JOURNAL OF GEOPHYSICALRESEARCH, VOL. 100,NO. B2, PAGES2069-2087, FEBRUARY 10, 1995
Significanceof seismicreflections beneath a tilted exposure of deep continental crust, Tehachapi Mountains, California
P. E. Malin,1,2 E. D. Goodman,3,4 T. L. Henyey,5 Y. G. Li,5 D. A. Okaya,5 andJ. B. Saleeby6
Abstract. The focusof this articleis a processwhereby lower crustalcrystalline and schistoserocks can riseto the surface,with the TehachapiMountains in Californiabeing the casein point. As a prime exam- ple of the lower crust,these mountains expose Cretaceous gneisses that formed25-30 km down in the SierraNevada batholith and appear to be underlainby the ensimaticRand schist. Integrated geophysical andgeological studies by the CALCRUST programhave produced a crosssection through this post-Mid- Cretaceousstructure and suggesta generalmodel for its development.Seismic reflection and refraction profiles show that the batholithicrocks dip northwardas a tilted slaband extendbeneath the southernend of the SanJoaquin Basin's Tejon embayment.Two southdipping reverse faults on the rim of the Tejon embaymentwere discoveredin the reflectiondata and verified in the field. The faultshave a combined separationof severalkilometers and cut throughan uppercrustal reflection zone that projectsto the sur- face outcropof the Rand schist.The upperand lower crustsare separatedby a zoneof laterallydiscon- tinuousreflectors. Reflections from the lower crustform a wedge, the baseof which is a nearlyflat Moho at 33 km. Regionalgeological relations and gravitymodels both suggestthat the reflectivezone correspondsto the Rand schistand the newly recognizedfaults account for its Neogeneexposure. Alternatively,the reflectivezone maybe part of the gneisscomplex, suggesting that the schisteither lies deeperor is not presentunder the gneisses.If the Rand schistunderlies the TehachapiMountains and Mojave regionto their south,a modelfor their evolutioncan be constructedfrom regionalgeological re- lations. It seemsthat duringLate CretaceousLaramide subduction the protolithof the schistwas thrust eastwardbeneath the Mojave. Along this portion of the Cordilleranbatholithic belt the subductionwas evidentlyat very low angles. The bottomof the batholithwas removed and replaced by a thick sectionof schist,fluids from which weakenedthe overlyingbatholith. This thickenedcrust collapsed by horizontal flow in the schistand faultingof the uppercrust into flat-lying slabs. When emplacementof the schist endedin latestCretaceous/earliest Paleocene, the underlyingmantle rose, compensating for the extension andproviding material for magmaticunderplating. In the Neogene,transpression and rotation of the up- per crustalong the San Andreasand Garlock faults resulted in the exposureof the schist.
Introduction andwhich may underlie both the Tehachapi Mountains and Tejon embayment,as well asthe Mojave Desert to theirsouth (Figure 2) The TehachapiMountains' gneiss complex dips northwestward [e.g., Ehlig, 1968; Silver, 1982, 1983; Plescia, 1985; Cheadle et underthe Cenozoic sediments of theSan Joaquin Valley's Tejon al., 1986; Carter, 1987; Lawson, 1989]. The processesand embayment,at the southernmosttip of the SanJoaquin Basin resultingcrustal structure that bringthe gneissand Rand schistto (SSJB)(Figure 1). Thegneisses are bounded on the south by the the surfaceare the focusof this paper(see also Salisburyand Garlockfault andan exposureof Randschist (Figure 2) [Wiese, Fountain [ 1990]). 1950]. Regionalgeology, petrology, geophysics, and seismology The TehachapiMountains' gneisses are thoughtto represent all suggestthat the Rand schist represents an ensimaticprotolith the Cretaceous mid-to-lower crust of the Sierra Nevada batholith that was thrustbeneath th• gneissesduring the Late Cretaceous [Saleebyet al., 1987;Saleeby, 1990; Pickett and Saleeby,1993]. Both the gneissesand the presumablyunderlying schist were 1DepartmentofGeology, Duke University, Durham, North Carolina. uplifted in the latestCretaceous/early Paleocene and exposedin 2FormerlyInstitute forCrustal Studies University ofCalifornia, Santa the Tertiary [Jacobsonet al., 1988; Goodman, 1989; Silver and Barbara. Nourse, 1986; Pickett and Saleeby, 1993]. The uplifted 3InstituteforCrustal Studies University ofCalifornia, SantaBarbara. Tehachapi block apparentlyexperienced Neogene clockwise 4NowatExxon Production Research Company, Houston, Texas. rotationand late Miocene-to-Recentthrusting along the White 5DepartmentofGeological Sciences, University ofSouthern California, Wolf fault (WWF), placing these basementrocks over the 12- Los Angeles. km-deep SSJB [e.g., McWilliams and Li, 1985; Goodmanand 6DivisionofGeological andPlanetary Sciences, California Institute of Malin, 1992]. Technology,Pasadena. The TehachapiMountains study described here was part of the CaliforniaConsortium for CrustalStudies (CALCRUST) effortto Copyright1995 by the AmericanGeophysical Union. understandthe relationshipsbetween surface exposures of high- gradeMesozoic metamorphicrocks, associated faults, and lower Paper number 94JB02127. crustalprocesses [e.g., Henyey et al., 1987]. In the Tehachapi 0148-0227/95/94JB-02127505.00 Mountains,CALCRUST acquireda 38-kmreflection profile and
2069 2070 MALIN ET AL.' REFLECTIONS BENEATH TILTED CRUST
OxSP1 x o IOm• t •S•erraNevada'•x 0 IOkm t, \\ ß\\ß \\ SOUTHERN SAN SanaSanLuciaGabrielR••D•A JOAQUIN VALLEY San Andreas Fault
EXPLANATION
CenozoicCover Pleito Thrust TEJON EMBAYMENT ,E•Tonalites of the Bearand ValleyGabbroids Suite
Tehachapi Mountains '• GneissComplex •J RandSchist
++++ Undifferentiated Mesozoic Pastoria + ++ x
Thrust +++++ [•] Intrusiveand Metamorphic + + + + Rocks '+++++ x MOJAVE •COCORP ___ Refraction Line and Shot Points (Fig. 3) DESERT LINE4 ReflectionLine (Fig. 6) 34ø45' 119o07.5' 118ø225• Figure 1. Index and location maps for the Tehachapi Mountains-Tejon embayment area in south-central California'sSan Joaquin Basin. Also shownis that portionof the CALCRUST seismicreflection profile containing deepreflections and discussedin this paper(between vibration points VP 352 to VP 987). Importantgeological featuresare the exposedMesozoic basementrocks; their Cenozoic sedimentarycover in the Tejon embayment (modifiedfi'om Sams and Saleeby[1988]); and the Rand schist outcrop and Rand thrust/north branch of the Garlockfault on its northside [e.g., Buwalda, 1954; Burchfiel and Davis, 1981]. Otherseismic survey lines are the overlying CALCRUST refraction profile [from Goodmanet al., 1989] and the most northwesternline of the COCORP Mojave Desertreflection survey, line 4 [from Cheadleet al., 1986].
a coincident110-km refraction profile [Malin et al., 1988;Ambos the refraction models cannot resolve the locations and amounts of and Malin, 1987]. Further constraintswere obtainedfrom newly dip in laterally varying structures.Thus the gravity and reflection compiled gravity data (J. Plescia, Jet PropulsionLaboratory, data add essentialconstraints to the refraction model (Figures 3, Pasadena, California, unpublished data, 1993) and industry 5, 6, and 7). As in the generalcases discussed by Barton [1986], reflection and well log measurementsin the Tejon embayment a relatively broad range of velocity-densityrelations was required [Goodman and Malin, 1988; Goodman et al., 1989; Goodman, to obtain a consistentmodel for both the seismicand gravity data 1989; Goodman and Malin, 1992]. The CALCRUST reflection from the TehachapiMountains. profile was begun a few kilometers north of the WWF. From The CALCRUST reflection profile aimed principally at there it was taken south, toward line 4 of the Consortium for addressingthe structuralrelationship of the gneissand schist, Continental Reflection Profiling (COCORP) Mojave Desert their distribution in the crust, and the responseof the crust to survey(Figure 2) [ Cheadleet al., 1986], until the projectbudget latest Mesozoic and Cenozoic tectonics. The depth-converted was exhaustedabout a kilometer north of the Rand schistoutcrop. commonmidpoint (CMP) stackof thesedata shows(1) a seriesof A central,22-km-long segmentof the CALCRUST datacontains northwest dipping reflections consistent with the refraction clear deep reflections(Figures 1 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 variable northwesterly 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 similar Moho 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 The refractiondata, which penetratedto midcrustallevels (<15 Mountainsmay be supportedat relatively shallowlevels, perhaps km), showseveral significant features. They include (1) velocity even within the crust. This might be the case,for example,if the discontinuities in the areas of the Garlock fault and WWF, (2) Tehachapi gneiss complex is underlain by a body of slightly high-velocity rock units at shallow depthsbeneath the site of the fasterbut slightly lessdense schist. reflection survey, and (3) a northwestdip in these units (Figure In total, the geology and geophysicsof the crust beneaththe 3c) [Goodmanet al., 1989]. Becauseof the effectsof averaging, Tejon embaymentand TehachapiMountains suggest an event of A) GarlockFault Tehachapi San Andreas Fault I Rand Mtns. / / Lancaster
MP / PR Mojave Desert .,,, SP SG
f'ARIZONA Los Angeles OS
ß •,,•CM •
,', PP?
