University of Nebraska - Lincoln
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USGS Staff -- Published Research
US Geological Survey
1-1-1989
THE STYLE OF LATE CENOZOIC
DEFORMATION AT THE EASTERN FRONT
OF THE CALIFORNIA COAST RANGES
Carl M. Wentworth
U.S. Geological Survey
Mark D. Zoback
Stanford University
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Wentworth, Carl M. and Zoback, Mark D., "THE STYLE OF LATE CENOZOIC DEFORMATION AT THE EASTERN FRONT
OF THE CALIFORNIA COAST RANGES" (1989). USGS Staff -- Published Research. Paper 471.
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TECTONICS,VOL. 8, NO. 2, PAGES237-246,
APRIL 1989
THE STYLE OF LATE CENOZOIC
DEFORMATION
AT
THE EASTERN
FRONT OF THE CALIFORNIA
COAST
RANGES
Carl M. Wentworth
U.S. GeologicalSurvey,Menlo Park,
California
Mark D. Zoback
Departmentof Geophysics,
Stanford
University,Stanford,California
Abstract. The 1983Coalingaearthquake
occurredat the
easternboundaryx)f theCaliforniaCoastRangesin response
to
northeast
directedthrusting.Suchmovements
overthepast
2 Ma haveproducedCoalingaanticlineby foldingabovethe
blind easterntip of the Coalingathrustzone. The 600-km
lengthof the CoastRangesboundarysharesa commonstructuralsettingthatinvolveswestwardupturnof Cenozoicand
Cretaceous
strataat theeasternfrontof theCoastRangesanda
major,southwest
facingstepin thebasementsurfacebeneath
thewesternGreatValley. Like Coalingaanticline,Pliocene
andQuaternaryfoldingandfaultingalongtherestof the
boundaryalsoresultfrom northeast-southwest
compression
actingnearlyperpendicular
to thestrikeof the SanAndreas
fault. We suggestthatmuchof thisdeformationis relatedto
activethrustsbeneaththe easternCoastRanges.The stepin
thebasementsurfacebeneaththe GreatValley seemsto have
controlledthe distributionof thisdeformationandthe shapeof
theCoastRangesboundary.
INTRODUCTION
The magnitude6.7 earthquake
thatoccurredbeneath
Coalingaanticlineon May 2, 1983, hasarousednew interest
in structurealongtheeastfrontof the CoastRangesin central
California(Figure1). Thisfeature,heretermedtheCoast
Rangesboundary,is a prominenttopographic
andstructural
boundary600 km longthatseparates
deformedrocksof the
CoastRangesfrom relativelyundeformed
rocksto the eastbeneaththeGreatValley.
The Coalingaearthquake
stimulateda varietyof geological,
geophysical,
andseismological
studies(SteinandKing [ 1984],
variouspaperscitedby RymerandEllsworth[ 1985, 1989],
andNamsonandDavis, [ 1988]). Of particularinterestis the
compressional
focalmechanism
determinedfor theCoalinga
main shock[Eatonet al., 1983;Eaton, 1989], whichrepresentsthrustor reversefaultingdirectedperpendicular
to the
CoastRangesboundary(Figures1-4). We haveexaminedthe
originof theearthquakein thecontextof a broaderstudyof the
CoastRangesboundarythat is foundedon a seriesof reflection
profilesacrosstheboundaryat differentlatitudes[Wentworthet
al., 1984, 1987;ZobackandWentworth,1986]. We present
herea tectonicinterpretation
of theCoalingaearthquake
thatis
foundedon seismicreflectionprofilesin theepicentral
region,
but that integratesmuchof thisotherwork as well.
We thenexaminethebroaderimplications
of ourCoalinga
interpretation.Similaritiesin structuralrelationsandcurrent
stressorientation
betweentheCoalingaareaandtherestof the
CoastRangesboundaryimply thatthe entireboundaryis undergoingactivethrustingandfolding. As discussed
by Page
[1981], Zobacket al. [1987], andMount andSuppe[1987],
theorientation
of thiscompression
andshortening
perpendicular to theSanAndreasfault is incompatiblewith deformation
in a wrenchtectonicsystem[MoodyandHill, 1956;Harding,
1976].
