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JOURNAL OF GEOPHYSICALRESEARCH,VOL. 100,NO. B2, PAGES2069-2087,FEBRUARY 10, 1995 Significanceof seismicreflectionsbeneatha tilted exposure of deep continentalcrust, Tehachapi Mountains, California P.E. Malin,1,2E. D. Goodman, 3,4T. L. Henyey, 5 Y. G.Li,5 D. A. Okaya, 5 andJ.B. Saleeby 6 Abstract. The focusof this articleis a processwherebylower crustalcrystallineand schistoserockscan riseto the surface,with the TehachapiMountainsin Californiabeingthe casein point. As a primeexample of the lower crust,thesemountainsexposeCretaceousgneissesthat formed25-30 km down in the SierraNevadabatholithandappearto be underlainby the ensimaticRandschist.Integratedgeophysical andgeologicalstudiesby the CALCRUST programhaveproduceda crosssectionthroughthispost-MidCretaceousstructureand suggesta generalmodel for its development.Seismicreflectionandrefraction profilesshowthat the batholithicrocksdip northwardas a tilted slaband extendbeneaththe southernend of the SanJoaquinBasin'sTejon embayment.Two southdippingreversefaultson the rim of the Tejon embaymentwere discoveredin the reflectiondataand verified in the field. The faultshave a combined separationof severalkilometersandcut throughan uppercrustalreflectionzonethatprojectsto the surfaceoutcropof the Randschist.The upperandlower crustsare separatedby a zoneof laterallydiscontinuousreflectors.Reflectionsfrom the lower crustform a wedge, the baseof whichis a nearlyflat Moho at 33 km. Regionalgeologicalrelationsandgravitymodelsbothsuggestthat the reflectivezone corresponds to the Rand schistandthe newly recognizedfaultsaccountfor its Neogeneexposure. Alternatively,the reflectivezonemaybepart of the gneisscomplex,suggesting thatthe schisteitherlies deeperor is not presentunderthe gneisses.If the Rand schistunderliesthe TehachapiMountainsand Mojave regionto their south,a modelfor theirevolutioncanbe constructed from regionalgeologicalrelations. It seemsthat duringLate CretaceousLaramidesubductionthe protolithof the schistwas thrust eastwardbeneaththe Mojave. Along this portionof the Cordilleranbatholithicbelt the subductionwas evidentlyat very low angles. The bottomof the batholithwasremovedandreplacedby a thick sectionof schist,fluidsfrom whichweakenedthe overlyingbatholith.This thickenedcrustcollapsedby horizontal flow in the schistandfaultingof the uppercrustinto flat-lying slabs. When emplacementof the schist endedin latestCretaceous/earliest Paleocene,the underlyingmantlerose,compensating for the extension andprovidingmaterialfor magmaticunderplating.In theNeogene,transpression androtationof the upper crustalongthe SanAndreasandGarlockfaultsresultedin the exposureof the schist. andwhichmayunderlieboththeTehachapi Mountains andTejon embayment, aswell astheMojaveDesertto theirsouth(Figure2) Introduction The TehachapiMountains'gneisscomplexdipsnorthwestward [e.g., Ehlig, 1968; Silver, 1982, 1983; Plescia, 1985; Cheadleet undertheCenozoic sediments of theSanJoaquin Valley'sTejon al., 1986; Carter, 1987; Lawson, 1989]. The processesand embayment, at the southernmost tip of the SanJoaquinBasin resultingcrustalstructurethatbringthe gneissandRandschistto (SSJB)(Figure1). Thegneisses arebounded onthesouthby the the surfaceare the focusof this paper(seealsoSalisburyand Garlockfault andan exposure of Randschist(Figure2) [Wiese, Fountain [ 1990]). 1950]. Regional geology, petrology, geophysics, andseismology The TehachapiMountains'gneissesare thoughtto represent all suggest thattheRandschistrepresents anensimatic protolith the Cretaceous mid-to-lower crust of the Sierra Nevada batholith that wasthrustbeneathth• gneisses duringthe Late Cretaceous [Saleebyet al., 1987;Saleeby,1990;PickettandSaleeby,1993]. Both the gneissesand the presumablyunderlyingschistwere Paleoceneandexposedin 1Department ofGeology, Duke University, Durham, North Carolina.upliftedin the latestCretaceous/early the Tertiary [Jacobsonet al., 1988; Goodman, 1989; Silver and 2Formerly Institute forCrustal Studies University ofCalifornia, Santa Barbara. Nourse, 1986; Pickett and Saleeby, 1993]. The uplifted 3Institute forCrustal Studies University ofCalifornia, Santa Barbara.Tehachapiblock apparentlyexperiencedNeogeneclockwise 4Now atExxon Production Research Company, Houston, Texas. rotationand late Miocene-to-Recentthrustingalongthe White Wolf fault (WWF), placing thesebasementrocks over the 125Department ofGeological Sciences, University ofSouthern California, km-deep SSJB [e.g., McWilliams and Li, 1985; Goodmanand Los Angeles. 6Division ofGeological and Planetary Sciences, California Institute of Technology,Pasadena. Malin, 1992]. The TehachapiMountainsstudydescribed herewaspartof the CaliforniaConsortium for CrustalStudies(CALCRUST) effortto understand the relationships betweensurfaceexposures of highgradeMesozoicmetamorphicrocks,associatedfaults,and lower crustalprocesses [e.g.,Henyeyet al., 1987]. In the Tehachapi Mountains,CALCRUST acquireda 38-kmreflectionprofileand Copyright1995by the AmericanGeophysical Union. Papernumber94JB02127. 0148-0227/95/94JB-02127505.00 2069 2070 MALIN ET AL.' REFLECTIONS BENEATH TILTED CRUST OxSP1 x IOm• o •S•erra Nevada t '•x 0 IOkm t,\\ß \\ß \\ Sana Lucia R••D•A San Gabriel SOUTHERN JOAQUIN SAN VALLEY San Andreas Fault EXPLANATION Cenozoic Cover Pleito Thrust TEJON ,E•Tonalites and Gabbroids of the Bear Valley Suite EMBAYMENT Tehachapi Mountains '• Gneiss Complex •J RandSchist Undifferentiated Mesozoic ++++ Pastoria [•] Intrusive andMetamorphic Rocks + Thrust + + + x '+++++ x MOJAVE ___ Refraction Line and Shot Points (Fig. 3) DESERT ReflectionLine (Fig. 6) •COCORP LINE4 34ø45' 119o07.5 ' Figure 1. ++ +++++ + 118ø22 5• Index and location maps for the Tehachapi Mountains-Tejon embayment area in south-central California'sSanJoaquinBasin.Also shownis thatportionof the CALCRUST seismicreflectionprofilecontaining deepreflectionsand discussedin this paper(betweenvibrationpointsVP 352 to VP 987). Importantgeological featuresare the exposedMesozoic basementrocks;their Cenozoicsedimentarycover in the Tejon embayment (modifiedfi'omSamsand Saleeby[1988]);andtheRandschistoutcropandRandthrust/north branchof the Garlockfault on its northside[e.g.,Buwalda,1954;BurchfielandDavis, 1981]. Otherseismicsurveylinesarethe overlying CALCRUST refraction profile [from Goodmanet al., 1989] and the most northwesternline of the COCORP Mojave Desertreflectionsurvey,line 4 [from Cheadleet al., 1986]. a coincident110-kmrefractionprofile [Malin et al., 1988;Ambos the refraction models cannot resolve the locations and amounts of dip in laterally varying structures. Thus the gravity and reflection data add essentialconstraintsto the refraction model (Figures 3, 5, 6, and 7). As in the generalcasesdiscussed by Barton [1986], a relatively broad range of velocity-densityrelationswas required to obtain a consistentmodel for both the seismicand gravity data from the TehachapiMountains. The CALCRUST reflection profile aimed principally at there it was taken south, toward line 4 of the Consortium for addressingthe structuralrelationshipof the gneissand schist, Continental Reflection Profiling (COCORP) Mojave Desert their distribution in the crust, and the responseof the crust to survey(Figure2) [ Cheadleet al., 1986], until the projectbudget latest Mesozoic and Cenozoic tectonics. The depth-converted was exhaustedabouta kilometernorth of the Rand schistoutcrop. commonmidpoint(CMP) stackof thesedatashows(1) a seriesof A central,22-km-longsegmentof the CALCRUST datacontains northwest dipping reflections consistent with the refraction clear deepreflections(Figures1 and 2). This segmentis entirely results, (2) a midcrustal change in reflection character, (3) a southof the WWF, whosemultiple strandsand complexfolding wedge of reflectorsin the lower crustwith variablenorthwesterly producea bad data area for deepersignals[e.g., Goodmanand dips and hinge points at or near the Moho, and (4) a relatively Malin, 1992]. The segmentalso lies 4 km north of the Rand flat, 33-km-deepMoho. A similarMoho structurewas previously schist, the southern 3 km of data being eliminated becauseof found in the earthquake tomography of Hearn and Clayton severe signal-generated noise, out-of-plane reflections, and [1986a, b] and simple gravity model of Plescia [1985]. The reflected refractions. gravity model suggeststhat the topographyof the Tehachapi Mountainsmay be supportedat relativelyshallowlevels,perhaps The refractiondata, which penetratedto midcrustallevels(<15 km), showseveralsignificantfeatures.They include (1) velocity even within the crust. This mightbe the case,for example,if the Tehachapi gneiss complex is underlain by a body of slightly discontinuities in the areas of the Garlock fault and WWF, (2) fasterbut slightly lessdenseschist. high-velocity rock units at shallow depthsbeneaththe site of the In total, the geology and geophysicsof the crust beneaththe reflection survey, and (3) a northwestdip in theseunits (Figure 3c) [Goodmanet al., 1989]. Becauseof the effectsof averaging, Tejon embaymentand TehachapiMountainssuggestan eventof and Malin, 1987]. Further constraintswere obtainedfrom newly compiled gravity data (J. Plescia, Jet PropulsionLaboratory, Pasadena, California, unpublished data, 1993) and industry reflection and well log measurementsin the Tejon embayment [Goodman and Malin, 1988; Goodman et al., 1989; Goodman, 1989; Goodman and Malin, 1992]. The CALCRUST reflection profile was begun a few kilometers north of the WWF. From A) Garlock Fault Tehachapi San Andreas Fault Rand I Mtns. / Lancaster / MP / PR Mojave Desert .,,, SP SG f'ARIZONA Los Angeles OS ß •,,•CM • ,', 0 PP? 1 O0 Km Pelona-Orocopia-Rand SchistOutcrops Vincent-Chocolate Mtn.-Rand ThrustSystem CALCRUST B) PA = Pastoria Thrust PROFILE p,L/'•_ --White Wolf--h/F, •...•••,• '• •/(•,, MOJAVE 3 ---I •,• DES .'.• -...i.ii, i• LINE6 ' 0 --I•'•l CRYSTALLINE ROCKS • I .. 