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Home , Coast Range ophiolite, Forearc, Great Valley Sequence, Nevadan orogeny

TECTONICS, VOL. 17, NO. 4, PAGES 558-570, AUGUST 1998

Ophiolitic to a forearc basin and implications for continental growth: The Coast Range/Great Valley ophiolit.e,

Nicola J. Godfrey• andSimon L. Klemperer Departmentof Geophysics,Stanford University, Stanford, California

Abstract. We presenta compilation of 18 publishedmodels material during the , when the dominantprocess at from the length of the Great Valley forearc basin, California, the North American margin was [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 . margin resultedin a typical subductiontriplet of accretionary The geophysicalmodels permit a 700-km-long, 70-km-wide, prism, forearc basin, and . In the , the complete 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 . 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 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 ( 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 of allochthonous 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: (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 • 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 (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 representsthe Late Jurassicto the ridge producing the subducting oceanic plate began Miocene accretionary prism of the Franciscan subduction interacting with the North American margin, creating two system [McLaughlin et al., 1982; Blake et al., 1988]. It is a triple junctions [Atwater, 1970] which migrated away from m•lange of material including graywacke, shale, one another, separated by the lengthening San Andreas metasedimentary rocks of varying grade, mafic volcanics, transformfault system(Figure 1). chert, limestone, and ophiolite fragments [Bailey et al., 1964; East of the system, outcrop geology Blake and Jones, 1974]. The Franciscan Complex is reflects the Mesozoic and Paleogene Andean-type margin; subdividedintc• three belts (Coastal,Central and Eastern) from west to east are exposed the Franciscan Complex which are further subdivided into and subterranes (accretionaryprism) of the Coast Ranges,Great Valley forearc [Blake et al., 1988; McLaughlin et al., 1993]. The basin and associatedCoast Range ophiolite,and Sierra Nevada metamorphicgrade and age of the Franciscanbelts generally magmatic arc, which in the north is associated with the increasefrom west (Coastal belt, prehnite-pumpellyitefacies Foothills MetamorphicComplex (Figure lb) [Dickinson and and Late Cretaceous to Miocene) to east (Eastern belt, Rich, 1972; Dickinson, 1981; Ernst, 1981]. blueschistfacies and Late Jurassicto mid-Cretaceous)[Blake et GODFREY AND KLEMPERER: GREAT VALLEY OPHIOLITE 561

