TECTONICS, VOL. 19, NO. 6, PAGES 1124-1143 DECEMBER 2000

Relations between hinterland and foreland shortening: Sevier orogeny,central North American Cordillera

WandaJ. Taylor •, JohnM. BartIcy2,Mark W. Martin3, John W. Geissmann, J.Douglas Walker •, Phillip A. Armstrong6,and Joan E. Fryxell7

Abstract. The tectonic relations between foreland and hinterland betweencentral Nevada and Utah thrusts,or that the Utah thrusts deformationin noncollisionalorogens are criticalto understanding persistinto southeastern Nevada but are located east of thelongitude theoverall development of orogens.The classiccentral Cordilleran of the central Nevada thrust belt. As a result of overall cratonward forelandfold-and-thrust belt in the United States(Late Jurassicto migrationof thrusting,the centralNevada thrust belt probably earlyTertiary Sevier belt) andthe moreinternal zones to the west formed the Cordilleran foreland fold-thrust belt early in the (centralNevada thrust belt) providedata critical to understanding shorteningevent but later lay in thehinterland of theSevier fold- the developmereof internaland externalparts of orogens. The thrustbelt of Idaho-Wyoming-Utah. GardenValley thrustsystem, part of the centralNevada thnkst belt, crops out in south-centralNevada within a region generally considered to be the hinterland of the Jurassic to Eocene Sevier 1. Introduction thrustbelt. The thrustsystem consists of at leastfour principal Manyinvestigations in the North American Cordillera and the thrustplates composed of strataas youngas Pennsylvanianin age Andes focus intereston tectonic relationsbetween deformationof that are unconformablyoverlain by rocks as old as Oligocene, internal and external zones of noncollisional orogens [e.g., suggestingthat contractionoccurred between those times. New Armstrong,1982; Jordan et al., 1983;Sheffels, 1990;/lllmendinger U/Pb dateson intrusionsthat postdatecontraction, combined with et al., 1990;/lllmendinger,1992; Schmitz, 1994; Kley, 1996; Lamb newpaleomagnetic data showing significant tilting of onearea prior and Hoke, 1997; Okayaet al., 1997; }Yells,1997]. Relations to intrusion,suggest that regionally these thrusts were active before betweenthe Late Jurassic to earlyTertiary Sevier foreland tlm•t belt -85-100 Ma. The thrustfaults are characterized by long,relatively in Idaho,Wyoming, and Utah and the more internal zones of the steeplydipping ramps and associated folds that are broad and open Cordi!leranorogen play a centralrole in evolvingthought regarding to close,upright and overturned.Although now fragmentedby foreland-hinterlandrelations [Armstrong,1968; Fleck, 1970; Cenozoiccrustal extension, individual thrusts can be correlatedfrom /lllmendingerand Jordan, 1984; Heller et al., ! 986;Lawton, 1986; rangeto rangefor tensto hundredsof kilometersalong strike. We Miller et al., 1988; }Yannamakeret al., 1997]. The areareaching correlatethe structurallylowest thrust of the GardenValley thrust from western Utah to central Nevada and northward into southern system,the GoldenGate-Mount Irish thrust,southward with the Gass Peak thrust of southern Nevada. This correlation carries the Idahohas long been regarded as the hinterland of theSevier thrust belt[e.g., Misch, 1960]. Interpretations of the structural style of the followingregional implications. At leastsome of the slip across Jurassic to mid-Cretaceous foreland thrusts in southern Nevada hinterlandand its tectonicrelationship to the forelandthrust belt varywidely. Armstrong [1968, 1972, 1982] concluded that during continuesnorthward along the centralNevada thrust belt rather than thin-skinnedforeland thrusting the hinterlandunderwent intense noaheastwardinto Utah. This continuationis consistentwith age ductiledeformation at middleto lowercrustal depths but mild upper relations,which indicate that thrusts in thetype Sevier belt in central crustaldeformation. This interpretation has been adopted by many Utah are synchronouswith or youngerthan the youngestthrusts in others[e.g., Jordan et al., 1983;Miller and Gans,1989, 1990; southernNevada. This in turn impliesthat geometricallysimilar Nelson,1990]. Folds,low-angle faults, and tectonic rock fabrics Sevierbelt thrustsin Utah mustdie out southwardbefore they reach recognizedin the region west of thefrontal Sevier thrusts [Nolan, Nevada,that slip alongthe southemNevada thrusts is partitioned 1935,1962; Misch, 1960; Cebull, 1967; Quinlivan et al., 1974;Hose and Blake, 1976; Tschanzand Pampeyan,1970] have been •Departmentof Geoscience, University of Nevada,Las Vegas. interpretedeither to affectonly deeper structural levels [e.g., Miller 2Departmentof Geology & Geophysics,University of Utah,Salt Lake et al., 1988;Miller andGans, 1989], to recordonly minor crustal city. shortening[/lllmendinger and Jordan, 1984; Miller andHoisch, 3U-PbGeochronology Laboratory, Massachusetts Institute of Technology, 1992;Miller andLush, 1994], or to haveformed during subsequent Cambridge. Mesozoicand/or Cenozoic crustal extension [e.g., Moores et al., 4Departmentof Earth & PlanetarySciences, University of NewMexico, Albuquerque. 1968;Armstrong, 1972]. In contrast,syntheses bySpeed [1983] and 5IsotopeGeochemistry Laboratory, University ofKansas, Lawrence. Speedet al. [1988]included significant contraction along a "Eureka 6Departmentof Geological Sciences, California State University, thrust belt" in centralNevada, but the nature, magnitude,and Fullerton. tectonicsignificance of thisthrust belt were largely unspecified. 7Departmentof Geological Sciences, California State University, San Resolutionof theuncertainties in age and kinematics of hinterland Bernardino. deformationalso is necessaryto future improvements in kinematic synthesisof Cordilleransubduction-related orogeny [e.g., Bird, Copyright2000 by theAmerican Geophysical Union 19981. Papernumber 1999TC001141. Major problemsin definingthe Sevierhinterland and in 0278-7407/00/1999TC001141 $12.00 elucidatingits structural history reflect difficulties in distinguishing

