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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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eCommons Citation McGrew, Allen J., "The Origin and Evolution of the Southern Snake Range Decollement, East Central Nevada" (1993). Geology Faculty Publications. 29. https://ecommons.udayton.edu/geo_fac_pub/29

This Article is brought to you for free and open access by the Department of Geology at eCommons. It has been accepted for inclusion in Geology Faculty Publications by an authorized administrator of eCommons. For more information, please contact [email protected], [email protected]. TECTONICS, VOL. 12, NO. 1, PAGES 21-34, FEBRUARY 1993

THE ORIGIN AND EVOLUTION OF INTRODUCTION THE SOUTHERN SNAKE RANGE The origin,kinematic significance and geometrical evolu- DECOLLEMENT, EAST CENTRAL tion of shallowlyinclined normal fault systemsare NEVADA fundamentalissues in extensionaltectonics. Regionally extensivefaults that juxtapose nonmetamorphic sedimentary Allen J. McGrew1 rocksin theirhanging walls againstplastically deformed Departmentof Geology,Stanford University, Stanford, crystallinerocks in their footwallscommand special California attentionbecause they offer rare opportunitiesto characterize kinematiclinkages between contrasting structural levels. Thesefaults, commonly known as detachmentfaults, are the Abstract.Regional and local stratigraphic, metamorphic, subjectsof muchcontroversy. This paper presents new data andstructural constraints permit reconstruction of the bearingon thenature and origin of onesuch fault, the southernSnake Range extensional deformational system in southernSnake Range dtcollement (SSRD) of eastcentral eastcentral Nevada. The dominant structure of therange, the Nevada[Misch, 1960; Whitebread, 1969]. Currently,debate southernSnake Range dtcollement (SSRD), operated during on thenature and origin of detachmentsfocuses on a few Oligoceneand Mioceneextensional deformation to exhumea criticalquestions such as thoseoutlined below. Whatmagnitudes of displacementtypify offsets across footwallof multiplydeformed metasedimentary and plutonicrocks. Intrusionof threeplutons (~ 160Ma, 79.1+ detachmentfaults ? Becausedetachment faults typically 0.5 Ma, and36+ 1 Ma, respectively)and development of two juxtaposedistinctly different structural levels, it is cleavagespreceded the onsetof extensionaldeformation. commonlydifficult or impossibleto identifythe piercing Plasticdeformation of lowerplate metasedimentary rocks pointsor cutoffsnecessary to constraindisplacements, accompaniedthe early phases of regionalextension and althoughseveral studies report relationships that seemto producedbedding-parallel grain shape foliations and WNW requiretens of kilometersof offset[e.g., Stewart,1983; trendingstretching lineations. These fabrics parallel the Reynoldsand Spencer, 1985, Davis and Lister, 1988; Wernicke SSRDeven in low-straindomains, suggesting that a et al., 1988]. Occasionallythis circumstance gives rise to significantcomponent of pureshear strain probably radicallydifferent estimates of displacementfor the same accompaniednoncoaxial deformation associated with motion faultsystem, as in thecase of thenorthern Snake Range on the SSRD, consistentwith other lines of evidence. dtco!!ement(NSRD) wherepublished estimates of Meanwhile,hanging wall rocks were greatly extended by at displacementvary from lessthan a few kilometers[Gans and Miller, 1983; Miller et al., 1983; Gans and Miller, 1985] to leasttwo generationsof tilt block-stylenormal faults soling into the SSRD, with the earlier faults antitheticto the SSRD approximately60 km [Bartleyand Wernicke, 1984; Wernicke andthe later faults dipping in thesame direction as the SSRD. andBartley, 1985]. Becausethe SSRD may originally have A retrodeformedregional cross-section sequence illustrates beenlaterally continuous with the NSRD, the retrodeformed plausiblealternative schemes for reconstructingthe southern cross-sectionpresented here may be relevantto the above SnakeRange extensional system. In onescheme, the SSRD controversy. Whatfundamental kinematic function do detachment formsas a crustalscale stretching shear zone separating an upperplate that extends on steeplyinclined normal faults faultsplay in accommodatinglithospheric extension ? One froma lowerplate that stretches by penetrativeflow. In the popularviewpoint holds that detachments represent discrete, other,lower plate deformation incorporates a component of through-goingcrustal shear zones [e.g., Wernicke, 1981; coaxialstretching, but the SSRD alsofunctions as a Davis,1983], while others see detachments as components in conventionalshear zone accommodating through-going dis- anastomosingshear zone networks [e.g., Kligfield et al., 1984; placementbetween opposing plates. In eithercase, as tectonic Hamilton,1987] or as regionallydeveloped decoupling zones unroofmgproceeds, differential isostatic unloading induces betweenthe seismogenicupper crust and deeper structural theSSRD to rotateto steeperdips as it migratesinto the levelswhere penetrative stretching and pluton emplacement frictionalsliding regime, thus enabling it to remainactive as a accommodateextension [e.g., Eaton, 1982;Miller et al., 1983; brittlenormal fault until it finallyrotates to its present Ganset al., 1985;Gans, 1987]. In maycase, because such shallowinclination. In eitherscenario, cross-section modelspredict contrasting strain patterns and deformational histories,observations in the southernSnake Range provide constraintssuggest that total extension accommodated by the SSRDwas probably between 8 km and24 km. importanttests of the viability of someof the competing interpretations. How doesthe inclinationof detachmentfaults vary during 1Now at Geologisches Institut, Eidgen'tssische Technische the kinematicevolution of extensionalsystems ? A particu- HochschuleZ'tkich (The SwissFederal Institute of larly importantissue in the debatesurrounding detachment Technologyat Zftrich),Zftrich, Switzerland. faultsconcerns whether they formedand were seismically activeat the dipsat whichthey are now observed,or whether Copyright1993 by the AmericanGeophysical Union. theysubsequently rotated to theircontemporary shallow Paper number92TC01713. inclinations(for example,Wemicke [ 1981]versus Jackson 0278-7407/93/92TC-01713510.00 andMcKenzie [1983]). Additionally,in recentyears so-called 22 McGrew:Origin of SouthernSnake Range Dtcollement, Nevada