The two seismicreflectionprofilesusedin our interpretation
of deepstructure
in theCoalingaareacross
Coalingaanticlineobliquelyjust to the northand southof the
mainshockepicenter(Figure2). Theywerecollectedby
WesternGeophysicalCompany(6 s, 24 fold, VIBROSEIS)
andlaterrecorrelatedto 12 s andreprocessed
for the U.S. GeologicalSurvey. The mostusefulreflectionprofile (SJ-19),
which crosses the anticline 7 km southeast of the main shock
Papernumber88TC03969.
epicenter(Figure 2), containsexcellentdetail to about5 s
(Figure3). A secondprofile northof the epicenter(SJ-3),part
of whichis illustratedby NamsonandDavis [ 1988,Figure7],
containsrelativelyfew reflectionsbelow the baseof the
Cenozoicsection,but doeshelpcorroborate
continuityof
structurebeneaththe lengthof the anticline. A 1977
COCORP line (Consortiumfor ContinentalReflection
Profiling)that crossesthe southerntip of the anticline
(Figure2) is similarto profile SJ-19but lacksits clarityand
027 8-7407/89/88TC-03969510.00
detail beneath the anticline.
Copyright1989
by theAmericanGeophysicalUnion.
This article is a U.S. government work, and is not subject to copyright in the United States.
2 38
WentworthandZoback:EasternFront,CaliforniaCoastRange
12o 0
1220
1240
I
I
I
I
KLAMATH
MOUNTAINS
Area of figure 4
40 o
O O O Coast
Ranges
boundary
ß
1983 Coalingamainshock
380
San
Area
figure
•(•'•%•
360
0
50
I
I
Co•
100 km
Fig.1. Indexmapof central
California
showing
majorstrike-slip
faultsandtheCoast
Ranges
boundary
between
the
CoastRangesandtheGreatValley.
COAST RANGES
BOUNDARY
The CoastRangesboundary(Figure1) separates
uplifted
andstronglydeformedrocksof theCoastRangeson thewest
fromdepressed
andrelativelyundeformed
stratabeneath
the
GreatValley ontheeast. Theseunreformedstrataoverliewest
tiltedcrystallinebasementthat,in its exposedeasternpart,
consists
of plutonicandmetamorphic
rocksof thewestern
SierraNevada. The deepestexposedrockin theeasternCoast
Ranges,in contrast,consists
of theaccretedFranciscan
assemblage. Beneaththe westernGreatValley, 1-4 km of Upper
Cretaceoussandstones
and shalesof the GreatValley sequence
overliecrystallinebasement.At theCoastRangesboundary,
thesestratamm upwardandthickenabruptlywestwardto the
typically8-15km of Jurassic
andCretaceous
sectionthatare
exposed
alongtheeasternedgeof theCoastRanges.This
thick sectionis coevalwith the Franciscanassemblage,
which
it stmcturallyoverlies.
Wentworth et al. [1984] have usedseismicreflection and
refractionevidenceat Coalingaandelsewhereto proposethat
thebasementrelationsconcealedbeneaththeGreatValley sequencealongtheCoastRangesboundaryinvolvea tectonic
wedgeof Franciscan
rockthathasbeenthrusteastwardonto
thecontinentalmargin. Principalemplacement
of this
obducted
Franciscan
wedgewasprobablycompleted
by early
Tertiarytime,for theFranciscan
assemblage
wasthenunroofed
andprovidingsedimentto nearbybasins[Dickinson,1966;
Berkland,1973; Bartowet al., 1985] andverticalloadwasbeingappliedto thecrustwestof theSanJoaquinValley
[Rentschler
andBloch, 1988]. FoldingwithintheCoast
Rangesandat theireasternmargininvolvesstrataasyoungas
PlioceneandQuaternary,
however,andregionalupliftof the
WentworthandZoback: EasternFront,CaliforniaCoastRange
239
36o30 '
120o30 '
120o15 '
/
NEW
DIAPIR
36 o
of cross section,
Namson and Davis, 1988
15'
120 o
30'
\
36 o
00'
120 o
15'
I
5
I
10
13.0
I
KILOMETERS
2.5
EXPLANATION
I
Epicenterof 1983Coalingaearthquake
----------
su'atain kilometersabovesealevel
I
kilometer marks
3 •
Structure
contourof thetopof Cretaceous
i
Seismicreflection
profile,showing
2.5•
Structure
contourof thetopof the
EoceneKreyenhagen
Formationin
kilometers
below sea level
Fig. 2. StructuralcontourmapshowingCoalingaanticlineandnearbyfolds,linesof reflectionprofiles,andwell
controlalongSJ-19(lettereddots). ModifiedfromZiglet et al. [1986].
presentCoastRangesis judgedto be of similarage
[Christensen,1965; Page, 1981].
COALINGA
ANTICLINE
AND THRUSTS
The Coalingaearthquake
occurredat theCoastRanges
boundarybeneathCoalingaanticline,whichis a youngfold
thatplungessoutheastward
off thesouthflankof Joaquin
Ridgeanticline(Figure2) andinvolvesstrataasyoungasthe
Pleistocenepart of the TulareFormation[Banow, 1989]
(Figure3).