1124 TAYLORET AL.: RELATIONBETWEEN FORELAND AND HINTERLANDSHORTENING 1125

amongdifferent ages and kinematichistories of structures(e.g., Mount Irish thrusts and associatedfolds. The system is compareMisch [1960] andCebull [1967] with Mooreset al. [1968] characterizedby thrustswith long,relatively steeply dipping ramps andArmstrong [1972, 1982]see also Smith et al. [1993]and Lawton andopen to closefolds that are typicallyupright in upperplates of et al. [1994]) and in unravelingtheir overprintingrelations. thrusts;overturned folds occur mainly in thrustfootwalls. Individual However,field mapping and isotopic dating [e.g., Allmendinger and thrust faults can be correlatedfrom range to range for tens to Jordan,1984; Lee et al., 1987;Fryxell, 1988;Bartley and Gleason, hundredsof kilometersalong strike. 1990;Wells et al., 1990;Taylor and Bartley, 1992; McGrew, 1993], Withinmany of the range-scalefault blocks where the thrusts are combinedwith betterunderstanding of the structuralgeometries of exposed(i.e., the TimpahuteRange-Worthington Mountains, and contractionaland extensionalbelts [e.g., Boyer and Elliott, 1982; MountIrish-Golden Gate Ranges), extensional deformation is small Wernickeand Burchfiel, 1982; Davis et al., 1983; Woodwardet al., in magnitudebut locallyintricate in geometry(e.g., Figure 2). Such 1985; Wernickeet al., 1988a],now permit clearerdistinctions Cenozoicextensional structures, including -N-S strikingnormal between structures related to crustal extension and contraction. faultsand E-W strikingfaults, variably overprint the GardenValley Geologicmapping indicates that thrusts with significant offset crop thrustsystem (Figure 2). The GoldenGate Range-MountIrish out in centraland eastern Nevada [e.g., Nolan, 1935;Tschanz and Cenozoicnormal-fault block is separatedfrom the Worthington Pampeyan,1970; Fryxell, 1988; Bartley and Gleason, 1990; Mountains-TimpahuteRange block by large-displacementnorth Armstrongand Bartley, 1993; Taylor et al., 1993; Dobbset al., strikingnormal faults. Tertiary volcanic rocks exposed between the 1993]. Thesestudies, including new field observationsreported southernWorthington Mountains and Timpahute Range (Figure 2) here,permit us to refinethe structural development of a significant arecut by numerousnormal faults that have been active episodically partof theSevier hinterland and to evaluatethe tectonic relationship since24 Ma [Taylor, 1992]. Eaststriking Cenozoic normal faults between the hinterland and the classic Sevier foreland thrust belt definethe southernboundary of thismore extended region with the (Figure 1). easttrending Timpahute Range fault block. An eaststriking fault Definedby Bartley and Gleason [1990], the Garden Valley thrust zonealso separates Paleozoic strata of the TimpahuteRange block systemis part of the centralNevada thrust belt and comprises frommore extended Tertiary volcanic rocks on its south(Figure 2). contractionalstructures exposed in severalranges southeast of RailroadValley (Figures 1 and2). Thepurposes of thispaper are (1) to describecontractional structures of theGarden Valley thrust 3. Stratigraphy systemwhich were previously not well understood,(2) to correlate 3.1. RelevantStratigraphy these contractional structureswithin the central Nevada thrust belt andregionally, and (3) to interpretthe relationshipbetween the Althoughcomprehensive stratigraphic descriptions lie outsidethe centralNevada thrust belt and the Sevierorogenic belt. Geologic scopeof this work [seeKellogg, 1963; Reso, 1963; Tschanzand mappingat 1:24,000and 1:12,000 scales of theGolden Gate Range, Pampeyan, 1970; Best and Grant, 1987; Best et al., 1989; WorthingtonMountains, Timpahute Range, and Mount Irish plus Hurtubise, 1989; Taylor, 1990; Best and Christiansen,1991; paleomagneticdata and U-Pb geochronology [also see Martin, 1987; Hurtubiseand du Bray, 1992],differences in stratigraphicrelations Armstrong,1991 ] areintegrated with published work [e.g., Tschanz of Paleozoicand Cenozoic rocks from one thrust plate to the next andPampeyan, 1970; Burchfiel et al., 1983;Wernicke et al., 1988b; are importantto structuralcorrelation. Therefore,we briefly Bartley and Gleason,1990; Jayko, 1990] to characterizethe introducethe regional stratigraphy. structuralstyle of thethrust belt, correlate structures along strike, The GardenValley thrustsystem deforms strata typical of the and reevaluatethe regional-tectonicframework for Mesozoic southernCordilleran continental margin that range from Late forelandthrusting in thissection of theCordilleran orogen. Precambrianto Pennsylvanianage [Kellogg, 1963; Stewart et al., 1977]. The strataare dominantlyclean and silty carbonate rocks 2. RegionalGeologic Setting with subordinatecalcite- and silica-cementedquartz sandstone, siltstone,and shale (Figure 3). This well-describedmiogeoclinal The centralNevada thrust belt is a north trendingbelt of sequencepermitted mapping and correlation of the sameformations contractionalstructures, continuous except where interrupted by throughoutmost of the area.Cenozoic rocks that overliethe thrust Cenozoicnormal faults,that can be tracedfor > 400 km northward systemare mostlyTertiary ash flow tuffs with subordinatelava fromthe Pahranagat shear zone (-37ø142q) in southernNevada to flows, freshwaterlimestone, and continentalsiliciclastic rocks of the northernAdobe Range (-41ø19•q). At its southernend the bothTertiary and Quaternary ages (Figure 3). centralNevada thrust belt projectstoward thrusts and folds of the Becausea regionaldecollement is notexposed, the oldest rocks Sevierorogenic belt in southernNevada (Figure 1) [Armstrong exposedin the thrustplates provide the only informationabout the 1968]. Part of the centralNevada thrust belt previouslywas depthof a regionalthrust decollement. In the Grantand Quinn identifiedas part of theEureka belt in thearea north and northwest CanyonRanges the LowerCambrian Prospect Mountain Quartzite of RailroadValley [Speed,1983; Speed et al., 1988]and the Garden is exposedin thrustplates. The oldest rocks exposed in otherranges Valleythrust system to thesoutheast of RailroadValley [Bartley and areMiddle Cambrian or younger.Therefore a regionaldecollement Gleason,1990; Armstrong and Bartley, 1993]. Although no specific wouldhave to lie belowthe Lower Cambrian rocks carried by the structurehas been correlated across Railroad Valley, contractional GardenValley thrustsystem. structureslocated northwest and southeastof RailroadValley lie approximatelyalong strike of eachother and have comparable styles 3.2. Sub-Tertiary Unconformity andcompatible timing brackets. Therefore the existence of a single, Theunconformity beneath Tertiary strata served as the principal once-continuous fold-thrust belt seems clear. datumfor reconstructingcross sections to their preextensional The Garden Valley thrust systemcomprises the Sawmill, geometry(Figure 4). The outcroppattem and angularity of thesub- SchofieldCanyon, Rimrock-Freiberg-Lincoln, and GoldenGate- Tertiary unconformityindicates that the unconformityascends 1126 TAYLORET AL.' RELATION BETWEEN FORELAND AND HINTERLAND SHORTENING

116øVV

,__ Canada 14Ow Railread Valley PACIFIC. OCEAN•

/ I Willow ( 1RANGE

Quinn Canyon GGR Andr•s

....•...:..

Lincoln Figure8

37o30'N

thrust

150 km :Spring _., Mountains 00 miles

Figure1. Locationsand names of ranges(stippled), valleys (white) and major contractional structures that are discussedin the text, including the Garden Valley thrust system. Box within the inset shows the location of the largermap. A, northern Adobe Range; CNTB, central Nevada thrust belt; EB, Eureka thrust belt [after Speed etal., 1988];EPF, East Pahranagat fault; Fs, Freiberg stock; GGR, Golden Gate Range; GPT, Gass Peak thrust; LCTS, locationofLast Chance thrust system; LD, Lincoln duplex; LT, Lincoln thrust; MI, MountIrish; MIT, MountIrish thrust;P,4, Pintwater anticline; PSZ, Pahranagat Shear Zone; ST, Schofieldthrust; TMA, Timber Mountain anticline;TR, Timpahute Range; and WM, WorthingtonMountains. TAYLOR ET AL.: RELATION BETWEEN FORELAND AND HINTERLAND SHORTENING 1127

115 ø 30'

Freiberg thrust •

numerous small

ß 37ø e ß

ß ß

ß

ß ß Lincoln

Schofield Pass fault

Lincoln thrust

Mt. Irish

lto4 Tikaboo:. Valley fault EXPLANATION

-• Quaternaryand 0 1 2 3 4 5 miles Tertiarybasin fill I 115 ø 30' 0 1 2 3 4 5kilometers *•...... TertiaryUnits _..• LincolnDuplex l :•..::. MountIrish [• TempiuteRidgeBlock ThrustHate Standardnormal fault (ball and bar), thrust fault (barb), normal-oblique :• Lincoln Thrust Plate Mt.FootwallIrish slipapproximately (arrows +balllocated, and bardotted )and where anticline concealed symbolsused, dashedwhere

Figure2. Tectonicmap showing major structures of part of the Garden Valley thrust system and traces of major Cenozoicnormal and strike-slip faults that overprim them.

