TECTONICS, VOL. 17, NO. 4, PAGES 558-570, AUGUST 1998 Ophiolitic basement to a forearc basin and implications for continental growth: The Coast Range/Great Valley ophiolit.e, California Nicola J. Godfrey• andSimon L. Klemperer Departmentof Geophysics,Stanford University, Stanford, California Abstract. We presenta compilation of 18 publishedmodels material during the Mesozoic, when the dominantprocess at from the length of the Great Valley forearc basin, California, the North American margin was subduction [Ernst, 1981]. based on seismic reflection, borehole, seismic refraction, California therefore is an ideal laboratory for investigating gravity, and aeromagnetic data to address long-standing continental accretionary growth. questionsabout the nature of the basementto the Great Valley, During the Mesozoic, convergencebetween the Pacific (and its origin, its tectonic history, and it's mechanism of earlier) oceanic plates and the North American continental incorporation into the North American continental margin. margin resultedin a typical subductiontriplet of accretionary The geophysicalmodels permit a 700-km-long, 70-km-wide, prism, forearc basin, and volcanic arc. In the Miocene, the complete ophiolite sequencebeneath the entire Great Valley. San Andreasfault system formed replacingthe subduction In the northern Great Valley, the ophiolite is overlain by margin with a transform margin. This resultedin Mesozoic ophiolitic breccia, the ophiolite crust is 7 - 8 km thick, and subductionunits becomingtrapped east of the San Andreas the mantle section is mostly unserpentinized. Beneath the fault system, where they are preservedtoday (Figure 1). southernGreat Valley, there is no ophiolitic breccia, the crust Subductioncontinues today north of the Mendocinotriple may be up to 10 - 12 km thick, and the mantle section, if junction along the Cascadia subduction zone which is presentat all, is serpentinizedto such a degree that it cannot associatedwith the Cascadesvolcanic arc (Figure l a). The be distinguishedfrom Sierran basement or mafic ophiolite Great Valley forearc basin in California is 700 km long and members on the basis of velocity or density data. 100 km wide. This paper focuseson the mode of formationof Geochemical, petrological, and paleomagnetic data support the forearc basin basement to understand how a 100-km-wide suprasubductionzone ophiolite formation at North American sectionof oceanic lithospherebecame incorporatedinto the paleolatitudes,and geologicaldata and geophysicalmodels are North American continent. consistent with ophiolite formation by back arc spreading A large magnetic(Plate 1) and gravityanomaly runs along behind an east facing arc. In the north, this was apparently the entire length of the Great Valley forearc basin [Cady, followed by obductionof back arc crust onto older continental 1975]. These anomalies,combined with penetrationof mafic basementduring the Late JurassicNevadan orogeny. In the lithologies by hydrocarbonexploration wells, were the first south, the newly formed intraoceanic arc and back arc evidencepresented for mafic and/or partially serpentinized apparently collided with the continental margin during the ultramafic material beneath the Great Valley. Since then, Nevadan orogeny but were not obductedonto it. Instead, the many other analysesof geophysicaldata have concludedthat arc and back arc "docked"with the continentalmargin leaving the 700-km-long,100-km-wide Great Valley basinis directly the arc itself to become the basement to the Great Valley underlainby mafic and ultramafic material that we refer to as basin. CretaceousSierran magmatismthen intruded plutons the Great Valley ophiolite [after Bailey et al., 1970] which beneaththe dockedophiolite and mafic arc. Irrespectiveof the may be genetically,and in placesalso physically,related to detailed accretionaryhistory, our cross sectionsshow a rapid the Coast Range ophiolite that cropsout along the western pulse of continental growth by ophiolite accretion of more margin of the Great Valley [Bailey et al., 1970; Griscom and than500 km3 km-1 in lessthan 10 Myr. Jachens, 1990; Jachens et al., 1995; Fliedner et al., 1996; Godfrey et al., 1997]. In this paper, we use the term, "ophiolite",to mean a completeoceanic crustal sequence that 1. Introduction is now part of the continent, rather than its conventional definition [Penrose Conference Participants, 1972]. New Much of western North America, and in particular velocity and density models from both the northern and California, formed as a result of volcanic-arc and subduction- southernGreat Valley require lower density/velocitymaterial terrane accretion and magmatic addition of batholithic beneaththe ophioliticsection [Fliedner et al., 1996;Godfrey et al., 