We distinguish
betweenJoaquinRidgeanticlineand
Coalingaanticline(Figure2) because
theyareseparate
folds
with different histories. Most workers have considered them to
be continuous[e.g.,Arnold andAnderson,1910;Namsonand
Davis, 1988], althoughit wasrecognized50 yearsagothat
Coalingaanticlineis a muchyoungerfold [ReedandHollister,
1936,p. 67]. JoaquinRidge anticlinestrikeswest-northwest
andhasmorethan4 km of structuralrelief,part of whichpredatesthe MioceneandPlioceneEtchegoinFormation[Dibblee,
1971; CaseyandDickinson,1976; Harding,1976;Wentworth
and Zoback, 1989]. Coalingaanticline,in contrast,strikes
northwestand hasa structuralrelief of only 1 1/2 km that has
developedlargelyin the past2 Ma (Figure3, Bartow [1989],
andWentworthandZoback[1989]).
The northwestterminationof Coalingaanticlineon the
2/40
WentworthandZoback: EasternFront,CaliforniaCoastRange
Coalinga Anticline
A
West
6 •
0
East
......
,
5
10
DISTANCE,
15
IN KILOME•RS
20
25
West
0
z
¸
1
m
z
2
<
4
East
A
B
•'<!•'
' -•--•'1
T ••
60 ,
C
,
i
i
'
•_
i
5
i
' __•-'-z-•
•_•
i
i
D
--•
i
ß
i
.....
•'•
i
10
i
E
N
•
i
•
•
•'
•--'
=•.
i ils I
••9•
t
I
DISTANCE,
IN KILOMETERS
ß
'
'
'c '
' ....... •'2' X ....
I
I
I
20
...
....
i
I
.....
...... .. -' '
...
iSouthwest
.......................
2.6
I
2•
•
4.6
s[_•x
I •
Cenozoic
strata
•
/_ '
M..... --------------
-
IFra
n an
or••••'•%• • Upper
part
of
Great
Valley
sequence
I GreatValley / :).rs/
.............
'-• _
•
•
2.7
4.5
cisc
I sequence
I
[
•
•
//•.....
/'//
Coalinga
thrustzo,,
I Franciscan
orGreat 58
'•
•
Lower
part
ofGreat
Valleysequence
?
'
10
•
•
I Valley
sequenc•
0
•
5.1/
Crystalline
basement
20
DISTANCE, IN KILOMETERS
30
40
Fig. 3. Structure
across
Coalingaanticline.(a) Migratedreflectionrecordof partof profileSJ-19. (b) Line
drawingof Figure3a. Stratigraphy
fromBartow[1988]. Markedhorizons:
C, stratigraphic
equivalent
of Friant
Pumicememberof TurlockLake Formation;J, top of S,'mJoaquinFormation;S, top of SantaMargaritaFormation,K, topof Kreyenhagen
Formation;M, topof MorenoFormationandapproximate
topof Cretaceous
strata.
Otherlettersexplainedin text. Wells: A, Standard302; B, RheemStandard28; C, Zwang2-14; D, Union(R.S.
Lyttle) USL68; E, UnionHelm, SumpfandSumpf,PleasantValley 8-14. (c) Crosssectionprojectedto a northeasttrendingline normalto Coalingaanticlineat its intersection
with profileSJ-19. Seismicvelocities,in kilometersper second,generalized
fromWentworthandZoback[1988]andWalter [ 1988]. Main shockfocalmechanism
of
Eaton et al. [1983] projectedand fitted to this crosssection.
Wentworth
andZoback:Eastern
Front,California
CoastRange
southflankof JoaquinRidgeanticlineandotheralignedstructural terminationsto the southleadusto identifya diffusefeature,the northnortheast
trendingPleasantValley crossstructure. We infer this featureto be a major tearfault in the subsurfaceacrosswhichtheprincipalDiablo Rangeuplift stepsto
the fight about30 km (Figures1 and 2). A few strike-slip
eventsin the Coalingaaftershocksequence
reinforcetheexistenceof thePleasantValley crossstructure:theseearthquakes
occurredalongthe crossstructureat depthsof 10 or more
kilometerswith theirright-lateralfocalplanesorientedparallel
to the crossstructure [Eaton, 1989].
Coalingaanticlineis clearlydefinedin thesubsurface
by
numerousoil wells [Zigler et al., 1986] and the seismic
reflectionprofiles(Figures2 and3). The crestandeastern
flank of theanticlineare separated
by a distinctflatteningof
the strata(Figure3, km 15) thatdividesthe fold intotwo distinctsteps(Figures2 and3). This fold configuration
extends
to a depthof severalkilometersthroughtheCenozoicsection
andanother3 km of Cretaceous
stratathatform theupperpart
of theGreatValley sequence
(Figure3). This lattersectionis
identifiedby correlationwith wells to the eastandoutcropto
the west.