structurallytothe west through successive thrust plates, as might be ashflow tuffs. Tertiarynormal faults commonly displace these tuffs expectedcrossing an east vergentthrust belt. The angular in theMount Irish Range and are rare in the southernGolden Gate discordanceacross the unconformitymainly reflects folding and Range.Also, in thisthrust plate, one prevolcanic normal fault crops tiltingof Paleozoicrocks during thrusting and only locally within out in the easternTimpahute Range. one thrustplate includesthe effectsof pre-Oligoceneextension Tertiarytuffs exposedat the southernend of the Worthington whichwas of smallmagnitude. Mountainsunconformably overlie Mississippian rocks that lie in a LateOligocene age tuffs unconformably overlie Devonian and duplexabove this thrust plate. The unconformityis lowto moderate Mississippianrocks in the GoldenGate-Mount Irish thrustplate, angle,dips gentlyeast, and is cut and tilted by west dipping whichis theeasternmost thnmt plate (Figures 1 and2). Restoration Cenozoic normal faults. of crosssections to thrust belt geometrymust accountfor the In thecentral and eastern parts of theTimpahute Range, shallow moderate(200-35 ø) westwarddip of compactionfoliation in these to moderatelywest dipping Tertiary ash flow tuffs overlie 1128 TAYLOR ET AL.: RELATION BETWEEN FORELAND AND HINTERLAND SHORTENING

Mt. Irish - Golden Gate Thrust Plate Rimrock- Freiberg- Lincoln _Qu_a•m__ary_all uvium Thrust Plate metersv_.v__ Tert. / Quat. alluvium_ 5000 ' Pahranaeat Tuff UpperStiingle Pass Tuff meters Tuffof Hancock Summit IVvvvvvvl LowerShingle Pass Tuff 3500- Guilmette Formation MonotonyTuff _. 4000'1"r" '.l I ElyLimestone ScottyWash Quartzite 3000- SimonsonDolomite ChainmanShale JoanaLimestone Oxyoke CanyonSandstone 3000- WestRange Limestone SevyDolomite 2500

LaketownDolomite Guilmette Formation •7s•h33-e• Dr716-n•t• • 2000- EurekaQuartzite 2000- Simonson Dolomite AntelopeValley OxyokeCanyon Sandstone Limestone • SevyDolomite Laketown Dolomite 1500- Kanosh Shale • • Fishhaven Dolomite ShingleLimestone '•'• 1000- ParkerSpring Fm.• Eureka Quartzite 1000- PogonipGroup GoodwinLimestoneJ- o-• •• 0t WindfallLimestone _• • Undifferentiated •

Figure3. Representativestratigraphy of the Garden Valley thrust system. Differences in theOrdovician units are importantfor distinguishing thethrust plates. Stratigraphy of the Rimrock-Freiberg-Lincoln thrust plate is modified from BartIcyand Gleason[1990]. The Mount Irish-GoldenGate thrustplate columnalso includesthe unconformablyoverlying Cenozoic units, which were deposited after thrusting. The standard lithologic symbols are used. Dark ovals indicate chert nodules.

Ordovician to Devonian rocks in the Lincoln plate, which fold strataas youngas Pennsylvanian.No known Permianor structurallyoverlies the duplexand the underlyingMount Irish Mesozoicstrata crop out in the area. The oldeststrata not involved thrustplate. The sub-Tertiaryunconformity is notpreserved on the in shorteningare Oligocene strata, which overlie the GoldenGate, northerncontinuation of this thrust plate in the Worthington Mount/fish, and Lincolnthrusts or thrustplates. However,farther Mountains,which causes greater uncertainty in reconstructingthe north in the central Nevada thrust belt, Permian and Cretaceous preextensionalgeometry of thisarea. rocks are deformed[e.g., Taylor et al., 1993; Vandervoortand Tertiaryvolcanic and sedimentaryrocks overlie Ordovician to Schmitt,1990]. Devonianstrata of theRimrock thrust plate and Ordovician strata of Foursmall plutons cut structuresor tilted stratain the Garden the Sawmill plate in the Grant and Quinn CanyonRanges. Pre- Valleythrust system. These plutons were suggested previously to Oligocene removal of mid-Paleozoicstrata appearsto be a be Cretaceousor Cretaceousto Tertiary in age, so improved distinctivefeature of the Sawmillthrust plate as a resultof its high geochronologycould constrain the timing of thrusting.The plutons structuralposition. eitherhave not beendated previously or weredated only by either Relationshipsacross the sub-Tertiaryunconformity on the the K/Ar or Rb/Sr method. We therefore conducted U/Pb SchofieldCanyon thrust plate, includingthe Timber Mountain geochronologyin an effortto bracketmore narrowly the ageof anticline, are uncertain. Field exposuresare insufficient to thrusting. The new data indicateplutons of both Tertiary and determineif Oligocenetuffs exposedin the Troy Canyon area Cretaceousages. [Cebull,1967, 1970;Fryxell, 1988]depositionally overlie Cambrian The Willow Creekstock cuts west vergent structures in theupper strata of the overturned limb of the Timber Mountain anticline or if plateof the Sawmillthrust in theQuinn Canyon Range [Bartley and theywere faultedinto that position. Gleason,1990]. Six zirconfractions were analyzed from a sample 4. Age of Contraction of thisstock (Table 1 andFigure 5). The zirconshave low U and radiogenicPb concentrations.For that reason, a K-feldsparseparate Theage of contractionon the Garden Valley thrust system is only wasanalyzed to determinethe isotopiccomposition of commonPb widelybracketed by stratifiedrocks. Thrustsin the systemcut or andthe correctionto the zircondata (Table 1). The analysesdefine TAYLOR ET AL.: RELATION BETWEEN FORELAND AND HINTERLAND SHORTENING 1129

t i i I

!

i t 1130 TAYLOR ET AL.' RELATION BETWEEN FORELAND AND HINTERLAND SHORTENING

D ! meters PahranagatRange 5200 westernpartof easternpartof 4200 hangingwall feet hanginganticline wall ]• •,anticline 3200 D ! 15,000 2200 MI/MDp{t•] • \ x Tv

1200 )g ß , -- 10,000

200 --5000

-800

-1800

-2800 •ez••,.•...... -.. ,..._,, -5000 -3800

-4800 ---10,000

VerticalExaggeration alluvium volcanic rocks Ely Ls W Bird SpringFm E LasVegas Range E'E meters •Opj•.•• • • feet Chainman Sh 3000- • --"'-"'-- '---. •10,000' limestone West RangeLs • • • • • / • • 18,000' = Pilot Sh Guilmette Fm 1500- . 6,000, Simonson Dolo SevyDolo Laketown Dolo 0- 000, Fish Haven Dolo 2 000' & EurekaQtzt

-1500- -4,000' AnteleopeValley Ls Middle PogonipGp • •bp -6,000' Goodwin Ls Nopah Fm -3000' ...... 7-,0,0o0 , DunderbergSh NoVeflical Exaggeration 12000' BonanzaKing Frn, BandedMtn Mbr, PapooseLake Mbr Carrata Fro, ZabriskiQtzt Wood Cyn Fm StiflingQtzt Johnnie Fm Figure 4. (continued)

a discordwith a lowerintercept of 31.9ñ 2.3Ma thatis interpreted ageis somewhat younger than typical basement ages for this region, asthe age of magmaticcrystallization. The Quinn Canyon Range butxenocrystic zircon could have been derived from Neoproterozoic includespart of a majorCenozoic volcanic center [e.g., Sargent and clasticstrata believed to underliethe area. Houser,1970], and this date is withinerror of Ar/Ar agesfrom Contactmetamorphism associated with eraplacementof the Tertiaryvolcanic strata near there [e.g., Taylor et aL, 1989;Best et Freibergstock, a granitestock near the northernend of the aL, 1993]. We thereforeinterpret the Willow Creek stock to bean WorthingtonMountains, overprints structures related tothe Freiberg Oligocenesubvolcanic intrusion. The upper intercept age is 1381 thrust[Martin, 1987]. Threeanalyzed zircon fractions define a ñ 27 Ma, interpretedasthe U/Pb age of xenocrysticzircon. This discordantlinear array on a concordia plotwith a lower intercept age TAYLOR ET AL.' RELATION BETWEEN FORELAND AND HINTERLAND SHORTENING 1 131

