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Department of Geology
2-1993
The Origin and Evolution of the Southern Snake
Range Decollement, East Central Nevada
Allen J. McGrew
University of Dayton, [email protected]
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McGrew, Allen J., "The Origin and Evolution of the Southern Snake Range Decollement, East Central Nevada" (1993). Geology Faculty
Publications. Paper 29.
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TECTONICS,
THE
ORIGIN
AND
THE
SOUTHERN
DECOLLEMENT,
VOL. 12, NO. 1, PAGES 21-34, FEBRUARY 1993
EVOLUTION
SNAKE
OF
RANGE
EAST CENTRAL
NEVADA
AllenJ. McGrew1
Departmentof Geology,StanfordUniversity,Stanford,
California
Abstract.Regionalandlocalstratigraphic,
metamorphic,
andstructuralconstraints
permitreconstruction
of the
southern
SnakeRangeextensional
deformational
system
in
eastcentral
Nevada.Thedominant
structure
of therange,the
southern
SnakeRangedtcollement
(SSRD),operated
during
Oligoceneand Mioceneextensionaldeformationto exhumea
footwallof multiplydeformed
metasedimentary
and
plutonicrocks.Intrusionof threeplutons(~160Ma, 79.1+
0.5 Ma, and36+ 1 Ma, respectively)
anddevelopment
of two
cleavages
precededtheonsetof extensional
deformation.
Plasticdeformation
of lowerplatemetasedimentary
rocks
accompanied
theearlyphases
of regionalextension
and
produced
bedding-parallel
grainshapefoliationsandWNW
trending
stretching
lineations.Thesefabricsparallelthe
SSRDevenin low-strain
domains,
suggesting
thata
significant
component
of pureshear
strainprobably
accompanied
noncoaxialdeformationassociated
with motion
on the SSRD, consistentwith otherlines of evidence.
Meanwhile,
hanging
wallrocksweregreatlyextended
by at
leasttwogenerations
of tilt block-style
normalfaultssoling
into the SSRD, with the earlier faults antitheticto the SSRD
andthelaterfaultsdippingin thesamedirectionastheSSRD.
A retrodeformed
regionalcross-section
sequence
illustrates
plausible
alternative
schemes
for reconstructing
thesouthern
SnakeRangeextensional
system.In onescheme,
theSSRD
formsasa crustal
scalestretching
shearzoneseparating
an
upperplatethatextendsonsteeplyinclinednormalfaults
froma lowerplatethatstretches
by penetrative
flow. In the
other,lowerplatedeformation
incorporates
a component
of
coaxialstretching,
buttheSSRDalsofunctionsasa
conventional
shearzoneaccommodating
through-going
displacement
betweenopposing
plates. In eithercase,astectonic
unroofmg
proceeds,
differential
isostatic
unloading
induces
theSSRDto rotateto steeper
dipsasit migrates
intothe
frictionalslidingregime,thusenablingit to remainactiveasa
brittlenormalfaultuntilit finallyrotatesto its present
shallowinclination.In eitherscenario,cross-section
constraints
suggest
thattotalextension
accommodated
by the
SSRDwasprobablybetween8 km and24 km.
1NowatGeologisches
Institut,
Eidgen'tssische
Technische
HochschuleZ'tkich (The SwissFederalInstituteof
Technologyat Zftrich),Zftrich,Switzerland.
Copyright1993 by the AmericanGeophysical
Union.
Paper number92TC01713.
0278-7407/93/92TC-01713510.00
INTRODUCTION
Theorigin,kinematicsignificance
andgeometrical
evolutionof shallowlyinclinednormalfault systems
are
fundamental
issuesin extensional
tectonics.Regionally
extensivefaultsthatjuxtaposenonmetamorphic
sedimentary
rocksin theirhangingwallsagainstplasticallydeformed
crystallinerocksin their footwallscommandspecial
attentionbecausethey offer rare opportunities
to characterize
kinematiclinkagesbetweencontrasting
structurallevels.
Thesefaults,commonlyknownasdetachment
faults,are the
subjects
of muchcontroversy.Thispaperpresents
newdata
bearingon thenatureandoriginof onesuchfault,the
southernSnakeRangedtcollement(SSRD) of eastcentral
Nevada[Misch,1960;Whitebread,1969]. Currently,debate
onthenatureandoriginof detachments
focuseson a few
criticalquestions
suchas thoseoutlinedbelow.
Whatmagnitudes
of displacement
typifyoffsetsacross
detachment
faults? Becausedetachment
faultstypically
juxtaposedistinctlydifferentstructurallevels,it is
commonlydifficult or impossibleto identifythe piercing
pointsor cutoffsnecessary
to constraindisplacements,
althoughseveralstudiesreportrelationships
thatseemto
requiretensof kilometersof offset[e.g.,Stewart,1983;
Reynolds
andSpencer,1985,DavisandLister,1988;Wernicke
et al., 1988]. Occasionally
thiscircumstance
givesriseto
radicallydifferentestimatesof displacement
for the same
faultsystem,asin thecaseof thenorthernSnakeRange
dtco!!ement(NSRD) wherepublishedestimates
of
displacement
varyfrom lessthana few kilometers[Gansand
Miller, 1983; Miller et al., 1983; Gans and Miller, 1985] to
approximately
60 km [BartleyandWernicke,1984;Wernicke
andBartley,1985]. Because
theSSRDmayoriginallyhave
beenlaterallycontinuous
with the NSRD, the retrodeformed
cross-section
presented
heremaybe relevantto theabove
controversy.
Whatfundamentalkinematicfunctiondo detachment
faultsplay in accommodating
lithospheric
extension
? One
popularviewpointholdsthatdetachments
represent
discrete,
through-going
crustalshearzones[e.g.,Wernicke,1981;
Davis,1983],whileothersseedetachments
ascomponents
in
anastomosing
shearzonenetworks[e.g.,Kligfield et al., 1984;
Hamilton,1987]or asregionallydeveloped
decoupling
zones
betweentheseismogenic
uppercrustanddeeperstructural
levelswherepenetrativestretchingandplutonemplacement
accommodate
extension[e.g.,Eaton, 1982;Miller et al., 1983;
Ganset al., 1985;Gans,1987]. In maycase,because
such
modelspredictcontrastingstrainpatternsanddeformational
histories,
observations
in thesouthernSnakeRangeprovide
importanttestsof the viability of someof the competing
interpretations.