The rocksthatunderliethisupperpartof theGreatValley
sequence
beneaththeanticlineare notpenetrated
by wells.
Refractionprofilingacrosstheanticline[Walter,1989]encountersbasementrock of highseismicvelocity(6.5 km/s) at
a depthof 15 km westof theanticline(Figure3c). This
basementsurfaceshallowsto theeastandis directlyoverlain
by theupperpartof theGreatValley sequence
eastofkm 35
on Figure3c. Beneaththeanticline,therockbetweentheupperpartof theGreatValleysequence
andthebasement
surface
is about10 km thick andhasa seismicvelocityof about
5.8 km/s; farther east,it is thinnerand has a velocity of about
5.1 km/s (Figure 3c).
Coalingaanticlineis rootedwithintheupperhalfof the 10kmothickintervalof 5.8-km/srock. The top of this interval
(horizonX) is foldedandfaultedaspartof theoverlying
Coalingaanticline,but theprominentreflectionslabeledT (at
about5 s,Figures3a and3b) do notparticipate
in thatfolding.
Theslightangulardiscordance
of thereflections
labeledZ relative to theunderlyingT reflectionssuggestthatT represents
a
faultzone. ReflectionsL andN splayupwardfromtheT zone
towardalignedterminations
of reflections
undertheeastlimbs
THE 1983 COALINGA EARTHQUAKE
The mainshockof theCoalingaearthquakesequence
occurredat a depthof about10 km beneathCoalingaanticline
[Eaton et al., 1983; Eaton, 1989], which placesit in the
Coalingathrustzone(Figure3c). Onefocalplaneof themain
shockmechanismstrikesN53øW, essentiallyparallelto the
anticlinalaxis,anddips23øto the southwest[Eaton,1989].
Thisfocalplanerepresents
northeast
directedthrusting
thatis
similarto, but slightlysteeperthan,thatof the Coalinga
thrustzone. Coseismicsurfacedeformationdocumented
by
Stein[ 1985] involvedgrowthof thecrestof the fold.
We thusconcludethattheCoalingamainshockresulted
from a northeastdirectedthrusteventin theCoalingathrust
zonethatsplayedupwardbeneaththecrestof thefold. The
hypocenter
appears
to be locatedin thecurvedjunctionbetweenthethrust(T) andreversesplay(Figures3b and3c), thus
accounting
for thedipof thethrustfocalplane.
Aftershocksof theCoalingamainshockdefineda complex
pattembeneathCoalingaanticlineandadjacentareas[Eaton,
1989;Eberhart-Phillips
andReasenberg,1989]. A numberof
aftershocks
occurredapproximately
alongtheCoalingathrust
zoneand theupdipreversesplaybeneaththefold crestthatinvolvedsubhorizontalcompression
similarto that of the main
shock. Othersimply thepresenceof thrustingfarthernorthwest. Althoughwe concludefrom the uplift patternthat main
shockruptureterminatedat thePleasantValley crossstructure,
numerousthrustaftershocks
nearthedepthof the Coalinga
thrust zone extended another 10 km to the northwest beneath
JoaquinRidgeanticline.
We havetestedthe ability of the Coalingathrustandreversesplayto producetheobserved
surfaceupliftusingdislocationmodeling[WentworthandZoback,1989]. The thrust
andreversefaultswererepresented
by gentlyandsteeplydippingplanesstrikingparallelto theanticlineandjoiningnear
the mainshockhypocenter.A fault lengthof 16 km wasdefinedbetweenthePleasantValley crossstructureon the
northwestandthe southeast
endof uplift andaftershocks.We
find a reasonable
fit to theobservedsurfaceuplift for thrustand
reverseplaneswith widths(dip extent)of 4 and7 km anddip
slipsof 2 and 1.2 m, respectively.This dislocationmodelis
similarto thatof Stein[1985,Figure3d] andyieldsan aggregateseismicmomentfor the bilateralruptureof about
of the crest and east flank of the fold.
8.6 x 102sdyn.cm. Thisresultis in fair agreement
with
We interprettheserelationstogetherto indicateeastward
thrusting
alongtheT zonethatsplayedupwardalongreverse
faultsandthendispersed
by foldingto formCoalingaanticline.
Similarbut lessdistinctrelationsarepresentin profileSJ-3,
thoseof Hartzell and Heaton [1983] and Kanamori [1983]
determined from seismic radiation
REGIONAL
SETTING
OF COALINGA
STRUCTURES
which reinforces the conclusion that the faults and the anticline
arecausallyrelated.The thrusts,whichwe nametheCoalinga
thrustzone, thusinvolve northeastdirectedmovementperpendicular to the fold axis.