0 1 O0 Km
Pelona-Orocopia-RandSchist Outcrops Vincent-ChocolateMtn.-Rand Thrust System PA = Pastoria Thrust
CALCRUST PROFILE B) p,L/'•_--White Wolf--h/F,
•...•••,• '• •/(•,, MOJAVE 3 ---I •,• DES
.'.• -...i.ii, i• LINE6 ' 0 20kin --I•'•l CRYSTALLINE ROCKS I .. , I • RAND SCHIST Figure 2. (a) Map showingmajor active structuralfeatures and present-dayexposures of P½lona-Orocopia-Rand schists. The Vincent-Chocolate-Randthrust system,indicated by teeth on the schistbodies, is of Mesozoic age. Using U.S. GeologicalSurvey, COCORP, and industryseismic data, Lawson [1989], for example,has arguedthat schistunderlies a large part of the westernMojave Desert. Besidesthose in the Tehachapiand Rand Mountains,the various schistoutcrops denoted are Mount Pinos (MP), Portal and Ritter ridges (PR), Sierra P½lona(SP), San Gabriel Mountains (SG), Orocopia Mountains (OM), ChocolateMountains (CM), and Picacho Peak (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 diagram is indicatedby the dashedpolygon in Figure 2a. Also shown are the locationsof the CALCRUST TehachapiMountains profile, the TehachapiMountains (TM), the Tejon ½mbayment(TE), the Pastoriathrust (PA), the Pl½itothrust (PL), the Rand Mountains(RM), and the Rand thrust(RT) in the Rand Mountains. The Mojave Desertreflection lines and their reflectionhorizons are shownin a cutawaysection along profile [after Cheadleet al., 1986]. The Rand schistmay projectunder much of the westernMojave Desert. Note the high-anglestructures interpretedas cuttingthrough reflection horizon A, the reflectorinterpreted as the Rand thrustat the top of the Rand schist. 2072 MALIN ET AL: REFLECTIONS BENEATH TILTED CRUST upwardtilt to the southeastby asmuch as 25ø . This processtook profile segment.With the possibleexception of requiringschist placebetween mid-Cretaceous and early Paleogenein association velocitiesseveral percent higher than those observed by Malin et with the emplacementand uplift of the Rand schist[Jacobson, al. [ 1981], this interpretationis consistent,within errors,with the 1990; Pickett and Saleeby, 1993]. The seismic data confirm refraction and gravity data. The tilted upper crust also steps borehole evidence that deep Sierran crystalline rocks extend upwardto the southacross two previouslyunrecognized, high- beneaththe Tejon embayment. At somewhatgreater depths, the angle, southeastdipping faults of late Neogene •ge. In the reflectiondata suggestthat the Rand schistunderlies the central present,transpression along the Garlockfault and WWF zones
(a) NW Distance (km) SE -50. 0. +50. 0
20-
4o-
6o-
+ Calculated 8O
(b) -50. WWF352 9871GF +50.
SL 0 2.35 2.33 BS 2.65 2.86 2.69
2.68 2.88 2.88 rs 2.66 10
-• 2.70
C• 20 2.83 2.79 f mM
2.93 Density in g/cm3
3O Moho m• 3.15
+50.
Figure 3. Forward crustalmodels of the gravity field and velocity structureover the CALCRUST Tehachapi Mountainssurvey profile. (a) The gravitydata and (b) gravitymodel are 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 refractionsurvey shown in Figure 1. This modelis a modifiedversion of the oneproposed by Ambosand Malin [1987] and Goodmanet al. [1989]. The surfacepositions of the refractionshot points are 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,densities are shownin gramsper cubic centimeter,and velocitiesin kilometersper second,with their range given by the values at the top and bottom of the layers. The most significantfeature of the gravity fit is the slightlylower densitybody below the gneissesof the Tehachapiblock. Our estimatesof theerror (1 standarddeviation) in depth,density, and velocity are 1 km, 0.1 g/cm3, and 0.15 km/s, respectively.Note that the thicknessof the crustis nearlyconstant in thesesimple models. In fact, at a depthof 32 km, the massesof the various crustal columns along this profile differ by less than these uncertainties. Dashed boundariesin the velocity model indicate the areaswhere these limit are likely to be exceeded. The annotated zonesare interpretedas basin and basin bottom (b), Rand schist(rs), crustalscale decollementor brittle/ductile transitionzone (ld), and Beno Springsfault (BS). MALIN ET AL ß REFLECTIONS BENEATH TILTED CRUST 2073
(c) WWF SP2 GF
SL0 - 2.6 - 3.9 J as I ' / /
6.2 -6.5 Velocityin km/s ..... Id *
20- 6.8-7.0 (?)
+50. NW Distance (km) SE
Figure 3. (continued) continuesto uplift the TehachapiMountains. Using these results the easternMojave are moresuggestive of suboceanicconditions to reconstructthe positionsof the gneissand schist in the past,it [Montana et al., 1991; Leventhal et al., 1992]. wouldseem that the wedgeof lowercrustal reflectors is relatedto The Tehachapigneisses were rapidly uplifted to 10-15 km the subcretion of the schist and subsequenttilting of the depthsometime between 100 and 85 Ma [Pickettand Saleeby, TehachapiMountains in latestCretaceous/early Tertiary time. 1993]. In the Rand Mountains to the east, age and structural relationsindicate emplacement of theRand schist beneath similar crystallinerocks in this sametime interval[Silver and Nourse, GeologicalBackground 1986]. In the TehachapiMountains, uplift anderosion continued Basementrocks north of the Gatlock fault are exposedin into the early Tertiary, so that Eocenemarine rocks of the Tejon scatteredoutcrops and consistof the 100-115 Ma Tehachapi Formation were deposited directly on the gneisses [e.g., gneisscomplex and Rand schist (Figures 1 and2) [Pickettand Goodmanand Malin, 1992]. Based on the presenceof schist Saleeby,1993]. Theserocks once occupied the lowercrust and 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 middleMiocene time Jacobson et al., 1988; Saleeby et al., 1987; Sharry, 1981]. [Goodman, 1989]. Postmagmaticductile fabricsin the gneissesare similar to those Geological and geophysicalevidence exists for three other seenin the upper platesof the Vincent and Rand thrusts. These Oligoceneto Miocenetectonic events in the Tehachapiregion faultsplace intrusive rocks over the schistin the San Gabrieland [e.g., Crowell, 1974, 1987; Goodman and Malin, 1988; Rand Mountains respectively[Ehlig, 1981; Silver et al., 1984; Goodman, 1989; Goodman and Malin, 1992]. The earliest event Silver and Nourse, 1986]. Evidently,this event and its associated is a late Oligocene/earlyMiocene period of extension,involving metamorphismtook place in Late Cretaceoustime [Silverand low- and high-angle normal faulting, volcanism, and the Nourse, 1986; Hamilton, 1988; Jacobson et al., 1988]. On its depositionof coarsefanglomerates. Uplift in the mid-Miocene north side, the Rand schist in the Tehachapi Mountains is exposedthe schistand appearsrelated to initial slip on the San boundedby a fault of variable exposureand dip that has been Andreasfault (SAF). Extensionand transtensionresumed after identified both as the "north branch" of the Gatlock fault and as thisperiod, resulting in obliqueslip faulting and rapid subsidence the "Rand thrust" [e.g., Buwalda, 1954; Davis and Burchfiel, of the SSJB to bathyal depths. During late Miocene time, 1973; Burchfieland Davis, 1981]. transtensionalternated with transpression,reflected by cyclesof Sharry [1981] has suggestedthat as in the case of the San subsidenceand uplift in the SSJB. Sincethat time, the SSJBhas Gabriel and Rand Mountains, the schist extends beyond its evolved from submarine, to sublacustrine, to subaerial, to outcrop, perhaps even under the Tejon embayment. This intermontane. Since the Pliocene, the entire region has been possibilitywas discountedto somedegree by Ehlig [ 1968], who dominatedby the transpressionalregime of the modern San suggestedthat only the gneissmay extend to the north. Even Andreasand Garlock faults, with local shorteningtaking place on farther to the north, however, the basement consists of intact, featuressuch as the WWF and Pleito thrust(Figures 1 and 2). shallow level batholithic crust, beneathwhich geophysicaland The left-lateral Garlock fault is the most significantand active deep-crustal xenolith data demonstratethat no significant crustaldiscontinuity exposed in the TehachapiMountains. Most underlyingschist terrane exists [Saleeby, 1986; Dodge et al., of its postulated64 km of displacementappears to 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[Goodman and 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, xenolith studies 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[Mukhopadhyay et al., 1988], while similar studiesin throughgoingcrustal discontinuity, any model of the crust 2074 MALIN ET AL.: REFLECTIONS BENEATH TILTED CRUST beneaththe TehachapiMountains must be consistentwith the duringthe Neogene[Graham et al., 1990; Plesciaand Calderone, structure and rocks beneath the Rand Mountains. 