"roUinghinge models" have introduced a new perspectiveon investigated,and abundant K-At, Rb-Sr,and U-Pb age data are the geometricalevolution of normalfault systems[e.g., available[Lee et al., 1968,1970, 1981, 1982, 1984, 1986a, Wernickeand Axen, 1988; Buck, 1988]. For instance,in the 1986b;Lee and Van Loenen, 1971; Lee and Christiansen, Wemickeand Axen model, detachment systems form as aseis- 1983a,b; Milleret al., 1988].For this study, the stmcturally mic, shallowlyinclined plastic shear zones in themiddle complexnortheastern flank of therange was re-mapped ata crustand rotate to steeperdips as the footwallis upliftedin scaleof 1:24,000by theauthor and by the1984 Stanford responseto negativeisostatic loading due to tectonicunroof- GeologicalSurvey led by E. L. Miller (mapreproduced here at ing. Consequently,the detachmentcan continue to functionas a scaleof 1:48,000,Plate 1). a steeplyinclined seismogenic normal fault until the hinge- Located in the hinterland of the Late Cretaceous Sevier line of isostaticflexure migrates through a givenfault orogenicbelt, the southern Snake Range exposes a broad, segmentrotating that segmentback to shallowdip. As in northtrending anticlinorium bounded to thewest by the mostdetachment systems, the rotationalhistory of the SSRD Buttesynclinorium and to theeast by theMesozoic-aged is particularlydifficult to constrain,but it is possibleto ConfusionRange structural trough (CRST, exposed in the evaluatethe geometricalviability of alternative BurbankHills; Figure 1). Thesouthern Snake Range interpretations. dtcollement(SSRD) forms the dominantstructural feattire of therange and dips 10ø-15 ø eastwardbeneath southern Snake GEOLOGIC SETTING Valley,a narrow(8 km wide)basin that separates the southernSnake Range from the relatively simple structure Situatedin eastcentral Nevada (Figtire 1), the southern andwell-lmown stratigraphy of theBurbank Hills to theeast SnakeRange (site of GreatBasin National Park) was mapped [Hintze,1960; Anderson, 1983]. Theupper plate of theSSRD previouslyat a scaleof 1:48,000by Whitebread([1969]. In exposesa severelyattenuated, normal-fault-bounded mosaic addition,plutonit rocks in thearea have been extensively of variousnonmetamorphic to slightly recrystallized

o,, •.o.•,oo KM NEVADA BASIN

LT LAKE CITY AND

RANGE

INSET PROVINCE COLORADO PLATEAU

Fig. 1. Localitymap showingthe studyarea relative to someimportant Mesozoic structural trends: the Sevierorogenic belt, the Buttesynclinorium, and the ConfusionRange structural trough (CRST). In par- titular,note the apparent deflection of theButte syuclinorium relative to theCRST, probably as a result of Cenozoic extension. McGrew:Origin of SouthernSnake Range D6collement, Nevada 23

sedimentaryrocks, including fragments of a thick,carbonate- dominatedmiogeoclinal sequence and overlying tuffaceous Tertiarymarls and conglomerates (Plate 1). By contrast,the lowerplate of the SSRDexposes a thick,stratified sequence of Late Proterozoicto Middle Cambrianquartzites and schistsoverlain by a thinhorizon of whitemarble mylonite. Intrudingthis sequence are threewell-dated plutons of Late Jurassic,Late Cretaceous,and early Oligocene age, respectively.Relationships between these plutons and countryrock are criticalin establishingthe chronologyof deformationaland metamorphic events in the studyarea (see below). Lower plate quartzitesin the easternpart of the map areawere strainedand recrystallizedduring Tertiary deformation,but relict sedimentaryfeatures including cross bedding,rare conglomeratichorizons, and primary compositionallayering are preserved. Grain shape foliation in theserocks is orientedperceptibly parallel to beddingand to the SSRD,and mean mineral elongation lineation (Le) is orientedapproximately 105 ø, 3ø (Plate1 andFigure 2a). Finally,the SSRD, lowerplate bedding, and lower plate foliationare all gentlyfolded about an axis oriented approximately102 ø , 6ø , essentiallyparallel to grainshape lineation(Figure 2a). In manyrespects the structuralframework outlined above resemblesthat of the northernSnake Range [Miller et al., 1983; Gans and Miller, 1983; J. Lee et al., 1987], but the rank of peakmetamorphism and the extentand intensity of mylon- itic deformationare muchlower in the southernSnake Range thanto the north. AlthoughMiller et al. [1983] suggested thatthe NSRD and SSRD are unrelatedstructures, subsequent mappingindicates that youngerfaulting in the Sacramento Passarea between the two rangesobscures the structuralrela- tionshipbetween the two surfaces,and they may oncehave beenlaterally continuous (E. L. Miller, personalcommunica- tion, 1986). Consequently,comparison of thesetwo ranges may offer importantinsights into the originof a regionally importantlow angle structure.