How doesthe inclinationof detachment
faults vary during
the kinematicevolutionof extensional
systems
? A particularly importantissuein the debatesurrounding
detachment
faultsconcerns
whethertheyformedandwereseismically
activeat thedipsat whichtheyarenow observed,
or whether
theysubsequently
rotatedto theircontemporary
shallow
inclinations
(for example,Wemicke[ 1981]versusJackson
andMcKenzie[1983]). Additionally,in recentyearsso-called
22
McGrew:Originof SouthernSnakeRangeDtcollement,Nevada
"roUinghingemodels"haveintroduced
a newperspective
on
thegeometrical
evolutionof normalfaultsystems[e.g.,
Wernickeand Axen, 1988; Buck, 1988]. For instance,in the
WemickeandAxenmodel,detachment
systems
formasaseismic, shallowlyinclinedplasticshearzonesin themiddle
crustandrotateto steeperdipsas the footwallis upliftedin
response
to negativeisostaticloadingdueto tectonicunroofing. Consequently,
thedetachment
cancontinueto functionas
a steeplyinclinedseismogenic
normalfault until thehingeline of isostaticflexuremigratesthrougha givenfault
segmentrotatingthat segmentbackto shallowdip. As in
mostdetachment
systems,the rotationalhistoryof the SSRD
is particularlydifficult to constrain,but it is possibleto
evaluatethe geometricalviability of alternative
interpretations.
GEOLOGIC
SETTING
Situatedin eastcentralNevada(Figtire1), thesouthern
SnakeRange(siteof GreatBasinNationalPark)wasmapped
previously
at a scaleof 1:48,000by Whitebread([1969]. In
addition,plutonitrocksin theareahavebeenextensively
o,, •.o.•,oo
KM
investigated,
andabundant
K-At, Rb-Sr,andU-Pbagedataare
available
[Leeetal., 1968,1970,1981,1982,1984,1986a,
1986b;LeeandVanLoenen,1971;LeeandChristiansen,
1983a,b; Milleret al., 1988].Forthisstudy,thestmcturally
complex
northeastern
flankof therangewasre-mapped
ata
scaleof 1:24,000
by theauthorandby the1984Stanford
Geological
Surveyledby E. L. Miller (mapreproduced
hereat
a scaleof 1:48,000,Plate 1).
Located in the hinterland of the Late Cretaceous Sevier
orogenic
belt,thesouthern
SnakeRangeexposes
a broad,
northtrending
anticlinorium
bounded
to thewestby the
Buttesynclinorium
andto theeastby theMesozoic-aged
Confusion
Rangestructural
trough(CRST,exposed
in the
Burbank
Hills;Figure1). Thesouthern
SnakeRange
dtcollement(SSRD) formsthe dominantstructuralfeattire
of therangeanddips10ø-15
øeastward
beneath
southern
Snake
Valley,a narrow(8 km wide)basinthatseparates
the
southern
SnakeRangefromtherelativelysimplestructure
andwell-lmownstratigraphy
of theBurbankHills to theeast
[Hintze,1960;Anderson,
1983]. Theupperplateof theSSRD
exposes
a severelyattenuated,
normal-fault-bounded
mosaic
of variousnonmetamorphic
to slightlyrecrystallized
NEVADA
BASIN
LT LAKE
CITY
AND
RANGE
INSET
PROVINCE
COLORADO
PLATEAU
Fig. 1. Localitymapshowingthestudyarearelativeto someimportantMesozoicstructuraltrends:the
Sevierorogenicbelt,theButtesynclinorium,
andtheConfusionRangestructural
trough(CRST). In partitular,notetheapparent
deflection
of theButtesyuclinorium
relativeto theCRST,probablyasa result
of Cenozoic extension.
McGrew:
Originof Southern
SnakeRangeD6collement,
Nevada
23
sedimentary
rocks,includingfragments
of a thick,carbonatedominated
miogeoclinal
sequence
andoverlyingtuffaceous
Tertiarymarlsandconglomerates
(Plate1). By contrast,
the
lowerplateof theSSRDexposes
a thick,stratifiedsequence
of Late Proterozoicto Middle Cambrianquartzitesand
schists
overlainby a thinhorizonof whitemarblemylonite.
Intrudingthissequence
arethreewell-datedplutonsof Late
Jurassic,
Late Cretaceous,
andearlyOligoceneage,
respectively.
Relationships
betweentheseplutonsand
countryrockare criticalin establishing
thechronology
of
deformational
andmetamorphic
eventsin thestudyarea(see
below). Lower platequartzitesin the easternpartof the map
areawerestrainedandrecrystallizedduringTertiary
deformation,but relict sedimentaryfeaturesincludingcross
bedding,rareconglomeratic
horizons,andprimary
compositional
layeringarepreserved.Grainshapefoliationin
theserocksis orientedperceptiblyparallelto beddingandto
theSSRD,andmeanmineralelongationlineation(Le) is
orientedapproximately
105ø,3ø (Plate1 andFigure2a).
Finally,the SSRD,lowerplatebedding,andlowerplate
foliationare all gentlyfoldedaboutan axisoriented
approximately
102ø, 6ø, essentially
parallelto grainshape
lineation(Figure2a).
In manyrespectsthe structuralframeworkoutlinedabove
resemblesthat of the northernSnakeRange[Miller et al.,
1983; Gansand Miller, 1983; J. Lee et al., 1987], but the rank
of peakmetamorphism
andtheextentandintensityof mylonitic deformationaremuchlower in thesouthernSnakeRange
thanto the north. AlthoughMiller et al. [1983] suggested
thattheNSRD andSSRD areunrelatedstructures,
subsequent
mappingindicatesthatyoungerfaultingin the Sacramento
Passareabetweenthetwo rangesobscures
thestructuralrelationshipbetweenthetwo surfaces,
andtheymayoncehave
beenlaterallycontinuous
(E. L. Miller, personalcommunication, 1986).Consequently,comparison
of thesetwo ranges
may offer importantinsightsinto theoriginof a regionally
importantlow anglestructure.
PRE-EXTENSIONAL
TECTONIC
HISTORY
In the easternpartof themap area,Tertiaryextensional
deformationlargely obscuresthe earlierdeformational
history,but in the westernpart of the studyareapreextensional
fabricrelationships
are well preserved.The
oldestdeformational
fabricin thesouthernSnakeRangeis a
penetrativecleavage(S1) thatis gentlyinclinedwith respect
to beddingand formsa northtrendingintersection
lineation
(L0xl). Andalusiteandpseudomorphosed
staurolite
porphyroblasts
in the contactaureoleof the SnakeCreekWilliamsCanyonplutonoverprintthis cleavage,and
hornfelstextureslocallyobliterateit. Datingof this zoned
tonaliticto graniticplutonby U-Pb zircon,K-Ar hornblende,
K-Ar muscovite,andRb-Srmethodsnarrowlyconstrains
its
ageat approximately160Ma (Figure3) [Leeet al., 1968,
1970, 1981, 1986b]. Consequently,
formationof theS1
cleavagemustat leastslightlypredate160 Ma, although
Miller et al. [ 1988]suggestthat S1 deformationbroadly
coincidedwith plutonemplacement.Metamorphicgradein
Fig.2. Stereoplots
of selected
structural
datafromthesouthernSnake
Range(Schmid
equal-area
net,lower-hemisphere
projection).