1986]. On the other side of the Garlock fault, at sites south of the Unfortunately,no definitiveevidence on the depthof the RandMountains, roughly25 ø of Mioceneclockwise rotation has Garlockfault presentlyexists. Basedon the continuityof an been measured [Golombeck and Brown, 1988]. It is not known apparentmidcrustal reflector observed in the Mojave Desert whether theserotations also apply to the Rand Mountains. In the COCORP data, Cheadleet al. [ 1986] proposedthat the Garlock Mojave extensionalbelt, Dokka [1989] has used paleomagnetic fault may not be deeplyrooted (Figure 2). Alternatively,this data and kinematic indicators to proposeearly Miocene north- reflectormay representa laterallydisplaced but flat horizon,an south extension, in contrast to the more east-northeasterly out-of-planereflection, or perhapsreflected refractions from a directionmore commonlysuggested for the Mojave region. subverticalfault [Serpaand Dokka, 1988]. Arrival times of Accepting the 40 ø to 60 ø rotations for the Tehachapi refractedPn waves show that the presentMoho under the Mountains, and undoingtheir Neogeneslip on the Garlock fault, TehachapiMountains is flat to withina few kilometers,with no Goodman and Malin [1992] argue that the mid-Tertiary apparentdifferences across the surface trace of theGarlock fault extensional structures seen there are related to coeval structures in [Hearnand Clayton,1986b]. Midcrustalrefracted Pg waves, the Mojave Desert and western California. In their view, mid- however,show large delays across this fault, even after correction Tertiary extensionwas regional in extent and predated,by many for low near-surfacevelocities [Hearn and Clayton, 1986a]. millions of years, deformationsassociated with the onshoreSAF. Paleomagneticdata suggest that the Tehachapi Mountains have In the Tehachapi Mountains, these mid-Tertiary features were rotated between 40 ø and 60 ø clockwise since the early Tertiary rotatedand reactivatedin Neogenetime. [McWilliamsand Li, 1985]. Data from volcanicflows of early Numerousreverse faults with differingamounts of obliqueslip Mioceneage show that as muchas 40 ø of thisrotation took place cut the gneissand schistas well as the Cenozoic sedimentsof the
5Kmi • (• /.•..__.--•EasternWhite WolfFault CentralWhite Wolf y Fault(blind thtrust ) • • ,•,,.•.•Comanche Fault(blindthrust) • • VP352•h Mounain
Springs Fault
• VP590• • • Basin/Basement / / "i"ontact • Badger• • • / • •VP740
• • Telon • • BenoSprings Fault • • Emb•yment• • / / / ...aseaanton
• •_•X••• /• Tehachapi • o•o • / ( ' Mountains • • •TunisFault
Figure4. Faultmap of theTejon embayment and Tehachapi foothills showing the segment ofthe reflection survey discussedin this paper (modified from Goodman et al. [1989]).The faults shown include ones identified in shallowreflection data by Goodmanand Malin [ 1992]. In theTejon embayment, which lies to the northwest ofVP 740,the traces of buriedfaults are shown with open barbs (reverse) and balls (normal); exposed faults are solid. The tracesof basement/sedimentcontact and the Beno Springs Fault (BSF) intersect at approximatelyVP 740, as indicated.The WWF is segmentedinto exposed and buried traces near Comanche Point [Goodman and Malin, 1992].The section of profileshown on this map and in Figure6 avoidsthe extreme dips near the WWF (
Tejon embayment(Figure 4). These include the WWF, Pleito a two-dimensional transform). A simple rejection filter, thrust,and Springsfaults, plus the recentlyrecognized Comanche analogousto a frequency-wavenumberor pie-slicefilter, was then Point, Beno Springs,and E1 PasoCanyon faults [Goodmanand appliedto thesetransformed data [e.g., Yilmaz, 1987]. Malin, 1992].These structures, some of whichmay be reactivated A filtered shotgather from 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 presentconfigurations by the combinedeffects of betweenthe WWF and Rand schist. It also showsthat the signal- Neogeneclockwise rotation and laterregional transpression. The generatedartifact was only partially removedby the specialfilter, CALCRUST data show that severalof thesefaults played major perhapsdue to the artifact'slong durationand aliasingin time. rolesin the present-dayexposure of the deep-seatedcrustal rocks. This residual noise, plus out-of-plane reflections and reflected refractions[e.g., Day and Edwards, 1983], createdproblems in Seismic Reflection Data: the subsequentprocessing. In part, becausethese signalshave Acquisition and Processing apparent moveouts similar to reflections in high-velocity basementrocks, many standardsignal-enhancement procedures The TehachapiMountains common midpoint (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] (seealso Figure 7b). In the equipmentused included a GEOSOURCE MDS-16 400-channel, end the combinationof theseproblems resulted in eliminationof 16-bitrecorder, and six 2xl 05 NT Littonvibrators with full 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 sectionresulted from (1) consisted of non uniformly consolidated alluvium and soil. deconvolutionof reverberationsand multiplesalong with source- Along the mountainousportion of the profile, accesswas along pulse time compressionand (2) frequency-wavenumber(F-K) crookedroads in steep-sidedcanyons with highly variableground filtering of Rayleigh waves,s-waves, out-of-plane reflections, and surfaces, all producing undesirable types of seismic signals. reflected refractions. Table 1 summarizesthe nominal acquisitionparameters that were After these steps,and muting of the first-break waveforms,a chosenafter field testingof theseconditions. suite of constant velocity stacks were generated every few Unfortunately, the tests revealed strong, site- and frequency- kilometersalong the profile. These stackshelped image 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-planeevents and 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 acquisition parameters, dipping features(Table 2). Both migration and depthconversion although numerous attempts were made. Thus even the tests were done on the final stack, and prestackmigration was uncorrelatedfield recordsof this surveybecame the subjectof an considered for critical features such as the faults and dipping independentsignal-processing investigation [Okaya et al., 1990, zones. While helping somewhat with interpretation, the 1992]. Fortunately,the time evolutionof this noise (originating migrations resulted in sectionswith lower contrastand greater possibly in 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 Interpretation of the Upper Crust and Moho frequency-timerepresentation. In this domain,the vibrator sweep and artifactsappear as linear functionswith differentslopes, like Vibration point VP 661 is located immediately southeastot direct and refracted waves in travel time plots. The second the Springsfault basementhigh and where the basementsurface transformoperated on both the frequencyand time variables(i.e., dips to the south (Figures 4 and 5) [Goodman et al., 1989;
Table 1. Data AcquisitionParameters for the TehachapiMountains Reflection Survey
Parameter Nominal Values Notes
Number of live groups 240-400 (floating) numberdetermined by cable roll-along rate Roll-along configuration split spread 40 trailing, 4 gap, 200-360 leading Station interval 33 m Geophonesper group 12 8-Hz geophones Geophone array in-line, 33 m long Numberof vibrators 6 at 2 x 105NT amplitude and phase force control Pad separation 13 m Vib point intervals 66-132 m set by signal-to-noiseratio and survey costs Vibrator array in-line stacked no move up due to noiseon pad pickup CMP fold 25-100 Sweeps per station 8-12 set by signal-to-noiseratio and survey costs Sweepfrequencies 8-32 Hz upsweep, set to reduce 18 to 26 hz harmonics Sweep length 32 s Taper 2 s Total record length 44 s 12-s full-bandwidth listen, correlated to 14 s. Except for geophoneresponse and channellimits, valueslisted were establishedby field testing. 2076 MALIN ET AL.: REFLECTIONS BENEATH TILTED CRUST
Table 2. Signal-processingSteps for the TehachapiMountains Seismic Reflection Survey
Step Comments 1. Hand-edit field data and set up field statics supplyfield geometryand correctionvelocities 2. Artifact removal for details see Okaya et al. (1990, 1992) 3. Despike-debursttraces 4. Time-frequency-amplitudeanalysis in preparationfor band-passfiltering 5. Global shot gather amplitudebalance 6. Time-exponentialtrace amplitudes 7. Correct sphericaldivergence 8. Band-passfiltering 9. Trace autocorrelationstudy in preparationfor deconvolution 10. Deconvolution 11. F-K filtering groundroll, out-of-planeand other unwantedsignals 12. Muting refractionsand sourcenoise; sort 13. Velocity analysisand normal moveouttests checksfor out of plane eventsand good reflectors 14. Mute and band pass 15. Surface-consistent residual statics testsa done using pilot traces 16. Stack tests and final stack final stackingvelocities selected from suite of CV stacks 17. Time-distancedip filtering suppressionof residualunwanted signals 18. Global CMP section amplitude balance 18. Migrationtests a poststackand prestacktests 18. Time to depth sectionconversion 19. Final display tests including coherencyfiltering, global gain, and thresholdtests
For referenceto the standardsignal processing steps listed here, see Yilmaz [1987]. Editing and artifact removal completedwith UCB DISCO-adaptedmodules. Postartifact removal processing with USC MERLIN-adaptedmodules. a. Tests fail to show clearly improvedstacked section due to residualartifacts.