PRE-EXTENSIONAL TECTONIC HISTORY

In the easternpart of themap area,Tertiary extensional Fig.2. Stereoplotsof selected structural data from the south- deformationlargely obscuresthe earlierdeformational ernSnake Range (Schmid equal-area net, lower-hemisphere history,but in the westernpart of the studyarea pre- projection).(a) Grainshape lineations (solid circles, N=94) extensionalfabric relationships are well preserved.The withmean lineation vector indicated by opencircle (105 ø, 3ø); oldestdeformational fabric in the southernSnake Range is a polesto grainshape foliation (crosses, N=162) with mean penetrativecleavage (S 1) that is gentlyinclined with respect orientationindicated (solid diamond), and with bestfit great to beddingand formsa northtrending intersection lineation circleand pole to bestfit greatcircle plotted (open diamond, (L0xl). Andalusiteand pseudomorphosedstaurolite 102ø, 6ø); and poles to SSRD(open squares, N=14). (b) L0x2 porphyroblastsin the contactaureole of the SnakeCreek- intersectionlineation (solid circles,N=22) with mean Williams Canyonpluton overprint this cleavage,and lineationvector indicated by opencircle (301 ø, 5ø); and poles hornfelstextures locally obliterate it. Datingof this zoned to S2crenulation cleavage (crosses, N=21) withbest fit great tonaliticto graniticpluton by U-Pb zircon,K-Ar hornblende, circleand pole to bestfit greatcircle plotted (solid diamond, K-Ar muscovite,and Rb-Sr methodsnarrowly constrains its 302% 15ø). ageat approximately160 Ma (Figure3) [Lee et al., 1968, 1970, 1981, 1986b]. Consequently,formation of the S1 the contactaureole of the SnakeCreek-Williams Canyon cleavagemust at leastslightly predate 160 Ma, although plutonincreases from greenschistfacies to amphibolitefacies Miller et al. [ 1988] suggestthat S1 deformationbroadly overa distanceof approximately1.5 km, representingthe peak coincidedwith plutonemplacement. Metamorphic grade in metamorphicgrade attained in themap area(Figure 3). 24 McGrew:Origin of SouthernSnake Range D6collement, Nevada

•++++ .,. \+ + + ++++ •- + I++ / I McGrew:Origin of SouthernSnake Range D6collement, Nevada 25

Overprimingthe penetrative S 1 cleavagedescribed above is linesof evidenceindicate an Oligoceneto Mioceneage for the a weak,sporadically occurring S2 crenulationfabric. L0x2 SSRD. Most significantly,the SSRD eithercuts or merges intersectionlineation trends consistently northwestward with upperplate normal faultsthat themselvescut and offset throughoutthe map area,but bedding to cleavageangles and Tertiary-agedstrata, including 29.5-30.6 Ma tuffsof the dip directionsof S2vary, suggesting subsequent fanning about NeedlesRange Group [Armstrong,1972; Best and Grant, an axisoriented approximately 301 ø, 5ø (Figure2b). At thin 1987].In addition,involvement of the36 Ma YoungCanyon- sectionscale, the S2 cleavage cuts and rotates chlorite porphy- KiousBasin pluton in deformationthat appears to be roblaststhat grewduring the 160 Ma metamorphicevent, associatedwith activityon the SSRDplaces an upperbracket thusplacing an upperbound on the ageof deformation.In on the age of footwallstrain (see discussion of lower plate contrast,andalusite porphyroblasts that grew in the contact strain below). aureoleof the 36 Ma YoungCanyon-Kious Basin pluton (see In additionto the abovelines of evidence,the southern below)statically overprint and therefore postdate the S2 SnakeRange exhibits a monotonicdecrease in K-At muscovite cleavage.Accordingly, the timing of S2 deformationis only agesfrom 79 Ma in the westernpart of the PoleCanyon-Can YoungCanyon pluton to a minimumof 17 Ma immediately broadlybracketed between 160 Ma and 36 Ma in the southern beneaththe SSRDin the YoungCanyon-Kious Basin pluton SnakeRange; however, Miller et al. [1988] suggeston the (Figure3) [Lee et al., 1970]. The youngestK-At muscovite basisof regionalrelationships that S2 fabricdevelopment agesoriginate from cataclasticallydeformed and probablycoincided broadly with Cretaceousplutonism. hydrothermallyaltered parts of the main phaseof the Consequently,a 79.1 + 0.5 Ma Rb-Srwhole rock isochron and Tertiarygranite [Lee et al., 1970, 1984],and anomalouslylow a 79.7 Ma K-At muscoviteage for the PoleCanyon-Can YoungCanyon pluton in the northcentral part of the map 8180and 8 D valuesfrom the samples yielding the youngest areamay give the best estimate for theage of S2cleavage K-Ar agessuggest that hydrothermalwhite micarather than primarymuscovite was dated [Lee et al., 1984]. Consequently, development[Lee et al., 1970, 1986a](Figure 3). In contrast theanomalously young K-Ar ageswithin the Tertiary granite to the Late Jurassicand Oligocene plutons, this distinctive probablyrecord mineral growth and/or Argon loss associated muscovitephenocrystic two-mica granite produced minimal with hydrothermalactivity that may haveaccompanied the contactmetamorphic effects, including sericitization of late stagesof movementon the SSRD. Jurassic-agedandalusite porphyroblasts and growthof new chlorite in the immediate contact aureole. Characterof Lower Plate Strain Thefinal stage of plutonismin thesouthern Snake Range involvedemplacement of theYoung Canyon-Kious Basin TheSSRD is well exposedin theeastern part of themap plutonin the northeasternpart of the maparea (Plate 1). A areawhere it typicallyoverlies a thin horizonof white 36 + 1 Ma U-Pb zirconage [Miller et al., 1988]and a 37.4 + marblemylonite that accommodated unquantified but 1.5 Ma Rb-Srwhole rock isochron [Lee et al., 1986b]constrain potentiallylarge shear strains. Beneath the marblemylonite the ageof thisintrusion (Figure 3). Garnet,poikiloblastic liesa thicksequence of lessintensely strained and mostly andalusite,and biotite aggregates forming pseudomorphs nonmyloniticquartzites and subsidiary schists characterized afterearlier chlorite porphyroblasts characterize peak by beddingparallel grain shape foliations and well-developed metamorphismin the immediatecontact aureole of this stretchinglineations oriented approximately 105 ø, 3ø, pluton,and, as noted above, these assemblages statically parallelto similarlineations in the northernSnake Range overprintearlier deformationalfabrics. In contrastto the thatare Oligocene to earlyMiocene in age(Figure 2a) [Lee relativelypristine older plutons to the west,the main phase andSutter, 1991]. Towardthe west these grain shape fabrics of theYoung Canyon-Kious Basin pluton is ratherseverely graduallygive way to ESE trendingslickenlineations on deformedand hydrothermally altered in the footwallof the beddingsurfaces, and the degree of transpositionof older SSRD(see discussion of lowerplate strain below). Lee and fabricsdiminishes such that S 1 andLlx0 (a northtrending Van Loenen[1971] mappeda fault contactbetween the intersectionlineafion) are increasingly preserved. In theeast severelydeformed main phase and a muchless deformed theseolder fabrics are preserved primarily by inclusiontrails "aplitic"phase exposed on the northwesternside of Kious in ESE rotatedchlorite porphyroblasts that grew prior to the Basin,but mapping conducted for thisstudy indicates that f'malstage of deformation(Figure 4a). boththe magnitude of deformationand the mineralogy of the The ageof the final stageof lowerplate strain is con- plutonchange gradationally across a narrowtransition zone strainedprimarily by the deformationof the 36 Ma Young (Plate 1) [McGrew, 1986]. Canyon-KiousBasin pluton. Distributed cataclastic deforma- tion alonga complexsystem of small-scaleshear surfaces EXTENSIONAL TECTONISM accomodatesmost of the strainwithin the 36 Ma pluton,but mylonificfabrics and solid statefoliations occur locally Overprintingand locally obscuringthe earlier within the outcroparea of this granite,and in somelocalities deformationalhistory outlined above is the major intrusiveapophyses into the deformedcountry rock carrythe extensionaldeformational event that is largelyresponsible sameWNW trendinggrain shape lineation that characterizes for the present-daystructural configuration of the southern surroundingquartzites. In thin section,the deformed SnakeRange, including the mostprominent structure of the Tertiarygranites typically show greenschist facies mineral range,the southernSnake Range d6collement (SSRD). Several assemblagesincluding chlorite, secondary epidote, and Fig.4. (a)ESE-lineated mylonitic chlorite schist froin Snake Creek window showing sigmoidal chlorite porphyroblaststhatgrew during earlier metamorphism andwere subsequently rotated inan apparent top- to-the-eastsense (dextral inthis photograph) during Oligocene deformation (plane light). (b) Plastically deformedTertiary granite (note sutured, polygonized, anddynamically recrystallized quartz; see text for additionaldiscussion) (crossed nicols). (c) Intensely conuninuted zone in cataclastically deformed Tertiarygranite (contrast with Figure 4b; see text for additional discussion) (crossed nicols). McGrew:Origin of SouthernSnake Range Dtcollement, Nevada 27