al., 1988; McLaughlin et al., 1993]. East-westshortening thickness variations in both CRO and GVO, we discount an across the Coast Ranges caused eastward wedging of alternativehypothesis that the thin crustalcomponent of the Franciscanmaterial beneath the Great Valley forearc basin CRO representsslow spreadingcrust [Lagabrielle and Cannat, [Wentworth et al., 1984; Wentworth and Zoback, 1990; 1990], whereasthe thicker crustal componentof the GVO Phipps and Unruh, 1992]. This wedging probablybegan in represents"normal" fast spreadingcrust [White et al., 1992]. the Late Cretaceous related to Franciscan subduction The UpperJurassic to UpperCretaceous Great Valley sequence [Wentworthet al., 1984] and is still ongoingtoday becauseof was depositedon oceaniccrust now representedby the CRO compressionacross the San Andreasfault [Unruh and Moores, beneaththe westernGreat Valley basin [Bailey et al., 1970; 1992] as a result of a changein plate motionsat about 5 Ma Evarts, 1977; Hopson et al., 1981]. Coeval strata are assumed [Cox and Engebretson,1985]. to have been depositedon the GVO in the centerof the basin and Mesozoic sedimentary rocks of the Great [Godfreyet al., 1997] (the deepestparts of the basinhave not Valley basincrop out from the latitudeof the Mendocinotriple beendrilled), and uppermost Cretaceous Great Valley sequence junction, the northern termination of the San Andreas fault, to was depositeddirectly on Foothills MetamorphicComplex the "Big Bend" in the San Andreas fault close to the Garlock beneaththe easternpart of the basin [Bailey and Blake, 1969; fault [Jennings, 1977] (Figure 1). The Great Valley forearc Harwood and Helley, 1987]. basin is divided into two subbasins: the Sacramento basin The Sierra Nevada batholith is a , first (northern Great Valley) and the San Joaquin basin (southern formedduring the (210 Ma) and continuinginto Great Valley). The Great Valley sequenceconsists of Upper the Late Cretaceous(80 Ma) [Kistler et al., 1971]. It consists Jurassic(deep ocean and slope environments)to Paleogene of a large numberof graniticplutons and in the west tonalitic (slope, shelf, and nonmarine environments) mudstones, and gabbroicplutons [Saleeby, 1981] that were intrudedinto ,and conglomerates[Bailey et al., 1970; Ingersoll, Paleozoic and Mesozoic rocks now represented by the 1982]. Cenozoic sedimentaryrocks (Oligoceneand younger) Foothills Metamorphic Complex [Bateman, 1981] and by depositedon the Great Valley sequenceinclude slope, shelf, diverseroof pendants. The FoothillsMetamorphic Complex and nonmarinesediments [Ingersoll, 1982]. includeshighly deformedPaleozoic active continentalmargin The Coast Range fault is the presentboundary between the sequences,early Mesozoic hemipelagiccontinental clastic and FranciscanComplex and the Coast Range ophiolite (CRO) (or arc-volcanic strata, and ophiolite fragments including the Great Valley sequencewhere there is no CRO) adjacentto the Smartville ophiolite [Schweickertand Cowan, 1975; Saleeby, Great Valley north of latitude 36.5øN [Jayko et al., 1987; 1981]. The Foothills Metamorphic Complex crops out over a Harms et al., 1992] (Figure lb). South of latitude 36.5øN, wider region (50- 80 km) in the north than in the south (0- there are also CRO outcrops west of the San Andreas fault 40 km) (Figure lb). (Figure lb). CRO is intermittently present along the entire westernmargin of the Great Valley, north of latitude 36øN. A complete CRO sequence can be reconstructed, but CRO outcropsare generally highly sheared,dismembered, missing 3. GeophysicalModels Along and sections, and highly serpentinized [Bailey et al., 1970; Acrossthe Great Valley Basin Hopson et al., 1981]. The missing sectionsof CRO suggest We presentmodels derived from geophysicaldata (seismic extension;the most recent motion on the Coast Range fault reflection, seismicrefraction, gravity, magnetic,and borehole was normal motion [Jayko et al., 1987; Harms et al., 1992]. data) collected in or acrossthe Great Valley over the last 14 •Basal Great Valley sequencerocks were depositeddirectly on , arranged from north to south, and aligned along the rocks of the CRO [Bailey et al., 1970; Hopson et al., 1981]. magnetic high (Plate 1). All the models discussedhave been The CRO formedduring the Middle to Late Jurassic(about presentedin detail elsewhere. The velocities in the models 173- 150 Ma) [Hopson et al., 1981; Saleeby et al., 1984; derived from seismicrefraction data are probably accurateto Mattinson and Hopson, 1992]. + 0.1 kms -1 in theupper crust and to +0.2km s-1 in thelower This paperfocuses on the Great Valley ophiolite(GVO), the crust, with 1990s data typically having greatestaccuracy, due ophiolite sequencepostulated to lie buried beneaththe Great to reducedstation spacingand increasedresolution. Density Valley sequence[Bailey et al., 1964, 1970; Schweickert and models, in principle, resolve density contrastsbut shouldbe Cowan, 1975; Page, 1981]. The GVO and CRO eastof the San goodto about+0.1 g cm-3 in absolutevalues. Magnetic Andreasfault may be or may have been physicallyconnected models provide better constraintson the top surface of a beneath the Great Valley [Schweickert and Cowan, 1975; magnetic body than on its base. Absolute magnetization Godfrey et al., 1997]. The CRO, however, is significantly values are far less important than the shape of magnetic thinner in outcrop than either the thickness of the GVO boundaries,because measured magnetizations vary by one to interpreted from geophysical models (this paper) or the two orders of magnitude in otherwise similar rock samples thickness of "normal" oceanic crust [White et al., 1992], (Table 1). Differences between models derived from different presumablybecause of its emplacementand postemplacement types of data along the same or similar profiles are apparent history [Jayko et al., 1987; Harms et al., 1992]. The Coast (comparemodels 2 and 3 or models 15 and 17, Plate 1). Most Range fault is believed to have moved as a normal fault 60 - models derived from geophysical data are inherently 70 Myr ago, resulting in missing CRO sections and nonunique,so more credenceshould be given to thosemodels juxtaposinglow-grade CRO and GVO directly on high-grade derivedfrom a combinationof data types,for examplemodels FranciscanComplex [Jayko et al., 1987; Harms et al., 1992]. 3 (derived from seismic,gravity, and magneticdata) and 17 Because of this structural evidence and because of large (derivedfrom gravity and magneticdata) (Plate 1). 562 GODFREY AND KLEMPERER: GREAT VALI•Y OPI-I]OL1TE

Table 1. Velocities, Densities, and Magnetizations of Rocks

Lithology Velocity,References Density, ReferencesMagnetization, References kms 'l g cm'- A m-1