(a)Grainshape
lineations
(solidcircles,
N=94)
withmeanlineationvectorindicated
by opencircle(105ø,3ø);
polesto grainshape
foliation(crosses,
N=162)withmean
orientationindicated(soliddiamond),andwith bestfit great
circleandpoletobestfit greatcircleplotted(opendiamond,
102ø,6ø);andpolesto SSRD(opensquares,
N=14). (b) L0x2
intersectionlineation(solid circles,N=22) with mean
lineation
vectorindicated
by opencircle(301ø,5ø);andpoles
to S2crenulation
cleavage
(crosses,
N=21) withbestfit great
circleandpoleto bestfit greatcircleplotted(soliddiamond,
302%15ø).
the contactaureoleof the SnakeCreek-WilliamsCanyon
plutonincreases
from greenschist
faciesto amphibolite
facies
overa distance
of approximately
1.5km,representing
thepeak
metamorphic
gradeattainedin themaparea(Figure3).
24
McGrew:Originof Southern
SnakeRangeD6collement,
Nevada
•++++ .,.
\+
+
+
++++
•-
/
I
I++
+
McGrew:Originof SouthernSnakeRangeD6collement,Nevada
OverprimingthepenetrativeS1 cleavagedescribed
aboveis
a weak,sporadically
occurringS2 crenulation
fabric. L0x2
intersection
lineationtrendsconsistently
northwestward
throughout
themaparea,butbeddingto cleavageanglesand
dip directions
of S2vary,suggesting
subsequent
fanningabout
an axisorientedapproximately
301ø, 5ø (Figure2b). At thin
sectionscale,theS2cleavagecutsandrotateschloriteporphyroblaststhatgrewduringthe 160 Ma metamorphic
event,
thusplacingan upperboundontheageof deformation.In
contrast,andalusiteporphyroblasts
that grew in the contact
aureoleof the 36 Ma YoungCanyon-Kious
Basinpluton(see
below)staticallyoverprintandthereforepostdatethe S2
cleavage.Accordingly,
thetimingof S2deformation
is only
broadlybracketedbetween160 Ma and36 Ma in the southern
SnakeRange;however,Miller et al. [1988]suggest
on the
basisof regionalrelationships
thatS2 fabricdevelopment
probablycoincidedbroadlywith Cretaceous
plutonism.
Consequently,
a 79.1+ 0.5 Ma Rb-Srwholerockisochron
and
a 79.7 Ma K-At muscoviteagefor thePoleCanyon-Can
YoungCanyonplutonin the northcentralpart of the map
areamaygivethebestestimatefor theageof S2cleavage
development
[Leeet al., 1970, 1986a](Figure3). In contrast
to theLateJurassic
andOligoceneplutons,thisdistinctive
muscovite
phenocrystic
two-micagraniteproducedminimal
contactmetamorphiceffects,includingsericitizationof
Jurassic-aged
andalusite
porphyroblasts
andgrowthof new
25
linesof evidenceindicateanOligoceneto Mioceneagefor the
SSRD. Most significantly,the SSRD eithercutsor merges
with upperplatenormalfaultsthat themselvescut and offset
Tertiary-aged
strata,including29.5-30.6Ma tuffsof the
NeedlesRangeGroup[Armstrong,1972;BestandGrant,
1987].In addition,involvement
of the36 Ma YoungCanyonKiousBasinplutonin deformationthatappearsto be
associated
withactivityon the SSRDplacesanupperbracket
on the ageof footwallstrain(seediscussion
of lower plate
strain below).
In additionto the abovelinesof evidence,the southern
SnakeRangeexhibitsa monotonicdecreasein K-At muscovite
agesfrom 79 Ma in thewesternpartof the PoleCanyon-Can
YoungCanyonplutonto a minimumof 17 Ma immediately
beneaththeSSRDin theYoungCanyon-Kious
Basinpluton
(Figure3) [Leeet al., 1970]. The youngest
K-At muscovite
agesoriginatefrom cataclastically
deformedand
hydrothermally
alteredpartsof the mainphaseof the
Tertiarygranite[Leeet al., 1970,1984],and anomalously
low
8180and8Dvalues
fromthesamples
yielding
theyoungest
K-Ar agessuggestthathydrothermal
whitemicaratherthan
primarymuscovite
wasdated[Leeet al., 1984]. Consequently,
theanomalously
youngK-Ar ageswithintheTertiarygranite
probablyrecordmineralgrowthand/orArgonlossassociated
with hydrothermal
activitythatmayhaveaccompanied
the
latestagesof movementon the SSRD.
chlorite in the immediate contact aureole.
Thefinalstageof plutonism
in thesouthern
SnakeRange
involvedemplacement
of theYoungCanyon-Kious
Basin
plutonin thenortheastern
partof themaparea(Plate1). A
36 + 1 Ma U-Pb zirconage[Miller et al., 1988]anda 37.4 +
1.5Ma Rb-Srwholerockisochron[Leeet al., 1986b]constrain
the ageof thisintrusion(Figure3). Garnet,poikiloblastic
andalusite,
andbiotiteaggregates
formingpseudomorphs
afterearlierchloriteporphyroblasts
characterize
peak
metamorphism
in the immediatecontactaureoleof this
pluton,and,asnotedabove,theseassemblages
statically
overprintearlierdeformationalfabrics. In contrastto the
relativelypristineolderplutonsto thewest,themainphase
of theYoungCanyon-Kious
Basinplutonis ratherseverely
deformedandhydrothermally
alteredin the footwallof the
SSRD(seediscussion
of lowerplatestrainbelow). Lee and
Van Loenen[1971]mappeda fault contactbetweenthe
severelydeformedmainphaseanda muchlessdeformed
"aplitic"phaseexposedon the northwestern
sideof Kious
Basin,butmapping
conducted
for thisstudyindicates
that
boththemagnitude
of deformation
andthemineralogy
of the
plutonchangegradationally
acrossa narrowtransitionzone
(Plate 1) [McGrew, 1986].
