This molluscan faunas in VP–Great Valley of each member, show the trajectories of corollary and its possible application to Group strata that correlate roughly individual segments of an oceanic plate the Coast Range Ophiolite is demon- with paleolatitude. Paleoinclinations moving from their origin through a suc- strated by the nearby Josephine Ophiolite of remanent magnetism in Coast Range cession of sedimentary environments of the western Klamaths (Fig. 1). The Ophiolite pillow lavas at Stanley Moun- toward the consuming plate margin (Fig. Josephine Ophiolite may be broadly tain (McWilliams and Howell, 1982), 3). We conclude that (1) the Coast Range correlative with the northern Coast Range Point Sal, and Llanada (Beebe, 1986; Ophiolite is exotic to the Jurassic North Ophiolite (Saleeby, 1981, 1992), but the Pessagno et al., 1996) were acquired in the American continental-margin arc; (2) the former is easier to interpret because it is Jurassic paleoequatorial region. Lower VP trackline assemblage cannot be fitted into preserved in its emplacement configura- strata that rest on the Coast Range Ophio- a backarc, forearc, or infant-arc associa- tion with little modification. In contrast, lite remnants consistently have Central tion; (3) the Coast Range Ophiolite–VP the Coast Range Ophiolite has been Tethyan radiolarian assemblages, whereas tracklines (plate motion) must be oriented severely modified by Franciscan under- progressively higher VP strata have North- approximately north-northeast–south- thrusting and extensional attenuation ern Tethyan and then Southern Boreal radi- southwest to fit the age–paleolatitude–fau- (Jayko et al., 1987). olarian assemblages, respectively (Fig. 2; nal province constraints; (4) the tracklines The Josephine Ophiolite formed in a Pessagno et al., 1996). Molluscans indicate dextral oblique subduction of transtensional basin that initially opened (Buchias) and radiolarians of the overlying oceanic lithosphere beneath the north- along the forearc edge of the Sierran- Great Valley Group strata are Southern west-trending Jurassic arc system, (5) the Klamath Middle Jurassic arc (Saleeby, Boreal. Boundaries between Central subduction zone lay between the arc and 1982; Harper and Wright, 1984; Wyld and Tethyan, Northern Tethyan, and Southern VP tephra fringe, the trench forming a Wright, 1988; Saleeby and Harper, 1993). Boreal provinces are placed at approxi- barrier to all but airborne volcaniclastic This arc was constructed in large part over mately lat 22°N and 30°N, respectively, on materials until the trench filled and was a polygenetic basement of older ensimatic the basis of global distributions of mollus- overlapped in late Tithonian time (Fig. 2; assemblages that were previously accreted can, radiolarian, and calpionelid faunas Llanada remnant) following the main (e.g., Pessagno et al., 1987). These data pulse of the Nevadan orogeny; (6) the Ophiolite continued on p. 7

6 GSA TODAY, February 1996 the margins of the valley indicate that at least the western part of the valley is floored by the Coast Range Ophiolite, and that the eastern margin of the valley is floored by coeval mafic submarine arc strata of the western Sierra Nevada. These arc rocks and their polygenetic basement are cut by swarms of sheeted and individ- ual dikes that are the same age as the Coast Range Ophiolite and Josephine Ophiolite, and which mark the waning of Middle Jurassic arc activity in this region (Saleeby, 1982, 1992; Saleeby et al., 1989). The forearc spreading model for the Coast Range Ophiolite considers these western- most Sierran rocks to be the inner bound- ary of the Coast Range Ophiolite–Joseph- ine Ophiolite basin system. As discussed below, the Josephine and Coast Range Ophiolites appear to have migrated north- ward shortly following spreading genesis, roughly placing the Josephine Ophiolite outboard of the western Sierra Nevada and the Coast Range Ophiolite farther south during basin formation. Unlike the Josephine Ophiolite segment of the basin system, the Coast Range Ophiolite–Great