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Radioisotopic and Biostratigraphic Age Relations in the Coast Range Ophiolite, Northern California: Implications for the Tectonic Evolution of the Western Cordillera

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Radioisotopic and Biostratigraphic Age Relations in the Coast Range Ophiolite, Northern California: Implications for the Tectonic Evolution of the Western Cordillera Radioisotopic and biostratigraphic age relations in the Coast Range Ophiolite, northern California: Implications for the tectonic evolution of the Western Cordillera John W. Shervais† Department of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84322-4505, USA Benita L. Murchey U.S. Geological Survey, 345 Middlefi eld Road, Menlo Park, California 94025, USA David L. Kimbrough Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA Paul R. Renne Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709 and Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA Barry Hanan Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA ABSTRACT Callovian to early Kimmeridgian (Unitary Keywords: ophiolite, age, CRO, cordillera, Association Zones 8–10) in the upper part. tectonics. The Coast Range ophiolite (CRO) in The SFVC sedimentary record preserves northern California includes two distinct evidence of a major faunal change wherein INTRODUCTION remnants. The Elder Creek ophiolite is a relatively small sized, polytaxic radiolarian classic suprasubduction zone ophiolite with faunas were replaced by very robust, oligo- The Coast Range ophiolite of California and three sequential plutonic suites (layered gab- taxic, nassellarian-dominated faunas that the tectonically subjacent Franciscan assemblage bro, wehrlite-pyroxenite, quartz diorite), a included Praeparvicingula spp. have played a pivotal role in plate tectonic theory mafi c to felsic dike complex, and mafi c-felsic We suggest that CRO formation began after since its inception and even now are considered a volcanic rocks; the entire suite is cut by late the early Middle Jurassic (172–180 Ma) colli- paradigm for active margin processes (Dickinson, mid-oceanic-ridge basalt (MORB) dikes and sion of an exotic or fringing arc with North 1971; Ernst, 1970; Ingersoll et al., 1999). None- overlain by ophiolitic breccia. The Stonyford America and initiation of a new or reconfi g- theless, the origin of the Coast Range ophiolite volcanic complex (SFVC) comprises three ured east-dipping subduction zone. The data (CRO) is still controversial, as is its relationship volcanic series with intercalated chert hori- show that the CRO formed prior to the Late to the Franciscan assemblage (e.g., Dickinson et zons that form a submarine volcano enclosed Jurassic Nevadan orogeny, probably by rapid al., 1996; Godfrey and Klemperer, 1998; Saleeby, in sheared serpentinite. Structurally below forearc extension above a nascent subduction 1997). Postulated origins include (1) formation at this seamount are mélange blocks of CRO zone. We infer that CRO spreading ended a Middle Jurassic equatorial midoceanic ridge similar to Elder Creek. with the collision of an oceanic spreading followed by rapid northward transport and Late U/Pb zircon ages from plagiogranite and center ca. 164 Ma, coincident with the oldest Jurassic accretion to North America (Hopson et quartz diorites at Elder Creek range in age high-grade blocks in the structurally underly- al., 1997); (2) formation as backarc basin crust from 165 Ma to 172 Ma. U/Pb zircon ages ing Franciscan assemblage. We further sug- behind a Middle Jurassic island arc that was obtained from CRO mélange blocks below gest that the “classic” Nevadan orogeny repre- sutured to the continental margin during the Late the SFVC are similar (166–172 Ma). 40Ar-39Ar sents a response to spreading center collision, Jurassic Nevadan orogeny (Godfrey and Klem- ages of alkali basalt glass in the upper SFVC with shallow subduction of young lithosphere perer, 1998; Schweickert, 1997; Schweickert et are all younger at ≈164 Ma. Radiolarians causing the initial compressional deforma- al., 1984); and (3) formation by forearc rifting extracted from chert lenses intercalated with tion and with a subsequent change in North above an east-dipping, proto–Franciscan subduc- basalt in the SFVC indicate that the sedimen- American plate motion to rapid northward tion zone during the Middle Jurassic, prior to the tary strata range in age from Bathonian (Uni- drift (J2 cusp) causing sinistral transpression Nevadan orogeny (Shervais, 1990; Shervais and tary Association Zone 6–6 of Baumgartner et and transtension in the Sierra foothills. These Kimbrough, 1985; Shervais et al., 2004; Stern al., 1995a) near the base of the complex to late data are not consistent with models for Late and Bloomer, 1992). There is abundant evidence Jurassic arc collision in the Sierra foothills or for arc-related geochemical signatures in the †E-mail: [email protected]. a backarc origin for the CRO. CRO (Evarts et al., 1999; Giaramita et al., 1998; GSA Bulletin; May/June 2005; v. 117; no. 5/6; p. 633–653; doi: 10.1130/B25443.1; 10 fi gures; 1 table; Data Repository item 2005079. For permission to copy, contact [email protected] © 2005 Geological Society of America 633 SHERVAIS et al. Shervais, 