EXTENSIONAL
TECTONISM
Overprintingandlocallyobscuringthe earlier
deformational
historyoutlinedaboveis themajor
extensional
deformational
eventthat is largelyresponsible
for the present-daystructuralconfigurationof the southern
SnakeRange,includingthemostprominentstructureof the
range,thesouthernSnakeRanged6collement(SSRD). Several
Characterof Lower Plate Strain
TheSSRDis wellexposed
in theeastern
partof themap
areawhereit typicallyoverliesa thin horizonof white
marblemylonitethataccommodated
unquantified
but
potentiallylargeshearstrains.Beneaththemarblemylonite
liesa thicksequence
of lessintensely
strained
andmostly
nonmylonitic
quartzites
andsubsidiary
schistscharacterized
by beddingparallelgrainshapefoliationsandwell-developed
stretching
lineationsorientedapproximately
105ø, 3ø,
parallelto similarlineations
in thenorthernSnakeRange
thatareOligoceneto earlyMiocenein age(Figure2a) [Lee
andSutter,1991]. Towardthewestthesegrainshapefabrics
graduallygiveway to ESE trendingslickenlineations
on
beddingsurfaces,
andthedegreeof transposition
of older
fabricsdiminishes
suchthatS1 andLlx0 (a northtrending
intersection
lineafion)areincreasingly
preserved.In theeast
theseolderfabricsarepreserved
primarilyby inclusiontrails
in ESE rotatedchloriteporphyroblasts
that grewprior to the
f'malstageof deformation(Figure4a).
The ageof thefinal stageof lowerplatestrainis constrainedprimarilyby the deformationof the 36 Ma Young
Canyon-Kious
Basinpluton.Distributedcataclastic
deformation alonga complexsystemof small-scaleshearsurfaces
accomodates
mostof the strainwithin the 36 Ma pluton,but
mylonificfabricsandsolidstatefoliationsoccurlocally
within the outcropareaof thisgranite,andin somelocalities
intrusiveapophyses
intothedeformedcountryrockcarrythe
sameWNW trendinggrainshapelineationthatcharacterizes
surrounding
quartzites.In thin section,the deformed
Tertiarygranitestypicallyshowgreenschist
faciesmineral
assemblages
includingchlorite,secondary
epidote,and
Fig.4. (a)ESE-lineated
mylonitic
chlorite
schist
froinSnake
Creek
window
showing
sigmoidal
chlorite
porphyroblasts
thatgrew
during
earlier
metamorphism
andwere
subsequently
rotated
inanapparent
topto-the-east
sense
(dextral
inthisphotograph)
during
Oligocene
deformation
(plane
light).
(b)Plastically
deformed
Tertiary
granite
(note
sutured,
polygonized,
anddynamically
recrystallized
quartz;
seetextfor
additional
discussion)
(crossed
nicols).
(c)Intensely
conuninuted
zone
incataclastically
deformed
Tertiary
granite
(contrast
withFigure
4b;seetextforadditional
discussion)
(crossed
nicols).
McGrew:Originof SouthernSnakeRangeDtcollement,Nevada
secondary
whitemica.In addition,quartzcommonlyshows
extensivesubgraindevelopmentandsuturedgrainboundaries
(Figures4b and4c). Finally,thinzonesof intensely
comminutedandcataclasized
rock frequentlycut earlier
plasticfabrics,implyinga plasticto brittle evolutionsimilar
to that of many otherCordilleranmetamorphiccore
complexes(Figure 4c).
These abundant small-scale cataclastic zones form a
complicated
kinematicsystem,with mostshearsurfaces
strikingapproximatelynorthwardand mostslickenlineationsorientednearlyparallelto dip, yieldinga mean
motionplaneorientedapproximately80ø, 77ø (Figure5; the
motionplaneis definedastheplanecontainingslickenlineationandthe theshearsurfacenormal). Wherestepped
slickensurfaces
preservea clearsenseof offset,normal-sense
displacement
appearsto predominate,
suggesting
an ENE
trendingextensiondirectionthat contrastssomewhatwith
theaverageslip line indicatedby theorientation
of Le in
surrounding
quartzites(105%3ø). Consequently,
the
extension
directionmayhavevariedby asmuchas30øduring
thecourseof the extensional
history. Approximately65%
of shearsurfacesin the granitedip towardthe eastwhile the
remainderdip westward,suggesting
thatthe bulk deformationpathmayhaveincorporated
a significantcomponent
of
puresheardeformation
aswell asa component
of east
directednoncoaxial
strain(an inferencereinforced
by
relationshipsdescribedbelow).
In orderto evaluatealternativeinterpretations
of the kinematichistoryof the SSRD,it is particularlyimportantto
45
27
-
120
40 I
lOO
35
30
80
25
l•
R
60
15
R
40
5
0
1
2
3
4
5
6
7
8
9
10
Fig. 6. Plot of foliationto shearzoneangle(0') as a function
of shearstrain(¾)and ellipticityof the strainellipse(R) for
thecaseof endmembersimpleshearstrain[cf. Kligfield and
others,1983]. Measuredfinite strainsin the lower plateof
thesouthernSnakeRangeshowvaluesof R < 5 (seeFigure3),
andevendirectlybeneaththeSouthernSnakeRangedtcollement (SSRD) thin sectionobservationson deformed
quartzitessuggestthat finite strainsprobablydid not
appreciably
exceedthisvalue. Consequently,
strainsin the
southern
SnakeRangewereprobablytoolow to giveriseto
the observedparallelismbetweenfoliationandthe SSRD by
strictsimpleshearstrainalone. A modestcomponentof pure
shear strain in addition to the noncoaxial strains that
undoubtedly
affectedlowerplaterockscouldgive riseto the
observed fabric relations.
constrain
thestrainpathcharacterizing
deformation
of lower
platerocks.Asnotedpreviously,
themylonitic
marbles
immediately
subjacent
totheSSRDprobably
record
significantamounts
of noncoaxial
strain,andsporadically
occurring
myloniticschists
at depthsasgreatas 100m beneath
the
SSRDexhibit
porphyroclast
asymmetries
thatmayrecord
ESEdirected
shear
(Figure4a). However,
despite
theevidence
forESEdirected
noncoaxial
strains
notedabove,
deformational
fabricrelationships
andquartzc axisfabric
datafromlowerplaterockssuggest
thatthedeformation
pathprobably
deviated
significantly
fromendmember
simple
shearstrain. Particularlyimportantis theobservation
that
bedding
anddeformational
fabrics
areoriented
perceptibly
parallelto theSSRDthroughout
thelowerplate.A simple
shearstraininterpretation
canexplaintheobserved
Fig. 5. Stereoplot
of polesto motionplanesfor shearsurfaces
in deformed
Tertiarygranite[MarshakandMitra, 1988].
Senseof displacement
onshearsurfaces
is oftenuncertain,
so
polesarenotplottedasdirectedlinesegments,
butnormalsensedisplacement
characterizes
mostsurfaceswheresenseof
slipcanbedetermined.