Thisinterpretation
contrasts
with thefault-bend-fold
interpretation
of NamsonandDavis[1988],whichwasfounded
on
surface
geologyanddrillholesacross
Joaquin
Ridgeanticline
(Figure2) andinterpretation
of reflectionprofileSJ-3[Namson
andDavis, 1988,Figure7]. Our ownstudyof profileSJ-3
finds relations in the critical area beneath the east limb of
Coalingaanticlineto be ambiguous.The clarityof relations
on profileSJ-19(Figure3) is morethansufficient,however,
to denythe possibilityof downwardterminationof stratain
theeastlimb of Coalingaanticlineagainstthefootwallflat
portionof a rampflat thrust[NamsonandDavis, 1988,Figures6 and7]. Our interpretation
of profileSJ-19 alsodiffers
fromthatof Fieldinget al. [1984],whichwasfoundedon the
nearbyCOCORP profile.
CoalingaanticlineandtheunderlyingCoalingathrustsoccurat the CoastRangesboundarywherethetop of deepcrystallinebasementbeginsto shalloweastward(Figure3c). The
basementsurfacerisesat nearly20ø froma depthof about
15 km west of the anticline to about 6 km eastof it (km 35,
Figure3c), wherethe surfaceabruptlyflattensto a more
gradualeastwardrisebeneaththe SanJoaquinValley (Figures
3c and4). This stepin thebasementsurfaceis definedprincipally by the seismicrefractionmodelof Walter [1989], but is
corroborated
by magneticandgravityinterpretation
[Griscom
andJachens,1989]andby coincidence
of therefractionbasementsurfacewith a strongreflectionat 3.8 s (5.6 km) on profile SJ-19at km 43 (Figures2 and 3).
The identityof the thick intervalof rock betweencrystallinebasementandthe baseof the upperpartof the Great
Valley sequence
is difficultto determine.Wells in theSan
JoaquinValley 20 km eastof theprofileencounterabout
2/42
Wentworth
andZoback:
Eastern
Front,California
Coast
Range
1200
122 o
1240
i
I
I
EXPLANATION
400
MAP UNITS
FQ-•-• Quaternary
(and
Phocene)
deposits
•-•
Tertiary
sedimentary
rocks
•
Cenozoic
volcanic
rocks
ø•k• Mesozoic
Great
Valley
sequence
•
•'"•'•
Mesozoic
Franciscan
assemblage
•
Maflc
and
ultramafic
rocks
SYMBOLS
Contact
D
--
Fault-- Dotted where concealed; Arrows
indicate direction of relative movement;
U on upthrownside,D on downthrownside
380
Thrustfault-- Sawteethon upperplate
SYMBOLS (continued)
oo,,o,
.........
SanJoaquinfault(Herd, 1979)
$
Structure contours on basement surface in kilometers
belowsealevel; compiledfromunpublished
well file
Line of maximumhorizontalgravitygradient(R.C. Jachens,
written communication,1986); inferred,from relationseast
of Coalinga(GriscomandJachens,1988), to markthe hne
of westwardsteepening
of thebasement
surface
360
Pointof westwardsteepening
of shallowsedimentary
section
Foldaxis;only anticlinesshownin GreatValley
Compression
axisinferredfromwellborebreakouts
•,
0
50
I.
I
100km
I
Compression
axisinferredfrom earthquakefocalmechanisms
I
I
I
I
Fig. 4. Compression
directions
andfoldaxesassociated
withtheCoastRangesboundary
in thecontextof
generalized
geologyin theCoastRanges
andthebasement
surface
beneath
theGreatValley. Geologymodifiedfrom
Jennings[ 1977] principallyusingthe worksby M. C. Blake,Jr. (writtencommunication,1987), Harwoodand
Helley[1987],Pampeyan
et al. [1981],WentworthandZoback[1988],andZigletet al. [1986]. Compression
directions
generalized
fromcompilation
of Zobacket al. [1987],MountandSuppe[1987],andWonget al. [1988].
Seetextfor sources
of pointsof westward
steepening
of shallowsectionin theGreatValley.