• • • • • • • o o o o o 1132 TAYLOR ET AL.' RELATION BETWEEN FORELAND AND HINTERLAND SHORTENING

0.0615 0.0425 LincolnStock Troy Granite 0.0515 EM3 (o) and L 0.0375

0.0325 0.0415

2 •.. 0.0275 0.0315 •C•0.0225 ,,• • 1628._7Ma+ 526.6 Ma 130s/' • _ _ _Interceptsat 0.0215 / • and 98.1 Ma_+ 80.8 Ma • • 1586.9Ma___ 58.6 Ma and 0.0175 /s" • 86.4Ma + 4.6 Ma

0.0115 0.1300 0.1800 0.2300 0.2800 0.3300 0.3800 0.4300 0.4800 0.0125 • (MSWD=10.6) 0.1300 0.1800 0.2300 0.2800 0.3300 207pb/235U 207pb/235U ! i i 0.0474 300•'" WillowCreek Stock • 0.012 - FreibergStock 7• _ QC41.1 0.0374 0.010 _WM-1 6• _ 200 c• 0.0274 .• 0.008- •zircon 45_ / / •'• - sphene / / • 0.006 cq 0.0174 •a / Interceptsat • • 1381.3Ma_+ 27.1 Ma and 25•/.• • 4212Ma_+l•30 Ma and •..--• 31.9Ma_+ 2.3 Ma 0.0074 (MSWD = 15) [• 25.1Ma+0.3Ma- [ • I I I 0.0200 0.0700 0.1200 0.1700 0.2200 0.2700 0.3200 0.01 0.03 0.05 0.07 0.09 207pb/235U 207pb/235U Figure5. U/Pbconcordia diagrams plotted from analyses of samples of plutonsthat cut structures in the Garden Valleythrust system. These plutons were suggested previously tobe Cretaceous orCretaceous-Tertiary inage, and thusU/Pb dating was done to better define the age of contraction orto determine whether the plutons would better constraintheage of contraction. Pluton locations areshown in Figure1. Zirconfractions were analyzed from all stocks.In addition,sphene was analyzed from the Freiberg stock. For the Lincoln stock the circles and squares indicateanalyses that came from two different samples from the same part of the stock. Note that the scales vary amongthe diagrams.

of24.9 + 3.4Ma andan upper intercept of 1490+ 480Ma (Figure t 2.7,95.0 t 2.9,and 97.9 + 3.0 Ma (Kruegerand Schilling [1971] 5). A singlesphene fraction yielded a concordantage of 25.1Ma. recalculatedwith constants of Steigerand Jaeger [1977]) suggest Thesphene and zircon data when regressed together yield lower and thatthe stockcooled through -250ø-300øC at ---90-98 Ma. The upperintercept ages of 25.1 + 0.3 Ma and 1420 + 100 Ma, relativelynarrow contact aureole and lack of regional metamorphism respectively(Figure 5). Thelower intercept and concordant sphene suggestthat the Lincolnstock was emplaced at a shallowcrustal ages overlapwithin uncertainties,and the patternof zircon level. Thereforethe K-Ar agesmay record cooling soon after discordanceis characteristicof inheritedxenocrystic zircon. emplacementof the pluton and may be close to thecrystallization Therefore,the lower intercept age defined by joint regression of the ageof thepluton. Two samples(EM-3 andLM-1) fromdifferent zirconand sphene data is interpretedasthe age of crystallizationof partsof the same phase of the Lincoln stock (Table 1) were analyzed theFreiberg stock. The age and location of theFreiberg stock are to determinea U/Pb age. In all, sevenzircon fractionswere consistentwith it alsobeing a subvolcanicintrusion related to the analyzed(Table 1 andFigure 5). Thedata yield a poorlydefined Tertiaryvolcanic center exposed in the adjacentQuinn Canyon lowerintercept age of 98 + 81 Ma. The scatterin the datais not Range[e.g., Martin and Bartley, 1989]. The upper intercept age is significantlyreduced when data from only one sample are plotted consideredto reflect an inheritedcomponent with an agethat is (Figure5). Theorigin of thescatter is uncertain,but given the statisticallyindistinguishable from that of theWillow Creek stock, ubiquitousevidence for xenocrysticzircon inheritedfrom a whichis consistent with the intrusions sharing a commonsource. Proterozoicsource, it is likelyto reflectoverprinting of Pbloss on Morerestrictive age limits are placed on contractionalstructures discordancedue to inheritedzircon. Heterogeneity ofthe original bythe Lincoln and Troy stocks. Paleozoic strata on Tempiute Ridge xenocrysticzircon populations also is possible. However, the lower weresteeply tilted, presumably byeast vergent folding related to the interceptage is consistentwith the K/Ar agesfrom the stock, and we Lincolnor a structurallyhigher thrust before the Lincoln stock was thereforefavor the interpretation that the pluton is mid-Cretaceous intruded(Appendix 1). K-Ar dateson biotite from the stock of 90.0 in age. TAYLOR ET AL.' RELATION BETWEEN FORELAND AND HINTERLAND SHORTENING 1 13 3

In the central Grant Range, the Troy stock cross cuts the containedwithin the upperplate 3-4 km to the westof the window overturnedlimb of the Timber Mountain anticline, a large east (Willow Creek backthrustzone) exposeNeoproterozoic-Lower vergentfold interpreted to haveformed at a relativelydeep structural CambrianProspect Mountain Quartzite. This relationindicates the level within the centralNevada thruststack (Figure 1) [Fryxell, absenceof a significantdecollement at any stratigraphiclevel from 1988]. The Troy stockdata yield a four-pointarray that defines a Lower Cambrian to Lower Devonian. Cross-sectionanalysis lowerintercept of 86.4 + 4.6 Ma (Figure5), whichis interpretedas indicatesat least5 km of thrustslip at theexposed ramp, but a larger thecrystallization age of thestock. The upper intercept is 1587ñ 59 valuecannot be excluded[Bartley and Gleason,1990]. The thinner Ma, consistentwith regionalbasement ages. and more basinal facies of Cambro-Ordovician rocks in the Sawmill On thebasis of theselimited data we proposethat much, perhaps thrustplate relative to otherthrust plates in the areais consistent all, thrustingin thecentral Nevada thrust belt was complete by early with but doesnot requirelarge displacementacross the Sawmill LateCretaceous time. Regionally,contraction in the centralNevada thrust. thrustbelt is bracketedbetween the Permianand Cretaceousby cross-cuttingrelations [e.g., Armstrong, 1968; Taylor et al., 1993]. 5.2. Schofield Canyon Thrust and the Timber Mountain The intrusionsdiscussed above provide the Late Cretaceousupper Anticline age limit of deformation. Severalranges in the northerncentral The SchofieldCanyon thrust and Timber Mountain anticline crop Nevadathrust belt exposecarbonate-dominated Lower to Middle out in the southernGrant Range (Figure 1). The thrust ramps Permianplatformal strata, some of whichare foldedand thrusted. southward from Ordovician Goodwin Limestone to Ordovician These strata include Pennsylvanian-PermianEly Limestone(as EurekaQuartzite in the footwalland throughthe CambrianLittle definedby Lawson[1906] and Steele[1960]), PermianArcturus Meadowsand Windfall formationsin its hangingwall. The throw Formation,Permian Carbon Ridge Formation, and Permian Riepe of the thrust is estimatedto be 2.7 km [Fryxell, 1991]. The SpringFormation [e.g., Taylor, 1990; Taylor et al., 1993;Overtoom, recumbentTimber Mountain anticline, located in the hangingwall 1994]. LatePermian tectonism in thearea cannot be excludedbased of the thrust,is interpretedto be geneticallyrelated to it. The on possiblePermian synorogenic conglomerate in the northern Timber Mountain anticlineis intrudeddiscordantly by the Troy PancakeRange [Perry, 1991, 1992] andPermian conglomerate of Granite(86.5 +/- 4.6 Ma) whichformed a relativelynarrow contact uncertaintectonic significance in the Eurekaarea to the north[e.g., metamorphicaureole that overprintsand thus postdatesboth Gallegosand Wardlaw,1992]. anticline-relatedstructures and regional metamorphicmineral assemblages. The SchofieldCanyon thrust and Timber Mountain anticline are 5. Garden Valley Thrust System locatedin the footwallof a largeCenozoic low-angle normal fault, TheGarden Valley thrust system originally included the Sawmill, the Troy Peakfault [Fryxell, 1988], whereasother mapped thrust Rimrock,Freiberg, and Golden Gate thrusts. Observations reported belt structuresin the Grant and Quinn Canyon rangeslie in its here allow us to correlate the Lincoln and Mount Irish thrusts in the hangingwall. Uncertaintiesin the directionand magnitude of slip Timpahute-MountIrish area to thrustsin theGarden Valley thrust acrossthe Troy Peak fault make structuralcorrelations across it systemand the Mount Irish thrust southward to thePahranagat and speculative. Becauseit is the only exposedthrust that involves GassPeak thrusts. These correlationsform a key step toward metamorphicrocks, the SchofieldCanyon thrust may be either a regionalcorrelation and regional tectonic interpretation of the thrust structurallylower thrustor a deeperexposure of one of the other system.In thefollowing sections we describebriefly the thrust belt thrusts. structuresexposed in the Quinn Canyon and Grant Ranges, Cole and Cashman[1999] suggestthat this deepestthrust may WorthingtonMountains, Timpahute and Golden Gate Ranges, and correlateto theBelted Range thrust exposed in theNevada Test Site MountIrish, proceeding generally from north and west to southand region. The structuralcomplexity in the Grant Range makes eastacross the belt. Thisdirection also is generallyfrom structurally correlationdifficult; however, the SchofieldCanyon thrust has less highestto structurallylowest through the thruststack with the stratigraphicseparation than the Belted Range thrust (2.7 compared exceptionof the SchofieldCanyon thrust. to 7 km) andmay be structurallylower.