Approximately
65% of shearsurfaces
dipeastward,
and35% dipwestward.Greatcircleindicates
orientation
of meanmotionplane.N--38.
parallelism
between
foliation
andtheSSRDonlyif shear
strains
aresystematically
highthroughout
thelowerplate
(Figure6). In relativelylow straindomains,
foliation(and
possibly
bedding)
should
beinclined
atdiscernible
angles
to
theshear
zoneboundary
unless
a component
ofpureshear
strain
issuperimposed.
Bycontrast,
coaxial
strains
operating
beneath
a shallowlyinclinedshearzoneshouldresultin
foliations
paralleling
theshearzoneboundary
evenin lowstraindomains.Consequently,
thepersistent
parallelism
28
McGrew:Originof SouthernSnakeRangeD6collement,
Nevada
betweengrainshapefabrics,bedding,
andtheSSRDevenat
1). The line of sectionshownin Plate 2 minimizesthe need
relativelydeepstructural
levelswheretherocksareneither
mylonificnorstrongly
strained
arguesagainata strictsimple
for projectionfromoutsidetheplaneof sectionand
represents
a compromise
betweenthesliplinessuggested
by
shearstrainoriginfor lowerplatedeformational
fabrics.
Unfortunately,
the finitestrainvaluesneededto quantify
tiffsargument
aredifficultto constrain
in thesouthern
Snake
Range.Nevertheless,
thinningof theCambrian
Pioche
Formation(Cpi) from approximately
120 m in the less
deformednorthwestern
partof the rangeto 80-90 m directly
beneath
theSSRDsuggests
thepossibility
of 25-33%tectonic
thinningof lowerplateunitsin themaparea(althoughit
shouldbe notedthat initial stratigraphic
variationscould
the various criteria outlined above. While the lack of a
also accountfor some or all of this thicknessvariation;
Stanford
GeologicalSurvey,unpublished
data,1982and
1984). A morerigorousestimateof finite strainin the lower
plateis providedby Rf-{' analysisof rareconglomeratic
horizonsfoundin quartzitesnorthof the Cretaceous
pluton,
yieldingfinitestrainvaluesof 3.1 ß1.0' 0.7 and2.4' 1.0' 0.7
corresponding
to k valuesof 5.25and3.5,respectively
(placingthemwell within the field of apparent
consfrictional
strainon a Flinn diagram)[RamsayandHuber,
1983](Figure3). Withoutmoreextensivedata,it is
impossible
to evaluatewhetherthesestrainvaluestruly
recordregionalconstrictional
strainor whethertheymerely
reflectlocalperturbations
in the boundaryconditionsof
plasticflow nearthe moreresistantCretaceous
pluton. In
anycase,despite
thenonmylonitic
character
andrelatively
low strainsof theserocks,the XY planeof the strainellipsoid parallelsbeddingasdoesfoliationthroughout
themap
area,suggesting
a significaatcomponent
of pureshearstrain
parallelto lowerplatebeddingasdiscussed
above(Figure6).
Thisinferenceaccordswell with the generallackof compositefabricsandotherasymmetric
microstinctures
in
metamorphic
tectonites
at depthbeneaththeSSRD. In addition,quartzc-axisfabricsfrombeneaththeSSRDgenerally
showwell developed
crossed
girdlepatternswithveryweak,
ESEdirectedsenses
of asymmetry,
suggesting
thata
significant
component
of pureshearstrainaccompanied
ESE
directednoncoaxialdeformation [McGrew, 1986].
Cross-section Construction and Restoran'on
Plate 2 presentsa sequentialcross-section
reconstruction
illustratingtwo contrastingscenariosfor the large-scale
geometrical
evolutionof thesouthernSnakeRangeextensionalsystem.The choiceof an appropriate
lineof section
presentsan immediateproblemin cross-section
construction
asseverallinesof evidencesuggest
thattheslipline maynot
havebeenconstantthroughout
deformation.As notedpreviously,the slip line impliedby the orientationof lowerplate
stretching
lineation(meanLe -- 105%3ø;Figure2) differs
fromthatimpliedby motionplaneanalysisof cataclastic
shearsurfaces
in theTertiarygranite(meanmotionplane-80%77ø;Figure5). In addition,upperplatenormalfaults
definetwoseparable
generations
characterized
by opposing
dipsand slightlydifferentstrikeorientations,
with the
earliergeneration
of westdippingfaultsgenerallystriking
155ø - 160ø whilethe later,east-dipping
faultsusuallystrike
approximately
N-S (seeWhitebread[1969]aswell asPlate
consistent
sliplineintroduces
a degreeof uncertainty
to
cross-section
restoration,
thisuncertainty
is probably
relativelysmallcomparedwith someof theuncertainties
discussed
below. In thesections
thatfollowI detailthekey
constraints
andassumptions
incorporated
into the
construction of Plate 2.
Pre-extensional
structuralgeometry. Severalobservations
suggestthat prior to the onsetof extensionaldeformationthe
southern
SnakeRangeprobablyresembled
theBurbankHills a
few kilometers
to theeast,i.e., gentlyfolded,butlacking
severestructuraldisruptionor profoundduplicationof section
at the structurallevelsnow exposed.
1. No significant
older-on-younger
faultrelationships
occuranywhere
in thesouthern
SnakeRange.
2. Andalusite-bearing
metamorphic
assemblages
in the
contactaureoles
of MesozoicandTertiaryplutonsare
compatiblewith stratigraphic
reconstruction
of the miogeoclinaloverburdenbut seemdifficult to reconcilewith
wholesaleduplication
of theuppercrust,at leastat
structural
levelsabovethesouthern
SnakeRange.
3. Despitestrikingcontrasts
in deformational
style,no
evidenceexiststo demonstrate
profoundomissionof either
stratigraphic
sectionor metamorphic
gradeacrosstheSSRD.
4. As emphasized
firstby Armstrong
[1972]andlaterby
GansandMiller [1983]pre-extensional
Tertiaryrocks
throughout
eastcentralNevadaareeverywhere
deposited
withminorangulardiscordance
onupperPaleozoic
or
youngerrocks.
5. As illustrated
in Plate2, upperplatefaultgeometries
canbe restoredwithoutinvokingseverepre-extensional
structuraldisruption.
However,it shouldbe notedthat the extentto whichthe
abovearguments
applytosurrounding
areassuchasthenorth-
emSnake
Range
isanissue
ofcontinuing
controversy
[Bartley
andWemicke,1984;GansandMiller,1985;Wernicke
and
Bartley,1985;Lewisetal., 1992].Furthermore,
noneof these
pointsbearonthepossibility
thatMesozoic
thrustfaults
mayexistbeneath
thedeepest
levelsexposed
in thesouthern
SnakeRange.In fact,theoccurrence
of Mesozoicdeforma-
tionalfabrics
in thelowerplatesuggests
increasing
structural
involvement
at depth,bothin thesouthern
Snake
Rangeandin surrounding
areas[Milleret al., 1988].