20kin , I RAND SCHIST Figure 2. (a) Map showingmajor active structuralfeaturesand present-dayexposuresof P½lona-Orocopia-Rand schists. The Vincent-Chocolate-Randthrustsystem,indicatedby teeth on the schistbodies,is of Mesozoic age. Using U.S. GeologicalSurvey,COCORP, and industryseismicdata,Lawson [1989], for example,has arguedthat schistunderliesa largepart of the westernMojave Desert. Besidesthosein the TehachapiandRand Mountains,the various schistoutcropsdenotedare Mount Pinos (MP), Portal and Ritter ridges (PR), Sierra P½lona(SP), San Gabriel Mountains (SG), Orocopia Mountains (OM), ChocolateMountains (CM), and PicachoPeak (PP) area (modified from Jacobson[1983]); (b) Block diagram and line drawingsof the reflection structureof the western Mojave Desert from the COCORP Mojave Desert reflection profiles. The approximate map location of this diagramis indicatedby the dashedpolygonin Figure 2a. Also shown are the locationsof the CALCRUST TehachapiMountainsprofile,the TehachapiMountains(TM), theTejon ½mbayment (TE), the Pastoriathrust(PA), the Pl½itothrust(PL), the Rand Mountains(RM), and the Rand thrust(RT) in the Rand Mountains. The Mojave Desertreflectionlines andtheir reflectionhorizonsare shownin a cutawaysectionalongprofile [after Cheadleet al., 1986]. The Rand schistmay projectundermuch of the westernMojave Desert. Note the high-anglestructures interpretedascuttingthroughreflectionhorizonA, the reflectorinterpretedasthe Randthrustat the top of the Rand schist. 2072 MALIN ET AL: REFLECTIONS BENEATH TILTED CRUST upwardtilt to the southeast by asmuchas25ø. Thisprocess took profilesegment.With the possibleexceptionof requiringschist by Malin et placebetweenmid-Cretaceous andearlyPaleogenein association velocitiesseveralpercenthigherthanthoseobserved with the emplacementand uplift of the Rand schist[Jacobson, al. [ 1981], this interpretationis consistent,within errors,with the refraction and gravity data. The tilted upper crust also steps 1990; Pickett and Saleeby, 1993]. The seismicdata confirm highborehole evidence that deep Sierran crystalline rocks extend upwardto the southacrosstwo previouslyunrecognized, angle, southeastdipping faults of late Neogene •ge. In the beneaththe Tejon embayment. At somewhatgreaterdepths,the alongthe Garlockfault and WWF zones reflectiondata suggestthat the Rand schistunderliesthe central present,transpression (a) NW SE Distance (km) -50. 0. +50. 0 20- 4o- 6o+ Calculated 8O (b) -50. SL 0 WWF 352 GF 2.35 2.33 BS 2.86 2.65 2.69 2.88 2.68 2.88 rs 10 -• 2.70 C• 20 2.83 2.66 2.79 f 2.93 +50. 9871 mM Density in g/cm3 3O Moho m• 3.15 +50. Figure 3. Forward crustalmodelsof the gravity field and velocity structureover the CALCRUST Tehachapi Mountainssurveyprofile. (a) The gravitydataand (b) gravitymodelare modifiedfrom Plescia [ 1985] and J.B. Plescia (unpublisheddata and forward models,1992). The gravity modelingfollowed the methodsof Barton [1986]. (c) The P wave velocity structure is from forward modelingof the CALCRUST TehachapiMountains refractionsurveyshownin Figure 1. This modelis a modifiedversionof the oneproposedby Ambosand Malin [1987] and Goodmanet al. [1989]. The surfacepositionsof the refractionshotpointsare indicatedby arrows,as are the White Wolf (WWF) and Garlock (GF) faults, which are depicted as south dipping features. In the uppermostcrust,where resolutionsof the gravity and refractiondata are poor, the structurewas taken from the reflectiondata. Depthsare in kilometers,densitiesare shownin gramsper cubiccentimeter,and velocitiesin kilometersper second,with their range given by the values at the top and bottom of the layers. The most significantfeatureof the gravity fit is the slightlylower densitybodybelow the gneissesof the Tehachapiblock. Ourestimates of theerror(1 standard deviation) in depth,density,andvelocityare1 km,0.1 g/cm3,and0.15km/s, respectively.Note that the thicknessof the crustis nearlyconstantin thesesimplemodels. In fact, at a depthof 32 km, the massesof the various crustal columns along this profile differ by less than theseuncertainties. Dashed boundariesin the velocity model indicate the areaswhere theselimit are likely to be exceeded. The annotated zonesare interpretedas basin and basinbottom (b), Rand schist(rs), crustalscaledecollementor brittle/ductile transitionzone (ld), and Beno Springsfault (BS). MALIN ET AL ßREFLECTIONS BENEATH WWF (c) SL0 SP2 TILTED CRUST 2073 GF - 2.6 - 3.9 J as I ' / / 6.2 -6.5 Velocityin km/s 20- ..... Id * 6.8-7.0 (?) +50. NW Distance (km) SE Figure 3. (continued) of suboceanic conditions continues to uplift theTehachapiMountains.Usingtheseresults the easternMojave are moresuggestive to reconstruct thepositionsof thegneissandschistin thepast,it [Montana et al., 1991; Leventhalet al., 1992]. wouldseemthatthewedgeof lowercrustalreflectorsis relatedto The Tehachapigneisseswere rapidly upliftedto 10-15 km the subcretion of the schist and subsequenttilting of the depthsometimebetween100 and85 Ma [Pickettand Saleeby, 1993]. In the Rand Mountains to the east, age and structural Tehachapi Mountainsin latestCretaceous/early Tertiarytime. relationsindicateemplacement of theRandschistbeneathsimilar crystallinerocksin this sametime interval[Silverand Nourse, GeologicalBackground 1986]. In the TehachapiMountains,uplift anderosioncontinued Basementrocks north of the Gatlock fault are exposedin into the early Tertiary,so thatEocenemarinerocksof the Tejon scatteredoutcropsand consistof the 100-115 Ma Tehachapi Formation were deposited directly on the gneisses [e.g., gneisscomplexandRandschist(Figures1 and2) [Pickettand Goodmanand Malin, 1992]. Based on the presenceof schist Saleeby,1993]. Theserocksonceoccupied thelowercrustand clastsin the "unnamedconglomerate,"which overliesa major weremetamorphosed at depthsof 25 to 30 km (7-10 kbar and550 angular unconformity, the Rand schist in the Tehachapi to 760øC) some 100 m.y. ago [Saleeby, 1990; Jacobson,1990; Mountainsfirst appearedat the surfacein middleMiocenetime Jacobson et al., 1988; Saleeby et al., 1987; Sharry, 1981]. Postmagmaticductilefabricsin the gneissesare similarto those seenin the upper platesof the Vincent and Rand thrusts. These faultsplaceintrusiverocksover the schistin the SanGabrieland Rand Mountainsrespectively[Ehlig, 1981; Silver et al., 1984; Silver and Nourse, 1986]. Evidently,thiseventand its associated metamorphism took place in Late Cretaceoustime [Silverand Nourse, 1986; Hamilton, 1988; Jacobson et al., 1988]. On its north side, the Rand schist in the Tehachapi Mountains is boundedby a fault of variable exposureand dip that has been identified both as the "north branch" of the Gatlock fault and as the "Rand thrust" [e.g., Buwalda, 1954; Davis and Burchfiel, 1973;Burchfieland Davis, 1981]. Sharry [1981] has suggestedthat as in the caseof the San Gabriel and Rand Mountains, the schist extends beyond its outcrop, perhaps even under the Tejon embayment. This possibilitywas discountedto somedegreeby Ehlig [ 1968], who suggestedthat only the gneissmay extend to the north. Even [Goodman, 1989]. Geological and geophysicalevidenceexistsfor three other Oligoceneto Miocenetectoniceventsin the Tehachapiregion [e.g., Crowell, 1974, 1987; Goodman and Malin, 1988; Goodman, 1989; Goodman and Malin, 1992]. The earliestevent is a late Oligocene/early Mioceneperiodof extension,involving low- and high-angle normal faulting, volcanism, and the depositionof coarsefanglomerates.Uplift in the mid-Miocene exposedthe schistand appearsrelatedto initial slip on the San Andreasfault (SAF). Extensionand transtensionresumedafter thisperiod,resultingin obliqueslipfaultingandrapidsubsidence of the SSJB to bathyal depths. During late Miocene time, transtensionalternatedwith transpression, reflectedby cyclesof subsidence anduplift in the SSJB. Sincethattime,the SSJBhas evolved from submarine, to sublacustrine, to subaerial, to intermontane. Since the Pliocene, the entire region has been dominatedby the transpressionalregime of the modern San Andreasand Garlock faults, with local shorteningtaking place on featuressuchas the WWF and Pleito thrust(Figures1 and 2). farther to the north, however, the basement consists of intact, The left-lateral Garlock fault is the most significantand active shallow level batholithic crust, beneathwhich geophysicaland deep-crustal xenolith data demonstratethat no significant crustaldiscontinuityexposedin the TehachapiMountains. Most underlyingschistterraneexists [Saleeby, 1986; Dodge et al., of its postulated64 km of displacementappearsto have taken 1986, 1988]. Basedon industryand COCORP seismicreflection place sincethe late Miocene [Carter, 1987], althoughthis fault data, Cheadle et al. [1986] and Lawson [1989] have suggested likely originatedin the mid-Tertiary[Goodmanand Malin, 1992]. that the schist terrane is present under much of the western Restoration of this slip brings the Tehachapi Mountains in Mojave Desert. In termsof their uppermantles,xenolithstudies proximity to the western Rand Mountains (Figure 2) [e.g., from the Sierra Nevada reveal subcontinental geochemical Crowell, 1979]. Thus, if the Garlock fault is taken to be a deep, signatures[Mukhopadhyayet al., 1988], while similarstudiesin throughgoingcrustal discontinuity, any model of the crust 2074 MALIN ET AL.: REFLECTIONS BENEATH TILTED CRUST beneaththe TehachapiMountainsmustbe consistent with the duringthe Neogene[Grahamet al., 1990;Plesciaand Calderone, structure and rocks beneath the Rand Mountains. 