Goodman, 1992]. The crystalline-sedimentcontact can be that the Springsfault, which outcropsnear VP 590 and is labeled identified in the VP 661 shot gatherat roughly 1 s, at the baseof S in Figure 6b, is part of a more complex set of structures the zone labeled b. Progressingdownward into the basement, associated with local transcurrent tectonics. Beneath VP 590 and there exists a zone of relatively flat subbasinreflectors, easily fartherto the southeast,the basinand subbasinreflectors appear visible below 2 s, and given the label ud. These reflectorsare to be truncatedby the transcurrentstructures, as is alsoseen in the alsoseen on industryseismic profiles discussed by Goodmanand CALCRUST profile. Near VP 661, both drill hole and shallow Malin [ 1992] andthe depthsections in Figure6. seismic data show buried thrust faults cutting much of the Beneath the subbasinreflectors is the first of two major northwest Tertiary section(these are indicatedby the letter P in Figure6b). dippinghorizons. The upperhorizon is labeledas rs in Figures5 Thesefaults are on strike with the currentlyactive Pleito thrust and 6. Near the top of this unit are phasesthat can be correlated and appearto be its northeastwardextension (Figure 4). over many tens of traces. Allowing for one or two waveform The bottomof the sedimentarybasin is mostprominent on the cycle skips, these phasescan be traced acrossthe entire shot northwestend of the seismicsection, and extendsto a depthof 2 gather(13.2 km of shotgather = 6.7 km of reflectionpoints). The km there. Below this, to a depth of at least 7 km, are the ud secondand deeperof thesetwo horizons,labeled mM in Figure 5, subbasinreflectors. Both the b and ud zones rise to the south, beginsjust above8 s. This secondhorizon is separatedfrom the wherethe sedimentarysection of the Tejon embaymentlaps onto first by two other features:a zone of residualsignal-generated the crystallinerocks of the TehachapiMountains, at roughlyVP artifacts (labeled a in Figure 5), and a zone of flat-lying, more 740 (Figure4). The ud reflectorsmay be relatedto mid-Tertiary segmentedreflectors (labeled ld in Figures 5 and 6). Shot extensionalstructures (the "detachments"of Goodman and Malin gathersto the northand southof VP 661 showthat the topsof the [1992]). Basement-involving,high-angle, normal- and oblique- rs and mM horizons dip consistentlynorthwest, moving up slip faults observedon industrydata in the Tejon embayment through the crust to the southeast. The dips of the reflectors appearto sole into similar reflectors. Outcrop data along the below the mM horizonappear to decreasewith depthand grade northernexposures of the Tehachapigneisses suggest that they downward to weaker, flat-lying reflections at roughly 11 s may follow older, shallow dipping, metamorphic,and ductile (labeledmT in Figures5 and 6). On othershot gathers as well as shear fabrics. the one at VP 661, the changein dip is seenas a shift of the Two additional shallow structures have been identified on the reflectionhyperbola to shorteroffsets with increasingtime. In depthsection and have been confirmedin the field as faults. One the depth section,this producesa wedge-shapedzone with a is locatedjust southeastof the sediment/basementonlap near VP fanlike internalstructure. The Pn velocitiesacross this region 740. The otheris in the gneisscomplex between VP 840 andVP [Hearn and Clayton, 1986b], suggestthat the mT reflectorsare 860. In the depth section,these features are associatedwith the the currentMoho. Both the Pn data andreflection data show the southeastdipping zones labeled BS and EP, which extend to Moho to be flat to within 3 km. depthsof roughly 7 km but are difficult to seedue to the scaleof Field mappingdata plus industry seismic profiles and well logs this section. The bestimages of thesezones are in the constant were particularly valuable in interpreting the upper part of the velocitystacks shown in Figure 7. This part of the profile also CALCRUST reflection profile. For example, thesedata show appears to contain numerous out-of-plane reflections and MALIN ET AL: REFLECTIONS BENEATH TILTED CRUST 2o77
NW DISTANCE SE BSF). This fault is locatedalong a seriesof ridgesdelineated on Sta 661 aerial photographs. It intersects the basin/basementcontact 0 5.O I0.0 • I I obliquelynear VP 740. In thegneiss complex the fault zone con- sistsof a 5- to 20-m-thickmylonite zone strikingN50øE and dipping50 ø SE. It is overprintedby cataclasitezones that dip more steeplyto the southeast.The gneissesshow structuraland lithologiccontrasts across the fault,with generallylow-dipping tonalitic-dioritic gneissesin the hanging wall and refolded graniticorthogneiss and paragneiss in the footwall. Fromsurface geologyalone, a majoramount of reversedisplacement appear to have taken place acrossthe 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 moredifficult to recognizein outcrop. Scatteredexposures show a changein the attitudeof foliationsin the areaof the reflections.West of the profile,the EPCF may form the poorlyexposed contact between the Tejon Creektonalite gneiss and the ComanchePoint paragneiss units of Samsand Saleeby[ 1988].
Interpretation of Reflectorsrs, ld, and mM The noahwestdipping rs zone, the subparallelm M zone, and the less inclined ld zone that separatesthem are imaged most clearlyin the depthsection between VP 550 andVP 815 (Figure 6). Along this portion of the profile, the top of the rs zone is &::..-'•f..: ?.%q:: - __::: ...... between5 and 8 km deepand has an apparentdip of 15ø to 30ø. From VP 450 to VP 550 this reflective zone maybe disruptedby -':2:.-.:.;./.;'.,. :.;'f...... - --:•%iC::u. ,."'=.... :::,L _.•- -, :- e:.Z:.L7,...':,x:,',,,',:-, , '.: .~, the extension of the Pleito thrust, the Springs fault, and other TT1T interpretedblind faults,such as thoseinterpreted beneath VP 525 and VP 400. Farther to the northwest the rs zone is identified as the weak reflectors seen in the middle crust. _ ß . "L%.': ,,. :.-- :..... ' , "'• ='='. - I ! • South of VP 815, recognition of the rs zone in the depth o ioo 200 300 sectiondepends on (1) its signalpattern, which consists of a sharp TRACE No. top reflector with subparallelunderlying reflectorsand (2) our Figure 5. Prominentfeatures in the filtered shot gatherfrom interpretation of the BSF and EPCF as reverse faults based on vibrationpoint VP 661 of the TehachapiMountains reflection field mapping. The resultingcorrelation is shownin Figure 7b, profile, as discussedand 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 thrust up over its equivalentreflectors to the be judged by following the signals above and below the noah. Accordingly,from both field and subsurfaceevidence, the annotations. The annotated reflection zones are interpreted as: BSF would have a vertical separationof almost 4 km. Farther basin and basin bottom (b), upper detachments(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 assumptionssuggest that the rs zone comes to brittle/ductile transition zone (ld), latest Mesozoic-earliest within 1 km of the surface. Cenozoicductile flow fabricor underplatingor thrustzone (mM), Giving a geologicalidentity to the rs zone is a critical issuein TertiaryMoho (mT). interpretation of the reflection data. In this regard there are several important lines of evidence that suggestthat the rs zone representsthe Rand schistextending northward from its outcrop reflectedrefractions (labeled RR and OP in Figure 7b). Both immediately to the south. First, geometricprojection of the rs thesetypes of signalshave high-velocitymoveouts like to those reflectors places them near the surface at the schist outcrop. of the basementrocks. Judgingfrom local topography,geology, Second, Silver [1982] has demonstratedthe presenceof stacked, shotgathers, common midpoint gathers,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 stacksection these signals create an appearanceof Emigdio Range immediately west of the profile, outcroppatterns crosscuttinghorizons [e.g., Yilmaz, 1987]. Taking this and the of schist and gneiss also show a similar structuralconfiguration field observationsinto account,the southeastdipping bandsare north of the Garlock fault [Ross, 1985]. Further, it should be seento be a seriesof coherentreflections that apparentlytruncate 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 mapping and examinationof aerial photosin the area of and the geologicalmapping data [Silver, 1982] show the same VP 740 suggest that the dipping reflective band there is a characteristic reflectors and rocks in this type of relationship previously unrecognizedfault, which we have called the Beno (reflectors A, B, and G in Figure 2b, as discussedlater in our Springsfault, (indicatedby BS in Figures6 and 7 and abbreviated reconstructionof the crustal tilting). 2078 MALIN ET AL.' REFLECTIONS BENEATH TILTED CRUST
STATION No. NW 661 SE
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s;:[':.•'• •.-"•::;'.: ': • -"•...,•;- "-. :: *"'•: ....,"" -''....: .:' .';'-C.5 .;7;3: '.:'::•":. ':.:'•.,:,: -..' ::-:•c-: ' •. ':"'%:::.:,T • ',•'• .:-.'.::;'• ' ::,•xX'. ? ':..::'T•:': ..,.:.:.,'::•: ':-'; -::;: •:' :::.:.?v :.':L, ':: ':,? ;• '-7:-'-:5:C J•'•': .:'
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i I I i o 5 IO 15 DISTANCE (km) Figure 6a. This Figureshows the TehachapiMountains common mid-point (CMP) depthsection. Plot parameters were chosen to best display the subbasinreflections discussedhere (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 degraded the clearerMoho reflectionseen in Figure5. Horizontalexaggeration is approximately2.25 to 1.
Another line of evidence comes from the refraction data and It was concludedfrom thesetests that the depthand shapeof regionalgravity field (Figure 3; modifiedfrom Ambosand Malin the Moho couldnot vary far from that suggestedby the Pn and [1987], Goodman et al [1989], Plescia [1985], and J.B. Plescia reflectiondata. If true, thenit is possiblethat the topographyof (personalcommunication, 1992)). These data have been modeled the TehachapiMountains is gravitationallybalanced within the using the approachsuggested by Barton [1986]. His method crustalone. One way in whichthis balance can come about is by usesboth seismicand gravity observations,but takesinto account the presenceof a slightly lighter schistbody beneaththe denser the limits of crustal velocity-densityrelationships. First, the gneiss,in a fashionconsistent with our interpretationof rs zoneof approximategeometry and composition of thesedimentary basins reflectorsbeing the schist.Assuming the uppermost crustal struc- were entered into the model, using the refraction data, the ture suggestedby the reflectiondata, the refractionand gravity CALCRUST andindustry reflection data, and the densities given data can be fit, within the limits of their resolution, with the by Plescia [1985]. Next the depth and shapeof the Moho velocitiesand densitiesshown in Figure3 (seecaption for error suggestedby the Pn andreflection data were used as a starting limits). An importantfeature of the densitiesand layer thickness point for severalmodels of the gravity over the Tehachapi of this modelis that at a depthof 32 km, the massesof the crustal Mountains. In these calculations, the near-surfacedensities as- columns along the profile vary by less than +1% from their sumedin the TehachapiMountains were thosesuggested by average, equivalent to the uncertainties in the densities and Plescia [1985], while the range of subsurfacedensities was thicknesses.Thus, while not uniqueor definitive,the resulting allowedto vary within the velocity-densitylimits setby Barton gravity model fits the observedgravity within errors and is [1986]. consistentwith the othergeophysical and geological data. MALIN ET AL.' REFLECTIONS BENEATH TILTED CRUST 2079
STATION No. NW 661 SE
i 400I I I I 500I I ! I 600I I I ! I 700I 800 900
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,T:'2::•',"•..-, .'.•: • .:.: :%•-:-." •" .... :'•-.. _;<::;:' •'"-•.,•:':",'; .... . :"/"<':".%'?"'¾ :"';-..:.'?:;'.'' ',--• •?::t:,;.'.' ,•':;',;;:•:"•'•': •-•:•'-, ..... ':',;"t:':'L:•;;':•:;/L:.::;,'?"
