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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
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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
•
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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
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•
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TILTED
I
600
I
I
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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
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-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
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slipontheSAFandGarlock
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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
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with the lowerplateschistexposures
(Figure9).
Burchfiel, B. C., Extension contemporaneous
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Rocksthataretypicalof theeastern
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theexpanded
segment
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extension
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moving
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1 and9).
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Moreover,the metamorphic
wall rocksof thebatholithic
belt
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appear
tobemissing
(Figure
9), soisthecorresponding
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basin. Destruction
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asa
resultof underthrusting
of the schistprotolithand westward
denudation
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extension.
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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
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E. D. Goodman,
ExxonProduction
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Co.,Box2189, Houston,
TX 77252-2189.
T. L. Henyey,Y. G. Li, andD. A. Okaya,Department
of Geological
Sciences,
Universityof Southern
California,LosAngeles,CA 90089.
(email:henyey;ygli; okaya;@coda.usc.edu)
P. E. Malin,Department
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NC 27708-0235.
(email:[email protected])
J. B. Saleeby,Division of GeologicalAnd PlanetarySciences,
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(Received
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Mexico,in Mesozoic
Crystalline
Rocks:Peninsular
Ranges
Batholith acceptedAugust 11, 1994.)