1986]. On the other side of the Garlock fault, at sites southof the Unfortunately,no definitiveevidenceon the depthof the RandMountains, roughly25øof Mioceneclockwiserotationhas Garlockfault presentlyexists. Basedon the continuityof an been measured [Golombeck and Brown, 1988]. It is not known apparentmidcrustalreflectorobservedin the Mojave Desert whethertheserotationsalso apply to the Rand Mountains. In the COCORP data, Cheadleet al. [ 1986] proposedthat the Garlock Mojave extensionalbelt, Dokka [1989] has usedpaleomagnetic fault may not be deeplyrooted(Figure2). Alternatively, this data and kinematic indicators to proposeearly Miocene northreflectormay representa laterallydisplacedbut flat horizon,an south extension, in contrast to the more east-northeasterly out-of-planereflection,or perhapsreflectedrefractions from a directionmore commonlysuggestedfor the Mojave region. Accepting the 40 ø to 60 ø rotations for the Tehachapi subverticalfault [Serpaand Dokka, 1988]. Arrival times of refractedPn waves show that the presentMoho under the Mountains,and undoingtheir Neogeneslip on the Garlock fault, Tehachapi Mountainsis flat to withina few kilometers, withno Goodman and Malin [1992] argue that the mid-Tertiary apparent differences across thesurface traceof theGarlockfault extensional structures seen there are related to coeval structures in [Hearnand Clayton,1986b]. Midcrustal refracted Pg waves, the Mojave Desert and western California. In their view, midhowever,showlargedelaysacrossthisfault,evenaftercorrection Tertiary extensionwas regional in extent and predated,by many for low near-surfacevelocities[Hearn and Clayton, 1986a]. Paleomagnetic datasuggest thattheTehachapi Mountains have rotated between 40 ø and 60ø clockwise since the early Tertiary [McWilliamsand Li, 1985]. Data from volcanicflowsof early Mioceneageshowthatasmuchas40øof thisrotationtookplace millions of years, deformationsassociatedwith the onshoreSAF. In the Tehachapi Mountains, these mid-Tertiary featureswere rotatedand reactivatedin Neogenetime. Numerousreversefaultswith differingamountsof obliqueslip cut the gneissand schistas well as the Cenozoic sedimentsof the 5Kmi • (• /.•..__.--• Eastern White Wolf Fault Central White Wolfy Fault(blind thtrust )• • ,•,,.•.• Comanche Fault (blind thrust) • • VP352• h Mounain Springs Fault • • • Badger• • • • / • • Emb•yment• / / • • • Telon VP590 • • • Basin/ Basement • • / • / "i"ontact • •VP740 Beno Springs Fault / ...aseaanton •_•X••• /• o•o • / ( ' Tehachapi Mountains • •Tunis Fault Figure4. Faultmapof theTejonembayment andTehachapi foothills showing thesegment ofthereflection survey discussed in thispaper(modified fromGoodman et al. [1989]).Thefaultsshown include onesidentified in shallow reflection databy Goodman andMalin[1992].In theTejonembayment, whichliestothenorthwest ofVP 740,thetracesof buriedfaultsareshownwithopenbarbs(reverse) andballs(normal); exposed faultsaresolid. Thetracesof basement/sediment contact andtheBenoSprings Fault(BSF)intersect at approximately VP 740,as indicated.TheWWF is segmented intoexposed andburiedtraces nearComanche Point[Goodman andMalin, 1992].Thesection of profileshown onthismapandinFigure6 avoids theextreme dipsneartheWWF(<VP352) andsevere sideswipein thenarrow canyons of thehighTehachapi Mountains (>VP987). MALIN ET AL.: REFLECTIONS BENEATH TILTED CRUST 2075 Tejon embayment(Figure 4). These include the WWF, Pleito a two-dimensional transform). A simple rejection filter, or pie-slicefilter, was then thrust,andSpringsfaults,plustherecentlyrecognizedComanche analogousto a frequency-wavenumber Point, Beno Springs,and E1 PasoCanyonfaults [Goodmanand appliedto thesetransformeddata [e.g., Yilmaz, 1987]. Malin, 1992].Thesestructures, someof whichmaybe reactivated A filtered shotgatherfrom vibrationpoint VP 661 is shownin extensional structuresformed in the Oligocene and Miocene, Figure 5. This gather contains all the major reflectors seen arrived at their presentconfigurationsby the combinedeffectsof betweenthe WWF and Rand schist. It also showsthat the signalNeogeneclockwiserotationandlaterregionaltranspression. The generatedartifact was only partially removedby the specialfilter, CALCRUST data showthat severalof thesefaults played major perhapsdue to the artifact'slong durationand aliasingin time. rolesin the present-dayexposureof the deep-seated crustalrocks. This residual noise, plus out-of-plane reflections and reflected refractions[e.g., Day and Edwards, 1983], createdproblemsin the subsequentprocessing. In part, becausethese signalshave Seismic Reflection Data: apparent moveouts similar to reflections in high-velocity Acquisition and Processing basementrocks, many standardsignal-enhancement procedures The TehachapiMountainscommonmidpoint(CMP) reflection were not as effective as in the caseof lower velocity sedimentary profile was acquired with a contract reflection crew. The basins(Table 2) [e.g., Yilmaz, 1987] (seealsoFigure7b). In the equipmentusedincludeda GEOSOURCE MDS-16 400-channel, end the combinationof theseproblemsresultedin eliminationof 16-bitrecorder,andsix2xl 05 NT Littonvibratorswithfull force the data immediately north of the Rand schist. The most control. In the low-relief Tejon embayment, the near surface significant improvementsin the final sectionresultedfrom (1) consisted of non uniformly consolidated alluvium and soil. deconvolutionof reverberationsandmultiplesalongwith sourceAlong the mountainousportionof the profile, accesswas along pulse time compressionand (2) frequency-wavenumber(F-K) crookedroadsin steep-sidedcanyonswith highly variableground filtering of Rayleighwaves,s-waves,out-of-planereflections,and surfaces, all producing undesirable types of seismic signals. reflected refractions. Table 1 summarizesthe nominal acquisitionparametersthat were After thesesteps,and muting of the first-breakwaveforms,a chosenafter field testingof theseconditions. suite of constant velocity stacks were generated every few Unfortunately, the testsrevealed strong, site- and frequency- kilometersalong the profile. Thesestackshelpedimage dipping dependent, source coupling problems in the 18-26 Hz band. horizons,such as the newly recognizedfault zones,and identify These problems produced a signal-generated artifact that out-of-planeeventsand reflectedrefractions.Once identified,an obscuredall reflectorsbelow 4 s of two-way travel time (all times effort was then made to suppressthe out-of-plane events and given here are two-way travel times). The artifact could not be reflected refractions without affecting reflections from the effectively suppressedwith changesin acquisitionparameters, dipping features(Table 2). Both migrationand depthconversion although numerous attempts were made. Thus even the tests were done on the final stack, and prestackmigration was uncorrelatedfield recordsof this surveybecamethe subjectof an consideredfor critical features such as the faults and dipping independent signal-processing investigation[Okayaet al., 1990, zones. While helping somewhat with interpretation, the 1992]. Fortunately,the time evolutionof this noise (originating migrationsresulted in sectionswith lower contrastand greater possiblyin the frame of the vibrator) differed from the vibrator distortionthan the depthsections.Thus,for reasonsof clarity and sweep. Becauseof this differenceit was possibleto suppressthe fidelity, the latter type of sectionis displayedin Figure 6. noise by filtering in a twice-transformeddata domain (Table 2). The first transform took each uncorrelated field trace to its frequency-timerepresentation.In this domain,the vibratorsweep and artifactsappearas linear functionswith differentslopes, like direct and refracted waves in travel time plots. The second transformoperatedon both the frequencyand time variables(i.e., Interpretation of the Upper Crust and Moho Vibration point VP 661 is located immediately southeastot the Springsfault basementhigh and where the basementsurface dips to the south (Figures 4 and 5) [Goodman et al., 1989; Table 1. Data AcquisitionParametersfor the TehachapiMountainsReflectionSurvey Parameter Nominal Values Number of live groups Roll-along configuration 240-400 (floating) split spread Station 33 m interval Notes numberdeterminedby cable roll-along rate 40 trailing, 4 gap, 200-360 leading Geophonesper group Geophone array 12 in-line, 33 m long 8-Hz geophones Numberof vibrators 6 at 2 x 105NT amplitude and phase force control Pad separation Vib point intervals Vibrator array 13 m 66-132 m in-line stacked set by signal-to-noiseratio and survey costs no move up due to noiseon pad pickup CMP 25-100 fold Sweepsper station Sweepfrequencies Sweep length Taper Total record length 8-12 8-32 Hz 32 s 2s 44 s set by signal-to-noiseratio and survey costs upsweep,set to reduce 18 to 26 hz harmonics 12-s full-bandwidth listen, correlated to 14 s. Except for geophoneresponseand channellimits, valueslisted were establishedby field testing. 2076 MALIN ET AL.: REFLECTIONS BENEATH TILTED CRUST Table 2. Signal-processing Stepsfor the TehachapiMountainsSeismicReflectionSurvey Step 1. Hand-edit field data and set up field statics 2. Artifact removal 3. Despike-debursttraces 4. Time-frequency-amplitude analysis 5. Global shot gather amplitudebalance 6. Time-exponentialtrace amplitudes 7. Correct sphericaldivergence 8. Band-passfiltering 9. Trace autocorrelationstudy Comments supplyfield geometryand correctionvelocities for detailssee Okaya et al. (1990, 1992) in preparationfor band-passfiltering in preparationfor deconvolution 10. Deconvolution 11. F-K filtering 12. Muting refractionsand sourcenoise;sort 13. Velocity analysisand normal moveouttests 14. Mute and band pass 15. Surface-consistent residual statics testsa 16. Stack tests and final stack 17. Time-distancedip filtering 18. Global CMP section amplitude balance groundroll, out-of-planeand other unwantedsignals checksfor out of plane eventsand good reflectors done using pilot traces final stackingvelocitiesselectedfrom suite of CV stacks suppression of residualunwantedsignals 18. Migrationtests a poststackand prestacktests 18. Time to depth sectionconversion 19. Final display tests including coherencyfiltering, global gain, and thresholdtests For referenceto the standardsignalprocessingstepslistedhere, see Yilmaz [1987]. Editing and artifact removal completedwith UCB DISCO-adaptedmodules.Postartifact removalprocessing with USC MERLIN-adaptedmodules. a. Tests fail