Great Valley rocks 1.6- 5.3 15 1.7- 2.7 2, 3, 18 0.1 19 FranciscanComplex 5.5 -6.2 14 2.21 - 2.99 2, 3, 18 N/A 2.5- 6.6 10, 12, 16 2.5- 2.99 2, 3, 12, 16-18 3 - 10 6, 11 Gabbro 6.0 - 7.7 10-12, 16, 17 2.7 - 3.22 3, 11, 12, 16-18 0.03 - 11.8 1, 2, 6-8 Unserpentinizedmantle 8.1 - 8.3 12, 16, 19 3.3 - 3.31 16, 19 0 - 4.0 1, 9, 19 10% serpentiizedmantle 7.6 - 7.8 13, 19 3.22 19 0.15 - 2.5 19 30% serpentinizedmantle 7.0- 7.4 13, 19 3.06 19 0.15 - 5 19 70% serpentinizedmantle 5.9 - 6.3 13, 19 2.78 19 1.0 - 20 19 100% serpentinite 4.9- 5.3 12, 16, 17, 19 2.21 - 2.57 3, 12, 16-18 0.4- 60 1-5, 19 Metavolcanics 5.3 - 5.6 16 2.61 - 2.67 2, 16, 18 0 - 35 1, 3 Granite 6.2 - 6.4 16 2.58 - 2.65 3, 16, 18 0.06 - 0.75 1

N/A is data not available. Referencesare as follows: 1, Cady [1975]; 2, Griscomand Jachens[1990]; 3, Griscomet al. [1993]; 4, Chapman[1975]; 5, Blakely and Stanley[1993]; 6, Wasilewskiand Castro [1990]; 7, Kikawa and Pariso [1991]; 8, Pariso et al. [1991]; 9, Laurent [1977]; 10, Whiteet al. [1992]; 11, Iturrino et al. [1991]; 12, Nicholset al. [1980]; 13, Clagueand Straley[1977]; 14, Stewart and Peselnick [1977]; 15, Schock et al. [1974]; 16, Christensen and Mooney [1995]; 17, Christensen [1978]; 18, Thompsonand Talwani [1964]; 19, Coleman[1971]; and 20, Christensen[1970].

All the geophysical data from the Great Valley permit sectionsof the Great Valley (Figure 3), basedon all the models interpretation of an ophiolite sequenceimmediately beneath available in each region (Plate 1) to addressthe questions the central part of the Great Valley along the entire length of posed above. the basin (models 1 - 18, Plate 1). One or more thrustwedges of Franciscanmaterial [Wentworthet al., 1984; Godfreyet al., 4.1. Thickness of Great Valley Ophiolite and 1997] can be interpreted immediately beneath the western Possible Inclusion of Oceanic Mantle as Well margin of the basin along the entire length of the Great Valley as Oceanic Crust (models 2 - 4, 6, 7, 11, 12, and 14 - 18, Plate 1), and lower In the Sacramentobasin, the Great Valley ophioliteappears velocity/density undifferentiated Sierran basement to be a complete ophiolite sequence with about 7- 8 km- (geophysicallyrepresented by the average velocity/densityof thickness of crust (1, Figure 3a) and about 7-8 km of the diverse Foothills Metamorphic Complex as well as the unserpentinizedmantle (velocitiesreaching 8.1 km s-1at Sierra Nevada batholith) is apparently present beneath the about 20 km depth (Table 1)) (2, Figure 3a). A 7- to 8-km- easternmargin of the basin (models 1 - 3, 5, 7, 11, 12, 13, and thick crust is reasonablefor "normal" oceanic crust [White et 17, Plate 1). Franciscan Complex and Sierran basement al., 1992]. Beneath the ophiolite is 15 km of lower extend east and west, respectively, to about the present-day density/velocitymaterial associatedwith the Sierran basement thickest sectionof Great Valley sediments. Magnetic material [Godfrey et al., 1997] which is also seenin the San Joaquin is present to some degree beneath the western margin of the basin (3, Figures 3a- 3c). In contrast,the compositecross basin (models 2, 3, 7, 11, 12, and 17, Plate 1) and may sectionsin the San Joaquin basin show no unserpentinized representa connectionbetween the CRO and GVO (see section ophiolite mantle sections(Figures 3b and 3c). Serpentinized 4.2). mantle has lower velocities/ densities than unserpentinized Some of the questionswe can addressby this compilationof mantle (Table 1), and we note that bodies interpretedas mafic models are as follows: (1) To what depth does ophiolitic from velocity/density data could also be interpreted as material extend? Is the material at the base of the preserved ultramaficmaterial that hasbeen serpentinizedto somedegree. ophiolite sectionoceanic crust or oceanicmantle? (2) Where The ophiolite section is at least 10 - 12 km thick beneath the is the ophiolite/Sierran basement contact? What orientation San Joaquinbasin (Figures3b and 3c). If this entire sectionis does it have? What is the nature of the contact? (3) What crust with very little or no mantle componentpreserved, the happensto the ophiolite beneaththe westernmargin of the San Joaquinbasin is underlainby a thickenedcrustal section Great Valley? (4) How do the answersto thesequestions vary compared with normal oceanic crust [White et al., 1992]. along the length of the Great Valley? Is there a significant Thickened crust might representintraoceanic island-arc crust differencebetween the northernand southernGreat Valley? or oceaniccrust (mid-oceanridge, forearc, or back arc) that has been thickened either petrologically or structurally(e.g., the 4. Discussion Oman ophiolite has been interpreted as overthickenedmid- ocean ridge crust [Nicolas, 1989]). If, however, some of the We divide the Great Valley into three regions(Sacramento 10- to 12-km sectionis serpentinizedmantle, there could be a basin, northernSan Joaquinbasin, and southernSan Joaquin "normal" thicknessof oceaniccrust overlying about 5 km of basin), and we construct generalized crustal-scale cross serpentinized mantle.