Valley segment survived the Nevadan orogeny. This difference may reflect ~100 km of eastward underthrusting of Josephine Ophiolite–related rocks beneath the central Klamaths following and partly in conjunction with northward transla- tion (Saleeby and Harper, 1993); analo- gous underthrusting is not directly observed, nor imaged geophysically, for the Coast Range Ophiolite–Great Valley Figure 3. Eastern Pacific–western North America region showing key tectonic elements for part of Mid- segment. The forearc spreading model dle to latest Jurassic time (~166–143 Ma). Tracklines of Point Sal (P), Cuesta Ridge (C), Stanley Mountain thus considers the modern morphologic (S), and Llanada (L) Coast Range Ophiolite segments trace their progression by sea-floor spreading from Great Valley as a partial remnant of the a 166 Ma paleoequatorial midocean ridge spreading center through deep-sea regions of (1) sub-CCD calcareous ooze starvation, (2) volcanopelagic sedimentation, to (3) their 143 Ma positions (arrowheads) original basin. just prior to burial beneath terrigenous clastic sediments (basal Great Valley Group) from the adjacent The forearc spreading model implies Nevadan orogen. Trackline positions and direction are constrained by data combined in Figure 2: the that the Coast Range Ophiolite formed in lithostratigraphy, biostratigraphy, Coast Range Ophiolite radiometric ages and paleomagnetic latitudes, a supra–subduction-zone (SSZ) setting, as and VP faunal provinces. Location of western North America at 143 Ma from Scotese and Denham suggested by abundant geochemical data (1988), modified to paleolatitudes of May et al. (1989). Triangles mark trend of Upper Jurassic arc (Shervais and Kimbrough, 1985; Shervais, volcanics and plutons. Jurassic subduction zone in front of the arc placed at the California Great Valley magnetic-gravity high (Fig. 1), where mafic high-velocity crust dips eastward beneath the Sierra Nevada 1990). An SSZ setting is further suggested (Mooney and Weaver, 1989); projection northward and southward is schematic. VP distal tuffaceous by the presence of arc-derived pyroclastic, facies (from airborne tephra and radiolarian ooze) accumulated outboard of the Jurassic trench. VP prox- volcaniclastic, and hypabyssal material imal sandy-fragmental facies (volcaniclastic turbidites and debris flows) accumulated inboard, bounded expressed mainly in the later phases of by the trench until it filled in latest Jurassic time (see text). JO, IO, and FO schematically depict Middle the Coast Range Ophiolite igneous and to Late Jurassic back-arc basins whose oceanic crust–mantle remnants are the Josephine, Ingalls, and sedimentary succession (referenced in Fidalgo ophiolites (Harper, 1984; Miller et al., 1993). SC marks the Smartville intra-arc igneous complex (Beard and Day, 1987). model 2). These later arc components are analogous to the constructional arc prod- ucts that migrated westward to the outer fringes of the Josephine Ophiolite basin. The absence of rifted screens of older Ophiolite continued from p. 6 terranes and part of the Josephine Ophio- basement in the Coast Range Ophiolite lite basin floor. The entire system was then (as noted in model 1) may stem from the to the Cordilleran plate edge; to the south- imbricated and crosscut by plutons during limited outcrop area of the Coast Range east the arc tracks onto North American the Late Jurassic Nevadan orogeny. The Ophiolite relative to the probable original continental lithosphere, which prior to forearc spreading generation, inter-arc basin size. active-margin tectonism had been thinned basin residence, and thrust imbrication The difficulties with model 1, which by passive-margin formation. As Josephine of the Josephine Ophiolite all occurred also envisions SSZ affinity, are outlined as Ophiolite forearc spreading progressed, within ~10 m.y. follows (after Saleeby and Busby-Spera et arc magmatism to the east waned. By the Geophysical and basement core data al., 1992): (1) The implied Late Jurassic cessation of spreading, arc magmatism indicate that the Great Valley is underlain (Nevadan) collisional suture within the relocated along the outer edge of the primarily by oceanic crust (Cady, 1975; western Sierras and Klamaths cannot be Josephine Ophiolite basin, capping both Saleeby et al., 1986). These geophysical rifted screens of older Klamath (or Sierran) data as well as stratigraphic relations along Ophiolite continued on p. 8