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DISTANCE (km) Figure 6b. Prominentfeatures in the CMP depth section. The annotatedreflection zones used here are the sameas thoseinterpreted in Figure 5. Additionalinterpreted structures are Springsfault (S), Pleito thrust(P), Beno Springs fault (BS), E1 PasoCanyon fault (EP).
The main discrepancyin our refractionmodel is with previous is that while the rs zone is the schist,it may exist as a thin sliver, work by Malin et al. [1981], who profiled the velocity structure only 2 or 3 km thick, sittingon an entirelyunknown or unrelated of the schistat Sierra Pelona in the San Gabriel Mountains. They basement. Suchinterpretations could also satisfythe gravity and found the schistP velocity to be around5.9 km/s as comparedto refractiondata, particularlyif the rocksbeneath the gneissesare the 6.2-6.5 km/s values shown in Figure 3. This difference relatively light and fast. These possibilitiesare kept in mind in maybe due to pressure,the 5.9 km/s being determinedat a depth our uplift model. As a final note on rs, if our correlation is of roughly 1 km and the 6.2-6.5 km/s at roughly 10 km. In fact, if correct,the steeplydipping contacts on the northside of the schist the schist extends below Sierra Pelona to these depths, where outcrop may be south dipping faults similar to the BSF and Malin et al. [1981] found similar velocities,then no discrepancy EPCF. exists. The difference may also reflect anisotropicvelocities in In the depth sectionsin Figure 6, the ld zone seemsto mark a the schist. transitionfrom a morereflective but lesslaterally coherent upper Thus the evidenceat hand suggeststhat the rs zone is the Rand crust to a less reflective but more laterally coherentlower crust. schist, that its top is the Rand thrust, and that its internal Below the ld zone, at VP 740, is the top of the mM zone, at a reflections are a function of changes in structural fabric, depthof roughly22 km anddipping 25øN. While less clear than compositionallayering, and tectonicimbrication and duplication. in the shotgathers, the reflectorsbelow this horizonappear to Nonetheless,because the reflectionline failed to carry the rs zone flatten with depth to the less distinct Moho mT. The CMP to outcrop,there remainssome doubt: the rs zone could represent stackingprocess degraded the mT zoneso much that shot gathers internal structurewithin the SierraNevada batholith or an entirely were used to help identify it in Figure 6. To somedegree, the unknown horizon. In particular, if the rs zone is within the wedge of lower crustal reflectorsbounded by mM and mT batholith, it could be associatedwith some type of imbrication, resembles a fan with its hinge point to the northwest of the placinghigher level rocksunder the gneisses.Another possibility profile. The same kind of reflection structurewas also observed 2080 MALIN ET AL.' REFLECTIONS BENEATH TILTED CRUST
NW STATION No. SE 725 750 775 800 825 850 875
12
6 0 1 2 3 4 DISTANCE (KM) Figure 7a. Expanded-scaleconstant velocity (5.6 km/s) stackof the CMP datafrom the areasof the BenoSprings and E1 PasoCanyon faults (VP 720 to VP 885). The horizontalexaggeration is approximately2.25 to 1. This sectionallows for the presenceof high-velocitygneiss, schist, and the moderatelyto steeplydipping fault zone reflectionsimmediately below VP 740 and VP 850. The sectioncontains out-of-plane reflections from this fault that appearto crosssome of the moreflat-lying reflectors.The sectionalso contains residual reflected-refractions above the rs horizon. at the easternend of Mojave Desert line 3, beneathand just south Exposure of the Gneiss and Schist of the Rand Mountains, but with a different geographic For a plausible geological explanation of the Tehachapi orientation [Cheadle et al., 1986] (reflectorsI and M, as shownin Mountainsreflection profile and other data, we have assumedthe Figure 2). following statementsto be true: As statedearlier, the Pn data of Hearn and Clayton [1986b] showmT to be a horizontalMoho. As in manyother places, this Basic Relationships particularMoho is a complexzone with numerousdiscontinuous 1. The Tehachapigneisses correspond to southwarddeepening phasesthat can be seen in shot gather but do not stack into a exposureof the CretaceousSierra Nevada batholith[e.g., Ross, single coherenthorizon [e.g., Mereu et al., 1989]. A common 1985, 1989; Saleeby1990]. model for the zone of flat, superimposedlenses of discontinuous 2. The Rand schistextends northward beneath the Tehachapi reflectorsis differentiationand underplatingof the lower crustby gneisses[e.g., Sharry, 1981]. partial melting of the upper mantle [e.g. Hauser et al., 1987]. 3. The schist is present under large parts of the western Likewise, the ld horizon has numerous analogs in which a Mojave Desert [e.g., Lawson, 1989]. laminated lower crust is separatedfrom a heterogeneousupper 4. The Garlockfault is a deep-rootedcrustal boundary that is crust[e.g., Mereu et al., 1989]. The differencesabove and below moreor lessvertical [e.g., Hearn and Clayton,1986a, b]. this horizonmay be productsof brittle versusductile deformation. The deformations could be related to the current seismotectonic Basic Sequenceof Events regime in southernCalifornia and/orto the eventsthat rotatedthe 1. The protolith of the schist was emplaced and TehachapiMountains earlier in the Neogene. As in the caseof metamorphosedunder the gneissin the Late Cretaceous[e.g., the upper detachment horizon, ud, which can account for Silver and Nourse, 1986]. differences in the structural styles of the sedimentarysection 2. Substantialuplift and coolingof the schistand overlying versus underlying basement, interpretation of ld as a lower gneissestook place during the Late Cretaceousand continued decollementhelps to explain somedifferences between the upper into Paleocenetime [Pickettand Saleeby,1993]. and lower crust. 3. Following erosion of the gneissesand overlying crust, MALIN ET AL.: REFLECTIONS BENEATH TILTED CRUST 2081
NW STATION NO. SE 725 750 775 800 825 850 875 • • • I.-t.:-..RR•"-I• ...... • " , ,,,.,:,.. . ,.,
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...... ::...... ' ,..'••...,.... ,::';...:'... '.'.",':-.,:,,.t,;rs...... ,::','.::'-:•:F::':'."?:',,.':"": ...... ::' ,'.:':.:.-...,.•:::,.,,,•.,:..•-... "¾:.:z•.,::-'.,•::••"./;i:i•:":'!'E.;:•?q"?•:i:;::,:,•,;::!:',:...... :.... .•.:,.?.:,.::.,..-...... •'... :'.,'.','..4 .",::"t'.'.'.,?•:.':?',,. '..:...;.;,•.' ,...•. ::'::,:'':,:?:,,:,.:,':•::;:":'.':,:'.-,.- ..-,":"-' '.... -:':•. '..:..':' "' ....:...... :...''•"••.__ - ""' .•'.,:i't:;::...... :::,..:,,,.,':•.,,., ,. ..:....:...... ,.."..,•"•::: :,,::::..-8 _i!ß ?,':""i") ..... :L,,,'. "7 ...... : ,:..... ,:,•"""',"'-:,'.,.,.:,.•.....',:'""""""'""'•,. -'-. ?.,., '.' ..... '":',":::,'•:,:..., .....% ...... •:...!.i:,)..:'.'.... ' ,4.':: ':.::.-,...i•,,"""-'-:.•...,•.::.,.:-::.-..':::: .;?:i'.,•.., iv::11!:!:' :'.... ::!:i:' :.?.•.'::?:'•:::•:; :::?:'.::i::.:.:':': :•...... z.,:,-:.:' . :•.:.::,':'":: ...... :.!.::,,':':',,:: ...... -:'::..::.,"::':'.:!':"',':' .::':"":::..,-',,?i:::-:;:'",:: .....: :,:,:-.:",..:.,:':'..':
12
-i -r T , 0 1 2 3 4 DISTANCE (km) Figure 7b. Interpretationof this CMP sectionafter applicationof a coherencyfilter and plotting signalsabove a minimum amplitude threshold. The annotatedreflection zones used here are the same as those in Figure 6: reflectors within the Rand schist and whose top would be the Rand thrust (rs), Beno Springs fault (BS), E1 Paso Canyon fault (EP). Out of plane reflections(OP) crossthe subhorizontalrs reflectorsat high anglesand are related to the BSF and EPCF. Reflectedrefractions (RR) producethe herringbone pattern at the top of the section.