GODFREY AND KLEMPERER: GREAT VALLEY OPHIOLITE 565

CRF (Plate 1), which intersects model 11 in the location of this a• 4•A:Sacramento basin aerofoil-shaped body, does not show the magnetic body / 0-•.GreatValle".•.:.-•'•.'. ' • GreatValley because model 8 is a velocity model based on seismic refraction data, and the aerofoil-shapedbody does not have a • 0• •"•e•:•••••::::J ...... !i t •• (FC)Franciscan Complex velocity signature. This is an exampleof the nonuniquenature • r'•'•• •...... '•• •S•e•r• ...... ['[' :' '/:] FoothillsMetamorphic of magnetic models, for example, magnetic models 2 and 3 •30 ' .'.'.'basement'.'.'.'.'•:•• ComplexNevada batholith&Sierra (Sierran both give a good fit to the same data [Godfrey et al., 1997]. Q ...... mantle -'-'.'.'-'-' ß basement(SB)) 4o Ophiolitecrust(GVO) Magnetic model 3 [Godfrey et al., 1997] has the advantage that the geometry of the magnetic model is constrainedby b• B:Northern San Joac•uin I Mantle velocity and density models, while the only part of model 2 [Jachenset al., 1995] that is constrainedby seismicdata is the '-'/ 0 ' reat•• ' ' ' CRF CoastRange fault base of the Great Valley basin. We therefore suggestthat all .cj• 20 ß•. •. •. '. •J;r•r•3 '. '. '.'.' .'.'. SAFSan Andreas fault existing data, however previously interpreted, are consistent • -' GVO-"-'-'_- basement '.'.'-' Verticalexaggeration x2 with continuityfrom the GVO to the CRO.

. . . . 5 o mantle ß ß 4.3. Location and Orientation of the 4, 3 Ophiolite/Sierran Basement Contact 0 C: SouthernSan Joaquin The ophiolite/Sierran basement (including the Foothills •C) •Great.,.Va.ß ":':"""•:'"::'•":'":'"":":•'"'"':"':"'"•""•'"'I!..ey'-:•,--••.'.'"' "-. '•. 'ß ßß Metamorphic Complex) contact underlies the Great Valley •.• • "•''.,,, ' 1: ...... " I•i'.'.'.'.'.'.' ©iSB ...... sedimentary section. The lateral position of the contact