Geometrical
evolution
oftheSSRD.Aswithother
major
detachment
faults,constraining
theinclination
of theSSRD
through
timepresents
a particularly
challenging
and
important
problem
incross-section
construction.
Atpresent,
theSSRDdips<15øeastward
throughout
thesouthern
Snake
Range,
butrequiring
theSSRDtomaintain
such
dips
throughout
itshistorywouldgiveriseto serious
mechanical
problems
andincongruency
withnumerous
observations
on
thecharacter
of active
normal-fault
seismicity
throughout
theworld[Jackson,
1987].Angular
relationships
withlower
platestrata
provide
someconstraints
onpossible
initial
orientations
of theSSRD.Throughout
theeastern
halfof the
range
theSSRDparallels
lowerplateunits,
andregional
McGrew:
OriginofSouthern
Snake
Range
D6collement,
Nevada
reflection
seismicdatasuggest
thatthispatterncontinues
towardtheeast,withtheSnakeRanged•collement
parallel-
ingthewestern
limboftheCRST(COCORP
UtahLine1)
[Allmendinger
etal.,1983]. Because
regional
stratal
dips
probably
didnotexceed
200-30
øattheonset
ofextension
(see
above),
thissegment
of theSSRDprobably
couldnothave
originated
withsteepdips.
Nevertheless,
mapping
byWhitebread
[1969]indicates
that
towardthewesttheSSRDcutsup-section
across
footwall
stratawithanangular
discordance
of approximately
30ø, thus
opening
thepossibility
thattheSSRDmayoriginally
have
steepened
toward
thewestasshown
inPlate2. Therotation
ofthissegment
of theSSRDtoshallower
dipcouldoccur
either
byrotation
in thehanging
walloftheSchell
Creek
fault severalkilometerstOthe west [Ganset al., 1985] or
because
of isostatic
adjustments
accompanying
deformation
[Buck,
1988,Wernicke
andAxen,1988].Bycontrast,
asimilar
isostaticmechanismcouldserveto rotatethe initially
shallowly
dipping
segment
oftheSSRDfirsttosteeper
angle
andthenbackto shallowdipastheinflectionpointof
isostatic
upliftmigrates
through
theregionof interest
[cf.
Spencer,
1984;Wernicke
andAxen,1988].
Therolethatisostasy
mayhaveplayedin thetectonic
evolutionof the SSRD is difficultto evaluatefroman empirical
pointofviewbecause
absolute
variations
inelevation
ofthe
Earth's
surface
duringthecourse
of orogenesis
are
notoriously
difficulttoconstrain
[England
andMolnar,
1990].However,
boththeTertiary
unconformity
data
referred
topreviously
[Gans
andMiller,1983]andthe
palinspastic
reconstruction
shown
inPlate
2 require
thatthe
CambrianPiocheFormationwasburiedto paleodepths
in
excessof 6 km beneaththe Earth'ssurfaceat the onsetof
Tertiaryextension.
At present,
thissameformation
is
exposed
atthecrestof thesouthern
Snake
Range,
atleast6 km
aboveitscontemporary
position
beneath
boththeBurbank
Hills to theeastandthesouthernSchellCreekRangeto the
west(Plate2) [HoseandBlake,1976].Mesozoic
folding
probably
accommodated
onlypartof thisdifferential
uplift,
withtheremainderoccurring
duringTertiaryexhumation,
mostlikelyasa resultof isostatic
adjustments.
In anycase,
reconstruction
of Plate2 withoutanyuplift of lowerplate
rocksin thesouthern
SnakeRangerelativeto surrounding
areaswouldresultin a retrodeformed
sectionshowing> 4 km
of verticalrelief over a lateraldistanceof < 5 km. For these
reasons,
Plate2 assumes
thatspaceproblems
between
the
hangingwall andthefootwallof theSSRDwereaccommodatedin partby footwallupliftaswellasby hanging
wall
collapse
[cfiWernickeandAxen,1988].
Geometry
of upperplatefaults. In thesouthern
Snake
Range,restoration
of complicated
upperplatefault
geometries
posesanother
challenging
problem.Map
relationships
in thesouthern
SnakeRangeimplytwofamilies
of upperplatefaults,anoldergeneration
of relatively
closely
spaced,
westdippingfaultsanda younger
generation
of more
widelyspaced,
eastdipping
faults.Although
theeastdipping
faultsdonotclearlycutthewestdippingfaultsin theareaof
Plate1, suchcrosscuttingrelationships
areexposed
h•
adjacent
areas
mapped
byWhitebread
[1969]tothesouth.
Because
of a lackof exposed
cutoffs,reconstruction
of upper
29
platefaultsreliesentirelyon restoration
of upperplatefault
blocksto inferredpre-extensional
stratigraphic
positions.
Fortunately,
involvement
of a well-knownpre-extensional
stratigraphy
facilitatesthismodeof reconstruction.
The involvementof a well-knownstratifiedsequencealso
placesimportantconstraints
on regionalfaultreconstruction.
Specifically,
theMiddleCambrianPoleCanyonlimestone
(Cpc)is preserved
bothin thehangingwallandin the
footwallof the SSRD (Plates1 and2). Consequently,
the
exposure
of a complete
sectionof thisformation
in thelower
platein thewesternpartof themapareaeffectivelydefinesa
footwallcutoff. As a result,the upperplatePole Canyon
limestonemusthaveoriginatedlessthan-14 km west'ofits
presentposition(Plate2).
Lackof controlon theprecisefault geometries
underlying
andbounding
southernSnakeValley presents
additional
problems
for regionalcross-section
construction
(Plate2).
Thenatureof crosscuttingrelationships
betweentherangeboundingfaultandthe SSRD is a particularlyimportant
uncertainty.AlamandPilger[ 1987]andAlam [1990]report
thatproprietary
seismicdataobtained
fromUnocalfor
southern
SnakeValley imagemoderately
dippingreflectors
thatmaycorrespond
totheSSRD.If so,thentheSSRDmust
steepen
beneath
thewestern
sideof SnakeValleyasshown
in
Plate2 (a possibility
thataccords
well withtherollinghinge
modeldescribedabove). However,a plausiblealternative
interpretation
is that thesereflectorsrecordthe geometryof
therangefrontfault ratherthanthe SSRD, in whichcasethe
SSRDmustbe tnmcatedby therange-bounding
structure.
Nevertheless,
this alternativeinterpretationprobablywould
not greatlyalter the grosscross-section
geometrybeneath
southern
SnakeValley, althoughthemagnitude
of extension
accommodated
by theSSRDwoulddecrease
slighfiy.