to show clearly improvedstackedsectiondue to residualartifacts. Goodman, 1992]. The crystalline-sedimentcontact can be identified in the VP 661 shotgatherat roughly 1 s, at the baseof the zone labeled b. Progressingdownward into the basement, there exists a zone of relatively flat subbasinreflectors, easily visible below 2 s, and given the label ud. These reflectorsare alsoseenon industryseismicprofilesdiscussed by Goodmanand Malin [ 1992] andthe depthsectionsin Figure6. Beneath the subbasinreflectorsis the first of two major northwest dippinghorizons.The upperhorizonis labeledasrs in Figures5 and 6. Near the top of this unit are phasesthat can be correlated over many tens of traces. Allowing for one or two waveform cycle skips, these phasescan be traced acrossthe entire shot gather(13.2 km of shotgather= 6.7 km of reflectionpoints). The secondand deeperof thesetwo horizons,labeledmM in Figure5, beginsjust above8 s. This secondhorizonis separatedfrom the first by two other features:a zone of residualsignal-generated artifacts(labeled a in Figure 5), and a zone of flat-lying, more segmentedreflectors (labeled ld in Figures 5 and 6). Shot gathersto the northand southof VP 661 showthatthe topsof the rs and mM horizons dip consistentlynorthwest,moving up through the crust to the southeast. The dips of the reflectors below the mM horizonappearto decreasewith depthandgrade downward to weaker, flat-lying reflections at roughly 11 s (labeledmT in Figures5 and6). On othershotgathersaswell as the one at VP 661, the changein dip is seenas a shift of the reflectionhyperbolato shorteroffsetswith increasingtime. In the depth section,this producesa wedge-shapedzone with a fanlike internalstructure.The Pn velocitiesacrossthis region [Hearn and Clayton, 1986b], suggestthat the mT reflectorsare the currentMoho. Both the Pn dataandreflectiondatashowthe Moho to be flat to within 3 km. Field mappingdataplusindustryseismicprofilesandwell logs were particularly valuable in interpretingthe upper part of the CALCRUST reflection profile. For example,thesedata show thatthe Springsfault, which outcropsnearVP 590 andis labeled S in Figure 6b, is part of a more complex set of structures associated with local transcurrent tectonics. Beneath VP 590 and fartherto the southeast, the basinand subbasinreflectorsappear to be truncatedby the transcurrentstructures,as is alsoseenin the CALCRUST profile. Near VP 661, both drill hole and shallow seismic data show buried thrust faults cutting much of the Tertiary section(theseare indicatedby the letterP in Figure6b). Thesefaults are on strike with the currentlyactive Pleito thrust andappearto be its northeastward extension(Figure4). The bottomof the sedimentary basinis mostprominenton the northwestend of the seismicsection,and extendsto a depthof 2 km there. Below this, to a depth of at least 7 km, are the ud subbasinreflectors. Both the b and ud zones rise to the south, wherethe sedimentarysectionof the Tejon embaymentlapsonto thecrystallinerocksof the TehachapiMountains,at roughlyVP 740 (Figure4). The ud reflectorsmaybe relatedto mid-Tertiary extensionalstructures(the "detachments"of Goodman and Malin [1992]). Basement-involving, high-angle,normal- and obliqueslip faults observedon industrydata in the Tejon embayment appearto sole into similar reflectors. Outcrop data along the northernexposuresof the Tehachapigneissessuggestthat they may follow older, shallow dipping, metamorphic,and ductile shear fabrics. Two additional shallow structures have been identified on the depthsectionand have been confirmedin the field as faults. One is locatedjust southeast of the sediment/basement onlapnearVP 740. The otheris in the gneisscomplexbetweenVP 840 andVP 860. In the depthsection,thesefeaturesare associated with the southeastdipping zones labeled BS and EP, which extend to depthsof roughly7 km but are difficult to seedue to the scaleof this section. The bestimagesof thesezonesare in the constant velocitystacksshownin Figure7. This part of the profile also appears to contain numerous out-of-plane reflections and MALIN NWSta ET AL: REFLECTIONS 0 5.O • I TILTED CRUST 2o77 BSF). This fault is locatedalonga seriesof ridgesdelineated on SE DISTANCE 661 BENEATH aerial photographs. It intersects the basin/basementcontact I0.0 obliquelynearVP 740. In thegneisscomplexthefaultzoneconsistsof a 5- to 20-m-thickmylonitezone strikingN50øEand I dipping50ø SE. It is overprinted by cataclasite zonesthatdip more steeplyto the southeast.The gneissesshowstructuraland lithologiccontrasts across thefault,withgenerally low-dipping tonalitic-dioriticgneissesin the hangingwall and refolded graniticorthogneiss andparagneiss in the footwall. Fromsurface geologyalone,a majoramountof reverse displacement appear to havetakenplaceacrossthe BSF. The EP zone, the E1 Paso Canyon fault (EPCF), reachesthe surface at VP 840. Reflections from this zone are similar to those from the BSF, but the fault itself is moredifficultto recognizein outcrop. Scatteredexposuresshow a changein the attitudeof foliationsin the areaof the reflections.West of the profile,the EPCF may form the poorlyexposedcontactbetweenthe Tejon Creektonalitegneissandthe ComanchePointparagneiss unitsof Samsand Saleeby[ 1988]. Interpretation of Reflectorsrs, ld, and mM The noahwestdippingrs zone, the subparallelmM zone, and the less inclined ld zone that separatesthem are imaged most &::..-' •f..: ?.%q:: - __::: -':2:.-.:.;./.;' .,. :.;'f......- --:•%iC::u. clearlyin the depthsectionbetweenVP 550 andVP 815 (Figure 6). Along this portion of the profile, the top of the rs zone is between5 and8 km deepandhasan apparentdip of 15øto 30ø. ................ ,."'= .... :::,L _.•- -, :- e:.Z:.L7,...':,x:, ',,,',:-, , '.: .~, TT1 T From VP 450 to VP 550 this reflective zone maybedisruptedby the extension of the Pleito thrust, the Springs fault, and other interpretedblind faults,suchasthoseinterpretedbeneathVP 525 and VP 400. ß . "L%.': ,,. :.-- :..... ' Farther to the northwest the rs zone is identified the weak reflectors _ , as seen in the middle crust. "'• ='='. - South of VP 815, recognition of the rs zone in the depth sectiondependson (1) its signalpattern,whichconsistsof a sharp TRACE No. top reflector with subparallelunderlying reflectorsand (2) our Figure 5. Prominentfeaturesin the filtered shot gatherfrom interpretationof the BSF and EPCF as reversefaults based on vibrationpoint VP 661 of the TehachapiMountainsreflection field mapping. The resultingcorrelationis shownin Figure 7b, profile, as discussed and interpretedin the text [Okayaet al., with the top of the rs zone at VP 815 now appearingat a depthof 1992]. The reflectionquality and continuityof the featurescan less than 2 km and thrustup over its equivalentreflectorsto the be judged by following the signals above and below the noah. Accordingly,from bothfield andsubsurface evidence,the annotations. The annotated reflection zones are interpreted as: BSF would have a vertical separationof almost 4 km. Farther basinand basinbottom (b), upperdetachments(ud), Rand schist south,on the hangingwall of the EPCF and southernend of the (rs), acquisition artifact (a), crustal scale decollement or profile, the same assumptionssuggestthat the rs zone comesto I ! • o ioo 200 300 brittle/ductile transition zone (ld), latest Mesozoic-earliest within 1 km of the surface. Cenozoic ductileflowfabricor underplating or thrustzone(mM), TertiaryMoho(mT). Giving a geologicalidentity to the rs zone is a critical issuein interpretationof the reflection data. In this regard there are several important lines of evidencethat suggestthat the rs zone representsthe Rand schistextendingnorthwardfrom its outcrop reflectedrefractions(labeled RR and OP in Figure 7b). Both immediatelyto the south. First, geometricprojectionof the rs thesetypesof signalshave high-velocitymoveoutslike to those reflectors places them near the surface at the schist outcrop. of the basementrocks. Judgingfrom localtopography, geology, Second, Silver [1982] has demonstratedthe presenceof stacked, shotgathers,commonmidpointgathers,and stackeddata,the out- north dipping thrusts with slivers of Rand schist along the of-plane reflections here seem to be related to the BS and EP Garlock fault east of the reflection profile. Third, in the San zones. In the stacksectionthesesignalscreate an appearance of Emigdio Range immediately west of the profile, outcroppatterns crosscuttinghorizons [e.g., Yilmaz, 1987]. Taking this and the of schistand gneissalso show a similar structuralconfiguration field observationsinto account,the southeastdippingbandsare north of the Garlock fault [Ross, 1985]. Further, it should be seento be a seriesof coherentreflectionsthatapparentlytruncate recalled that palinspastic reconstruction of the Tehachapi the more flat-lying reflectors, including those of the rs zone Mountains joins them to the Rand Mountains. In the latter below VP 815. mountains, the COCORP reflection data [Cheadle et al., 1986] Field mappingand examinationof aerial photosin the area of VP 740 suggest that the dipping reflective band there is a previously unrecognizedfault, which we have called the Beno Springsfault, (indicatedby BS in Figures6 and 7 and abbreviated and the geologicalmappingdata [Silver, 1982] show the same characteristicreflectors and rocks in this type of relationship (reflectors A, B, and G in Figure 2b, as discussedlater in our reconstructionof the crustaltilting). 2078 MALIN ET AL.' REFLECTIONS BENEATH STATION NW I CRUST No. 661 400 I I I 500 I l, I I • _ TILTED I 600 I I I SE I I 700 I I I ! 800 I . I :'•i•7:•!.'•'•:.•'• -:•'--"•'• +• : *i•':,'.•--. :::;:;:[:•:: :..•' •%,:'.:i::-',r;. •.*..'..... ' ., .z.-.,.* :'? .;':•.' ;r:-,•,• "•.: I I 900 I I i :?•:':•.:•:<-'•'•. .,;i•:i• ..•:•.•.-;"2, -..:..•.:•.-:•"'.•z ?",';,'**:•'.;. :.::: ...:...; •,..-..•..:•.•:•,•-.•.•-•.-.:•,•.•:,•,.•-,.•;.r,,: • .. ;•;'•,•-•:'•2,'..'.:::•'• ..... s;:[': .•'• •.