1 km of GreatValley sequence
thatunconformably
overlies
dipping
reflections
(notillustrated
inFigure3) andhasa re-
crystallinebasement,whereasin the mountainsto the west
about8 km of GreatValley sequence
areexposedthatstmcrurallyoverlietheFranciscan
assemblage.
fractionvelocity(5.0-5.2 km/s) thatis only slightlyhigher
thanthe4.8 km/s observedin GreatValley sequence
at shallowerdepthelsewhere[WalterandMooney,1982]. Belowand
westof the anticline,in contrast,the seismicvelocityin this
intervalis higherandhasvalues(5.7-6.1km/s)thataretypical
of Franciscan
rock elsewherein theDiabloRange[Stewartand
Rock east of the anticline in the interval between basement
andtheupperpartof theGreatValleysequence
isprobably
lowerGreatValley sequence,
asit yieldspoorlydefined,west
WentworthandZoback:Eastern
Front,CaliforniaCoastRange
Peselnick,1978;Walter andMooney,1982]. It is thepresenceof sucha bodyof relativelyhigh-velocityrockseparating
GreatValley sequence
abovefroman essentiallycontinuous
basementsurfacebelow, both hereandelsewhere,that led to
theproposalof a tectonicwedgeof Franciscan
assemblage
at
the CoastRangesboundary[Wentworthet al., 1984].
The whole 10-km thicknessof 5.8-km/s rock beneath and
westof Coalingaanticlinecannotbe a tectonically
emplaced
wedgeof Franciscan
rock,however,because
emplacement
of
sucha thickness
of rockwouldhaveraisedtheoverlyingsection appreciablymorethanis nowevident(Figure3c). We
concludethattheintervalis half lowerGreatValley sequence
andhalfFranciscan
assemblage
[WentworthandZoback,
1989]. If the upperpart of theintervalis GreatValley sequenceandthelowerpartFranciscan
assemblage,
thenthe
continuous,
layeredreflectionsin theX-T interval(Figure3)
arewell accounted
for, althoughtheseismicvelocityis
anomalously
highfor GreatValley sequence
at thatdepth. In
thiscasethe Coalingathrustsfollow the CoastRangethrustat
theroof of theFranciscan
wedge(Figure3c). Conversely,
if
the lowerhalf of thisintervalis GreatValley sequence
andthe
upperhalf Franciscan
assemblage,
thedeepburialof theGreat
Valley sequence
helpsaccountfor thehighseismicvelocity.
In thiscasethe Coalingathrustsrepresent
a continuation
of
eastwardthrustmovementalongthebaseof theFranciscan
wedge. Resolutionof this uncertaintyis not essentialto the
presentdiscussion.
Althoughuplift of the CoastRangeswestof PleasantValley begaaby earlyTertiarytime,asindicatedby angularunconformityat thebaseof the Cenozoicsection(km 1 on Figure3b), Coalingaanticlineis muchyounger.Thinningacross
thecrestof the fold beganonly with upperPliocenestrataof
theSanJoaquinFormation[Bartow,1989]andis prominentin
SJ-19 only abovethe top of that formation,which is about
2 Ma old [Sarna-Wojcicki
et al., 1985]. Continuedfolding
involvedhorizonC, whichis thestratigraphic
equivalentof
the 0.6 Ma Friant Pumice Member of the Turlock Lake
Formation. This equivalence(J. A. Bartow,oral communication,1984)is basedon correlation
betweengeophysical
logsof oil wells anddepthto theCorcoranClay memberin
shallowwells [e.g.,Page, 1986].
Deformationassociated
with Coalingaanticlinethusbegan
in latePliocenetime,whenwe inferthattheFranciscan
wedge
advancedbeneathPleasantValley andraisedtheoverlyingstrata. MovementthenproceededalongtheCoalingathrustzone
andformedthebalanceof thefold. The geometryof thefold
suggests
thatseveralkilometersof thrustoffsetwererequired
to form it. Over the 2 million yearsof fold growth,thisrepresentsshorteningof about4 km, whichis severalpercentof
therateof lateralmovementalongthe SanAndreasfault.
SIMILARITIES
BOUNDARY
ALONG
THE COAST
RANGES
Theprincipalelements
of theregionalsettingat Coalinga
arecommonto thewholelengthof theCoastRangesboundary. Mostevidentis theparallelism
alongtheboundary
betweentheabruptwestwardupturnof CenozoicandCretaceous
strataandthe stepin the basementsurfaceto the eastbeneath
theGreatValley(Figure4). Thatbasement
surface
dips
gentlywestward
beneath
theeastern
twothirdsof thevalley(to
thehachured
lineonFigure4) beforestepping
downmore
steeplytowardtheleadingedgeof theCoastRanges.There,it
flattensandcontinues
beneath
theedgeof theCoastRanges
(seeFigure3c). A third,lesscertain,parallelfeatureis the
buffedtip of theFranciscan
wedge,whichis inferredto have
2/43
been thrust eastward acrossthis basement surface
approximately
to theCoastRanges
boundary
[wentworthet
al., 1984].
