View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by University of Dayton University of Dayton eCommons Geology Faculty Publications 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] Follow this and additional works at: https://ecommons.udayton.edu/geo_fac_pub Part of the Geology Commons, Geomorphology Commons, Geophysics and Seismology Commons, Glaciology Commons, Hydrology Commons, Other Environmental Sciences Commons, Paleontology Commons, Sedimentology Commons, Soil Science Commons, Stratigraphy Commons, and the Tectonics and Structure Commons 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 number 92TC01713. 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-