Eocene marine sediments were deposited onto the gneisses; beyonda reasonabledoubt, it is alsonecessary to considerthe sediment onlap was to the east in terms of present-day followingpossibilities to be true. coordinates[e.g., Goodman,1989; Goodmanand Malin, 1992]. 4. Formation of Oligocene/Miocene basins by extension/ Alternative Relationships 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 Tehachapigneisses or large partsof the Mojave [e.g., Ehlig, the modem San Andreas fault became active in central California 1968, 1981]. The crust beneath these regions consistsof [Goodman and Malin, 1992]. imbricated high level intrusive rocks of the Sierra Nevada 6. The TehachapiMountains were apparentlyrotated between Batholith or some other unit. 30ø and 45 ø clockwise during the Neogene,after substantial uplift 2. The Garlock fault is not a deep-rooted,vertical crustal of the schistand gneisses,and possiblyanother 30 ø earlier [e.g., boundary. Beginningwith our basicset of assumptionsand McWilliams and Li, 1985; Plescia and Calderone, 1986]. movingbackward in time, the followingreconstruction of the 7. The Rand schist in the Tehachapi mountains was first crustbelow the TehachapiMountains is proposed.Undoing the exposedin mid-Miocene, as indicatedby clastsin the Unnamed Neogeneleft-lateral slip of theGarlock fault brings the reflection conglomerate[Goodman, 1989]. structureof the TehachapiMountains adjacent to thatof the Rand 8. Left-lateral slip on the Garlock fault in the late Miocene Mountains (Figure 2) [Carter, 1987]. By their timing, the followed the rotations and separated the schist bodies now Neogenetectonic and depositionalevents in the SSJBseem re- exposed in the Tehachapi and Rand Mountains [e.g., Carter, lated to the rotations establishedby paleomagnetismfor the 1987]. generalMojave region [e.g., Grahamet al., 1990]. In the same 9. Regional contractionand local transpressionbeginning in vein, faults like those shown in Figure 4 and/or a midcrustal the late Pliocene further exhumedthe schist[e.g., Graham et al., decollementmay haveaccommodated the necessary upper crustal 1990]. The schistis now exposedat scatteredoutcrops along the extensionand shorteningas the rotationsprogressed [Goodman San Andreas, San Gabriel, and Garlock faults (Figure 2a) [e.g., and Malin, 1992; Jacksonand Molnar, 1990, Figure 3; Ingersoll, Ehlig, 1968]. 1988]. Owingto therotations and onset of SAF-relatedtectonics, Since basicrelationships 2, 3, and 4 have not been established north directed reverse faulting became active and stepped 2082 MALIN ET AL: REFLECTIONS BENEATH TILTED CRUST northwardtoward the Tejon embayment. The structurallyhighest field evidencefor normalfaults of thisage does not yet exist,but reverse faults locally exhumedthe Rand schistin their hanging they may not have been recognizedbecause of their reactivation walls in mid-Miocene time. The pre-Garlock Neogenerotations in the Neogene (e.g., Pastoriathrust, Figure 1; as discussed may have occurredabove the interpreteddecollement ld so that below). the deeper reflectors mM and m T are in their original In the secondscenario, which is more in keepingwith the orientations. If so, they may directly relate to the deep reflectors alternativerelationships listed above, the reflectors m M andI (or observed south of the Garlock fault and beneath the Rand possiblyF, as proposedby Cheadle et al. [1986]) represent Mountains. The most straightforwardcorrelation is to identify regionalthrust ramps that sole into the reflection Moho and along horizonsld, mM, andm T of this studywith COCORPhorizons F, which the gneissand schistwere carried to the surface. In such I, and M respectively(Figure 2b) [Cheadleet al., 1986]. fold-and-thrustbelt modelsfor exposureof the deepcrust, the Since the rotational history of the Rand Mountains is not overlyingcrust is usuallyconsidered to havebeen removed by known, direct correlation of upper crustal features acrossthe erosion. The currentreflection Moho, mT and M, would then Garlock are difficult. We suggest that the rs zone in the alsobe tectonicboundaries, since m M and I appearto soleinto Tehachapi Mountains and reflectors A, B, and G in the Rand them. Finally,if theGarlock is nota deepcrustal boundary, 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 thrustsystem and the gneissand schistbodies could be of only COCORP reflector A. Both rs and A horizons have been local significance(i.e., structurallyunrelated to any of the other interpreted as being cut by high-angle structures,which in the schistexposures shown in Figure2). A majorproblem with this Tehachapi Mountains profile have been establishedas reverse interpretationis thata thrustramp of thismagnitude should have faults. somesurface geological expression, (an obviousoutcrop or In any case,it appearsthat the Rand schistand overlyingrocks terrane boundary) none of which has been found to date. There in the Tehachapiand Rand Mountainswere uplifted togetherin a would also be a major puzzle as to what underliesthe schistand single event that took place near the Cretaceous/Paleocene allows for the degreeof observeduplift. Also missingis a boundary[Silver and Nourse, 1986; Jacobsonet al., 1988; Pickett sedimentaryrecord of theproper age and paleogeographic setting and Saleeby, 1993]. The questionsraised by this event include for the depositionof the erodedoverburden. (1) the mechanics of the uplift and (2) the removal of the overlying upper crust. If the schist is present under the batholithicbasement of the westernMojave thereis the additional A Crustal-ScaleTilting Model questionof (3) the removal of the original lower crustand mantle. Regionaltectonic relationships in southernCalifornia help Central to these issues are the nature of the lower crustal constrainthe crustal unloading and tilting model that we propose reflectorsthat lie beneaththese mountain ranges. for the exposureof deep crustalrocks in the Tehachapi Presently,interpretations of the deep reflectionstructure along Mountains. All the available evidence indicates that the Rand the Tehachapi Mountains and Mojave Desert profiles offer two and correlativeschists were thrustbeneath Tehachapi-style end-member scenarios for the unroofing of the schist and gneissesand intrusive rocks of the Cordilleran batholithic belt in overlying gneiss. The first scenariois our preferred working Late Cretaceoustime [Silverand Nourse, 1986: Hamilton, 1988; model for explaining the exposureof the deep-crustalrocks and Jacobsonet al., 1988]. The east-westcontinuity of theupper its related lower crustal evolution. This model consists of Late plategeology in southernCalifornia requires emplacement of the Cretaceouseastward underthrustingof the Rand schistwith re- ensimaticschist terrane by underthrustingfrom the west. This sulting crustalthickening, followed immediatelyby gravitational subcretionevent correspondsin time with the initiation of collapseand isostaticcrustal unloading [cf. Burchfiel, 1992]. The Laramide,rapid, low-angle subduction [Dickinson and Snyder, late Cretaceous/earlyTertiary unloadingevent would have been 1978;Bird, 1988;Hamilton, 1988]. In ourproposed crustal-scale accompaniedby brittle and ductile deformation,including low- tilting model, emplacementof the schistterrane initiated crustal angle faulting, large-magnitudelower crustalflow and, possibly, collapseby theaddition of a buoyantplate of varyingthickness to magmatic underplating (modified from Wernicke and Axen thelower crust. The observed low-angle ductile flattening fabrics [1988] and Block and Royden [1990]). The reflectionstructures withinschist exposures, conflicting kinematic indicators along the between mM and mT in the CALCRUST data (Figure 6) and I faults above the schist, and the evidence for latest and M in the COCORP data (Figure 2) are interpreted as Cretaceous/Paleocenelow-angle normal faulting suggest that the developing along the margins of thick sectionsof subcreted crustcollapsed shortly after the emplacement of theschist [e.g., schist, and hence along the margins of extendedcrust. These Haxel et al., 1985; Nourse and Silver, 1986; Hamilton, 1988; wedges of lower crustal reflectors are related to the crustal-scale Jacobsonet al., 1988;Nourse, 1989]. Tilting and doming of the tilting pattern observedin the Tehachapi and Rand mountains, deep-level,Tehachapi/Rand basement rocks occurredin this including intracrustal support of their topography. We propose setting.Given the disappearance of these rocks to thenorth [e.g., that the internal fan characterof the wedges is a result of either Ross,1985, 1989; Saleeby,1986; Dodge et al., 1986, 1988; ductile flow layering and/or progressivemagmatic underplating Saleeby,1990], they also appear to siton its northern edge. This of the tilting margin. In the caseof ductile flow, the reflection picture is also supportedby differencesin upper mantle fabric developed as material moved up and away from the geochemistry north and south of the Garlock fault margin, while in the case of underplating successivemagmatic [Mukhopadhyayet al., 1988; Montana et al., 1991;Leventhal et unitsparticipated in the tilting. In eithercase, the schistwas then al., 1992]. broughtto the surfaceat a muchlater time. In the uppercrust, Figure 8 summarizesin a seriesof schematiccross sections our extensionwas probably accommodated along stacks of low-angle modelfor subcretionof the schist,the extensionalunloading normal faults [cf. Wernicke and Burchfiel, 1982]. Conclusive event, the tilting of the Tehachapi Mountains, and Eocene A. (90-85 Ma) "Great Valley" forearcbasin sed,ments Cretaceousbatholith -o Felsic botholithic crust -12 --.- Metamorphic pendants -24 E
-36 (•:of the Tehachopi Mtns. sub-bat oh lithic-"'--,•.•• _----•-
- 48 -.-- Sub-continental mantle A pproximate intersection B. (85-80Ma) of SW-NE sections SW •, NE NW ,, SE ..,_,•=, ,-,,,<• ,-_-'•,,,,•; 0 , i/•/ ,'..-,,-',-,.,;,- ;• '..',•'•,;),':' • .-, 12 , ,,, ..... )' ;5."K•.-"/ •'I',: ' J '---/; "-",: ' ";;'/;'[" • ; •' u-,.•--'•'.'-%• '?-- .'0',5 ;z'. -•,:,:...•r' ', ' ; -,'"' /•-•; / :,; ,! ; ?; •,, ".' • "•.",: •2436 ';;.2, •:T (RandThrust) 48
hment) •:• ...... oflower mantle.'/• ?/?' Sub-oceanic'"••'"" '"' RSDI Subcretion/ •u•b•ofL•CIregional •(• I•Y•e• g•c t bathollthlcremoval Sub-continentol(/•: )' mantle schistlerrone Approximateintersection crust of NW-SE sections •' tear?