'.'.' Sierran ..... (4, Figure 3) is about 75 km east of the present western • 30 . ß basement ..... margin of the basin beneath the Sacramentobasin and 60 km -.mantle ...... east of the margin beneath the San Joaquin basin. The 40 east geophysicalmodels generally show the contact dipping east west1' 20 km (4, Figure 3), although model 3 [Godfrey et al., 1997] SAF suggestsit is near vertical down to 18 km depth. The nature Figure 3. Generalized cross sections across the Great of the contactis enigmatic. In the northernSacramento basin, Valley basedon all the modelsin Plate 1. (a) Profile A uses the shallow expressionof the contact is currently associated models 1 - 7 (Plate 1), (b) Profile B uses models 8 - 12 with east dipping faults (models 1 and 5, Plate 1) that offset (Plate 1), and (c) Profile C uses models 13 - 18 (Plate 1). the Great Valley sequenceand therefore postdate(at least in Numbersin squaresare subsurfaceregions referred to in the part) the GVO/Sierran basement contact. All the seismic text.The profilesalong which these models are constructedare reflection data presentedin this paper are concentratedon the shown in Figure lb. western margin of the basin, and the contact within the basement beneath the eastern margin has not, to our knowledge, been imaged. The subsurface location of the ophiolite/Sierran basementcontact is inferred from borehole data and projectionsof surface fault traces (models 1 and 5, 4.2. Great Valley Ophiolite Beneath Western Plate 1). Margin of the Great Valley Basin Although geologistshave long assumeda connection,there 4.4. Structural Variations Along the Great Valley is no velocity/densitysignature to suggestthat the GVO and CRO are connectedbeneath the western margin of the basin. The major difference between the northern and southern Magnetic models, however, suggest that highly magnetic GVO is that unserpentinizedophiolite mantle extendsbeneath mafic and/or serpentinizedultramafic material may connectthe the whole width of the Sacramento basin (2, Figure 3a). two ophiolite segments [Jachenset al., 1995; Godfrey et al., Unserpentinizedophiolite mantle is not present beneath the 1997]. Model 3 [Godfrey et al., 1997] (Plate 1) shows a San Joaquin basin (2, Figures 3b and 3c). Either the San -continuousmagnetic body beneath the westernmargin of the Joaquin section has a thicker crustal section and the mantle Great Valley. Godfrey et al. [1997] suggestthat tectonic sectionis missing, or the San Joaquinmantle sectionhas been wedging of Franciscanmaterial beneath the Great Valley massively serpentinized. [Wentworth et al., 1984; Wakabayashi, 1992] may have Beneath the Sacramento basin, mafic/ultramafic material formed a m61angeof Franciscanmaterial and GVO/CRO crust west of the presentwestern margin of the basin occursat deep and serpentinized mantle that have a velocity/density levels in the crust (20-30 km depth) (5, Figure 3a), with signature indistinguishable from Franciscan Complex but a unserpentinizedGVO mantle dipping west to merge with the strong magnetic signature due to mafic and serpentinized present-day mantle. In contrast, beneath the San Joaquin ultramafic components of the m61ange. All the magnetic basin, mafic material extends beyond the western margin of models of Jachens et al. [1995] and Griscom and Jachens the basin at midcrustal (about 15 km) depths (5, Figures 3b [1990] have an "aerofoil" shapedmagnetic body at less than and 3c). This mafic material could have one of two origins. It 5 km depth beneath the western margin of the Great Valley may be relatedto mafic materialdetected beneath the coastand (models 2, 7, 11, 12, and 17, Plate 1) rather than a continuous offshore California and interpreted as stalled subductedslab magnetic body. Model 8 [Blumling and Prodehl, 1983] [Meltzer and Levander, 1991; Trdhu, 1991] that is now 566 GODFREY AND KLEMPERER: GREAT VALLEY OPHIOLITE