An equallyimportantuncertaintyconcernsthe geometry
of upperplatefaultsbeneathsouthern
SnakeValley.
Fortunately,thenarrowness
of southernSnakeValley andthe
relativelywell knownstructureandstratigraphy
of the
adjacentBurbankHills provideconstraints
amelioratingthis
problem. Plate2 illustratesan upperplatefault geometry
beneathsouthern
SnakeValley thatis plausiblebut
necessarily
speculative.Nevertheless,
whatevertheprecise
faultgeometryunderlyingSnakeValley, thenarrowness
of
thebasinservesto limit the magnitudeof errorassociated
with thisuncertaintybecauseextensionwithintheupper
plateacrossSnakeValley canbe no lessthan0 km andno
greaterthanthe widthof the valley (8 kin).
.Natureand geometryof lowerplate strain. Regardless
of
how theuncertainties
discussed
aboveareresolved,differing
cross-sections
arepossibledepending
on thekinematicbehavior attributedto the SSRD. For instance,if a strict in situ
extension
modelis invoked[e.g.,Miller et a1.,1983],then
thinningandnearlycoaxialstretchingof lowerplateunits
directlybeneaththe SSRDmustfully accommodate
upper
plateextensionat any givenpointalongthedetachment.On
theotherhand,if a strictsimpleshearmodelis invoked[e.g.,
Wernicke,1981;BartleyandWernicke,1984],thenthefinal
geomett3'
of lower plateunitsdependson the distributionand
magnitude
of shearstrainandthe initialorientation
of lower
platestratarelativeto the shearzoneboundary.As outlined
30
McGrew:
Originof Southern
Snake
RangeD6collement,
Nevada
earlier,lowerplate strainrelationships
in the southernSnake
Rangeseemto precludeeitherend-member
interpretation,
but
betweenthesetwo extremesa broadspectrumof plausible
scenarios
exists,includingsomethatresembletheendmemberinterpretations.
In short,it seemslikely thatsomebut not all of the
extensionin upperplaterockswasaccommodated
in situby
penetrative
stretchingdirectlybeneaththe SSRD.
Consequently,
theSSRDmaywell haveactedasa "stretching
shearzone"in the senseof Means [ 1989], with the detachment
functioningat leastin part to maintainstraincompatibility
betweena relativelyrigid upperplateanda lowerplatethat
wasstretching
plasticallyas deformation
proceeded.Plate2
illustratesbothan end-memberstretchingshearzone
interpretation
of the SSRD (solidlines)anda hybrid
interpretation
in whichtheSSRDfunctionsin partas a
stretching
shearzoneandin partasa moreconventional
through-going
shearzone(brokenlines). In thefast case
stretching
of lowerplaterocksby homogeneous
planestrain
completely
accommodates
displacement
of upperplaterocks,
whereasin the secondcaselowerplatestretching
accommodates
onlypartof thedisplacement
on theSSRD,
withtheremainderbeingtakenupby through-going
shear.
In eithercase,asextensionmigrateseastwardthrough
time,portionsof the lowerplatelying to the westexperience
unroofing
andhencecoolingandcessation
of plasticdeformationat an earlierstagethando areasto theeast(compatible
with theK-Ar coolingagepatternsrecognized
by Lee et at.
[1970]). Accordingly,
themoreprotracted
historyof plastic
deformation in the east results in a west-to-east strain
gradientin lowerplateunitssuchasthatdepictedfor theendmemberstretchingshearzoneinterpretation
in Plate2.
While exposures
of lowerplaterocksare not sufficiently
extensiveto documentsucha straingradientin the southern
SnakeRange,a strikinglysimilarstrainpatternis well
documented
in thenorthernSnakeRange[J.Leeet al., 1987].
While'bearing
muchin common
within situextension
interpretations
previouslyadvanced
for thenorthernSnake
Ranged6cottement
[GansandMiller, 1983;Miller et at.,
1983], it shouldbe notedthat the end-memberstretching
shearzoneinterpretation
developedabovediffersin thatit
requiresa largecomponent
of uniform-sense
noncoaxial
strainparallel to the SSRD. For example,in the westernpart
of thecross-section
theupperplateaccommodates
at least
120% extensionwhereasthe lower plateaccommodates
no
morethan65% extension,thusrequiringsubstantial
downto-the-eastdisplacement
on the SSRD.
Cross-section reconstruction.
The cross-section
restorationillustratedin Plate 2 is balancedwith respectto
areabut doesnot conserveline lengthsor stratigraphic
thicknesses due to the deformationat mechanisms assumed.
To wit, lowerplateunitswere asstuned
to stretchandthin
plasticallyduringdeformation
andsocannotconserve
line
length. In addition,lateraldifferencesin verticaluplift
accompanying
deformation
wereaccommodated
by vertical
simpleshear.In upperplaterocks,spaceproblemsdeveloping
betweenadjacentfaultblocksduringextension
were
accommodated
insofaraspossible
by rigidbodyrotations
(a
line lengthconservative
process
resembling
the tilt block
modelsof Proffett[1977], Gansand Miller [1983], andother
authors).However,asnumerousauthorshavepointedout,it
is not possibleto completelyaccommodate
spaceproblems
betweenfaultblocksby rigid bodyrotationalone[e.g.,
WernickeandBurchfiel,1982],andsointernaldeformation
by
inclinedsimpleshearwasutilizedto accommodate
residual
spaceproblemsasnecessary
(a processthatdoesnotconserve
line lengths)[Whiteet al., 1986;McGrewandCrews,1990].
The reconstruction
shownin Plate2 yieldsan estimateof
totalextensionof approximately19 km betweenthe central
partof thesouthern
SnakeRangeandthehingeline
of the
CRST. Assumingdifferentkinematicmechanisms
or
contrasting
cross-section
parameters
cangiveriseto different
estimatesof extension,but extensionprobablyliesbetween8
km and24 km providedthat the footwallcutoffof the
Cambrian
PoleCanyonFormationis honoredandthatan
upperplatefaultreconstruction
resembling
thatshownin
Plate2 is adopted.
DISCUSSION
AND
CONCLUSIONS
In thepreferredscenariodevelopedin Plate2, theSSRD
initatesasa plasticstretchingshearzoneseparating
an upper
platethatdeformsby frictionalslidingon steeplyinclined
normalfaultsfrom a lower plate that deformsby penetrative
stretchingandplutonemplacement[cf. Miller et al., 1983;
Means,1989]. As illustrated,a majoreastdippingnormal
fault resemblingmodernrangefront fault systemsgoverns
upperplatedeformation
duringthe earlyphasesof extension,
with an arrayof closelyspaceddown-to-the-west
normal
faultsantitheticto the masterfault accommodating
hanging
wall collapse[cf. White et al., 1986]. As extensionproceeds,
tilt block rotationof the antitheticsystemresultsin
dramaticattenuation
of upperplaterocks,thusapplyinga
negativeisostaticloadto footwallrocksandultimately
resultingin relativeuplift of the lower plate and backrotationof the masternormal fault in the westernpart of the
extendedterrain[cf. ZandtandOwens,1980, Buck, 1988].