-"•:: ;'.:':• -"•...,•;"-.:: *"'•: ....,"" -''....: .:' .';'-C.5 .;7;3: '.:'::•":. ':.:'•.,:,: -..' ::-:•c-: ' •. ':"'%:::.:,T • ',•'• .:-.'.::;' • ' ::,•xX'. ? ':..::' T•:': ..,.:.:.,'::•: ':-';-::;: •:':::.:.?v :.':L,'::':,? ;• '-7:-'-: 5:C J•'•': .:' I0- 15-- 75 ø 25. " 30- ....... , .• -,.:,:':. -,._, • ..... .. . ; .... :;. ':'.',,'.: ,:,-.. ,.. •' -•:..:•.': .. ,.2 ' , • • , - .:: .:, :55- i I I o 5 IO i 15 DISTANCE (km) Figure 6a. This FigureshowstheTehachapiMountainscommonmid-point(CMP) depthsection.Plot parameters were chosento best display the subbasinreflectionsdiscussedhere (for data in the basin, see Goodman et al. [1989], Goodman [1989], and Goodmanand Malin [1992]). In this sectionand the one in Figure 6b, both the stackingand displayprocesses degradedthe clearerMoho reflectionseenin Figure5. Horizontalexaggeration is approximately2.25 to 1. Another line of evidence comes from the refraction data and regionalgravity field (Figure 3; modifiedfrom Ambosand Malin [1987], Goodman et al [1989], Plescia [1985], and J.B. Plescia (personalcommunication,1992)). Thesedata have beenmodeled using the approachsuggestedby Barton [1986]. His method usesboth seismicand gravity observations,but takesinto account the limits of crustalvelocity-densityrelationships.First, the approximate geometryandcomposition of thesedimentary basins were entered into the model, using the refraction data, the CALCRUST andindustryreflectiondata,andthedensities given by Plescia [1985]. Next the depthand shapeof the Moho suggested by the Pn andreflectiondatawereusedas a starting point for severalmodelsof the gravityover the Tehachapi It was concludedfrom theseteststhat the depthand shapeof the Moho couldnot vary far from that suggested by the Pn and reflectiondata. If true,thenit is possiblethatthe topography of the TehachapiMountainsis gravitationallybalancedwithin the crustalone. Oneway in whichthisbalancecancomeaboutis by the presenceof a slightlylighter schistbody beneaththe denser gneiss,in a fashionconsistent with ourinterpretation of rs zoneof reflectorsbeingthe schist.Assumingtheuppermost crustalstructure suggestedby the reflectiondata, the refractionand gravity data can be fit, within the limits of their resolution, with the Mountains. In these calculations, the near-surfacedensitiesas- velocitiesand densitiesshownin Figure3 (seecaptionfor error limits). An importantfeatureof thedensitiesandlayerthickness of this modelis that at a depthof 32 km, the massesof the crustal columns along the profile vary by less than +1% from their sumedin the TehachapiMountainswere thosesuggested by average, equivalent to the uncertainties in the densities and Plescia [1985], while the range of subsurfacedensitieswas thicknesses.Thus,while not uniqueor definitive,the resulting allowedto vary within the velocity-density limits setby Barton gravity model fits the observedgravity within errors and is [1986]. consistent with the othergeophysicalandgeologicaldata. MALIN ET AL.' REFLECTIONS STATION NW TILTED CRUST 2079 No. 661 i 400 I I •.z:• .':.:.. U I I 500 I I ! I 600 I I I 7-:,•,•':C'•;•:':'•:-:•?" ;*j,•.z:-".•':, •:.':." ;•'. L',"::.'•--• ..:•'• .... ,T:'2: :•',"•..-, 15- BENEATH SE ! I 700 I 800 900 ..•.", ..... : --,. ., ......... ,•:f•,•,-.:'t:'.........'.;' .'.•: • .:.: :%•-:-." •" .... :'•-.. _;<::;:'•'"-•.,•:':",'; .... . :"/"<':". %'?"'¾:"';-..:.'?:;'.'' ',--••?::t:,;.'.' ,•':;',;;:•:"•'•': •-•:•'-, ..... ':',;"t:':'L:•;;':•:;/L:.::; ,'?" '. :,. I 20- ,.-. .. Q_ ..,.. ... ..... •s.• '.-'":...: • .'-- '•:- -. ,;", ' • ....':';'--• 25•,•.•.,..; _. ,.:., ..,, ,•. ........ .... . 30- ';.;.":S-:7' •, ,... ,•.•; ........ .7. ':-•. , -'. -•.,. • ' ' ...•,. • , ';' ;•. ., .--•: . .,..• ,, ' ' ":::'"'; ' ,':'? 4:-..... •,:•' '..... , ..- ,. .. 35- DISTANCE (km) Figure 6b. Prominentfeaturesin the CMP depthsection. The annotatedreflectionzonesusedhere are the sameas thoseinterpretedin Figure 5. Additionalinterpretedstructuresare Springsfault (S), Pleito thrust(P), Beno Springs fault (BS), E1 PasoCanyonfault (EP). The main discrepancyin our refractionmodel is with previous work by Malin et al. [1981], who profiled the velocity structure of the schistat Sierra Pelonain the San Gabriel Mountains. They found the schistP velocity to be around5.9 km/s as comparedto the 6.2-6.5 km/s values shown in Figure 3. This difference maybe due to pressure,the 5.9 km/s being determinedat a depth of roughly 1 km and the 6.2-6.5 km/s at roughly 10 km. In fact, if the schist extends below Sierra Pelona to these depths, where Malin et al. [1981] found similar velocities,then no discrepancy exists. The difference may also reflect anisotropicvelocities in is that while the rs zone is the schist,it may exist as a thin sliver, only 2 or 3 km thick, sittingon an entirelyunknownor unrelated basement. Suchinterpretationscould also satisfythe gravity and refractiondata, particularlyif the rocksbeneaththe gneissesare relatively light and fast. Thesepossibilitiesare kept in mind in our uplift model. As a final note on rs, if our correlationis correct,the steeplydippingcontactson the northsideof the schist outcrop may be south dipping faults similar to the BSF and the schist. transitionfrom a morereflectivebut lesslaterallycoherentupper Thus the evidenceat hand suggeststhat the rs zone is the Rand schist, that its top is the Rand thrust, and that its internal reflections are a function of changes in structural fabric, compositionallayering, and tectonicimbricationand duplication. Nonetheless,becausethe reflectionline failed to carry the rs zone to outcrop,there remainssomedoubt:the rs zone could represent internalstructurewithin the SierraNevadabatholithor an entirely unknown horizon. In particular, if the rs zone is within the batholith, it could be associatedwith some type of imbrication, placinghigherlevel rocksunderthe gneisses.Anotherpossibility crustto a less reflective but more laterally coherentlower crust. Below the ld zone, at VP 740, is the top of the mM zone, at a EPCF. In the depth sectionsin Figure 6, the ld zone seemsto mark a depthof roughly 22kmanddipping 25øN.Whilelessclearthan in the shotgathers,the reflectorsbelow this horizonappearto flatten with depth to the less distinct Moho mT. The CMP stackingprocessdegradedthemT zonesomuchthatshotgathers were usedto help identify it in Figure 6. To somedegree,the wedge of lower crustal reflectorsboundedby mM and mT resembles a fan with its hinge point to the northwest of the profile. The same kind of reflection structurewas also observed 2080 MALIN ET AL.' REFLECTIONS NW 725 BENEATH TILTED STATION No. 750 775 800 CRUST SE 825 850 875 12 6 0 1 2 3 4 DISTANCE (KM) Figure 7a. Expanded-scale constantvelocity(5.6 km/s) stackof theCMP datafromthe areasof theBenoSprings and E1 PasoCanyonfaults (VP 720 to VP 885). The horizontalexaggeration is approximately2.25 to 1. This sectionallows for the presenceof high-velocitygneiss,schist,and the moderatelyto steeplydippingfault zone reflectionsimmediatelybelow VP 740 and VP 850. The sectioncontainsout-of-planereflectionsfrom this fault that appearto crosssomeof the moreflat-lyingreflectors.The sectionalsocontainsresidualreflected-refractions above the rs horizon. at the easternend of Mojave Desert line 3, beneathandjust south of the Rand Mountains, but with a different geographic orientation [Cheadle et al., 1986] (reflectorsI and M, as shownin Figure 2). As statedearlier, the Pn data of Hearn and Clayton [1986b] showmT to be a horizontalMoho. As in manyotherplaces,this particularMoho is a complexzone with numerousdiscontinuous phasesthat can be seen in shot gatherbut do not stackinto a singlecoherenthorizon [e.g., Mereu et al., 1989]. A common model for the zone of flat, superimposedlensesof discontinuous reflectorsis differentiationand underplatingof the lower crustby partial melting of the upper mantle [e.g. Hauser et al., 1987]. Likewise, the ld horizon has numerous analogs in which a laminated lower crust is separatedfrom a heterogeneousupper crust[e.g., Mereu et al., 1989]. The differencesaboveandbelow this horizonmay be productsof brittle versusductiledeformation. The deformations could be related to the current seismotectonic regimein southernCalifornia and/orto the eventsthat rotatedthe TehachapiMountainsearlier in the Neogene. As in the caseof the upper detachment horizon, ud, which can account for differences in the structural styles of the sedimentarysection versus underlying basement, interpretation of ld as a lower decollementhelpsto explain somedifferencesbetweenthe upper and lower crust. Exposure of the Gneiss and Schist For a plausible geological explanation of the Tehachapi Mountainsreflectionprofile and otherdata, we have assumedthe following statementsto be true: Basic Relationships 1. The Tehachapigneisses correspond to southwarddeepening exposureof the CretaceousSierraNevada batholith[e.g., Ross, 1985, 1989;Saleeby1990]. 2. The Rand schistextendsnorthwardbeneaththe Tehachapi gneisses[e.g., Sharry, 1981]. 3. The schist is present under large parts of the western Mojave Desert [e.g., Lawson, 1989]. 4. The Garlockfault is a deep-rooted crustalboundarythatis moreor lessvertical[e.g.,Hearn and Clayton,1986a,b]. Basic Sequenceof Events 1. The protolith of the schist was emplaced and metamorphosedunder the gneissin the Late Cretaceous[e.g., Silver and Nourse, 1986]. 2. Substantialuplift and coolingof the schistand overlying gneissestook place during the Late Cretaceousand continued into Paleocenetime [Pickettand Saleeby,1993]. 3. Following erosion of the gneissesand overlying crust, MALIN ET AL.: REFLECTIONS NW TILTED CRUST STATION NO. 725 750 • • •'"•,•.'"' BENEATH 775 800 , .--.,,•' SE 825 • I.-t.:-.. RR•"-I• ...... 2081 • 850 " 875 , ,,,.,:,.. . ,., .':•.... '•'•'' • ".,-'--'-.;•' '• "',•-':2,.• ,4,';:;'*' ' '":,,', ' ....: :,• '"'"""',.' "'"' .•'. •" :":',:'""'" ...." ' - •...,. :,.:: ?:!?,:(.:.:.:;.:;;:.!:•. %..,../.,..•:,: ,:;::,:;: ::.;:.;, .,: •..-..__,,....,...