separated from the GVO by the San Andreas fault. Mid-Jurassic '•--- Late Jurassic Cret. Alternatively,the mafic materialwest of the San Joaquinbasin Bath. Call. Oxford. Kimm. Tith. may be a continuationof GVO from the east rather than stalled slab. In this latter case, its continuation farther west and at shallowerdepths than in the Sacramentobasin may be related to a north-southdifference in the amountand depthof eastward I Nevadd½ wedgingof Franciscanmaterial. At present,the driving force for ongoing Franciscan wedging is compressionacross the San Andreastransform [Wentworth et al., 1984; Wakabayashi, 1992]. In the north, the San Andreas fault is up to 100 km west of the Sacramento basin, leaving a 100-km-width of Franciscan material available between the San Andreas fault and the CoastRange fault to be involvedin eastwardthrusting at all levels in the crust (Figure 3a). In the south, the San 175 169 163 156 150 144 IVa Andreasfault is the westernmargin of the San Joaquinbasin, leaving only the Franciscan material beneath the western Figure 4. Timeline of major events discussedin the text. margin of the Great Valley (and the small lateral extent of Timescale based on Hadand et al. [1982]. The Sierra Nevada FranciscanComplex that cropsout at the surfacein the Diablo plutonsare only those from the Late Jurassicafter subduction stepped westward forming the "new" Sierra Nevada Range)to be involvedin eastwardwedging at relativelyhigh levels in the crust. [Schweickert and Cowan, 1975]. Shading indicates well- constrained ages, and no shading indicates less well- constrained ages. Sources are as follows: Coast Range ophiolite, Hopson et al. [1981], Phipps [1984], Ingersoll and 5. Origin of the Great Valley Ophiolite Schweickert [1986], McLaughlin et al. [1988], Robertson If the Great Valley ophiolite and Coast Range ophiolite are [1989, 1990], and Mattinson and Hopson [1992]; Nevadan orogeny, Schweickert et al. [1984], Ingersoll and Schweickert related (physically or genetically) [Godfrey et al., 1997], we [1986], McLaughlin et al. [1988], Blake et al. [1989], and can use information about the origin of the Coast Range Robertson [1989, 1990]; ophiolitic breccias,Phipps [1984], ophiolite, for which geological, petrological, and Robertson [1989, 1990], and Moxon [1990]; Great Valley geochemical data are available, in conjunction with sparser sequence, Ingersoll and Schweickert [1986] and Jayko and petrological and lithological information from boreholes Blake [1986]; FranciscanComplex, Ingersoll and Schweickert within the Great Valley to address the origin of the Great [1986], Jayko and Blake [1986], Blake et al. [1988], Valley ophiolite. There are three main schoolsof thought as McLaughlin et al. [1988], Blake et al. [1989], and Moxon to the origin of the CRO/GVO which are summarized by [1990], and Sierra Nevada, Schweickert and Cowan [1975]. Dickinson et al. [1996]. A timeline (Figure 4) showsthe age abbreviations are as follows: Bath, Bathonian; information available for the main components involved. Call, Callovian; Oxford, Oxfordian; Kimm, Kimmeridgian; Tith, Tithonian; and Cret, Cretaceous. Model I (after W. R. Dickinson [Dickinson et al., 1996]) proposesthat during the Late Jurassic, the CRO formed by seafloorspreading in a back arc settingbehind an east facing intraoceanic island arc (Figures 2a and 2b). This island arc collided with the west facing Sierran continental arc after the west leavingthe CRO/GVO in a forearcsetting (Figure 2h). intervening oceanic lithosphere was consumedduring a Late ModelIII (afterJ. Saleeby[Dickinson et al., 1996])proposes Jurassic collisional event (the Nevadan orogeny). The that the ophioliteformed in situ by seafloorspreading in a geophysical models (Figure 3) all suggest GVO overlies forearcsetting in responseto transtensiondue to slab rollback Sierranbasement. We can modify modelI by requiringthat the (Figures 2i and 2j), and was therefore not accreted to the Nevadan orogenynot only dock the ophiolite againstthe continentduring a collision (Figure 2j). In this case, the older continentalmargin but also obductthe ophioliteonto Nevadan orogeny, which resulted in significant crustal the Sierran basement(Figure 2c). A new subductionzone shorteningin the Sierra Nevada [Schweickertet al., 1984], (Franciscansubduction) initiated to the west leaving the must be viewed as a highly compressiveevent, rather than a CRO/GVO in a forearcsetting (Figures 2c and2d). Subduction collisional event. To fit regional geological data, the continuedat the new trench,forming the FranciscanComplex, extensionthat allowed the ophiolite to form by seafloor with the subductingslab dipping beneath the continental spreadingmust have been relatively local and relatedto slab marginand obductedophiolite. Model II (after C. A. Hopson rollback, whereas compressionassociated with the Nevadan [Dickinsonet al., 1996]) proposesthat the CRO formed at a orogenywas regional and occurredafter ophiolite formation paleoequatorialmid-ocean ridge (Figures 2e and 2f). It was (Figure 4). To get Sierran basementbeneath the GVO in this then transported north to the latitude of the North American modelrequires that plutons associated with Sierranmagmatism continentalmargin along strike-slip faults, where it docked are emplacednot only beneaththe axis of the Sierra Nevada arc with the continentduring the Nevadanorogeny. Again, in but also westward beneath the newly formed ophiolite order to obtain the geometryseen in the geophysicalmodels, (J. Saleeby,personal communication, 1996) (Figure2k). we needto modifythis model by obductingthe CRO/GVO onto Hagstrumand Murchey[1996] revisitedthe paleomagnetic the older continentduring the Nevadanorogeny (Figure2h). data from the Coast Range ophiolite and concludedthat Franciscansubduction initiated as the subductionzone jumped previous paleolatitude determinations were based on a GODFREY AND KLEMPERER: GREAT VALLEY OPHIOLITE 567