secondarywhite mica.In addition,quartz commonly shows 45 - 120 extensivesubgrain development and suturedgrain boundaries 40 I (Figures4b and4c). Finally,thin zonesof intensely lOO comminutedand cataclasizedrock frequentlycut earlier 35 plasticfabrics, implying a plasticto brittle evolutionsimilar 30 80 to that of many otherCordilleran metamorphic core complexes(Figure 4c). 25 These abundant small-scale cataclastic zones form a l• R 60 R complicatedkinematic system, with mostshear surfaces strikingapproximately northward and mostslicken- 15 40 lineationsoriented nearly parallel to dip, yieldinga mean motionplane oriented approximately 80 ø , 77ø (Figure5; the

motionplane is definedas theplane containing slickenlin- 5 eationand the the shearsurface normal). Wherestepped slickensurfacespreserve a clearsense of offset,normal-sense displacementappears to predominate,suggesting an ENE 0 1 2 3 4 5 6 7 8 9 10 trendingextension direction that contrastssomewhat with the averageslip line indicatedby the orientationof L e in Fig. 6. Plot of foliationto shearzone angle (0') as a function surroundingquartzites (105% 3ø). Consequently,the extensiondirection may have varied by asmuch as 30 ø during of shearstrain (¾) and ellipticity of the strainellipse (R) for the courseof the extensionalhistory. Approximately65 % the caseof endmember simple shear strain [cf. Kligfield and of shearsurfaces in the granitedip towardthe eastwhile the others,1983]. Measuredfinite strainsin the lower plate of remainderdip westward,suggesting that the bulk deforma- the southernSnake Range show values of R < 5 (seeFigure 3), tionpath may haveincorporated a significant component of andeven directly beneath the SouthernSnake Range dtcolle- pureshear deformation as well asa componentof east ment (SSRD) thin sectionobservations on deformed directednoncoaxial strain (an inferencereinforced by quartzitessuggest that finite strainsprobably did not relationshipsdescribed below). appreciablyexceed this value. Consequently,strains in the In orderto evaluatealternative interpretations of the kine- southernSnake Range were probably too low to giverise to matichistory of the SSRD, it is particularlyimportant to the observedparallelism between foliation and the SSRD by strictsimple shear strain alone. A modestcomponent of pure shear strain in addition to the noncoaxial strains that undoubtedlyaffected lower plate rocks could give riseto the observed fabric relations.