1990; Shervais and Kimbrough, 1985), For purposes of comparison between radioiso- (Murchey and Blake, 1993), predating the Coast but other data, such as sedimentary cover associa- tope and biostratigraphic ages, we use the recent Range ophiolites. The oldest clastic rocks in tions and seismic imaging of the continental mar- Jurassic time scale of Palfy et al. (2000) and the the Franciscan slightly postdate the base of the gin, have been interpreted as support for the open radiolarian zonation of Baumgartner (1995). Great Valley Sequence in northern California ocean or backarc basin models (e.g., Godfrey and Taken together with geochemical data for rocks (Imlay, 1980). Some high-grade metamorphic Klemperer, 1998; Hull et al., 1993; Robertson, of the ophiolite and their fi eld relations, these blocks in the Franciscan assemblage probably 1989). Resolving this controversy is central to data allow us to construct a synthesis for the formed ca. 160–165 Ma (Mattinson, 1988), but our understanding of the tectonic evolution of origin and evolution of the ophiolite, its rela- regional metamorphism of strata in the Eastern western North America during the Jurassic and to tionship to high-grade metamorphism in the belt, including Valanginian metagraywacke, our understanding of how ophiolites form. Franciscan assemblage, and the tectonic evolu- may have occurred in the late Early Cretaceous Much of this debate hinges on age relations tion of the western Cordillera during the Middle (115–120 Ma) (Blake and Jones, 1981). within the CRO, and between the CRO, the and Late Jurassic. Franciscan assemblage, and the Sierra Nevada COAST RANGE OPHIOLITE foothills metamorphic belt. Previous work GEOLOGIC SETTING suggests that the CRO ranges in age from ca. The Coast Range ophiolite in northern Cali- 163 Ma at Point Sal to ca. 154 Ma at Del Puerto The Mesozoic geology of California south of fornia is represented by a thin selvage of serpen- Canyon (Hopson et al., 1981), although there the Klamath Mountains comprises three main tinite matrix mélange along most of the bound- is reason to believe the upper age limit may provinces, from east to west: The Sierra Nevada ary separating forearc sedimentary rocks of the be even older (Mattinson and Hopson, 1992). magmatic arc province, the Great Valley fore- Great Valley Sequence from blueschist facies These ages for CRO formation are similar to arc province, and the Franciscan accretionary metamorphic rocks of the Franciscan Eastern the oldest age found for high-grade (amphibolite complex (Fig. 1). The Sierra Nevada province belt or shale-matrix mélange of the Franciscan facies) metamorphism in the northern Franciscan consists of island arc volcanic, plutonic, and Central belt. This boundary was originally inter- assemblage, as determined by U-Pb isochron sedimentary rocks that range in age from Paleo- preted as a fossil subduction zone (“the Coast and 40Ar/ 39Ar dating on high-grade metamorphic zoic to Late Cretaceous. Many of these rocks Range thrust,” e.g., Bailey et al., 1970), but later blocks (Mattinson, 1986, 1988; Ross and Sharp, formed more or less in place along the western work showed that this boundary may have been 1986; Ross and Sharp, 1988), leading to the margin of North America; others in the Sierra modifi ed by later low-angle detachment faults suggestion that high-grade metamorphic blocks Nevada Foothills metamorphic belt may rep- (Harms et al., 1992; Jayko et al., 1987; Platt, in the Franciscan assemblage formed during resent exotic or fringing arc terranes that were 1986), out-of-sequence thrust faults (Ring and subduction initiation beneath an existing backarc welded to the continental margin during Late Brandon, 1994; Ring and Brandon, 1999), and basin (= CRO) (Wakabayashi, 1990). Jurassic or older collisions (Girty et al., 1995; by east-vergent Neogene thrust faults (Glen, Further, it has been suggested that radiolar- Saleeby, 1983a; Schweickert et al., 1984). 1990; Unruh et al., 1995). The current bound- ian cherts deposited unconformably on top of The Great Valley province represents a Late ary is a complex high-angle fault that may have the CRO document a hiatus of some 8–11 m.y. Jurassic through Cretaceous forearc basin that components of strike-slip, normal faulting, and between ophiolite formation in the Middle was underlain by the Coast Range ophiolite reverse faulting. Jurassic and deposition of overlying chert in the (e.g., Bailey et al., 1970). The Great Valley There are two large ophiolite remnants pre- Late Jurassic (Oxfordian–Tithonian) (Hopson et Sequence consists largely of distal turbidites served in the northern Coast Ranges west of the al., 1992; Hopson et al., 1981; Pessagno et al., near its base, which become more proximal and Sacramento Valley: The Elder Creek ophiolite 2000). This proposed hiatus is cited as primary more potassic upsection (Dickinson and Rich, and the Stonyford volcanic complex (Fig. 1). evidence for an open-ocean origin to the CRO, 1972; Ingersoll, 1983; Linn et al., 1992). The Despite their close proximity (they are separated far from any source of arc detritus or terrigenous lower part of the Great Valley Sequence near by only 60 km along strike) these remnants are sediment.
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