GSA TODAY, February 1996 7 Ophiolite–Josephine Ophiolite basin system may have represented ~2000 km of the forearc (and ephemeral inter- to intra-arc) region along the Cordilleran Tuffaceous radiolarian chert within the tuff/radiolarite plate edge. Available constraints on facies of the Upper Jurassic spreading kinematics recorded within volcanopelagic succession, the Josephine Ophiolite, and locally lying unconformably on pil- within the Coast Range Ophiolite, are per- low lava (not shown) of the missive of a strong spreading component Middle Jurassic Coast Range subparallel to the plate edge, suggesting ophiolite. Point Sal Coast Range Ophiolite–VP remnant, that tangential transport was dynamically Santa Barbara, California. linked to spreading (Harper et al., 1985; Saleeby, 1992). Furthermore, regional lin- ear gravity-magnetic anomalies oriented along the axis of the Great Valley (Fig. 1) may be modeled as a fossil transform sys- tem within the basin floor. Such longitudi- nal transform(s) could have served as Ophiolite continued from p. 7 zone along which subduction nucleated. zones of terrane removal as well as ophio- The details of this mechanism encounter lite accretion and translation (Saleeby and delineated with confidence; the most difficulty for the Coast Range Ophiolite, as Busby-Spera, et al., 1992). Stratigraphic likely structures are pre-Callovian (>169 discussed in model 2. An alternative, yet differences between Josephine Ophiolite Ma) in age, as indicated not only by local fundamentally similar slab rollback mech- and Coast Range Ophiolite can be recon- crosscutting relations of plutons but also anism for the Coast Range Ophiolite is the ciled with the northward transport model. by the occurrence of a regional belt of subduction of old, cold Panthalassan Overlapping volcanic-poor turbidites are Middle Jurassic dioritic to peridotitic arc lithosphere inherited from the Pangea of Oxfordian-Kimmeridgian age above the plutons that cut across the depositional regime (Saleeby and Busby-Spera et al., Josephine Ophiolite and its fringing arc, basement of both hypothetical east- and 1992). Upper-plate extension along the and similar strata of the lowermost Great west-facing arcs. (2) The entire subduction southwest Cordilleran plate edge may be Valley Group young southward from late complex and forearc region of the postu- recorded as far back as time Kimmeridgian to Tithonian age above lated east-facing arc is missing; Tethyan by earlier phases of forearc magmatism the Coast Range Ophiolite and its over- limestone-bearing melange units (Saleeby, 1992) as well as a tendency for lapping arc strata. In the western Sierra purported to represent a sandwiched much of the arc magmatism to have Nevada, similar strata locally range back subduction complex reside as depositional expressed itself by silicic ignimbrite pond- to Callovian in age. These units are inter- basement for Jurassic arc rocks and thus ing within a largely submarine graben preted as different parts of a regional represent an earlier phase of tectonic depression system (Busby-Spera, 1988). progradational submarine fan system accretion. (3) Likewise, the Middle Jurassic The single largest pulse of ignimbrite derived from northerly Middle and Late forearc and subduction complex for the ponding along the eastern Sierra Nevada Jurassic highlands and spread southward, implied west-facing system are missing; corresponds precisely in time with the for- first across the western Sierran belt and the best candidates for such rocks are cut mation of the Coast Range and Josephine then sequentially across the Josephine by copious Middle Jurassic arc plutons Ophiolites as