constrainthe strain path characterizing deformation of lower platerocks. As notedpreviously, the mylonitic marbles immediatelysubjacent tothe SSRD probably record signifi- cantamounts of noncoaxialstrain, and sporadically occurring myloniticschists at depthsas great as 100m beneaththe SSRDexhibit porphyroclast asymmetries that may record ESEdirected shear (Figure 4a). However,despite the evidence forESE directed noncoaxial strains noted above, deformationalfabric relationships and quartz c axisfabric datafrom lower plate rocks suggest that the deformation pathprobably deviated significantly from end member simple shearstrain. Particularlyimportant is the observationthat beddingand deformational fabrics are oriented perceptibly parallelto theSSRD throughout the lower plate. A simple shearstrain interpretation can explain the observed parallelismbetween foliation and the SSRD only if shear Fig. 5. Stereoplotof polesto motionplanes for shearsurfaces strainsare systematically high throughout the lower plate in deformedTertiary granite [Marshak and Mitra, 1988]. (Figure6). In relativelylow straindomains, foliation (and Senseof displacementon shearsurfaces is oftenuncertain, so possiblybedding) should be inclined at discernible angles to polesare not plotted as directed line segments,but normal- theshear zone boundary unless a componentof pure shear sensedisplacement characterizes most surfaces where sense of strainis superimposed. By contrast, coaxial strains operating slipcan be determined.Approximately 65 % of shearsurfaces beneatha shallowlyinclined shear zone should result in dip eastward,and 35% dip westward.Great circle indicates foliationsparalleling the shear zone boundary even in low- orientationof meanmotion plane. N--38. straindomains. Consequently, the persistent parallelism 28 McGrew:Origin of SouthernSnake Range D6collement, Nevada betweengrain shape fabrics, bedding, and the SSRD even at 1). The line of sectionshown in Plate 2 minimizesthe need relativelydeep structural levels where the rocks are neither for projectionfrom outsidethe planeof sectionand mylonificnor stronglystrained argues againat a strictsimple representsa compromise between the slip lines suggested by shearstrain origin for lower platedeformational fabrics. the various criteria outlined above. While the lack of a Unfortunately,the finite strainvalues needed to quantify consistentslip line introducesa degree of uncertaintyto tiffsargument are difficult to constrainin thesouthern Snake cross-sectionrestoration, this uncertainty is probably Range.Nevertheless, thinning of theCambrian Pioche relativelysmall compared with someof the uncertainties Formation(Cpi) from approximately120 m in the less discussedbelow. In thesections that follow I detailthe key deformednorthwestern part of the rangeto 80-90 m directly constraintsand assumptionsincorporated into the beneaththe SSRDsuggests the possibility of 25-33%tectonic construction of Plate 2. thinningof lowerplate units in the maparea (although it Pre-extensionalstructural geometry. Severalobservations shouldbe notedthat initial stratigraphicvariations could suggestthat prior to the onsetof extensionaldeformation the also accountfor some or all of this thicknessvariation; southernSnake Range probably resembled the Burbank Hills a StanfordGeological Survey, unpublished data, 1982 and few kilometersto theeast, i.e., gentlyfolded, but lacking 1984). A morerigorous estimate of finite strainin the lower severestructural disruption or profoundduplication of section plateis providedby Rf-{' analysisof rareconglomeratic at the structurallevels now exposed. horizonsfound in quartzitesnorth of the Cretaceouspluton, 1. No significantolder-on-younger fault relationships yieldingfinite strain values of 3.1 ß1.0' 0.7 and2.4' 1.0' 0.7 occuranywhere in thesouthern Snake Range. correspondingto k valuesof 5.25 and3.5, respectively 2. Andalusite-bearingmetamorphic assemblages in the (placingthem well within the field of apparent contactaureoles of Mesozoicand Tertiary plutons are consfrictionalstrain on a Flinn diagram)[Ramsay and Huber, compatiblewith stratigraphicreconstruction of the mio- 1983](Figure 3). Withoutmore extensive data, it is geoclinaloverburden but seemdifficult to reconcilewith impossibleto evaluatewhether these strain values truly wholesaleduplication of the uppercrust, at leastat recordregional constrictional strain or whetherthey merely structurallevels above the southernSnake Range. reflectlocal perturbations in the boundaryconditions of 3. Despitestriking contrasts in deformationalstyle, no plasticflow nearthe moreresistant Cretaceous pluton. In evidenceexists to demonstrateprofound omission of either anycase, despite the nonmylonitic character and relatively stratigraphicsection or metamorphicgrade across the SSRD. low strainsof theserocks, the XY planeof the strainellip- 4. As emphasizedfirst by Armstrong[ 1972] and later by soid parallelsbedding as doesfoliation throughout the map Gansand Miller [1983]pre-extensional Tertiary rocks area,suggesting a significaat component of pureshear strain throughouteast central Nevada are everywhere deposited parallelto lowerplate bedding as discussedabove (Figure 6). withminor angular discordance on upperPaleozoic or This inferenceaccords well with the generallack of com- youngerrocks. positefabrics and other asymmetric microstinctures in 5. As illustratedin Plate2, upperplate fault geometries metamorphictectonites at depthbeneath the SSRD. In addi- canbe restoredwithout invoking severe pre-extensional tion,quartz c-axis fabrics from beneath the SSRD generally structuraldisruption. showwell developedcrossed girdle patterns with veryweak, However,it shouldbe notedthat the extentto whichthe ESE directedsenses of asymmetry,suggesting that a abovearguments apply to surroundingareas such as the north- significantcomponent of pureshear strain accompanied ESE emSnake Range is an issue of continuing controversy [Bartley directednoncoaxial deformation [McGrew, 1986]. andWemicke, 1984; Gans and Miller, 1985; Wernicke and Bartley,1985; Lewis et al., 1992].Furthermore, none of these Cross-section Construction and Restoran'on pointsbear on thepossibility that Mesozoic thrust faults mayexist beneath the deepest levels exposed in thesouthern Plate 2 presentsa sequentialcross-section reconstruction SnakeRange. In fact,the occurrence of Mesozoicdeforma- illustratingtwo contrastingscenarios for the large-scale tionalfabrics in thelower plate suggests increasing geometricalevolution of the southernSnake Range exten- structuralinvolvement at depth,both in thesouthern Snake sionalsystem. The choiceof an appropriateline of section Rangeand in surroundingareas [Miller et al., 1988]. presentsan immediateproblem in cross-sectionconstruction Geometricalevolution ofthe SSRD. As with other major asseveral lines of evidencesuggest that the slip line maynot detachmentfaults, constraining the inclination of theSSRD havebeen constant throughout deformation. As notedprevi- throughtime presents a particularly challenging and ously,the slip line