I C. (80-70Ma)
SW SE
12
• 24 •4•E
36
Continued schist subcretion
mega thrust) D. (70-60M0) SW • NE NW ...... :-.,•, .'.•••'•ß ...... ß':,..ß..'.v:',:':'.::'-:...... ,:..:.:..:.'v.::..:..::.,'::.:.,:.:..:i:..;.•':v.:..:.,::.. '""'"'''"' . ',':'-';"'-"
I•-callyrising'sub-oceanic mantle NW E. {PRESENTDAY) SE Tejon Tehochopi Mojave Emboyment Mtns •,GorlockF. Desert SYMBOLS; --- / ß 0 --&_ Major active EocenetoRecent strata thrust zone ^ Major inactive thrust detachment and/.__.•z • • '"'"" '.'-•""...... '"'.... •..... "•••-"'• '••'•'••"•••••••-'-•.'.':'-':'-".• ß Second order Neogene ..:.-•'.•':...... 24 active thrust orbrittle- ductile ,ransition ,.:.::::::¾::.':F?:?:'...... •::•i.::;•_•.'.:'.h..:i.i•.'• '•'••••••••:.•:•'-"'"'•"'••'''"'' ' fault Moh o ::'::':':":'"'"":"? ' ...... "•'•' •' "' .... '"' -'"'•-' '-' '•• '•" -•"•"'•••••'•••• 36 ,• Second order l_'owser rctau I f•n trsucture J inactive thrust fault 0 25 50 75 IOOkm inheritedfrom extensional 48 • © Motion inond out t i , ...-..! i .J collapse deformationof D. of cross section Figure 8. This Figureshows a schematicsummary of the subcretionand extensionalcollapse model for the restoredTehachapi and Rand Mountains regions. (a) Generalizedstarting conditions of thebatholithic crust with theapproximate location of thedeep-level Tehachapi rocks indicated. (b)-(d) Orthogonal views during the crustal evolutionthat hasexposed the deep-crustalmetamorphic core of the Tehachapiand Rand Mountains. (e) The currentconfiguration of the crust. The intersectionsof viewsare as indicated in Figure9. Extensionalcollapse structureswere favorablyoriented for reactivationas reversefaults during either Tertiary crustalrotations or Plioceneto Recentcontraction. (For additionalevidence for NW-SE differencesshown in the crustsee Ross [ 1985, 1989],Saleeby et al. [1987],Dodge et al. [1986, 1988],and Saleeby [1990].) 2084 MALIN ET AL.: REFLECTIONS BENEATH TILTED CRUST
exposureof their metamorphiccore. The modelbegins 90 to 85 Implications for Regional Reconstruction Ma with a Sierra Nevadan batholith that was flanked to the west by a forearc basin. The thickness of the batholithic crustal is As type localities for gneissic and schistoserocks, the assumedto be -50 km, with an increasein maficrocks with depth Tehachapiand Rand Mountains present a uniqueopportunity for [e.g.,Saleeby, 1990]. The increasein maficrocks is indicatedby reconstructingthe mannerin which suchrocks might be uplifted the presenceof maficlower crustal xenoliths in theupper parts of from the lower crust. South of the Tehachapi and Rand thebatholith [Dodge et al., 1986,1988] and deep exposures along Mountains, exposuresof deep plutonic rocks and coincident its tilted north-southaxis [Saleeby,1990]. The structureof the lower plate schist are found in southeasternCalifornia and batholithicbelt during this time interval is assumedto have been southwesternArizona (Figure 2). Structuralrelations and seismic two-dimensionalalong the presentSierra Nevada and Mojave datasuggest that largetracts of the schistunderlie the Mojave Desert. Desert [e.g., Silver, 1982; Cheadle et al., 1986; Lawson, 1989]. Figure 8 showstwo orthogonalcross sections of the initial In contrast,no schistexposures occur in the PeninsularRanges underthrustingof the Rand schist at 85 to 80 Ma [Jacobson, batholith,southwest of the currentSAF in the Mojave Desert.A 1990; Silver and Nourse, 1986]. A low-anglesegment of the subductingoceanic plate carriedthe ensimaticschist protolith beneaththe batholithicbelt at lower crustallevels. The deepest Sierra Nevada Western metamorphic batholith and most mafic layers of the Sierran batholith were thus rocks displacementdown dip along the subductionzone. Regional Eastern zone magmaticactivity may haveended in this initial underthrusting rocks phase. Likewise, the beginning of broad uplift with stern zone compressionalfaulting might havedestroyed the adjacentforearc rocks basin. To accountfor the northwarddisappearance of the schist, zonationof the batholith,and geochemicaland geophysicaldif- ferences, the NW-SE cross sections show the northern terminus of the subcretedschist terrane at the latitudeof the Tehachapi Mountainsand Tejon embayment. Bakersfield FromLate Cretaceous (80 to 70 Ma) to latestCretaceous/early Pastario pproximate traces Paleocene(70 to 60 Ma), we showthe schistprotolith as a water- "thrust" of Fig.8 B-D sections rich, accretionarybody beneaththe upperplate batholithic crust. Under theseconditions, the subcretedrocks were highlyductile; Monterey• Exposures of the fluidsthat escapedupward from themweakened the quartz- Schist of regional lower rich upperplate and promoted retrograde metamorphic reactions Sierra de plate schist in it [e.g.,Wernicke, 1990]. The tectonicallythickened crust then Salinas terrane collapsedunder its own burden.This collapsewas facilitated by horizontalflow withinthe newly formed schist and fragmentation of the upper plate into shallow dipping slabs boundedby Restored ductile/brittleshear zones. We proposethat the integrated Salinia displacementof theseslabs was to the west(modified from Silver [1983] andMay [1989]). Duringthis westwardtransport, the Transverse crystalline rocks of the Tehachapi Mountains may have Ranges undergoneup to 30ø of clockwiserotation [Burchfieland Davis, 1981;McWilliams and Li, 1985;Plescia and Calderone,1986]. Duringthe collapsephase, magmatic underplating may have occurred,at least locally. Igneous-textured,lower crustalmafic PeninsularRanges xenolithswith mid-oceanridge basalt (MORB) isotopic signature i i . .. i andlatest Cretaceous/early Paleocene ages are present in eastern 0 100 km rocks Mojave volcanicflows [Montana et al., 1991;Leventhal et al., Westernmemmorpn•c 1992]. This is consistent with subcreted oceanic mantle supplyingthe melts. Figure8 showsthe underplatingas an Figure9. A diagrammaticmap restoration showing pre-Neogene importantfeature during the late phasesof extensionalcollapse, distributionof upper plate sialic crystalline rocks and lower plate subsequentto the underthrustingof the oceanicplate. schist exposures relative to the Sierra Nevada and Peninsular In summary,we proposethat crustal-scaletilting of the Rangesbatholiths (modified from Burchfiel and Davis [1981]), TehachapiMountains is relatedto the removalof the deepest as comparedto the present-dayschist distributionshown in levels of the Sierranbatholith by subcretionand imbricationof Figure2. Saliniais suggestedto havebeen dispersed westward the schistand subsequentextensional collapse and magmatic froma subcretion-extensionzone[after Silver, 1983; May, 1989; underplating.The regionalsetting suggests that the tilting took Silverand Nourse,1986] with the lower plate schistterrane placealong the northern boundary of thisprocess. Thus the m M possiblyrepresented by the schistof Sierrade Salinas[Ross, to mT wedge of reflectorsmay be analogousto the ductile flow 1976]. The transversepetrochemical zonation pattern of and shearlayering commonlydeveloped around the edgeof a batholithicbelt is that of Silver et al. [1979], Mattinson and ductile baudin, in this case, a crustal-scalebaudin of subcreted James[ 1985], Silver and Mattinson [ 1986], and Saleeby [ 1990]. schist. Progressive,late stage underplating, which should The marginsof the Mesozoicschist bodies later servedto localize accompanythe extensionalcollapse, may also contributeto the major Neogenestructures that dismemberedand exposed reflectioncharacter of the wedge. relativelysmall portions of the schistitself. MALIN ET AL.: REFLECTIONS BENEATH TILTED CRUST 2085 reconstructionof upper plate, sialic crystalline rocks and lower References plateschists before slip on the SAF and Garlock fault places the Ambos, E. L., and P. E. Malin, Combined seismic reflection/refraction Saliniancrystalline terrane west of ourpostulated extensional investigationin the TehachapiMountains, southern California: Results collapsezone (Figure 9; modifiedfrom Burchfiel and Davis from the 1986 CALCRUST experiment,Eas Trans AGU, 68 (44), [1981]). SinceSalinia appears on its north end to bepart of the 1360, 1987. SierraNevada and on its southend part of the PeninsularRanges Barazangi,M., and B. L. Isacks,Spatial distribution of earthquakesand [Silverand Mattinson, 1986], our preferred model suggests that it subductionof the Nazca plate beneathSouth America, Geology,4, wasbrought westward from them by the extension following the 686-692, 1976. subcretionof the schist(modified from Silver [1983], May Barton,P. J., The