well as the western Sierra Ophiolite and Coast Range Ophiolite as and, in the northern Sierra, are part of the dike swarms. We thus suggest that the the various segments of the basin system depositional basement of Lower and Mid- broadly extensional arc-forearc region migrated into their resting sites (Saleeby dle Jurassic arc strata. (4) Rock assemblages intensified in its extensional deformation and Busby-Spera, et al., 1992). along the Sierran crest and farther east, toward the end of the Middle Jurassic, considered to be the axial Jurassic arc, are resulting in the production of ophiolitic REFERENCES CITED dominated by silicic ignimbrites and by forearc crust in the wake of the foundering plutonic suites with scattered backarc slab. This analysis considers the dynamics Bailey, E. H., and Blake, M. C., 1974, Major chemical characteristics of Mesozoic Coast Range Ophiolite in geochemical affinities; the axis of the arc of the subducting plate to be the prime California: U.S. Geological Survey Journal of Research, more likely lay farther west, represented factor in promoting forearc spreading. v. 2, p. 637–656. in part by the regional belt of dioritic to As mentioned above, the Josephine Bailey, E. H., Irwin, W. P., and Jones, D. 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G., ed., The geotectonic development of California: Englewood Cliffs, New Jer- Society of America Bulletin, v. 101, p. 1420–1433. Mankinen, E. A., Gromme, C. S., and Williams, K. M., sey, Prentice-Hall, p. 132–181. Edelman, S. H., Day, H. W., and Bickford, M. E., 1989, 1991, Concordant paleolatitudes from ophiolitic Implications of U-Pb zircon ages for the tectonic set- sequences in the northern California Coast Ranges, Saleeby, J. B., 1982, Polygenetic ophiolite belt of the tings of the Smartville and Slate Creek complexes, U.S.A.: Tectonophysics, v. 198, p. 1–21. California Sierra Nevada: Geochronological and tectonostratigraphic development: Journal of northern Sierra Nevada, California: Geology, v. 17, Mattinson, J. M., and Hopson, C. A., 1992, U/Pb ages of Geophysical Research, v. 87, p. 1803–1824. p. 1032–1035. the Coast Range Ophiolite: A critical reevaluation based Evarts, R. C., 1977, The geology and petrology of the on new high-precision Pb/Pb ages: American Associa- Saleeby, J. B., 1992, Petrotectonic and paleogeographic Del Puerto ophiolite, Diablo Range, central California tion of Petroleum Geologists Bulletin, v. 76, p. 425. settings of U.S. Cordilleran ophiolites, in Burchfiel, B. C., et al., The Cordilleran orogen: Conterminous Coast Ranges, in Coleman, R. G., and Irwin, W. P., eds., May, S. R., Beck, M. E., and Butler, R. F., 1989, North U.S.: Boulder, Colorado, Geological Society of America, North American ophiolites: Oregon Department of American apparent polar wander, plate motion, and Geology of North America, v. G-3, p. 653–682. Geology and Mineral Industries Bulletin 95, p. 121–139. left-oblique convergence: Late Jurassic–Early Cretaceous Evarts, R. C., and Schiffman, P., 1982, Submarine orogenic consequences: Tectonics, v. 8, p. 443–451. Saleeby, J. B., and Harper, G. D., 1993, Tectonic rela- tions between the Galice Formation and the schists of hydrothermal metamorphism of the Del Puerto ophio- McWilliams, M. O., and Howell, D. G., 1982, Exotic ter- Condrey Mountain, Klamath Mountains, northern lite, California: American Journal of Science, v. 283, ranes of western California: Nature, v. 297, p. 215–217. p. 289–340. California, in Dunne, G., and McDougall, K., eds., Menzies, M., Blanchard, D., and Xenophontos, C., Mesozoic paleogeography of the western United Gradstein, F., Agterberg, F., Ogg, J., Hardenbol, J., van 1980, Genesis of the Smartville arc-ophiolite, Sierra States—II: Pacific Section SEPM, Book 71, p. 61–80. Veen, P., Thierry, J., and Huang, Z., 1994, A Mesozoic Nevada foothills, California: American Journal of Saleeby, J. B., Harper, G. D., Snoke, A. W., and Sharp, time scale: Journal of Geophysical Research, v. 99, Science, v. 280-A, p. 329–344. p. 24,051–24,074. W. D., 1982, Time relations and structural-stratigraphic Miller, R. B., Mattinson, J. M., Funk, S. A. G., Hopson, Hagstrum, J. T., and Murchey, B. L., 1993, Deposition of C. A., and Treat, C. L., 1993, Tectonic evolution of Upper Jurassic cherts (Coast Range Ophiolite) at Stanley Mesozoic rocks in the southern and central Washington Ophiolite continued on p. 10