5.1. Sawmill Thrust 5.3. Rim rock, Freiberg, and Lincoln Thrusts and Related Structures The Sawmillthrust is thestructurally highest known thrust of the GardenValley system,and is exposedin structuralwindows in 5.3.1. Rimrockthrust. The Rimrockthrust [Bartley and Sawmilland Hooper canyons in the QuinnCanyon Range (Figure Gleason,1990] cropsout on the eastside of the southernGrant 1). Oligocenevolcanic rocks unconformably overlie upper plate Range(Figure 2) andperhaps locally on thewest side. The thrust Ordovicianstrata at both windows. Bartley and Gleason[1990] wasfirst mapped by Cebull [1970] and Sainsbury and Kleinhampl describedthe thrusts and interpreted their modification by Cenozoic [ 1969]who did not distinguish among the primary thrust contact and normalfaults. They concluded that the lower plate of the Sawmill twomajor Tertiary normal faults that excise the thrust plane: the thrustis theRimrock thrust plate. In the SawmillCanyon window TroyPeak fault [Fuvxell, 1988, 1991; Taylor et al., 1993]and the [Murray, 1985] the thrust placesCambrian and Ordovician WadsworthRanch fault [Bartley and Gleason, 1990]). The Tertiary limestoneupon Devonian dolostone. The SawmillCanyon window faultsobscure the geometryand kinematicsof the thrust.In the providesa strike-parallelexposure of the thrustand yields little southernGrant Range the thrust dips gently to thenorthwest and informationabout fault geometryor vergence. In the Hooper placesOrdovician Pogonip Group limestone on Devonian dolostone Canyonwindow the Sawmillthrust cuts up sectionto thesoutheast with a stratigraphicthrow of-• 1.2 kin. It cuts down section throughboth footwall Silurian and Devonian dolostones and upper northwardalong strike in bothits footwall and hanging wall to place plateUpper Cambrian strata. West vergent minor thrusts and folds OrdovicianGoodwin Limestone over SilurianLaketown Dolomite. 1 134 TAYLOR ET AL.: RELATION BETWEEN FORELAND AND HINTERLAND SHORTENING

Acrossstrike the thrustcuts up sectionto the southeastin bothits The Lincoln duplexin the WorthingtonMountains comprises hangingwall andfootwall, indicating southeast vergence. rocks rangingfrom the DevonianGuilmette Formation to the 5.3.2. Freiberg thrust. The northstriking Freiberg thrust is the MississippianChainman Shale that are foldedand imbricatedby maincontractional structure exposed in the WorthingtonMountains severalgently west dipping faults. Folds measured in Mississippian [Tschanzand Pamœeyan,1970; Martin, 1987]. The Freibergthrust limestoneplunge gently north (Figure 6), suggestinga maximum rampsup sectionto theeast at a moderateangle (300-50 ø) through shorteningdirection that trends roughly N80øE. Low-anglefaults bothhanging wall and footwallstrata. The thrustcuts gradually variouslyplace younger rocks on older and older on younger. upsectionto the southin its hangingwall, progressivelyplacing However, rocks within the duplex are overturnedlocally, and OrdovicianPogonip Group limestone and Eureka Quartzite upon a thereforeyounger-over-older relations across faults may reflect footwallof Devoniandolostone. Stratigraphic throws range from either thrustingin an overturnedsection or overprintingby low- 0.6 to 1.2 km (Figure4). Mesoscalefolds above the thrustverge anglenormal faults. eastward. Along its southernmostexposure a complexarray of Our correlationof imbricatedrocks in the southernWorthington upperplate splays and breakback thrusts above the basalFreiberg Mountainswith the Lincoln Duplex is not uniquebecause in the thrustplane suggests a complexand perhaps prolonged history of southernWorthington Mountains the roof thrust, if present,has been slip [Martin, 1987]. erodedaway. The imbricatezone in the WorthingtonMountains 5.3.3. Lincoln thrust. The Lincoln thrust [Tschanz and thereforemay have lackeda roof thrustand may have been an Pampeyan, 1970] crops out in the westernTimpahute Range imbricatefan. However, the low-angle faults in the southern (Figures1 and3) andplaces Ordovician Pogonip Group or Eureka WorthingtonMountains converge eastward, suggesting that the Quartziteover the Lincolnduplex. Cambrianrocks are exposedin thrustsmerged updip to form a duplex rather than separately stratigraphiccontinuity with Ordovicianrocks that are cut by the intersectingthe surfaceas an imbricatefan. thrust,which requires that the thrust cuts up sectionto theeast in the hangingwall. Rocks within the Lincoln thrust plate define an upright 5.4 Tempiute Ridge Block and the SchofieldPass Fault opensyncline. The axial surfacestrikes north-northwest and dips steeplywest. A smallthrust emerging from the core of the syncline 5.4.1. TempiuteRidge block. Northtrending Tempiute Ridge structurallydisrupts the axial surface.The thrustpresently strikes forms the westernTimpahute Range and containshomoclinal N20ø-25øE,dips 37ø-45øW, and rampsacross both footwall and Pennsylvanianto Ordovicianbeds dipping 30ø-60øE (Figure 2). hangingwall strata. Removal of 20ø-25øEof eastwardCenozoic tilt ThesePaleozoic strata are cut by theCretaceous Lincoln stock. The indicatesa postcontraction thrust dip of 57ø-70øW. homoclineis boundedon the eastby the SchofieldPass fault and on 5.3.4. Lincolnduplex. TheLincoln duplex (named here) is well the westby the westdipping normal fault that definesthe range front. exposedin the TimpahuteRange and the southernWorthington Mountains.We interpretthe duplex to endnorthward at a location southof exposuresof the Freibergthrust because of the relatively Scatter Plot of Fold Axes strongcorrelation of the Lincolnand Freiberg thrusts, as presented N in Section6. A largestratigraphic separation, east striking high- anglefault that separatesthe duplexfrom the main part of the WorthingtonMountains appears to be the northernterminus of the duplex. In theTimpahute Range the duplex is flooredby theSchofield or MonteMountain thrusts and roofed by the Lincolnthrust (Figures 2 and4). Althoughthese thrusts were recognizedand namedby Tschanzand Pampeyan [1970] andChamberlain and Chamberlain [ 1990],our mapping differs from theirs. The duplexcontains rocks as old as the OrdovicianPogonip Group and as young as the MississippianChainman Shale (Figure 3) thatare deformed by north striking,gently to moderatelywest dippingthrust faults, north trendingfolds, and east striking faults that may be tearfaults. The Schofieldthrust places Ordovician rocks (Eureka Quartzite in the south and Pogonip Group in the north) upon Mississippian (ChainmanShale and Joana Limestone). The footwallimmediately below the Schofieldthrust is folded into an overturnedsyncline. The MonteMountain thrust crops out east of theSchofield thrust in the easternTimpahute Range and dies out northward. At its maximumexposed stratigraphic separation the Monte Mountain thrust places sandstoneand carbonateof the lower Devonian GuilmetteFormation over Mississippian limestone. Relations of the BEST-FIT FOLD AXIS: 1ø,171ø Monte Mountain thrust to the Mount Irish anticline and thrust are obscuredby northstriking normal faults of bothprevolcanic and late Figure 6. Lower-hemisphereequal-area plot showingthe Cenozoicages and a Cenozoicanticline-syncline pair (Figure4). attitudesof fold axesmeasured in Mississippianlimestone within The smallstratigraphic separation and decrease of slip northward the Lincolnduplex in the southernWorthington Mountains. The acrossthe thrustwithin the range both suggestthat the Monte overall trend of the fold axes suggestsroughly east-west Mountainthrust is not regionallycontinuous. shorteningwithin the duplex. TAYLOR ET AL.: RELATION BETWEEN FORELAND AND HINTERLAND SHORTENING 1 13 5