relationshipbetween seismic velocity and densityin the [ 1989],and Silver and Nourse [ 1986]). continentalcrust- A useful constraint?,Geaphys. J. R. Astron. Sac., In supportof this idea,we notethat afterunslipping the 87, 195-208, 1986. Bird, P., Formation of the Rocky Mountains, westernUnited States:A Neogenefaults, the widthof the Cretaceousbatholith in the continuumcomputer model, Science, 239, 1501-1507, 1988. Mojaveis at leasttwice that of thesouthernmost Sierra Nevada Block, L., andL. H. Royden,Core complexgeometries and regional scale andthe northernmostPeninsular Ranges. This expandedsegment flow in the lower crust, Tectonics, 9, 557-567, 1990. is coincidentwith the lowerplate schist exposures (Figure 9). Burchfiel, B. C., Extension contemporaneouswith shorteningwithin Rocksthat are typical of theeastern zones of thebatholithic belt mountain belts, Geol. Sac. Aust. Abstr., 32, 6-8, 1992. appearin thewest along the expanded segment [Mattinson and Burchfiel, B.C., and G. A. Davis, Mojave Desert and environs,in The James,1985; Silver and Mattinson,1986]. An exampleof this GeotectonicDevelopment of California, Rubey Volume 1, editedby extensionmay be preserved in theTehachapi Mountains. Here, W. G. Ernst, pp. 217-252, Prentice-Hall, EnglewoodCliffs, N.J., younger,eastern zone, high-level granitic rocks lie aboveolder, 1981. westernzone, deep-level tonalitic and dioritic gneisses along the Buwalda, J.P., Geology of the TehachapiMountains, California, Bull. Calif. Div. Mines, 170, 100 pp., 1954. Pastoriafault [Crowell, 1974; Ross, 1989]. The geometryof the 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. dramaticupper plate/lower plate contrasts across it areconsistent Paleontol.and Mineral., Pac. Sect., Bakersfield,Calif., 1987. withlarge normal separation, resulting in theupper-plate rocks Cheadle, M. J., B. L. Czuchra, T. Byrne, C. J. Ando, J. E. Oliver, L. D. movingwestward in theearly Tertiary (Figures 1 and9). Brown, S. Kaufman, P. E. Malin, and R. A. Phinney,The deep crustal Moreover,the metamorphicwall rocksof the batholithicbelt structureof the Mojave Desert, California, from COCORP seismic that shouldnow lie on the westernmargin of the expandedbelt reflection data, Tectonics, 5, 293-320, 1986. appearto bemissing (Figure 9), sois thecorresponding forearc Crowell, J. C., Origin of late Cenozoic sedimentarybasins in California, basin. Destructionof the metamorphicbelt andthe forearcbasin Spec.Publ. Sac. Ecan. Paleantal. Mineral., 22, 190-204, 1974. Crowell, J. C., The San Andreas fault systemthrough time, J. Geol. Sac. alongthis segment of thebatholithic belt may have occurred as a London, 136, 293-302, 1979. resultof underthrustingof the schistprotolith and westward Crowell, J. C., Late Cenozoic basins of onshore southern California: denudationduring postsubcretion extension. Complexity is the hallmark of their tectonic history, in Cenozoic In our model,the expandedsegment of the SierraNevada Developmentof Coastal California, Rubey Volume VI, edited by R. batholithicbelt corresponds to the region of schistsubcretion and V. Ingersoll and W. G. Ernst, pp. 207-241, Prentice-Hall,Englewood extensionalcollapse and is a productof thatprocess. We suggest Cliffs, N.J., 1987. thatthe rapidlysubducting oceanic plate was segmented into Davis, G. A., and B.C. Burchfiel, Garlock fault: An intra-continental fault-boundeddomains of different dips, similar to the modern transform structure, southern California, Geol. Sac. Am. Bull., 84, Nazcaplate [Barazangi and Isacks,1976]. In thismodel, the 1407-1422, 1973. schist-subcretionzone in the lower crustcorresponds to a shal- Day, G.A. and J.W.F. Edwards, Reflected refractedevents on seismic sections,First Break, September1983, 14-17, 1983. low-dippingsegment of thesubducting plate. Thecrust of the Dickinson, W. R., and W. Snyder, Plate tectonics of the Laramide modernTehachapi and Rand mountains was apparently situated orogeny,Mere. Geol. Sac.Am., 151, 355-366, 1978. nearthe northern edge of thislow-dipping plate segment. As a Dodge, F. C. W., K. C. Calk, and R. W. Kistler, Lower crustal xenoliths, resultof schistemplacement and extensional collapse, this region Chinese Peak lava flow, central Sierra Nevada, J. Petrol., 27, 1277- wasstrongly tilted, exposing the roots of theSierras and later the 1304, 1986. lowerplateschist itself. Dodge, F. C. W., J.P. Lockwood, and L. C. Calk, Fragmentsof the mantle and crust from beneath the Sierra Nevada batholith: Xenoliths Acknowledgments.This work was supported under NSF grants EAR83- in a volcanic pipe near Big Creek, California, Geol. Sac. Am. Bull., 19254 to CALCRUST, EAR87-08266and 89-04063to J. Saleeby,and 100, 938-947, 1988. EAR91-19263 and 91-19263 to P. Malin. J. Plesciaof the Jet Propulsion Dokka, R. K., The Mojave extensional belt of southern California, Laboratory,Pasadena, California, provided gravity data and several early Tectonics, 8, 363-390, 1989. modelsfrom which Figure 3 wasdeveloped. We aregrateful for his open Ehlig, P. L., Causesof distributionof Pelona,Rand, and Orocopiaschist andkind help. Theauthors have benefited from conversations with J. along the San Andreas and Garlock faults, in Proceedingsof the Crowell,T. McEvilly,S. Richard,and J. Sharryand many others. We Conferenceon GeologicalProblems of the San AndreasFault System, would like to thank ARCO for providingtheir "vibratorbuster" test edited by W. R. Dickinson and A. Grantz, Stanford Univ. Publ. equipmentto check the vibrators before the survey and the Tejon Ranch Geasci., 11, 294-305, 1968. Companyfor accessto theirproperty in theTehachapi Mountains. We Ehlig, P. L., Origin and tectonichistory of the basementterrane of the San thankE. Karageorgifor generatingthe filtered field dataat theEarth Gabriel Mountains, central Transverse Ranges, in The Geotectonic ScienceDivision, Lawrence Berkeley Laboratory. Comments on the Development of California, edited by W. G. Ernst, pp. 253-283, originalmanuscript byL. Serpa,T. Broecker,and an anonymous reviewer Prentice-Hall,Englewood Cliffs, N.J., 1981. aregratefully acknowledged. Further constructive reviews, resulting in Golombeck, M., and L. Brown, Clockwise rotation of the western thepresent manuscript, were provided by E. Hauser,G. Fuis,and T. Pratt. Mojave Desert,Geology, 16, 126-130, 1988. Threethoughtful and knowledgeable reviews by G. Fuiswere especially Goodman, E. D., The tectonics of transition along an evolving plate appreciated. margin -- Cenozoic evolution of the Southern San Joaquin Basin, 2086 MALIN ET AL.: REFLECTIONS BENEATH TILTED CRUST
California, Ph.D. thesis, 225 pp. and 3 plates, Univ. of Calif., Santa refraction profiling, paper presented at the Third International Barbara, 1989. Workshop and Symposium on Seismic Probing of Continentsand Goodman,E. D., and P. E. Malin, Commentson the geologyof the Tejon Their Margins, AustralianNational University, Canberra,Australia, embaymentfrom seismicreflection, boreholeand subsurfacedata, in 1988. Studiesof the Geologyof the San JoaquinBasin, Publ. 60, editedby Mattinson,J. M., andE. W. James,Salinian block U/Pb ageand isotopic S. A. Graham, pp. 89-108, Pacific Section, Society of Economic variations:Implications for origin and emplacementof the Salinian Paleontologistsand Mineralogists,B akersfield,Calif., 1988. terrane, in TectonostratigraphicTerranes of the Circum-Pacific Goodman,E. D., and P. E. Malin, Evolution of the southernSan Joaquin Region,Earth Sci. Ser., vol. l, editedby D. G. Howell, pp. 215-226, Basin and mid-Tertiary transitional tectonics, central California., Circum-PacificCouncil for Energyand Mineral Resources,Houston, Tectonics, 11, 478-498, 1992. Tex., 1985. Goodman, E. D., P. E. Malin, E. R. Ambos, and J. C. Crowell, The May, D. J., Late Cretaceousintra-arc thrustingin southernCalifornia, southern San Joaquin Valley as an example of Cenozoic basin Tectonics, 8, 1159-1173, 1989. evolutionin California, in The Origin and Evolutionof Sedimentary McWilliams, M., and Y. Li, Tectonicoroclinal bending of the southern Basins and their Energy and Mineral Resources,Geophys. Monogr. Sierra Nevada batholith,Science, 230, 172-175, 1985. Set., vol. 48, editedby R. A. Price, pp. 87-107, AGU, Washington,D. Mereu, R. F., S. Mueller, and D. M. Fountain(Eds.), Properties and C., 1989. Processesof Earth's Lower Crust, Geophys.Monogr. Ser., vol. 51, Graham, S. A., P. G. Decelles, A. R. Carroll, and E. D. Goodman, Middle AGU, Washington,D.C., 1989. 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