GSA TODAY, February 1996 9 Ophiolite continued from p. 9 WASHINGTON REPORT patterns in ophiolite accretion, west-central Klamath Mountains, California: Journal of Geophysical Bruce F. Molnia Research, v. 87, p. 3831–3848. Saleeby, J. B., Blake, M. C., and Coleman, R. G., 1984, Washington Report provides the GSA membership with a window on the activities of the Pb/U zircon ages on thrust plates of west-central federal agencies, Congress and the legislative process, and international interactions that Klamath Mountains and Coast Ranges, northern California and southern Oregon: Eos (American could impact the geoscience community. In future issues, Washington Report will present Geophysical Union Transactions), v. 65, p. 1147. summaries of agency and interagency programs, track legislation, and present insights into Saleeby, J. B., and 12 others, 1986, Continent-ocean Washington, D.C., geopolitics as they pertain to the geosciences. transect, corridor C2, Monterey Bay offshore to the Colorado Plateau: Geological Society of America Map and Chart Series TRA-C2, 2 sheets, scale 1:500,000, 87 p. text. Saleeby, J. B., Shaw, H. F., Niemeyer, S., Moores, E. M., Farewell, U.S. Bureau of Mines and Edelman, S. H., 1989, U/Pb, Sm/Nd and Rb/Sr geochronological and isotopic study of northern Sierra Nevada ophiolitic assemblages, California: Contribu- “The U.S. Department of Interior will recognize the accomplishments and tions to Mineralogy and Petrology, v. 102, p. 205–220. honor the contributions of the U.S. Bureau of Mines in a commemorative Saleeby, J. B., and seven others, 1992, Early Mesozoic tectonic evolution of the western U.S. Cordillera, in ceremony Wednesday, December 13, 1995. The ceremony will focus on the Burchfiel, B. C., et al., eds., The Cordilleran orogen: Conterminous U.S.: Boulder, Colorado, Geological research and achievements made by the 85 year old agency, slated for Society of America, Geology of North America, v. G-3, p. 107–168. closure January 8, 1996, as a result of GOP budget cuts.” Schweickert, R. A., 1976, Shallow-level plutonic —Department of Interior Media Advisory, December 11, 1995 complexes in the eastern Sierra Nevada, California, and their tectonic implications: Geological Society of America Special Paper 176, 58 p. “When I accepted the responsibilities and challenges of being the 19th Schweickert, R. A., and Cowan, D. S., 1975, Early Mesozoic tectonic evolution of the western Sierra Director of the U.S. Bureau of Mines, I did so as a scientist who believes Nevada, California: Geological Society of America Bulletin, v. 86, p. 1329–1336. that scientists should invest in scientific leadership. It is critical that the Schweickert, R. A., Bogen, N. L., Girty, G. H., Hanson, focus of our research investments be on solving problems, rather than lost in R. E., and Merguerian, C., 1984, Timing and structural expression of the Nevadan orogeny, Sierra Nevada, conflict and chaos. I have observed threats to that focus for our nation that California: Geological Society of America Bulletin, v. 95, p. 967–979. are truly staggering. We are a nation who relies on science and technology, Scotese, C. R., and Denham, C. R., 1988, User’s manual for Terra Mobilis: Plate tectonics for the Macintosh: yet we run away from science and technology leadership. First, the Office Scotese and Denham. of Technology Assessment; second, the U.S. Bureau of Mines; third, who Shervais, J. W., 1990, Island arc and ocean crust ophiolites: Contrasts in the petrology, geochemistry knows? Given the impasse in budget resolution over the past two and a and tectonic style of ophiolite assemblages in the California Coast Ranges, in Malpas, J., et al., eds., half months, we should ask, ‘How are we going to