a)Wall rock b) Stock c)Wall rock d) Stock •o[U.••mtl •[ LW.i•..••• Up / LM2K•! LM3E'x•1360 ø'•wm 24-90] 1

LM1G W• W'o.oo5•Wm3• ' 1•20}6 16 9 W 9y •0.05•m/ _l 130C I Sløøwn 6 1 1.1 I 3mT• 130•[• I S Do• • • 7• •

e) N!

I Irvingand x••••ing (1988)

L Irving(1982) G6r•on eta1.1(1984) . E W'--• I '=60ø 30 •, E!mid-Cretaceous • expecteddirection• s Figure7. Palcomagneticresults from the Lincoln stock and carbonate wall rocks. (a-d) Zijderveld-type orthogonal plotsillustrating sample responses to progressive demagnetization, where the endpoint of themagnetization vector measuredafter each demagnetization step is projectedonto the horizontal (solid circles) and vertical (open circles) planes.Demagnetization treatments are indicated along the vertical projections. Samples LM1G andLM3E are carbonatewall rock samples that we interpretto havebeen remagnetized by intrusionof theLincoln stock. LM2K andLM4J are samplesfrom the Lincolnstock; most samples from the intrusionare heavily contaminated by a strongviscous remanance carded in low-coercivityphases that madeit difficultto isolatea stableprimary remanance.(e) Lower-hemisphereequal-area projection of sitemean directions for the four sites (circles) and mid- Cretaceousexpected directions (squares) derived from three compilations of North America palcomagnetic poles, bothwith associatedprojected cones of 95% confidence.

Palcomagneticdata (Appendix 1) from the Lincolnstock and 5.4.2. SchofieldPass fault. The SchofieldPass fault, locatedin carbonate wall rocks indicate no more than 15ø of southeast side the western Timpahute Range, places rocks as young as downtilting of the stockand its wall rockssince intrusion (Figure Pennsylvanianin the TempiuteRidge block on the west against 7). This impliesthat wall rockbedding was steeply tilted priorto strataas old as Cambrian(Table 2) on the east, a stratigraphic intrusion. Because the Schofield Pass fault truncates the contact separationof-• 4000 m (Figure3). The straighttrace of the aureole of the stock and therefore was active after intrusion, we SchofieldPass fault acrosstopography indicates a steepdip, but concludethat the majorityof tiltingof theTempiute Ridge block is poorexposure prevents determination of the dip direction. The significantlyolder than and unrelatedto displacementacross the SchofieldPass fault cutsthe contactmetamorphic aureole of the Schofield Pass fault. CretaceousLincoln stock that intrudes the northem Tempiute Ridge 1136 TAYLOR ET AL.: RELATION BETWEEN FORELAND AND HINTERLAND SHORTENING

Table 2. FossilsFrom Cambrian Rocks in theTimpahute Range a

MetersBelow Sample Fossil Fossil(s) Zone Ordovician Type Goodwin Limestone 37 TR-95-4 inarticulate Lannarssonellasp. Elvinia brachipods

55 TR-95-3 trilobites Kindbladiasp. Elvinia

100 TR-95-2 thiobites Dunderbergia Dunderbergia variagranulaPalmer 100 TR-95-2 trilobites Pterocephalia Dunderbergia compareconcava Palmer 100 TR-95-2 trilobites Homagnostustumidosus Dunderbergia (Hall and Whitfield).

113 TR-95-1 trilobites Meteoraspissp. Crepicephalus 113 TR-95-1 trilobites Llanoaspissp. Crepicephalus 113 TR-95-1 trilobites Kingstoniasp. Crepicephalus aFossilscollected from a sectionthrough the lowest stratigraphic unit in theLincoln thrust plate, just east of the Schofield Pass fault, indicate theunit age (Figure 2). Thebiostrafigraphic zones range from uppermost Middle Cambrian (Crepicephalus) to Middle Upper Cambrian (Elvinia) [Palmer,1998]. Theyshow that (1) theLincoln thrust must cut down into the Cambrian and (2) the SchofieldPass fault cuts Cambrian rocks onits east side. Fossils were collected under the direction of M. N. Reesand were identified by A. R. Palmer(personal communication, 1998).