•..•.._•::.,• •..:•.;I::.':;:;; ?:: :;•;-!::" ::':::.:¾: :;:•L"•? : ,...r,, ', ' ',.,,.': '..:.,:,...:,'.::,:½i,•'" :':"':'.t .:•:....,::,'.'.;2-4•'. :-:v: F• ..::",.,:..:',::: v..:,':'_'" :•L: :..-.:.?, :'•.,.:".:?:':,'T :-"'=':" '.'•• "-•.. • '",•.•.'.'" :•:''::,,:.;, "D',;,'-": '.•'::,:," ;"':•'; ""-'• ' ,,? b,';;.:v, .•:?::i. %::::::.:::'"(:T" ":,,.,•:•:'::: ,':,'.'".•,•,': ...... ', ',,.',:i ,.,..:,:::.!:::.•;':..i: .-::':2•"•W:.,' ...... .- :.;::•," :,".':::',,'.t':'.:'.';':, '.:.;-.•''.•..•'::,::,-4 - ",:':,i'::: .....":::: '" '.'"'.•.',. '"".':' :i:'::.:.:'• i::'",'::.'•,'•'• i";".:•.': .,.•", :•':4, '2;:'•.: •".'•i• ,-:7"$ :"'":' '"'.:'::' :':::':. :2'.':::'::":':..•::"::""' '"; ,::.:'?::',,':: :'.,:•',,.:•:•i?,.t '.::..'.:,..;:',,,"•.;,,:½ ,.', :::D'(::;":,:;,?.::,;5":,.5 ..... :,::.:.,..,-.....ud ,, :....::':., •',..... ,:,-•.•: ..... ,,,. :D:'•, •'. ....:..',:'.. [-...:.. ,....:.'; ..... ;:..:',"• :..... :;•:":'?:' .::":?":<.::,::'!•::",i:",::;,:',: ::::::::::::::::::::: .:::::?,i:,x:• :':..:'i•:::, •.Z::,:,.. • ".:;'..:,' i':.,:' ::: .... . '.:':.: .... ,,"":':".',': ::::i: '•,',?,-::::•":::2: :":':'"':,:", •':•:..%: ,':";?•:. :;.:' ,,i.";4;.: .... '::::':':t .'::i"-':::5. _ '•,,,....:.::. ;, -,'.;"... ,,•:,.,.:, ..'......: .",:'. ......... :.,:.?,.,.,,: :,....;,... :, .::,,::,,:.,. :',. ........ ::...... ' ,..'••..., .... ,::' ;... :'... '.'.",':-.,:,,.t,;rs ...... .,::','.::'-:•:F::':'."?:',,.':"": ...... ::' ,'.:':.:.-...,.•: ::,.,,,•.,:..•-... "¾: ,•::••"./;i:i•:":'!'E.;:•?q"?•:i:;::,:,•,;::!:' ,:.......... :.... .•.:,.?.:,.::.,..-. .",::"t'.'.'.,?•:.':?',,. ::'::,:' :,:?:,,:,.:,':•::;:":'.':,:'.-,..':' "' .:z•.,::-'. ..... •'...:'.,'.','..4 '..:...;.;,•.' ,...•..... ' ..-," :"-' '....-:':•. '..:. ....:........... :...' '•"••.__ - ""' .•'.,:i't:;:: ....... :::,..:,,,.,':•.,,., ,. ..:....:.. ,.."..,•"•::: :,,::::..8 ß?,..... :L,,,"7..........: ,:..... ,:,•"""',"'-:,'.,.,.:,.•.....',:' """"""'""'•,. -'-. ?.,., '.' ..... '":',":::,'•:,:..., .....%...... •.... ' ,4.':: _i!':""i") '. :...!.i:,)..:'.' ':.::.-,...i•,,"""-'-:.•... ,•.::.,.:-::.-..'::::.;?:i'.,•.., iv::11!:!:' :'.... ::!:i:' :.?.•.'::?:'•:::•:; :::?:'.::i::.:.:':': :• ..... .z.,:,-:.:' . :•.:.:: ,':'":: ........ :.!.::,,':':',,:: ...... -:'::..::.,"::':'.:!':"',':' .::':"":::..,-',,?i:::-:;:'",:: ..... : :,:,:-.:",..:.,:':'..': 12 -i 0 -r T , 1 2 3 4 DISTANCE (km) Figure 7b. Interpretationof this CMP sectionafter applicationof a coherencyfilter and plotting signalsabovea minimum amplitude threshold. The annotatedreflection zones used here are the same as those in Figure 6: reflectors within the Rand schistand whose top would be the Rand thrust (rs), Beno Springsfault (BS), E1 Paso Canyonfault (EP). Out of planereflections(OP) crossthe subhorizontalrs reflectorsat high anglesand are related to the BSF and EPCF. Reflectedrefractions(RR) producethe herringbonepatternat the top of the section. Eocene marine sediments were deposited onto the gneisses; beyonda reasonable doubt,it is alsonecessary to considerthe to be true. sediment onlap was to the east in terms of present-day followingpossibilities coordinates[e.g., Goodman,1989; Goodmanand Malin, 1992]. Alternative Relationships 4. Formation of Oligocene/Miocene basins by extension/ transtension[e.g., Crowell, 1987]. 1. The schist is not present immediately beneath the 5. Major uplift in the mid-Miocene (-17 Ma) at aboutthe time Tehachapigneissesor large partsof the Mojave [e.g., Ehlig, the modem San Andreas fault became active in central California [Goodmanand Malin, 1992]. 1968, 1981]. The crust beneath these regions consistsof imbricatedhigh level intrusive rocks of the Sierra Nevada 6. The TehachapiMountainswere apparentlyrotatedbetween Batholith or some other unit. 30øand45øclockwiseduringtheNeogene,aftersubstantial uplift of the schistand gneisses,and possiblyanother30øearlier [e.g., 2. The Garlock fault is not a deep-rooted,vertical crustal conglomerate[Goodman, 1989]. 8. Left-lateral slip on the Garlock fault in the late Miocene followed the rotations and separated the schist bodies now exposed in the Tehachapi and Rand Mountains [e.g., Carter, structureof theTehachapiMountainsadjacentto thatof theRand Mountains(Figure 2) [Carter, 1987]. By their timing, the Neogenetectonicanddepositional eventsin the SSJBseemre- 1987]. generalMojaveregion[e.g.,Grahamet al., 1990]. In the same 9. Regional contractionand local transpressionbeginningin the late Pliocenefurther exhumedthe schist[e.g., Graham et al., 1990]. The schistis now exposedat scatteredoutcropsalongthe San Andreas, San Gabriel, and Garlock faults (Figure 2a) [e.g., Ehlig, 1968]. Sincebasicrelationships2, 3, and 4 have not been established vein, faults like those shown in Figure 4 and/or a midcrustal decollement mayhaveaccommodated thenecessary uppercrustal extensionand shortening as the rotationsprogressed [Goodman and Malin, 1992; Jacksonand Molnar, 1990, Figure 3; Ingersoll, boundary. Beginningwith our basicset of assumptions and McWilliams and Li, 1985; Plescia and Calderone, 1986]. movingbackwardin time, the followingreconstruction of the 7. The Rand schist in the Tehachapi mountains was first crustbelow the TehachapiMountainsis proposed.Undoingthe exposedin mid-Miocene, as indicatedby clastsin the Unnamed Neogene left-lateralslipof theGarlockfaultbringsthereflection lated to the rotations establishedby paleomagnetismfor the 1988]. Owingto therotations andonsetof SAF-related tectonics, north directed reverse faulting became active and stepped 2082 MALIN ET AL: REFLECTIONS northwardtoward the Tejon embayment. The structurallyhighest reversefaults locally exhumedthe Rand schistin their hanging walls in mid-Miocene time. The pre-GarlockNeogenerotations may have occurredabove the interpreteddecollementld so that the deeper reflectors mM and mT are in their original orientations. If so, they may directly relate to the deepreflectors observed south of the Garlock fault and beneath the Rand BENEATH TILTED CRUST field evidencefor normalfaultsof thisagedoesnotyet exist,but they may not have beenrecognizedbecauseof their reactivation in the Neogene (e.g., Pastoriathrust, Figure 1; as discussed below). In the secondscenario,which is more in keepingwith the alternativerelationshipslistedabove,thereflectorsmM andI (or possiblyF, as proposedby Cheadleet al. [1986]) represent regionalthrustrampsthatsoleintothereflectionMohoandalong Mountains. The most straightforwardcorrelationis to identify horizonsld, mM, andmT of thisstudywith COCORPhorizonsF, which the gneissand schistwere carriedto the surface. In such I, andM respectively(Figure2b) [Cheadleet al., 1986]. fold-and-thrust belt modelsfor exposureof the deepcrust,the Since the rotational history of the Rand Mountains is not overlyingcrustis usuallyconsidered to havebeenremovedby known, direct correlation of upper crustal features acrossthe erosion. The currentreflectionMoho, mT and M, wouldthen Garlock are difficult. We suggest that the rs zone in the alsobe tectonicboundaries,sincemM and I appearto soleinto Tehachapi Mountains and reflectors A, B, and G in the Rand them. Finally,if theGarlockis nota deepcrustalboundary, these Mountainsare related(Figures 2 and 6). We have alsoproposed reflectorsmay not evenbe spatiallyrelated. that the top of the rs zone correspondsto the Rand thrust. In this model, the reflectorswithin rs couldbe partsof the Cheadle et al. [1986] have made the same correlation with thrustsystemandthe gneissand schistbodiescouldbe of only COCORP reflector A. Both rs and A horizons have been interpretedas being cut by high-angle structures,which in the Tehachapi Mountains profile have been establishedas reverse faults. In any case,it appearsthat the Rand schistand overlyingrocks in the Tehachapiand Rand Mountainswere upliftedtogetherin a single event that took place near the Cretaceous/Paleocene boundary[Silverand Nourse, 1986;Jacobsonet al., 1988;Pickett and Saleeby, 1993]. The questionsraisedby this event include (1) the mechanics of the uplift and (2) the removal of the overlying upper crust. If the schist is present under the batholithicbasementof the westernMojave thereis the additional questionof (3) the removal of the original lower crustandmantle. Central to these issues are the nature of the lower crustal reflectorsthat lie beneaththesemountainranges. Presently,interpretationsof the deepreflectionstructurealong the TehachapiMountains and Mojave Desert profiles offer two end-member scenarios for the unroofing of the schist and overlying gneiss. The first scenariois our preferred working model for explainingthe exposureof the deep-crustalrocksand its related lower crustal evolution. This model consists of Late Cretaceouseastwardunderthrustingof the Rand schistwith resulting crustalthickening,followed immediatelyby gravitational collapseand isostaticcrustalunloading[cf. Burchfiel, 1992]. The late Cretaceous/earlyTertiary unloadingevent would have been accompaniedby brittle and ductile deformation,including lowangle faulting, large-magnitudelower crustalflow and, possibly, magmatic underplating (modified from Wernicke and Axen [1988] and Block and Royden[1990]). The reflectionstructures between mM and mT in the CALCRUST data (Figure 6) and I and M in the COCORP data (Figure 2) are interpreted as developing along the margins of thick sectionsof subcreted schist, and hence along the margins of extendedcrust. These wedgesof lower crustalreflectors are related to the crustal-scale tilting pattern observedin the Tehachapi and Rand mountains, including intracrustal supportof their topography. We propose that the internal fan characterof the wedgesis a result of either ductile flow layering and/or progressivemagmaticunderplating of the tilting margin. In the caseof ductile flow, the reflection fabric developed as material moved up and away from the margin, while in the case of underplating successivemagmatic unitsparticipatedin the tilting. In eithercase,the