refocus our science and Ophiolites, oceanic crustal analogues: Nicosia, Cyprus Geological Survey Department, p. 507–520. technology spending so that the national interest is the common ground?’” Shervais, J. W., and Kimbrough, D. L., 1985, Geochemi- —Bureau of Mines Director Rhea L. Graham, December 13, 1995 cal evidence for the tectonic setting of the Coast Range ophiolite: A composite island-arc–oceanic crust terrane in western California: Geology, v. 13, p. 35–38. Short, P. F., and Ingersoll, R. V., 1990, Petrofacies and On December 13, 1995, the last ued programs include pollution preven- provenance of the Great Valley Group, southern workday before the start of the prolonged tion and control, environmental waste Klamath Mountains and northern Sacramento Valley, in Ingersoll, R. V., and Nilsen, T. H., eds., Sacramento shutdown of the Federal Government, remediation, minerals land assessment, Valley symposium and guidebook: Pacific Section SEPM the Department of the Interior (DOI) and minerals availability. Book 65, p. 39–52. paid tribute to the employees of the U.S. Facilities and offices that were closed Stern, R. J., and Bloomer, S. H., 1992, Subduction zone Bureau of Mines (USBM) for 85 years of are located at Spokane, Washington; Reno, infancy: Examples from the Eocene Izu-Bonin-Mariana outstanding public service and dedication Nevada; Salt Lake City, Utah; Denver, and Jurassic California arcs: Geological Society of America Bulletin, v. 104, p. 1621–1636. to improving technology and protecting Colorado; Tuscaloosa, Alabama; Rolla, Stern, T.W., Bateman, P. C., Morgan, B. A., Newell, human resources. Missouri; Minneapolis, Minnesota; and M. F., and Peck, D. L., 1981, Isotopic U-Pb ages of In October 1995, as part of the budget Washington, D.C. The 90-day time line zircon from the granitoids of the central Sierra appropriations process, the Congress expired on January 8, 1996. At the com- Nevada, California: U.S. Geological Survey Professional Paper 1185, 17 p. voted to terminate all of the USBM memoration, Secretary of the Interior programs in 90 days. It also directed that Bruce Babbitt stated, “The Bureau of Mines Suchecki, R. K., 1984, Facies history of the Upper Juras- sic–Lower Cretaceous Great Valley sequence: Response the USBM’s health and safety research has pioneered award-winning research to structural development of an outer-arc basin: Journal program activities be transferred to the and developed technologies to improve of Sedimentary Petrology, v. 54, p. 170–191. Department of Energy, that some of its the life for the country in many areas. Wakabayashi, J., 1992, Nappes, tectonics of oblique information analysis activities be trans- Their research and development helped to plate convergence, and metamorphic evolution related to 140 million years of continuous subduction, ferred to the U.S. Geological Survey, and detect and prevent fires, reduce silica and , California: Journal of Geology, that the Mineral Land Assessment in coal dust exposure, prevent mine cave-ins, v. 100, p. 19–40. Alaska be transferred to the U.S. Bureau and reengineer dangerous practices and Wyld, S. J ., and Wright, J. E., 1988, The Devils Elbow of Land Management. USBM’s helium equipment to create a safer environment. ophiolite remnant and overlying Galice Formation: program will be administered by the The Bureau has played key roles in New constraints on the Middle to Late Jurassic evolu- tion of the Klamath Mountains, California: Geological Secretary of the Interior until its proposed improving and protecting the health and Society of America Bulletin, v. 100, p. 29–44. privatization is completed by 1997. safety of mine operators.” Secretary Bab- Manuscript received July 1, 1995; revision received Septem- Almost $100 million of USBM 1995 bitt also stated, “As concerns for a cleaner ber 19, 1995; accepted September 20, 1995. ■ programs and activities were eliminated and 1200 employees separated. Discontin- Bureau of Mines continued on p. 11

10 GSA TODAY, February 1996

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