impliedby the orientationof lower plate importantproblem in cross-sectionconstruction. At present, stretchinglineation (mean L e -- 105%3ø; Figure 2) differs theSSRD dips < 15ø eastward throughout the southern Snake from thatimplied by motionplane analysis of cataclastic Range,but requiring the SSRD to maintain such dips shearsurfaces in theTertiary granite (mean motion plane -- throughoutits historywould give rise to seriousmechanical 80%77ø; Figure 5). In addition,upper plate normal faults problemsand incongruency with numerous observations on definetwo separablegenerations characterized by opposing thecharacter of active normal-fault seismicity throughout dipsand slightlydifferent strike orientations,with the theworld [Jackson, 1987]. Angular relationships with lower earliergeneration of westdipping faults generally striking platestrata provide some constraints onpossible initial 155ø - 160ø while the later,east-dipping faults usually strike orientationsof the SSRD. Throughout the eastern half of the approximatelyN-S (seeWhitebread [1969] as well as Plate rangethe SSRD parallels lower plate units, and regional McGrew:Origin of SouthernSnake Range D6collement, Nevada 29 reflectionseismic data suggest that this pattern continues platefaults relies entirely on restorationof upperplate fault towardthe east, with the Snake Range d•collement parallel- blocksto inferredpre-extensional stratigraphic positions. ingthe western limb of theCRST (COCORP Utah Line 1) Fortunately,involvement of a well-knownpre-extensional [Allmendingeretal., 1983]. Becauseregional stratal dips stratigraphyfacilitates this mode of reconstruction. probablydid not exceed 200-30 øat the onset of extension(see The involvementof a well-knownstratified sequence also above),this segment of theSSRD probably could not have placesimportant constraints on regionalfault reconstruction. originatedwith steepdips. Specifically,the Middle CambrianPole Canyon limestone Nevertheless,mapping by Whitebread [1969] indicates that (Cpc)is preservedboth in thehanging wall andin the towardthe west the SSRD cuts up-section across footwall footwallof the SSRD (Plates1 and 2). Consequently,the stratawith an angular discordance of approximately 30ø , thus exposureof a completesection of thisformation in thelower openingthe possibility that the SSRD may originally have platein thewestern part of themap area effectively defines a steepenedtoward the west as shown in Plate2. Therotation footwallcutoff. As a result,the upperplate Pole Canyon of thissegment of theSSRD to shallower dip could occur limestonemust have originated less than -14 km west'ofits eitherby rotation in thehanging wall of theSchell Creek presentposition (Plate 2). fault severalkilometers tO the west [Ganset al., 1985] or Lack of controlon the precisefault geometriesunderlying becauseof isostaticadjustments accompanying deformation andbounding southern Snake Valley presentsadditional [Buck,1988, Wernicke and Axen, 1988]. By contrast, a similar problemsfor regionalcross-section construction (Plate 2). isostaticmechanism could serve to rotatethe initially The natureof crosscutting relationships between the range- shallowlydipping segment of theSSRD first to steeper angle boundingfault andthe SSRD is a particularlyimportant andthen back to shallowdip asthe inflection point of uncertainty.Alam andPilger [ 1987]and Alam [ 1990]report isostaticuplift migrates through the region of interest[cf. thatproprietary seismic data obtained from Unocal for Spencer,1984; Wernicke and Axen, 1988]. southernSnake Valley imagemoderately dipping reflectors Therole that isostasy may have played in thetectonic evo- thatmay correspond to the SSRD. If so,then the SSRD must lutionof the SSRD is difficultto evaluatefrom an empirical steepenbeneath the western side of SnakeValley as shown in pointof view because absolute variations inelevation ofthe Plate2 (a possibilitythat accords well with therolling hinge Earth'ssurface during the course of orogenesisare modeldescribed above). However,a plausiblealternative notoriouslydifficult to constrain[England and Molnar, interpretationis that thesereflectors record the geometryof 1990].However, both the Tertiary unconformity data therange front fault ratherthan the SSRD, in whichcase the referredto previously [Gans and Miller, 1983] and the SSRDmust be tnmcatedby therange-bounding structure. palinspasticreconstruction shown in Plate 2 requirethat the Nevertheless,this alternativeinterpretation probably would CambrianPioche Formation was buried to paleodepthsin not greatlyalter the grosscross-section geometry beneath excessof 6 km beneaththe Earth'ssurface at the onsetof southernSnake Valley, althoughthe magnitude of extension Tertiaryextension. At present,this same formation is accommodatedby the SSRDwould decrease slighfiy. exposedatthe crest of thesouthern Snake Range, at least6 km An equallyimportant uncertainty concerns the geometry aboveits contemporary position beneath both the Burbank of upperplate faults beneath southern Snake Valley. Hills to the eastand the southernSchell Creek Range to the Fortunately,the narrownessof southernSnake Valley and the west(Plate 2) [Hoseand Blake, 1976]. Mesozoicfolding relativelywell knownstructure and stratigraphy of the probablyaccommodated only part of thisdifferential uplift, adjacentBurbank Hills provideconstraints ameliorating this with theremainder occurring during Tertiary exhumation, problem. Plate2 illustratesan upperplate fault geometry mostlikely as a resultof isostaticadjustments. In anycase, beneathsouthern Snake Valley thatis plausiblebut reconstructionof Plate2 withoutany uplift of lowerplate necessarilyspeculative. Nevertheless, whatever the precise rocksin thesouthern Snake Range relative to surrounding fault geometryunderlying Snake Valley, the narrownessof areaswould result in a retrodeformedsection showing > 4 km the basinserves to limit the magnitudeof errorassociated of verticalrelief over a lateral distanceof < 5 km. For these with thisuncertainty because extension within the upper reasons,Plate 2 assumesthat space problems between the plateacross Snake Valley canbe no lessthan 0 km andno hangingwall andthe footwall of theSSRD were accommo- greaterthan the width of the valley (8 kin). datedin partby footwalluplift as well as by hangingwall .Natureand geometryof lowerplate strain. Regardlessof collapse[cfi Wernicke and Axen, 1988]. how the uncertaintiesdiscussed above are resolved,differing Geometryof upperplate faults. In thesouthern Snake cross-sectionsare possible depending on thekinematic behav- Range,restoration of complicatedupper plate fault ior attributedto the SSRD. For instance,if a strict in situ geometriesposes another challenging problem. Map extensionmodel is invoked[e.g., Miller et a1.,1983],then relationshipsin the southern Snake Range imply two families thinningand nearlycoaxial stretching of lower plateunits of upperplate faults, an oldergeneration of relativelyclosely directlybeneath the SSRDmust fully accommodateupper spaced,west dipping faults and a youngergeneration