homocline.Several east striking Cenozoic normal faults that Cenozoichigh-angle normal fault that was reactivated as a strike- probablypostdate early slip on the Schofield Pass fault transfer slip slipfault. However,we cannotexclude the possibilitythat the ontoit, reactivatingit as a strike-slipfault (Figure 4). We therefore SchofieldPass fault was once a steeplyeast dipping reverse fault. inferat leastsome post mid-Cretaceous and postintrusion slip across the fault. 5.5. Golden Gate, Mount Irish, and Pahranagat Thrusts 5.4.3. Structuralposition of the TempiuteRidge block. The structuralposition of theTempiute Ridge block in thethrust stack 5.5.1. GoldenGate thrust. The GoldenGate thrust is exposed andthe type and structural significance of theSchofield Pass fault for •6.5 km on the easternflank of the GoldenGate Range where it areuncertain. Roeder [1989] interpreted the SchofieldPass fault to strikesgenerally north and dips 41ø-56øW after removing 24 ø of bethe Lincoln thrust, synformally folded such that it reintersectsthe westwardCenozoic tilt (Armstrongand BartIcy [ 1993],on which the Earth'ssurface to the westof its frontalexposure, and therefore followingsummary is based).The thrust slip goes to zerowhere it interpretedthe TempiuteRidge blockto be a windowinto the terminatesnorthward in a complexpattern of openfolds and high- underlyingMount Irish thrustplate. However,on the basisof anglefaults. Net slipincreases southward to -• 2.5 km suchthat the geometricevidence and reasoning presented below, we interpretthe thrustplaces Ordovician rocks upon Mississippian Joana Limestone SchofieldPass fault to be a westside down normal or oblique-slip in the southernmostpart of the range. The thrustdefines a steep faultalong which the Tempiute Ridge block was emplaced from a footwallramp through gently dipping, weakly deformed strata of the structurallyhigher position. footwallblock. The mainhanging wall structureis a gentlynorth Rocksof theTempiute Ridge block range up to Pennsylvanianin plunging,upright to steeplyinclined anticline that opens northward age,yet the youngest rocks in thefootwalls of boththe Mount Irish intoa boxfold. The westernlimb andsubhorizontal top of thebox andLincoln thrusts are Mississippian.This relationshiprequires fbld projectwestward under Tertiary cover, leaving much of the thateither the thrustsystem cuts down section in the directionof backlimb of theGolden Gate anticline unexposed. Imbricate splays transportor the TempiuteRidge block is partof the Lincolnor a alongthe thrust bound-horses that contain subvertical bedding. higherthrust plate. Because thrust faults normally cut up sectionin 5.5.2. Mount Irish Thrust. Althoughthe Mount Irish thrust the directionof transport[e.g., Dahlstrom, 1969], and because no eitherhas been excised by or reactivatedas a Cenozoicfault along evidencecontradicts this assumption in the GardenValley thrust muchof its exposedlength (Figure 2) [Bartleyand Taylor,1992], system,Taylor et al. [1994] inferreda thrustbeneath Tempiute its geometryand footwall relations resemble those of the southern Ridgewhich they suggested was structurally higher than the Lincoln part of the Golden Gate thrust. The youngerfault diverges thrust. However,the geometriesof stratain the TempiuteRidge southwardfrom the thrust, leaving the Mount Irish thrust relatively blockand the Lincolnthrust plate alsopermit derivation of the tinmodifiedon the southside of therange. At thislocation the thrust Tempiuteblock from the Lincoln plate (Figure 4). placesuppermost Cambrian or lowermostOrdovician limestone PreLate Cretaceous tilting of theTempiute Ridge block probably upona faultedand folded footwall of Mississippianand Devonian occurredduring displacement across either the Lincoln or a higher rocks(Figure 2). Thefault presently dips 39øW in thesouth where thrust,whereas west side down movement across the SchofieldPass the fault is unmodified. Removal of 200-25ø of westward Cenozoic fault occurredsignificantly later. Field andregional relations are tiltingsuggests that the thrust originally dipped - 20øWat theend consistentwith the SchofieldPass fault being a west dipping of thrustmovement, Splays along the thrust-boundhorses contain TAYLOR ET AL.: RELATION BETWEEN FORELAND AND HINTERLAND SHORTENING 1137

Wianernucca Bdt4 i:'?- •? ß

•5 km o 5 lO miles

Pahranagat• Ran -, '.':':' '.•...:;. 37' 30' ,•.'"i

[•] Tertiary-Quaternary basindeposits [•] Tertiary,mostly volcanic rocks [• Tertiary [• Triassic •l• Pennsylvanian- Permian I• Mississippian ![] Devonian [• Silurian [• Cambrian- Ordovician