TheCoastRanges
boundary
isprominently
marked
by
westwardupturningof strataasyoungasPlioceneand
Pleistocene,
by upliftof theCoastRanges
to thewest,andby
parallelfoldingalongandeastof theboundary
(Figure4).
Theorientation
of thesefoldsandtheshapeof theCoast
Ranges
boundary
itselfcloselyfit theshapeof thelineof
westward
steepening
of thebasement
surface,
whichtypically
lies 15 to 20 km eastof theboundary.
In thesouthern
GreatValley,anapparent
line of slightbut
distinct,
westward
steepening
of theshallowsedimentary
sectionabovethatlineof basement
steepening
seemsto mark
another
featureparallelto theCoastRanges
boundary.Fourof
fivepointsof controlnearlycoincidein planviewwiththe
inferredlineof basement
steepening
(Figure4). Thisshallow
steepeningis evidenton seismicrefectionlinesat latitudes
35.8,36.2,and37.3 [wentworthet al., 1983,Figure3,
km 68; WentworthandZoback, 1988, Plate la, km 38;
Wentworthet al., 1987,Figure2, km 47] andon detailedcross
sectionsat latitudes36.7 and37.1 [Lettis, 1982]. Thesedetailed cross sectionsshow this deformation to have continued
at leastintoearlyRiverbanktime (300-450Ka; Lettis[1982,
1988]).
Manyof thefoldsalongandeastof theCoastRanges
boundary
areyoung,particularly
southeast
of Coalinga
along
theKettleman
Hillsanticlinaltrendandnearby,
butalsofarther
north[seeJennings,
1977]. Themostprominent
youngfold
alongthenorthern
partof theboundary
is Dunnigan
Hills
anticline(Figures1 and4), whereupperQuaternary
strataare
involvedin thefolding[Harwood
andHelley,1987]. In contrastto theKettlemantrend,Sitesanticline,whichsimilarly
occursat theCoastRangeboundarybutnearits northernend,
is unconformably
overlainby PlioceneTehamaFormation
(3.4Ma) andshowsnoQuaternary
folding[Helleyand
Harwood,1985]. To theeastof it, however,foldsalongthe
Cominganticlinaltrend(Figures1 and4) doinvolveupper
Quaternary
strataandareassociated
withsteepfaultsthatshow
reverseoffsetof thebasement
surface[HarwoodandHelley,
1987;Harwood,1984,Figure5]. Similarly,youngfoldsin
thesouthern
GreatValley eastof theKettlemantrendare
probablyalsorelatedto reversefaultsin basement[wentworth
et al., 1983; Wentworth andZoback, 1989].
At theeasternfrontof thecentralDiabloRange,theCoast
Rangesboundaryis distinguished
by theearlyQuaternary
San
Joaquinfault (Figure4), which exhibitsdown-to-the-east
offsetlocallygreaterthan 140-220m [Herd, 1979;Lettis,
1982]. Althoughconsidered
a normalfaultby Herd [1979],
thisfaultprobablyalsoresultsfromcompressive
deformation
across
theCoastRangesboundary
andis nowconsidered
likely
to be a thrustor reversefaultby Lettis [1988].
Westof thesouthern
partof the SanJoaquinfault,Lettis
[1982]hasmappednumerous
down-to-the-west
reversefaults
thatdocument
continuing
eastward
tiltingof theGreatValley
sequence
in theeasternDiabloRangeduringQuaternary
time.
The easternmarginof the upliftedFranciscancoreof the
northern
CoastRangesandcentralDiabloRangeis alsoparallel to the CoastRangesboundary(Figure4). It lies 15 to
20 km westof theboundaryandtypicallyseparates
high,
ruggedtopographyin the Franciscantermnefrom more
subdued
topography
on GreatValley sequence.Thiscontact
haslongbeenconsidered
thesurfacetraceof theCoastRange
thrust.In mostplaces,however,it actuallyconsists
of steep
Cenozoicfaults(CoastRangefault,Tesla-Ortigalitafault) that
haveundergone
dip slipassociated
with rangeupliftand,in the
21414
WentworthandZoback: EasternFront,CaliforniaCoastRange
Diablo Range,right-lateralstrikeslip as well [Raymond,
1973;Page, 1981; Andersonet al., 1982;JaykoandBlake,
1988, also oral communication, 1988]
STATE OF STRESS ALONG
BOUNDARY
THE COAST RANGES
theboundaryandtheline of basement
steepening
andperpendicularto thelocalcompression
direction.Along thesouthern
partof theboundary,the compression
is producinga 100-kmlongzoneof anticlineslocatedat theboundary.We have
shownthatthe youngfoldingat Coalingaanticlineis the
shallowexpression
of deeperthrusting.Structurethere,at
Kettleman South Dome [Wentworth et al., 1983], and at Lost
The directionof maximumhorizontalcompression
indicated by earthquake
focalmechanisms
andwell borebreakouts
alongthe lengthof the CoastRangesboundaryis consistently
northeast-southwest
(Figure4). Mount and Suppe[1987]
showthat the meanorientationof fold axesin the Coalinga
regionis essentially
perpendicular
to themaximumhorizontal
compressive
stress.In fact, thisrelationholdsin detailalong
the wholelengthof the CoastRangesboundary,bothwith the
foldsandwith the line of basementsteepening:
wheretheir
orientationvarieslocally,sotoodoesthecompression
direction (Figure 4).