1997; H. Benz, personalcommunication, 1997]. This slower,lower-density material is most likely Sierranbasement 1Now at Departmentof Earth Sciences, University of Southern (Sierra Nevada batholith and/or Foothills Metamorphic California,Los Angeles. Complex) [Godfrey et al., 1997]. Tectonic scenarioswhich could result in an ophiolite overlying Sierran basement Copyright1998 by theAmerican Geophysical Union. include obduction of allochthonous oceanic crust over older Papernumber 98TC01536. continentalbasement [Suppe, 1979; Jayko and Blake, 1986; 0278-7407/98/98TC-01536512.00 Saleeby, 1986; Blake et al., 1989; Dickinson et al., 1996] or 558 GODFREY AND KLEMPERER: GREAT VALLEY OPI-[IOL1TE .55;9 fault 124 ø 122 ø 120øW ß activetrench b) paleotrench ; ophioliticbreccia ; stateline ; a) 40 ø 135 ø 130ø 125ø I ; 55øN 'iSabrament• 50 ø 38 ø P• • -..... -.•:-.:.. ß Plate SanFrancisco •":' '"'"-'-'"'-"..-.-. 45 ø Montere' 36 ø 40 ø Mendocino EZ. Plate Monterey Pacific Morro Ocean 35 ø Arguello CROoutcrops Pacific 30 ø Plate I::J Sierran(volcanicbasement arc) Guadalupe .,,•;'--•(accretionaryFranciscan Complexprism) Shirley F.Z., i:.::...::...::.:.1Great(forearcValley basin) 25 ø 135 ø 130 ø 125 ø 120 ø 115 ø 110 ø 105 ø 1ooøw Figure 1. Location maps. (a) Large-scale map of western North America showing present-day plate boundaries. (b) Map of California showingthe geologicalprovinces discussed in this paper. Abbreviations are as follows: FZ, fracture zone; Gor, Gorda plate; Ex, Explorer plate; CRF, Coast Range fault; SAF, San Andreasfault; CSZ, Cascadiasubduction zone; JdF, Juande Fuca plate; GF, Garlock fault; SNF, Sur Nacimiento fault; HF, Hosgri fault; SGF, San Gregorio fault; FMC, Foothills MetamorphicComplex; SN, Sierra Nevada batholith; K, Klamath terrane; V, Tertiary and Quaternaryvolcanics; B, Basin and Range; S, Salinian block; and M, Mendocino triple junction. younger batholithic intrusions beneath in situ or Valley (San Joaquinbasin). We also use our compilationto allochthonousoceanic crust [Saleeby, 1986; Dickinsonet al., addresslong-standing questions about the origin and tectonic 1996] (Figure 2). evolutionof the Great Valley and Coast Rangeophiolites Motivated by the recentpublication of the first full-crustal [Dickinson et al., 1996]. By understandingthe evolution of sectionsacross the Great Valley to be constrainedby seismic the forearc basement, we learn more about how the forearc data [Fliedher et al., 1996; Godfreyet al., 1997], we compile becamepart of the North Americancontinent. all-previously publishedgeophysical models from the Great Valley (Plate 1). We conclude that similar ophiolite 2. Tectonic and GeologicSetting componentsare representedalong the length of the Great Valley basin, with relatively minor differencesbetween the From Mesozoic through Paleogene (about 210- 29 Ma) northernGreat Valley (Sacramentobasin) and southernGreat time, oceanic crust was being subductedbeneath the North 560 GODFREY AND KLEMPERER:GREAT VALLEY OPHIOLITE MODEL I: MODEL I1: MODEL II1: MODEL IV: BACK ARC OPHIOLITE MOR OPHIOLITE FOREARC OPHIOLITE BACK ARC/ ARC OPHIOLITE (no obduction) (no obduction) '.•;-..:•=••:•..,.:•::., Map view: Late Jurassic (before Nevadan orogeny) I NAM Cross section: f) o.o•¾o. Prior to Late Jurassic Nevadan orogeny After Late Jurassic Nevadan orogeny ß.r-• pre-MiddleJurassic continent ::'/;•CRO/GVO (oceanic crust & intraoceanicarc) Plan view: r• oceaniccrust (undifferentiated) Early Cretaceous • Franciscancomplex r--'] SierraNevada plutons 100 km Figure 2. Four tectonic models for the origin of the Coast Range/GreatValley ophiolite (models I -III are basedon Dickinson et al. [1996] but adjustedfor the geometryshown by our geophysicalmodels). (a) - (d) Model I back arc spreadingmodel after W. R. Dickinson[Dickinson et al., 1996] with ophioliteobduction. (e) - (h) (and Figure 2d) Model II mid-oceanridge model after C. A. Hopson[Dickinson et al., 1996] with ophiolite obduction. (i) - (k) (and Figure 2d) Model III forearcspreading model after J. Saleeby[Dickinson et al., 1996] with no ophioliteobduction. (1) - (n) (and Figure2d) Model IV back arc spreading,where the oceanarc itself is part of the ophiolite with no ophiolite obduction(based on Blake et al., [1989]). Figures2b, 2c, 2f- 2h, 2j, 2k, 2m, and 2n are cross-sectionalviews before and after the Nevadanorogeny. Figures2a, 2e, 2i, and 21 are map views for eachmodel in the Late Jurassicbefore the Nevadanorogeny. Figure 2d is a map view for the Late Cretaceous(valid for all models). Abbreviationsare as follows: CRO, Coast Range ophiolite; GVO, Great Valley ophiolite; SN, pluton emplacementassociated with the Sierra Nevada batholith;MOR, mid-oceanridge; and NAM, North American plate. American plate [Hamilton, 1969] (Figure 2). At about29 Ma, The Franciscan