remagnetized component. Analysis of the primary may be an extensionalfeature related to forearcspreading. The magnetization, which had previously been overlooked, east dipping contact shown in our three profiles (Figure 3) suggests that the CRO formed at North American possiblyresembles the thrust wedging now well documented paleolatitudes(about 35øN) rather than at paleoequatorial [Fuis, 1998] in the Franciscan on the west side of the Great latitudes.A revisionto modelII thereforeno longerrequires a Valley (Figure 3) and so may be indicative of a collisional or large northward translation component. Geochemical at least accretionalsetting, rather than an extensionalsetting, analysesof CRO samples suggestthat the southernCRO suggestingsupport for model I rather than model III. (Diablo Range and farther south,Figure lb) and partsof the Model I requires major uplift of the ophiolite onto the northernCRO are calc-alkalinerocks comparablewith island- continent during obduction. This would cause erosion of the arc rocks, while samplesfrom the remainderof the northern uplifted oceanic crust, the debris from which might be CRO sites are comparablewith enrichedmid-ocean ridge expectedto accumulatein the newly forming forearcbasin. An basaltsor ocean-islandtholeiites [Shervais and Kimbrough, ophiolitic breccia zone up to 100 m thick is documented 1985]. Petrologicalanalyses also suggesta suprasubduction between the uppermostCoast Range ophiolite members and zone setting rather than a mid-ocean ridge setting [Evarts, the lowermost Great Valley sequence rocks at the western 1977; Saleeby, 1986; Blake et al., 1989; Wentworth et al., margin of the Sacramento basin. Phipps [1984] reports 1995]. Geochemicaland petrologicaldata thereforeappear to olistostromes(including the Pope Creek breccia of Wagner supporta suprasubductionzone settingrather than an open- [1975]) that consist of ophiolitic debris mixed with Great ocean mid-ocean ridge. In the light of these data, we omit Valley sequencemudstones. These olistostromesare believed further discussionof model II as a possibleorigin for the to have been depositedon the serpentinitesrepresenting the CRO/GVO. Blake et al. [1989] suggestedthat the southern CoastRange ophiolite in this areaand gradeupward into well- CRO (Diablo Rangeand south,Figure lb) representsan island bedded Great Valley sequence rocks. Stratigraphic arc that formed above oceanic basement, consistent with the relationships demonstratethat the chaotic olistostrome unit thicker mafic sections shown in our San Joaquin models formedwithin 20 Myr of the formationof the ophioliteitself (Figures 3b and 3c), while the northern CRO has a more [Phipps, 1984] (Figure 4), implying that the obducted oceanic character (forearc, back arc, or mid-ocean ridge), lithospherewas young, presumablyhot and buoyant and consistent with the 7- to 8-km mafic section above resistantto subduction,facilitating obduction. A similar age unserpentinizedmantle shown in our model (Figure 3a). We relationshipis seenin the Oman ophiolite,where ophiolitic next attempt to use our geophysical models to distinguish conglomeratesderived from erosionof the obductedophiolite betweenCRO/GVO formationin a back arc (model I), forearc were depositedwithin 10 Ma of the formation of the oceanic (model III), or as part of an island arc [Blake et al., 1989]. crust [Nicolas, 1989]. Hopson et al. [1981], Robertson [1989], and Moxon [1990] also report an ophiolitic breccia 5.1. Geophysical Models With Additional zone that was deposited on the uppermost members of the Geological Information: Distinguishing CoastRange ophiolite at Paskenta,Thomes Camp, South Fork Which Model(s) Best Describes the Origin or Elder Creek, Elk Creek, and Wilbur Springs. This breccia of the GVO/CRO and Its Incorporation zone also has lowermost (Upper Kimmeridgian- Lower Into the North American Continent Tithonian)Great Valley sequencerocks deposited directly on Model 1 which requires collision and obduction of the top of it at the westernmargin of the Sacramentobasin. In ophiolite onto the continent (Figure 2) can explain the contrast, neither the breccia zone nor the ophiolitic ophiolite overlying Sierran basementby thrustingthe entire olistostromesare seenin the San Joaquinbasin, where well- ophiolite section onto the older continentalmargin. In this beddedGreat Valley sequencemudstones directly overlie Coast case, the velocity/densityinversion (at about 20 km depth) in Range ophiolite [Hopson et al., 1981]. our geophysical models (models 3, 8, 10, 13, 14, and 17, This indirect evidence for greater ophiolite uplift in the Plate 1) representsa major thrust surface,similar to the north than in the south is consistentwith much geological zonesinterpreted as a detachmentin the ultramafic sectionsof evidence that the Nevadan orogeny, widely interpretedas a the exposed 400-km-long, 50- to 100-km-wide Oman collisional event [Schweickert et al., 1984], resulted in more intense deformation in the northern Sierra Nevada than in the ophiolite [Nicolas, 1989]. In this model, the eastern east dipping ophiolite/Sierrancontact would be a major collisional southernSierra Nevada [Schweickertet al., 1984]. suture zone. Model III (Figure 2) requiresCretaceous Sierran 5.2. Summary and Model magmatismto have been responsiblefor intruding the lower- velocity/density material not only beneath the axis of the We heresummarize all the key datawe wish to explain. magmaticarc but also west of the main magmaticarc beneath In the northern Great Valley we contendthe following: (1) the ophiolite that formed in a forearc setting (J. Saleeby, The Nevadanorogeny was an intensecompressional, probably personal communication, 1996). This may be plausible collisional event, (2) plutons of the main Sierra Nevada beneath the San Joaquin basin, which formed adjacent to batholith occur only east of the easternmargin of the Great granitic and tonalitic intrusive bodies, but is scarcely so for Valley, (3) ophiolitic breccia overlies the Coast range the Sacramentobasin, which formed adjacentto the Foothills ophiolite, (4) the Great Valley ophiolite crust is interpreted Metamorphic Complex which representsthe basement into from geophysicalmodels to be about7- to 8-km thick, and (5) which the Sierran magmasintruded, with the intrusivesfor the petrology and geochemistry suggest a suprasubduction most part cropping out farther east. In the case of model III, affinity for the CRO/GVO, and the ophiolitecould be back arc, the eastern east dipping ophiolite/Sierran basement contact arc, or forearc crust. 568 GODFREY AND KLEMPERER: GREAT VALLEY OPHIOLITE