Meanwhile,asareasto thewestarerelativelyuplifted,adjacentsegments
of the SSRD thatwereoriginallyshallowly
dippingmustbecome
moresteeplyinclinedandthuscan
remainactiveastectonicunroofingcarriesthemupwardinto
thefrictionalslidingregime[cf. WernickeandAxen, 1988].
Perhapsnet lateralflow of deepcrustalmaterialout from
beneathlessextended
adjacentareascouldhelpaccommodate
suchdifferential
verticalmotions[cf.Gans,1987]. In any
case,asunroofingproceeds
from westto east,upperplate
faultsandthe SSRDitselfeventuallyrotateto sufficiently
shallowanglesthatfurtherslip becomesimpossible
dueto
frictionalresistance,
andasa restfitnewsteeplydipping
faultsmustform, trackingunroofingtowardthe east.
Meanwhile,portionsof the lowerplatein the westernpartof
theareaexperience
unroofingearlierandpassthroughAr
closuretemperatures
earlierthanin theeast(possibly
contributing
to the eastwarddecreasein K-At muscovite
coolingagesrecognized
by Leeet at., 1970]. In otherwords,
updipportions
of theextensional
system
become
defunct
as
down-dipportionsof thesystemtowardtheeastcontinueto
accommodate extension.
McGrew:Originof Southern
SnakeRangeDtcollement,
Nevada
AlthoughPlate2 illustratesa plausiblescenariofor the
structuralevolutionof the southernSnakeRange,it shouldbe
evidentthatcertainelementsin this scenarioare more speculative•_han
others.Amongthemostpoorlyconstrained
elementsin thisscenarioare theinitial dip of theSSRD in the
westernpartof thecross-section,
thenatureof crosscutting
relationships
betweenthe SSRDandtherangefrontstructure,
andtheprecisegeometryof lowerplateunitsto theeastof
thesouthern
SnakeRange.Thedip of theSSRDat inter-
mediatestages
in itsevolutionary
historyandtheprecise
effectof isostaticcompensation
on the evolutionof this
geometrypresentothermajorelementsof uncertainty.
Nevertheless,
despiteimportantuncertainties,several
pointscanbe madewithsomeconfidence.The SSRDformed
asa plasticshearzonesometime afteremplacement
of the 36
Ma YoungCanyon-Kious
Basinpluton,butby thetimeit
ceased
to be active(probablybefore17 Ma, at leastin the
SnakeRangeproper)it wasprobablyfunctioningas a brittle
normalfault. While the initial dip of the SSRD is uncertain
alongthewesternflank of therange,it very probablyformed
at shallowinclinationparallel to lower plate stratain the
east. In theupperplate,an earlygeneration
of westward
dippingfaultsand a later generationof eastwarddipping
faultsclearlyaccommodated
greatextension,
andattenuation
of hangingwall rocksultimatelyproduceddramatic
unroofingandrelativeuplift of lowerplaterockscompared
to surrounding
areas. Suchdifferentialverticalmovements
are difficult to explainexceptin the contextof isostasy.
Finally,penetrativestretchingof lowerplaterocksdirectly
33
beneath
theSSRDappears
tohaveaccommodated
at leastpart
of upperplateextension,
buta substantive
component
of ESE
directedshearis alsopresent.Consequently,
theSSRD
probably
playeda dualkinematic
role,actingin partto
transfer
displacement
downdiptowardtheeastandin partto
resolverelativelylocalizedstrainmismatches
betweena
relatively
rigidupperplateanda plastically
stretching
lower
plate.
Acknowledgements.
ElizabethL. Miller provided
particu-
larlyinvaluable
advice
andsupport
during
thecourse
of this
work. In addition,the 1984StanfordGeologicalSurveycon-
tdbutedmuchbasicdataandmanyimportant
insights
to this
study.Excluding
theauthor,
theparticipants
in thisfield
campwere:Bill Aaron,LisaBaugh,AdamBryer,Shana
Brokken,
TonyCampbell,
DougClark,ClaireDuncan,
Phil
Gans,CareyGazis,Stanley
Grant,PeterLewis,KarenMerino,
EricMiller,SteveMunn,KirbyShanks,
DavidStahl,Andy
Tomlinson,JackWhittimore,andElizabethMiller (leaderof
SGS).PhilGans,JeffLee,andJimWrightprovided
many
usefuldiscussions,
andKarenMerinoandHeidi S. McGrew
provided
especially
important
fieldandotherassistance.
Peg
Brownprovided
vainable
assistance
in drafting.JohnBartley,
TeklaHarms,andRichardAllmendinger
providedthorough
andusefulreviews.Fundingfor thisprojectcamein part
fromgeneral
grantsfromARCOandSOHIO,in partfrom
NSFgrantEAR 8418678to E. L. Milleret al.,andin part
froma Stanford
GeologyDepartment
graduate
fellowship.
REFERENCES
Alam, A.H.M. Shah,Crustalextensionin the
southern
SnakeRangeandvicinity,
Nevada-Utah:An integratedgeological
andgeophysical
study,Ph.D.thesis,La.
StateUniv., BatonRouge,126pp.,1990.
Alam, A.H.M. Shah,andR.H. Pilger,Jr.,
Extensionof the southernSnakeRange
Dtcollement beneath the southern Snake
Valleybasedon reflectionseismicdata:
BasinandRangeProvince,Nevada-Utah,
Geol.Soc.Am.Abstr.Programs,19, 568,
1987.
Allmendinger,
R.W., J.W.Sharp,D. VonTish,
L. Serpa,
L. Brown,S. Kauffinan,
J.Oliver,
andR.B. Smith,CenozoicandMesozoic
structure of the eastern Great Basin
province,Utah,fromCOCORPseismicreflectiondata,Geology,I1,532-536,
1983.
Anderson,R.E., Cenozoicstructuralhistory
of selectedareasin the easternGreat Basin,
Nevada-Utah,U.S. Geot Surv.OpenFile
Rep.,83-504,47 pp.,1983.
.Armstrong,
R.L., Low-angle(denudational)
faults,hinterlandof the SevierOrogenic
Belt, easternNevadaand westernUtah,
Geol. Soc.Am. Bull., 83, 1729-1754, 1972.
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