of more plateextension at any givenpoint along the detachment.On widelyspaced, east dipping faults. Although the east dipping theother hand, if a strictsimple shear model is invoked[e.g., faultsdo not clearly cut the west dipping faults in thearea of Wernicke,1981; Bartley and Wernicke, 1984], then the final Plate1, suchcross cutting relationships are exposed h• geomett3'of lower plateunits depends on the distributionand adjacentareas mapped by Whitebread [1969] to the south. magnitudeof shearstrain and the initial orientationof lower Becauseof a lackof exposedcutoffs, reconstruction of upper platestrata relative to the shearzone boundary. As outlined 30 McGrew:Origin of SouthernSnake Range D6collement, Nevada earlier,lower plate strainrelationships in the southernSnake modelsof Proffett [1977], Gans and Miller [1983], and other Rangeseem to precludeeither end-member interpretation, but authors).However, as numerousauthors have pointed out, it betweenthese two extremesa broadspectrum of plausible is not possibleto completelyaccommodate space problems scenariosexists, including some that resemble the end- betweenfault blocks by rigid bodyrotation alone [e.g., memberinterpretations. Wernickeand Burchfiel, 1982], andso internaldeformation by In short,it seemslikely that somebut not all of the inclinedsimple shear was utilized to accommodateresidual extensionin upperplate rocks was accommodated in situby spaceproblems as necessary (a processthat does not conserve penetrativestretching directly beneath the SSRD. line lengths)[White et al., 1986;McGrew andCrews, 1990]. Consequently,the SSRD may well haveacted as a "stretching The reconstructionshown in Plate2 yieldsan estimateof shearzone" in the senseof Means [ 1989], with the detachment total extensionof approximately19 km betweenthe central functioningat leastin part to maintainstrain compatibility partof thesouthern Snake Range and the hingeline of the betweena relativelyrigid upperplate and a lowerplate that CRST. Assumingdifferent kinematic mechanisms or wasstretching plastically as deformationproceeded. Plate 2 contrastingcross-section parameters can give rise to different illustratesboth an end-memberstretching shear zone estimatesof extension,but extensionprobably lies between8 interpretationof the SSRD (solidlines) and a hybrid km and24 km providedthat the footwallcutoff of the interpretationin whichthe SSRD functionsin partas a CambrianPole Canyon Formation is honoredand that an stretchingshear zone and in partas a moreconventional upperplate fault reconstructionresembling that shown in through-goingshear zone (broken lines). In the fast case Plate2 is adopted. stretchingof lower platerocks by homogeneousplane strain completelyaccommodates displacement of upperplate rocks, DISCUSSION AND CONCLUSIONS whereasin the secondcase lower plate stretching accommodatesonly part of thedisplacement on theSSRD, In thepreferred scenario developed in Plate2, the SSRD with theremainder being taken up by through-goingshear. initatesas a plasticstretching shear zone separating an upper In eithercase, as extensionmigrates eastward through plate that deformsby frictionalsliding on steeplyinclined time,portions of the lowerplate lying to the westexperience normalfaults from a lower plate that deformsby penetrative unroofingand hence cooling and cessation of plasticdeforma- stretchingand pluton emplacement [cf. Miller et al., 1983; tionat an earlierstage than do areasto theeast (compatible Means,1989]. As illustrated,a majoreast dipping normal with theK-Ar coolingage patterns recognized by Lee et at. fault resemblingmodern range front fault systemsgoverns [1970]). Accordingly,the more protracted history of plastic upperplate deformation during the earlyphases of extension, deformation in the east results in a west-to-east strain with an arrayof closelyspaced down-to-the-west normal gradientin lowerplate units such as thatdepicted for theend- faultsantithetic to the masterfault accommodatinghanging memberstretching shear zone interpretation in Plate2. wall collapse[cf. White et al., 1986]. As extensionproceeds, While exposuresof lower plate rocksare not sufficiently tilt block rotationof the antitheticsystem results in extensiveto documentsuch a straingradient in the southern dramaticattenuation of upperplate rocks, thus applying a SnakeRange, a strikinglysimilar strain pattern is well negativeisostatic load to footwallrocks and ultimately documentedin thenorthern Snake Range [J. Lee et al., 1987]. resultingin relativeuplift of the lower plate and back- While'bearing much in commonwith in situextension rotationof the masternormal fault in the westernpart of the interpretationspreviously advanced for thenorthern Snake extendedterrain [cf. Zandt andOwens, 1980, Buck, 1988]. Ranged6cottement [Gans and Miller, 1983;Miller et at., Meanwhile,as areas to thewest are relatively uplifted, adja- 1983], it shouldbe notedthat the end-memberstretching centsegments of the SSRD that were originallyshallowly shearzone interpretation developed above differs in thatit dippingmust become more steeply inclined and thus can requiresa largecomponent of uniform-sensenoncoaxial remainactive as tectonicunroofing carries them upward into strainparallel to the SSRD. For example,in the westernpart thefrictional sliding regime [cf. Wernickeand Axen, 1988]. of the cross-sectionthe upperplate accommodatesat least Perhapsnet lateralflow of deepcrustal material out from 120% extensionwhereas the lower plate accommodatesno beneathless extended adjacent areas could help accommodate more than65 % extension,thus requiring substantial down- suchdifferential vertical motions [cf. Gans,1987]. In any to-the-eastdisplacement on the SSRD. case,as unroofing proceeds from westto east,upper plate Cross-section reconstruction. The cross-section faultsand the SSRDitself eventually rotate to sufficiently restorationillustrated in Plate 2 is balancedwith respectto shallowangles that further slip becomesimpossible due to areabut doesnot conserveline lengthsor stratigraphic frictionalresistance, and as a restfitnew steeplydipping thicknesses due to the deformationat mechanisms assumed. faultsmust form, trackingunroofing toward the east. To wit, lower plateunits were asstunedto stretchand thin Meanwhile,portions of the lowerplate in the westernpart of plasticallyduring deformation and so cannotconserve line thearea experience unroofing earlier and pass through Ar length. In addition,lateral differencesin verticaluplift closuretemperatures earlier than in the east(possibly accompanyingdeformation were accommodated by vertical contributingto the eastwarddecrease in K-At muscovite simpleshear. In upperplate rocks, space problems developing coolingages recognized by Lee et at., 1970]. In otherwords, betweenadjacent fault blocksduring extension were updipportions of theextensional system become defunct as accommodatedinsofar as possible by rigidbody rotations (a down-dipportions of the systemtoward the eastcontinue to line lengthconservative process resembling the tilt block accommodate extension.