schistwasthen local significance(i.e., structurallyunrelatedto any of the other schistexposures shownin Figure2). A majorproblemwith this interpretation is thata thrustrampof thismagnitude shouldhave somesurfacegeologicalexpression, (an obviousoutcropor terraneboundary) none of which has been found to date. There would alsobe a major puzzle as to what underliesthe schistand allows for the degreeof observeduplift. Also missingis a sedimentary recordof theproperageandpaleogeographic setting for the depositionof the erodedoverburden. A Crustal-ScaleTilting Model Regionaltectonicrelationships in southern Californiahelp constrain thecrustalunloading andtiltingmodelthatwe propose for the exposureof deep crustalrocksin the Tehachapi Mountains. All the available evidence indicates that the Rand and correlativeschistswere thrustbeneathTehachapi-style gneissesand intrusive rocks of the Cordilleran batholithic belt in LateCretaceous time [SilverandNourse,1986:Hamilton,1988; Jacobson et al., 1988]. The east-west continuity of theupper plategeologyin southern Californiarequiresemplacement of the ensimaticschistterraneby underthrusting from the west. This subcretionevent correspondsin time with the initiation of Laramide,rapid,low-anglesubduction [Dickinson and Snyder, 1978;Bird, 1988;Hamilton,1988].In ourproposed crustal-scale tilting model,emplacementof the schistterraneinitiatedcrustal collapse by theaddition of a buoyant plateof varyingthickness to thelowercrust.Theobserved low-angle ductileflattening fabrics withinschist exposures, conflicting kinematic indicators alongthe faults above the schist, and the evidence for latest Cretaceous/Paleocene low-angle normalfaultingsuggest thatthe crustcollapsed shortlyaftertheemplacement of theschist[e.g., Haxel et al., 1985; Nourse and Silver, 1986; Hamilton,1988; Jacobson et al., 1988;Nourse,1989].Tiltinganddoming of the deep-level,Tehachapi/Randbasementrocks occurredin this setting.Giventhedisappearance of these rockstothenorth[e.g., Ross,1985, 1989; Saleeby,1986;Dodge et al., 1986, 1988; Saleeby, 1990],theyalsoappear to sitonitsnorthern edge.This picture is also supportedby differencesin upper mantle geochemistry north and south of the Garlock fault [Mukhopadhyayet al., 1988;Montana et al., 1991;Leventhalet al., 1992]. Figure 8 summarizesin a seriesof schematiccrosssectionsour broughtto the surfaceat a muchlater time. In the uppercrust, extensionwasprobablyaccommodated alongstacksof low-angle modelfor subcretion of the schist,the extensional unloading normal faults [cf. Wernicke and Burchfiel, 1982]. Conclusive event, the tilting of the Tehachapi Mountains, and Eocene A. (90-85 "Great Valley" forearc basin sed,ments Ma) Cretaceous batholith -o Felsic botholithic crust -12 --.- Metamorphic pendants -24 (•:of the Tehachopi Mtns. sub-bat oh lithic-"'--,•.•• E -36 _----•- - 48 -.-- Sub-continental mantle A pproximate intersection of SW-NE B. (85-80Ma) SW 0 •, NE , NW 12 ,-,.)' ;5."K•.-"/ ..... •'I',: ' J '---/; : ' ";;'/;'[ '•'.'-%• ;z'. ,:,:...•r'-, "-"," •; •' u-,.•-'?--.'0',5 -• ', ' ; '"' ';;.2, •: Thrust) 36 T (Rand 48 SE ,-_-'•,, ,,•; ,'..-,,-' ,;,-;• '..',•'•,;) ,':' • .-, , ,,, •24 ,, ..,_,•=, ,-,,,<• i/•/ sections /•-•; / :,; ,!; ?; •,,".' • "•.",: mantle.'/• '"•• '"" '"' RSDI ?/?' Sub-oceanic hme nt) •:• ........ oflower Sub-continentol(/•: / •u•b• L •CI •(• I•Y •e• g •c t removal Subcretion of regional bathollthlc schistlerrone Approximate intersection crust of NW-SE sections )' mantle •' tear? I C. (80-70Ma) SE SW 12 •4•E • 24 36 Continued schist subcretion mega thrust) D. (70-60M0) SW • ..... ....:-.,•, NE NW '""'"'''"' ',':'-';"'-" .'.•••'• ß ...... ß ':,.. ß ..'. v:',:':'.::'-:. ........ .,:..:.:..:.'v.::..:..::.,'::.:.,:.:..: . I•-cally rising'sub-oceanic mantle NW E. {PRESENT DAY) Tejon Tehochopi Emboyment Eocene toRecent strata --- Mtns •,Gorlock F. / ß SE Mojave SYMBOLS; Desert 0 --&_ detachment and/.__.•z..:.-•'.•': • • ........... '"'"" '.'-•"" ........ '"'.... •..... "•••-"'• '••'•'••"•••••••-'-•.'.':'-':'-".• Neogene 24 ß orbrittleductile ,ransition ,.:.::::::¾::.':F?:?:'. .............. . .•::•i.::;•_•.'.:'.h..:i.i•.'• '•'••••••••:.•:• '-"'"'•" '••' ''"''' Moho::'::':':":'"'"":"? ' ......."•'•' •'"'....'"' -'"'•-''-''••'•" -•"•"'•••••'•••• 36 l_'ow se r rctau I f•n trsucture J ,• 0 t 25 i 50 , ...-..! 75 i Major active thrust ^ IOOkm .J inherited fromextensional collapsedeformation of D. 48 • © zone Major inactive thrust Second order active thrust fault Second inactive fault order thrust Motion inond out of cross section Figure 8. This Figureshowsa schematicsummaryof the subcretion andextensional collapsemodelfor the restored TehachapiandRandMountainsregions.(a) Generalized startingconditions of thebatholithiccrustwith theapproximate locationof thedeep-level Tehachapi rocksindicated. (b)-(d)Orthogonal viewsduringthecrustal evolutionthathasexposedthe deep-crustal metamorphic coreof the TehachapiandRandMountains.(e) The currentconfiguration of thecrust.The intersections of viewsareasindicated in Figure9. Extensional collapse structures were favorablyorientedfor reactivationas reversefaultsduringeitherTertiarycrustalrotationsor Plioceneto Recentcontraction.(For additionalevidencefor NW-SE differences shownin thecrustseeRoss[ 1985, 1989],Saleebyet al. [1987],Dodgeet al. [1986,1988],andSaleeby[1990].) 2084 MALIN ET AL.: REFLECTIONS BENEATH exposureof their metamorphiccore. The modelbegins90 to 85 TILTED CRUST Implications for RegionalReconstruction Ma with a Sierra Nevadan batholith that was flanked to the west As type localities for gneissic and schistoserocks, the by a forearc basin. The thickness of the batholithic crustal is TehachapiandRandMountainspresenta uniqueopportunity for reconstructing the mannerin whichsuchrocksmightbe uplifted from the lower crust. South of the Tehachapi and Rand assumed to be -50 km, with an increasein maficrockswith depth [e.g.,Saleeby,1990]. The increasein maficrocksis indicatedby thepresence of maficlowercrustalxenolithsin theupperpartsof thebatholith[Dodgeet al., 1986,1988]anddeepexposures along its tilted north-southaxis [Saleeby,1990]. The structureof the batholithicbelt during this time interval is assumedto have been Mountains, exposuresof deep plutonic rocks and coincident lower plate schist are found in southeasternCalifornia and southwestern Arizona (Figure2). Structuralrelationsand seismic two-dimensionalalongthe presentSierraNevadaand Mojave datasuggestthat largetractsof the schistunderliethe Mojave Desert [e.g., Silver, 1982; Cheadle et al., 1986; Lawson, 1989]. In contrast,no schistexposures occurin the Peninsular Ranges Figure 8 showstwo orthogonalcrosssectionsof the initial of the currentSAF in the MojaveDesert.A underthrustingof the Rand schist at 85 to 80 Ma [Jacobson, batholith,southwest 1990; Silver and Nourse, 1986]. A low-anglesegmentof the subductingoceanicplate carriedthe ensimaticschistprotolith Sierra Nevada beneaththe batholithicbelt at lowercrustallevels. The deepest Western metamorphic batholith and most mafic layers of the Sierran batholith were thus rocks Desert. displacementdown dip along the subductionzone. Regional magmaticactivitymay haveendedin thisinitial underthrusting phase. Likewise, the beginning of broad uplift with compressional faultingmight havedestroyedthe adjacentforearc basin. To accountfor the northwarddisappearance of the schist, zonationof the batholith,and geochemicaland geophysicaldif- Eastern zone rocks stern zone rocks ferences, the NW-SE cross sections show the northern terminus of the subcreted schistterraneat the latitudeof the Tehachapi MountainsandTejon embayment. Bakersfield FromLateCretaceous (80 to 70 Ma) to latestCretaceous/early Paleocene(70 to 60 Ma), we showthe schistprotolithasa waterrich, accretionarybodybeneaththe upperplatebatholithiccrust. Undertheseconditions,the subcreted rockswerehighlyductile; the fluidsthatescaped upwardfrom themweakened the quartzrich upperplateandpromotedretrograde metamorphic reactions in it [e.g.,Wernicke,1990]. The tectonicallythickenedcrustthen collapsed underits ownburden.Thiscollapsewasfacilitatedby horizontalflow withinthenewlyformedschistandfragmentation of the upper plate into shallow dipping slabsboundedby ductile/brittleshearzones. We proposethat the integrated pproximate traces of Fig.8 B-D sections Pastario "thrust" Monterey • Exposures of regional lower plate schist Schist of Sierra de Salinas terrane Restored Salinia displacementof theseslabswasto the west(modifiedfrom Silver [1983] andMay [1989]). Duringthis westwardtransport, the crystalline rocks of the Tehachapi Mountains may have undergone up to 30øof clockwiserotation[BurchfielandDavis, 1981;McWilliamsandLi, 1985;Plesciaand Calderone,1986]. Duringthe collapsephase,magmaticunderplating mayhave Transverse Ranges PeninsularRanges occurred,at least locally. Igneous-textured,lower crustalmafic xenoliths withmid-ocean ridgebasalt(MORB)isotopicsignature andlatestCretaceous/early Paleocene agesarepresentin eastern Mojave volcanicflows [Montanaet al., 1991;Leventhalet al., 1992]. This is consistent with subcreted oceanic mantle i i i 0 km 100 . .. rocks Western memmorpn•c supplyingthe melts. Figure8 showsthe underplating as an Figure9. A diagrammatic maprestoration showing pre-Neogene importantfeatureduringthe late phasesof extensional collapse, distribution of upperplatesialiccrystalline rocksandlowerplate subsequent to the underthrusting of the oceanicplate. In summary,we proposethat crustal-scaletilting of the schist exposuresrelative to the Sierra Nevada and Peninsular Ranges batholiths (modified fromBurchfiel andDavis [1981]), TehachapiMountainsis relatedto the removalof the deepest as comparedto the present-dayschistdistributionshownin levels of the Sierranbatholithby subcretionand imbricationof the schistand subsequent extensionalcollapseand magmatic underplating.The regionalsettingsuggests that the tilting took placealongthenorthern boundary