In the southernGreat Valley we contendthe following: (1) 6. Conclusions The Nevadan orogeny was a less intense compressiveevent, Geophysicaldata from the entire Great Valley permit a 700- (2) SierraNevada plutons are found adjacentto and beneaththe km-long and 70-km-wide complete ophiolite section as easternmargin of the Great Valley, (3) there is no ophiolitic basementto the Great Valley forearc basin. In the northern breccia overlying the Coast range ophiolite, (4) geophysical Great Valley, the GVO includes an unserpentinizedmantle models suggestthe Great Valley ophiolite crust may be up to section, whereas in the southern Great Valley the mantle 10- to 12-km thick, and (5) petrology and geochemistry section, if present, is serpentinizedto such a degree that it suggestsuprasubduction formation of the GVO/CRO and may be indicative of arc material rather than forearc or back arc cannot be distinguishedfrom Sierran basementor ophiolite crust. crust on the basis of velocity and density. Wedging of We now presenta model for the formationand emplacement Franciscanmaterial has truncated the western margin of the of the Great Valley ophiolite. We favor seafloorspreading in ophiolite at middle- to lower-crustaldepths in the north, but a back arc environment associatedwith an east facing arc for wedging was confined to upper crustal depths in the south, ophioliteformation, similar to that shownby model I (Figures allowing the ophiolite to extend west beyond the western 2a and 2b). We envisagedifferent modesof emplacementfor margin of the Great Valley at midcrustal depths in the San Joaquin basin. the ophiolite, however, from north to south. In the north, we We favor the formation of oceanic crust in a back arc basin suggestthat the eastfacing arc and back arc crustcollided with (model I of Dickinsonet al. [1996]) for the origin of the Great and were obductedonto the older continental arc (Figure 2c). Erosion of the ophiolite during obductioncaused ophiolitic Valley/Coast Range ophiolite. We propose a new hybrid model for Nevadan ophiolite incorporation into the North brecciasto be depositedon top of the ophiolite as the earliest American continent,with obductionof a back arc ophiolite in depositsof the Great Valley basin. In the south, where the the north and collision but not obduction of oceanic arc and Nevadanorogeny was lessintense, the eastfacing arc and back back arc ophiolite in the south. After the Nevadan orogeny, arc crust (Figures 21 and 2m) collided with the older the Franciscan subduction zone initiated west of the Coast continentalmargin but were not obductedonto it (Figure 2n), Range/Great Valley ophiolite, and Sierra Nevada magmatism explaining the lack of ophiolitic breccias in the southern GreatValley. •he arc becamethe basementto the newly emplacedplutons beneath the ophiolite in the southbut only' eastof the ophiolitein the north. In a shortperiod of the Late forming Great Valley basin. After the Nevadan orogeny, Jurassic,the entire Coast Range/Great Valley ophiolite was Franciscansubduction initiated west of the Coast Range/Great incorporated into the North American continent to serve as Valley ophiolite, and the associatedSierra Nevada magmatism basementto the nascentGreat Valley basin. was responsible for intruding plutons beneath the newly formed ophiolite in the southernGreat Valley (Figures2n and 2d). By the ,the entire Coast Range/Great Valley ophiolitehad been assimilatedinto the North American continent (Figure 2d), representinga very rapid (< 10 Myr) Acknowledgments. We thank R. Coleman and S. Graham for discussionand H. Benz for his seismic-tomographyresults ahead of pulseof mafic/ultramaficaccretionary growth. North America publication.Thorough reviews by M. C. Clark, R. J. McLaughlin,and continuedto grow by the accretionof Franciscanterranes until D. W. Scholl made this a bettermanuscript. Research was fundedby at 29 Ma tt-,eSan Andreasfault systeminitiated. NSF gr•tntEAR-9218209 (ContinentalDynamics).

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