The breakoutstressorientationsshownin Figure4 were
mostlyobtainedby Mount andSuppe[1987] from wellsthat
rangein depthfrom 0.2 to 4.6 km in sedimentary
rock. The
closeagreement
betweentheseorientations
andthosededuced
from thefocalmechanisms
of earthquakes
at depthsof about
3 to 12 km (Figure 4) showsthat the sedimentarystrataare
foldingin response
to the samestressfield thataffectsthe
seismogenic
thickness
of thecrust[Zobacket al., 1987].
Thusa consistent
relationbetweendeepfaultingandshallow
folding,whichwe havedocumented
at Coalinga,probablyexistsalongmuchof the CoastRangesboundary.
A surprising
featureof theobserved
directionof maximum
horizontalcompression
alongtheCoastRangesboundaryis
thatit is nearlyperpendicular
to thestrikeof theSanAndreas
fault, suggesting
thatthe SanAndreasis movingundervery
low shearstress[Zobacket al., 1987; Mount and Suppe,
1987]. This northeastdirectedcompression
probablybeganat
about3.6 Ma [HarbertandCox, 1986], whena slightclock-
wisechangein thedirectionof Pacificplatemotionrelativeto
North Americaintroduceda smallcomponentof convergence
acrossthe SanAndreasfault [Cox andEngebretson,
1985].
Zobacket al. [1987] arguethatthe highangleof maximum
horizontalstressacrossthe fault is actuallycausedby low
shearstrengthalongthe fault.
CONCLUSIONS
Moderncompression
directionsdetermined
from earthquakes
andwell borebreakoutsin the easternmost
CoastRangesand
the Great Valley indicatethat the entirelengthof the Coast
Rangesboundaryis undergoingcompression
andresultant
shortening
thatis essentiallyperpendicular
to thatboundary.
Althoughthiscompression
probablyderivesfrom plateinteractionsat the SanAndreastransform,its orientationnearly
perpendicular
to thetransformis not compatiblewith conventional wrench tectonics.
A line of abruptwestwardsteepening
of thebasementsurfacebeneaththe GreatValley, inferredlargelyfrom geophysical data,lies about20 km eastof the CoastRangesboundary.
This line of steepening
andtheCoastRangesboundaryextend
in closelyparallelfashionfor almostthe whole600-km length
of theCaliforniaCoastRanges.Near coincidence
with the
apparentline of westwardsteepening
in the shallow
sedimentarysectionin the southernGreatValley impliesthat
thisbreakin basementslopehasexertedcontrolon late
Quaternarydeformation.
Foldsat the CoastRangesboundaryandin the western
GreatValley are orientedparallelto the localorientationsof
Hills [Medwedeffand Suppe,1986] all suggestthatthiswhole
anticlinaltrendresultsfrom blind thrustingbeneaththe folds.
Coalingaaftershocks
indicatethatthrustingextendsbeneath
JoaquinRidgeanticline,andlocalfoldingat theboundaryfarthernorthmayalsoresultfromdeeperthrusting.Nearthe
northendof theboundary,however,theprominentfoldingat
Sitesanticlineseemsto predatethecurrenttectonicregime.
Youngfoldingeastof the CoastRangesboundaryis a
productof the samecompression.Wherebasementis deep
thisfoldingmay alsoreflectunderlyingthrusts,but to theeast
wherebasementis shallowerthe foldingprobablyoverliesreverse faults.
Faulting,eastwardtilting,andassociated
uplift of theeastern CoastRangesmay alsobe relatedto underlyingthrusts
movingundernortheast-southwest
compression.
We suggest
thatmuchof theyoungfoldingandfaulting
alongthe CoastRangesboundaryis the shallowexpression
of
deeperthrustingthatis drivenby northeast-southwest
compression.Thedetailedgeometric
association
betweenthisdeformationandthe line of abruptwestwardsteepening
of basementimpliesfundamental
controlof thatdeformation
by the
stepin thebasement
surfacebeneaththewesternGreatValley.
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(ReceivedJuly 12, 1988;
revisedAugust26, 1988;
accepted
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