McGrew:Origin of SouthernSnake Range Dtcollement, Nevada 33

AlthoughPlate 2 illustratesa plausiblescenario for the beneaththe SSRD appears to haveaccommodated at least part structuralevolution of the southernSnake Range, it shouldbe of upperplate extension, but a substantivecomponent of ESE evidentthat certainelements in this scenarioare more specu- directedshear is alsopresent. Consequently, the SSRD lative•_han others. Among the most poorly constrained probablyplayed a dualkinematic role, acting in partto elementsin thisscenario are the initial dip of the SSRD in the transferdisplacement down dip towardthe east and in partto westernpart of thecross-section, the nature of crosscutting resolverelatively localized strain mismatches between a relationshipsbetween the SSRD and therange front structure, relativelyrigid upper plate and a plasticallystretching lower andthe precise geometry of lowerplate units to theeast of plate. thesouthern Snake Range. The dip of theSSRD at inter- mediatestages in its evolutionaryhistory and the precise Acknowledgements.Elizabeth L. Miller providedparticu- effectof isostaticcompensation on the evolutionof this larlyinvaluable advice and support during the course of this geometrypresent other major elements of uncertainty. work. In addition,the 1984 StanfordGeological Survey con- Nevertheless,despite important uncertainties, several tdbutedmuch basic data and many important insights to this pointscan be madewith some confidence. The SSRDformed study.Excluding the author, the participants in this field asa plasticshear zone some time afteremplacement of the 36 campwere: Bill Aaron,Lisa Baugh, Adam Bryer, Shana Ma YoungCanyon-Kious Basin pluton, but by thetime it Brokken,Tony Campbell, Doug Clark, Claire Duncan, Phil ceasedto be active(probably before 17 Ma, at leastin the Gans,Carey Gazis, Stanley Grant, Peter Lewis, Karen Merino, SnakeRange proper) it wasprobably functioning as a brittle EricMiller, SteveMunn, Kirby Shanks, David Stahl, Andy normalfault. While the initial dip of the SSRD is uncertain Tomlinson,Jack Whittimore, and ElizabethMiller (leaderof alongthe western flank of the range,it very probablyformed SGS). PhilGans, Jeff Lee, and Jim Wright provided many at shallowinclination parallel to lower plate stratain the usefuldiscussions, and Karen Merino and Heidi S. McGrew east. In theupper plate, an earlygeneration of westward providedespecially important field and other assistance. Peg dippingfaults and a later generationof eastwarddipping Brownprovided vainable assistance in drafting. John Bartley, faultsclearly accommodated great extension, and attenuation Tekla Harms,and Richard Allmendinger provided thorough of hangingwall rocksultimately produced dramatic anduseful reviews. Funding for thisproject came in part unroofingand relativeuplift of lower platerocks compared fromgeneral grants from ARCO and SOHIO, in partfrom to surroundingareas. Suchdifferential vertical movements NSFgrant EAR 8418678to E. L. Milleret al., andin part are difficult to explainexcept in the contextof isostasy. froma StanfordGeology Department graduate fellowship. Finally,penetrative stretching of lowerplate rocks directly

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