of thisprocess.ThusthemM Figure2. Saliniais suggested to havebeendispersed westward froma subcretion-extension zone[afterSilver,1983;May,1989; Silverand Nourse,1986] with the lowerplateschistterrane possiblyrepresented by the schistof Sierrade Salinas[Ross, 1976]. The transversepetrochemical zonationpatternof to mT wedgeof reflectorsmay be analogousto the ductileflow and shearlayeringcommonlydevelopedaroundthe edgeof a batholithicbelt is that of Silver et al. [1979], Mattinsonand ductile baudin, in this case, a crustal-scalebaudin of subcreted James[1985],SilverandMattinson [1986],andSaleeby [1990]. schist. Progressive,late stage underplating,which should accompanythe extensionalcollapse,may also contributeto the reflectioncharacterof the wedge. The marginsof the Mesozoicschistbodieslater servedto localize major Neogenestructures that dismembered and exposed relativelysmallportionsof the schistitself. MALIN ET AL.: REFLECTIONS reconstruction of upperplate,sialiccrystalline rocksandlower BENEATH TILTED CRUST 2085 References plateschists before slipontheSAFandGarlock faultplaces the Ambos, E. L., and P. E. Malin, Combined seismicreflection/refraction Saliniancrystalline terranewestof ourpostulated extensional investigationin the TehachapiMountains,southernCalifornia:Results collapse zone(Figure9; modified fromBurchfiel andDavis from the 1986 CALCRUST experiment,Eas Trans AGU, 68 (44), [1981]). SinceSaliniaappears onitsnorthendto bepartof the 1360, 1987. SierraNevadaandon its southendpartof the PeninsularRanges [SilverandMattinson, 1986],ourpreferred modelsuggests thatit wasbrought westward fromthembytheextension following the subcretionof the schist(modifiedfrom Silver [1983], May [ 1989],andSilverandNourse[1986]). In supportof this idea,we notethat afterunslipping the Neogene faults,the widthof theCretaceous batholith in the Mojaveis at leasttwicethatof thesouthernmost SierraNevada andthe northernmost PeninsularRanges.This expandedsegment Barazangi,M., and B. L. Isacks,Spatialdistributionof earthquakesand subductionof the Nazca plate beneathSouthAmerica, Geology,4, 686-692, 1976. Barton,P. J., The relationshipbetweenseismicvelocityanddensityin the continentalcrust- A useful constraint?,Geaphys.J. R. Astron. Sac., 87, 195-208, 1986. Bird, P., Formation of the Rocky Mountains,westernUnited States:A continuumcomputermodel,Science,239, 1501-1507,1988. Block,L., andL. H. Royden,Corecomplexgeometries andregionalscale flow in the lower crust, Tectonics,9, 557-567, 1990. is coincident with the lowerplateschistexposures (Figure9). Burchfiel, B. C., Extension contemporaneous with shorteningwithin Rocksthataretypicalof theeastern zones of thebatholithic belt mountain belts, Geol. Sac. Aust. Abstr., 32, 6-8, 1992. Burchfiel, B.C., and G. A. Davis, Mojave Desert and environs,in The GeotectonicDevelopmentof California, RubeyVolume 1, editedby W. G. Ernst, pp. 217-252, Prentice-Hall, EnglewoodCliffs, N.J., appear in thewestalong theexpanded segment [Mattinson and James,1985;SilverandMattinson,1986]. An exampleof this extension maybepreserved in theTehachapi Mountains.Here, younger, eastern zone,high-level granitic rockslie above older, western zone,deep-level tonalitic anddioriticgneisses alongthe Pastoria fault[Crowell,1974;Ross,1989]. Thegeometry of the 1981. Buwalda,J.P., Geology of the TehachapiMountains,California, Bull. Calif. Div. Mines, 170, 100 pp., 1954. Carter, B., Quaternaryfault-line featuresof the central Garlock fault, Pastoria fault and its kinematic indicators are complex, but Kern County,California,Field Trip Guideb.57, 67 pp., Soc.of Econ. dramatic upperplate/lower platecontrasts across it areconsistent Paleontol.and Mineral., Pac. Sect., Bakersfield,Calif., 1987. withlargenormalseparation, resulting in theupper-plate rocks Cheadle,M. J., B. L. Czuchra,T. Byrne, C. J. Ando, J. E. Oliver, L. D. Brown, S. Kaufman, P. E. Malin, and R. A. Phinney,The deepcrustal moving westward in theearlyTertiary(Figures 1 and9). structureof the Mojave Desert, California, from COCORP seismic Moreover,the metamorphic wall rocksof thebatholithic belt that shouldnow lie on the westernmarginof the expandedbelt appear tobemissing (Figure 9), soisthecorresponding forearc basin. Destruction of themetamorphic belt andtheforearcbasin alongthissegment of thebatholithic beltmayhaveoccurred asa resultof underthrusting of the schistprotolithand westward denudation duringpostsubcretion extension. In our model,the expandedsegmentof the SierraNevada batholithic beltcorresponds to theregionof schistsubcretion and extensional collapse andis a product of thatprocess. We suggest thatthe rapidlysubducting oceanicplatewassegmented into fault-boundeddomainsof differentdips, similar to the modern Nazcaplate[Barazangiand Isacks,1976]. In thismodel,the schist-subcretion zone in the lower crustcorresponds to a shal- low-dipping segment of thesubducting plate. Thecrustof the modern Tehachapi andRandmountains wasapparently situated nearthenorthernedgeof thislow-dipping platesegment.As a resultof schist emplacement andextensional collapse, thisregion wasstrongly tilted,exposing therootsof theSierras andlaterthe lowerplateschistitself. reflection data, Tectonics, 5, 293-320, 1986. Crowell, J. C., Origin of late Cenozoic sedimentarybasinsin California, Spec.Publ. Sac. Ecan. Paleantal. Mineral., 22, 190-204, 1974. Crowell, J. C., The San Andreasfault systemthroughtime, J. Geol. Sac. London, 136, 293-302, 1979. Crowell, J. C., Late Cenozoic basins of onshore southern California: Complexity is the hallmark of their tectonic history, in Cenozoic Developmentof Coastal California, Rubey Volume VI, edited by R. V. Ingersoll and W. G. Ernst, pp. 207-241, Prentice-Hall,Englewood Cliffs, N.J., 1987. Davis, G. A., and B.C. Burchfiel, Garlock fault: An intra-continental transform structure, southern California, Geol. Sac. Am. Bull., 84, 1407-1422, 1973. Day, G.A. and J.W.F. Edwards, Reflected refractedevents on seismic sections,First Break, September1983, 14-17, 1983. Dickinson, W. R., and W. Snyder, Plate tectonics of the Laramide orogeny,Mere. Geol. Sac.Am., 151, 355-366, 1978. Dodge, F. C. W., K. C. Calk, and R. W. Kistler, Lower crustal xenoliths, Chinese Peak lava flow, central Sierra Nevada, J. Petrol., 27, 12771304, 1986. Dodge, F. C. W., J.P. Lockwood, and L. C. Calk, Fragmentsof the mantle and crust from beneath the Sierra Nevada batholith: Acknowledgments. Thisworkwassupported under NSFgrants EAR83- Xenoliths in a volcanic pipe near Big Creek, California, Geol. Sac. Am. Bull., 100, 938-947, 1988. 19254to CALCRUST, EAR87-08266and 89-04063to J. Saleeby,and EAR91-19263 and 91-19263 to P. Malin. J. Plesciaof the JetPropulsion Dokka, R. K., The Mojave extensional belt of southern California, Tectonics, 8, 363-390, 1989. Laboratory, Pasadena, California, provided gravitydataandseveral early models fromwhichFigure3 wasdeveloped. We aregrateful forhisopen Ehlig, P. L., Causesof distributionof Pelona,Rand, and Orocopiaschist along the San Andreas and Garlock faults, in Proceedingsof the andkindhelp. Theauthors havebenefited fromconversations withJ. Conferenceon GeologicalProblemsof the San AndreasFault System, Crowell,T. McEvilly,S. Richard, andJ. Sharryandmanyothers.We would like to thank ARCO for providingtheir "vibratorbuster"test edited by W. R. Dickinson and A. Grantz, Stanford Univ. Publ. equipment tocheck thevibrators before thesurvey andtheTejonRanch Geasci., 11, 294-305, 1968. Company foraccess to theirproperty in theTehachapi Mountains. We Ehlig, P. L., Origin andtectonichistoryof the basementterraneof the San thankE. Karageorgi for generating thefilteredfielddataat theEarth ScienceDivision,LawrenceBerkeleyLaboratory.Commentson the Gabriel Mountains, central Transverse Ranges, in The Geotectonic Developmentof California, edited by W. G. Ernst, pp. 253-283, Prentice-Hall,EnglewoodCliffs, N.J., 1981. original manuscript byL. Serpa, T. Broecker, andananonymous reviewer aregratefully acknowledged. Further constructive reviews, resulting in Golombeck, M., and L. Brown, Clockwise rotation of the western Mojave Desert,Geology,16, 126-130, 1988. thepresent manuscript, wereprovided byE.Hauser, G.Fuis,andT. Pratt. Threethoughtful andknowledgeable reviews by G. Fuiswereespecially Goodman, E. D., The tectonics of transition along an evolving plate appreciated. margin -- Cenozoic evolution of the SouthernSan Joaquin Basin, 2086 MALIN ET AL.: REFLECTIONS BENEATH California, Ph.D. thesis,225 pp. and 3 plates,Univ. of Calif., Santa TILTED CRUST refraction profiling, paper presented at the Third International Workshop and Symposium on Seismic Probing of Continentsand Their Margins, AustralianNational University, Canberra,Australia, Barbara, 1989. Goodman,E. D., and P. E. Malin, Commentson the geologyof the Tejon embaymentfrom seismicreflection, boreholeand subsurfacedata, in Studiesof the Geologyof the San JoaquinBasin, Publ. 60, editedby S. A. Graham, pp. 89-108, Pacific Section, Society of Economic Paleontologistsand Mineralogists,B akersfield,Calif., 1988. Goodman,E. D., and P. E. Malin, Evolution of the southernSan Joaquin Basin and mid-Tertiary transitional tectonics, central California., Mattinson,J. M., andE. W. James,SalinianblockU/Pb ageandisotopic variations:Implicationsfor origin and emplacementof the Salinian terrane, in TectonostratigraphicTerranes of the Circum-Pacific Region,Earth Sci. Ser., vol.l, editedby D. G. Howell, pp. 215-226, Circum-PacificCouncilfor Energyand Mineral Resources, Houston, Tectonics, 11, 478-498, 1992. Goodman, E. D., P. E. Malin, E. R. Ambos, and J. C. Crowell, The May, D. J., Late Cretaceousintra-arc thrustingin southernCalifornia, 1988. Tex., 1985. Tectonics, 8, 1159-1173, 1989. southern San Joaquin Valley as an example of Cenozoic basin evolutionin California, in The Origin and Evolutionof Sedimentary Basinsand their Energy and Mineral Resources,Geophys.Monogr. 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