37 ø

Figure 8. This generalizedtectonic map shows ranges, rock ages, and fault locations from the southeastern Garden Valley thrustsystem to the Las Vegasarea and emphasizesthe geometricand spatial basis for correlation of the Gass Peak, Pahranagat,Mount Irish, andGolden Gate thrusts(marked by pointsA- H). Note that small outcropsof the footwall upper Paleozoicrocks suggest that the GassPeak thrust continues northward past B, althoughsome slip may be transferredto theeast (see Section 6). Locationsof cross sectionsin Figure 4 are shown. CNTB, central Nevada thrust belt; DT, Delamar thrust;EPF, East Pahranagatfault; GGR, Golden Gate Range;LT, Lincoln thrust; MIR, Mount Irish Range; IV[IT, Mount Irish thrust;PSZ, Pahranagatshear zone; PT, approximatelocation of thePahranagat thrust;ST, Schofieldthrust. 115' 30' 115' 1138 TAYLOR ET AL.: RELATION BETWEEN FORELAND AND HINTERLAND SHORTENING steeplydipping to overturnedDevonian and Mississippianbeds. 6. Discussion The steepdip andthe moderate- to high-angletruncation of footwall beddingboth indicate that the exposed part of thethrust is a footwall 6.1. Structural Correlations ramp. 6.1.1. Golden Gate-Mount Irish-Pahranagat-Gass Peak The upperplate structure is dominatedby a largeanticline here thrust. Tschanz and Pampeyan[1970] and Armstrong[1991 ] namedthe Mount Irish anticline(Figure 2). The fold is broad- correlated the Golden Gate thrust with the Mount Irish thrust. We hingedand open,with a steeplydipping axial surfaceand a short concurwith this interpretationfor severalreasons. First, the eastern limb that ends at the thrust. Northward truncation of the structuralstyles and geometries of thethrusts are similar. Each has Mount Irish anticlineagainst the Mount Irish thrust where it is a broad-hingedhanging wall anticline with the easternlimb stronglymodified by Cenozoicfaulting suggests that the younger imbricatedby backwardbreaking thrust splays to form horsesof •hult cut out at leastparts of the thrustrather than reactivatedit. steeplydipping east facing strata. Second,the Mississippian Removalof 200-25ø of Cenozoic-agewestward tilt suggeststhat the limestone section above the thrusts includes a thin-bedded facies beddingon the easternlimb generallywas nearly horizontalor that is uncommonelsewhere. Finally, correlatingthe two thrusts dippedgently east (except immediately near the thrust) and bedding resultsin a systematicincrease in stratigraphicseparation passing on the westernlimb dippedmoderately to steeplywest just after southwardfrom the exposedthrust tip in the GoldenGate Range. contraction. We correlate the Golden Gate-Mount Irish thrust southward with The geometryand structuraldevelopment of the Mount Irish thePahranagat thrust, which also has a broad-hingedhanging wall thrustand anticline resemble relations exposed in the GoldenGate anticline above footwall and hangingwall ramps and complex Range to the north and the PahranagatRange to the south. imbricationalong the thrust plane. However, the bedding in footwall Implicationsof thisresemblance are examinedin Section6. horsesalong the Pahranagatthrust, in part definedby the East 5.5.3. Pahranagatthrust. The Pahranagatthrust is cut out or Pahranagatfault, dips more gently than in horsesalong the Golden reactivatedby a Cenozoicnormal fault or coveredby Cenozoic Gate-MountIrish thrust. UpperPaleozoic strata on the east side of stratathroughout the PahranagatRange (Figures 1 and4). Several the East Pahranagatfault are deformedinto an asymmetricto linesof evidenceindicate the existenceof thisthrust. (1) A fault on overturned,east vergent syncline, which we correlatewith footwall the easternside of the rangeoffsets Tertiary volcanic rocks in a synclinesin similarage stratabeneath the Mount Irish and Gass normal sensebut placesolder Paleozoicrocks (Devonian) over Peak thrusts. The Paleozoic strata that define this fold can be traced younger (Mississippian). This relationshipwould result if a northwardin nearlycontinuous outcrop to withina few kilometers Cenozoicnormal fault cut out or reactivateda preexistingthrust of footwallstrata of the Mount Irish thrust. In contrast,no specific fault(the Pahranagat thrust) if thenormal fault had less slip than the structuralcontext for thisfold is providedby previousinterpretations thrust fault. However, a similar relationshipmight result if a [e.g.,Jayko, 1990]. This thrust correlation continues the systematic postvolcanicrock normalfault cut throughand displacedan east increasein stratigraphicseparation to the southalong the thrusts. dippingMesozoic or earlyTertiary normal fault that moved before Finally,we correlatethe Mount Irish-GoldenGate-Pahranagat emplacementof Oligocenestrata in the PahranagatRange [Taylor thrustsouthward with the GassPeak thrust in the Las VegasRange et al., 1989; Taylor and Bartley, 1992; Axen et al., 1993]. (2) (Figure 8) [Longwellet al., 1965; Burchfiel,1965; Guth, 1980, Reversefaults, including the EastPahranagat thrust (Figures 1 and 1981]. The GassPeak thrust is a majorthrust of theSevier orogenic 8), and an associatedsyncline crop out east of the fault just belt in southernNevada and may accommodatethe largestslip of .described[Tschanz and Parnpeyan,1970; Jayko, 1990]. These anySevier belt thrust at thatlatitude [Guth, 1981, 1990;Wernicke reversefaults and the synclineresemble faults that cut a syncline et al., 1984]. The MountIrish, Golden Gate, Pahranagat, and Gass ,eastof the Golden Gate and Mount Irish thrusts. However, this Peakthrusts all lie nearlyalong strike of eachother in a regionthat .synclineis betterpreserved than in areasto thenorth. (3) The three under-wentonly small-magnitude Cenozoic extension. The tip line large panels of Paleozoicrocks in the PahranagatRange have of thisregional thrust system lies nearpoint H in the GoldenGate differentdips, implying the presenceof a hangingwall anticline Range(Figure 8). Passingsouthward, stratigraphic separation similarto thatin the hangingwalls of the GoldenGate and Mount increasesto Ordovicianover Devonian(point G), Cambrianover Irish thrusts(Figure 4d). In the westernpanel, strataof the Mississippianand Devonian (F), LowerCambrian on Pennsylvanian CambrianBonanza King Formation through Mississippian limestone (B), and,finally, Neoproterozoic on Pennsylvanian (A). Threesmall areexposed dipping 30ø-50øENE. These rocks form the western part leftsteps occur near 37øN latitude between points E andB (Figure of the anticline. A westdipping nonplanar normal fault (Badger 8). The stepsreflect Cenozoicmovement across strands of the Mountainfault of Jayko,1990) separatesthe westernand central dominantlyleft-lateral Pahranagat shear zone. Between points B and panels,and tilted the western panel -30ø-50øENE. In the central A theGass Peak thrust surface is concealedby alluvitunon the west panel,strata of the CambrianBonanza King Formationthrough sideof the Las VegasRange, but its locationand northward strike Mississippianlimestone are exposeddipping 70ø-30øNE. The arewell definedby exposuresof upperand lower plate strata as far centralpanel forms the easternpart of the anticline. The eastern northas the traceof the late Neogeneto QuaternaryPahranagat panel, interpretedto be the footwall of the Pahranagatthrust, shearzone near 37øN latitude (Figure 8). Our suggestedthrust exposesDevonian and Mississippian strata that dip 20ø-30øWSW. correlationplaces Cambrian and younger Paleozoic rocks that crop As constructedin Figure4, the Pahranagatthrust places Upper outwest of theEast Pahranagat fault in thePahranagat Range in the Cambrianrocks over Devonian Guilmette Formation, a stratigraphic GassPeak thrust plate. separationthat is slightlylarger than that on the southern end of the The commonalternative interpretation of the traceof the Gass MountIrish thrust. In addition,the Pahranagat thrust has a hanging Peakthrust [e.g., Armstrong, 1968] is that it tumseast near 37øN wall anticline,a footwallramp, and footwallsplays or imbricate latitudeand continues into Utah (Figure8). The Delamarttmast faults. faultlies in thegeneral area of thiseastward deflection [cf. Tschanz TAYLOR ET AL.: RELATION BETWEEN FORELAND AND HINTERLAND SHORTENING 1 139 and Pampeyan,1970; Page, 1992]. We suggestthat correlationof Alternatively,the Lincolnthrust may correlate southward with theGass Peak and Delamar thrusts is inappropriatefor threereasons. theSpotted Range thrust, which places Middle Cambrian strata on (1) In outcropsnearest the GassPeak thrust, Devonian rocks lie in Mississippianrocks (Figure l) [Cole and Cashman,1999], a the footwall of the Delamar thrust. If the Gass Peak and Delamar stratigraphicseparation similar to butslightly larger than that of the thrusts correlate, then the thrust would have to cut down section Lincolnthrust at its southernexposures. Footwall rocks of the from Pennsylvanianto Devonian in the footwall in a direction SpottedRange thrust contain tight to isoclinal,southeast vergent subparallelto the transportdirection. (2) The Pahranagatshear zone folds[Barnes et al., 1982]that are not observed below the Lincoln accommodatedlet•-normal oblique slip [Tschanzand Pampeyan, thrust.The SpottedRange thrust also does not lie preciselyalong 1970;Ekren et al., 1977;Jayko, 1990]. Thereforethe traceof the strikeof the Lincoln thrust,but Cenozoicextension in the vicinity Gass Peak thrust should be offset to the southwest not to the of the SpottedRange probably is greaterthan in the Timpahute northeast,passing northward across the Pahranagat shear zone. (3) Rangeand could account for thedifference [cf. Guth,1990]. The Delamarthrust fault placesrocks as old as Cambrianin a 6.2. Timing of Deformation in the Cordi!!eran Fold hangingwall synclinerather than an anticlineover Devonianto and Thrust Belt Pennsylvanianfootwall rocks. Althoughwe cannotexclude the possibilityof an along-strikechange in fault-foldrelations, we Correlation of the Golden Gate-Mount Irish thrust with the Gass regard the consistencyof fault-fold patternsprovided by our Peakthrust implies that the Garden Valley thrust system overlaps in correlationas more likely. However, our suggestedcorrelation agewith the Sevier thrust belt. Agebrackets on deformation in the permitsthe possibilitythat someslip relatedto the GassPeak or GardenValley tl'm•t system(Pennsylvanian to mid-Cretaceous;ca. Pahranagatthrusts was transferred to theeast, particularly along the 85 Ma) are sufficientlywide to be easilycompatible with sucha basaldecollement of thethrust system. Indeed, the relatively rapid correlation.In thiscase it is worthwhileto examineimplications for northwardtermination of the thrust in the Golden Gate Range thetime-space pattern of deformation. probablysuggests that slip wasrelayed to more frontalthrusts in Thetiming of Sevierthrusts in southernNevada and southeastern Utah. Californiais largelybased on ages of prekinematicand synkinematic In summary,present data favor continuation of the GassPeak intrusionsand on the age of the synorogenicLavinia Wash thrustalong its northwardstrike to link withthe Pahranagat,Mount conglomeraticsequence [Burchfiel and Davis, 1971, 1988;Carr, Irish, and GoldenGate thruststo a lateraltip in the GoldenGate 1980, 1983;Cowan and Bruhn, 1992; Burchfiel et al., 1992]. This Range. No direct field evidencefavors the conventionalinter- evidence indicates that contraction acrossthe southern Sevier thrust pretationthat the GassPeak thrust tums noaheastward into Utah, belt occurredbetween- 200 and 92 Ma, i.e., Middle Jurassicto althoughsome slip transfer in thisdirection is likely. Thisproposed earlyLate Cretaceous time [Burch•eland Davis, 1975; Walker et thrustcorrelation implies a directstructural tie betweenthe Garden al., 1995]. Valley thrustsystem and the GassPeak-Wheeler Pass thrust [e.g., Theage of initiationSevier thrusting is controversial.However, Armstrong,1968; Burchfiel et al., 1992]of the Sevierthrust belt. all data are consistentwith activitybetween latest Jurassic and 6.1.2. Lincoln-Freiberg-Rimrockthrusts. Bartleyand Gleason Eocenetimes. DeCelleset al. [ 1995] usedclast provenance to tie [1990] correlatedthe Rimrockthrust southward with the Freiberg forelandbasin conglomerate deposits to specificthrust sheets and, thrust. Thiscorrelation was based on along-strikelocations within onthat basis, interpreted thrusting in theSevier belt to rangefrom the same Tertiary normal-faultblock, similar stratigraphic Early Cretaceousto Paleocene.On the basisof the geometry, separations,and a similarfacies of themiddle Pogonip Group in the contactrelations, facies patterns,and provenanceof foredeep hangingwall. We retainthe correlation and, for similarreasons, depositsand overlap assemblages, Lawton et al. [1997]inferred that correlatethe Freiberg thrust with the Lincoln thrust. In particular, theemplacement of thrustsheets in south-centralUtah began in the PogonipGroup facies in theTimpahute Range resemble those in the latestJurassic and persisted into Eocene time. In contrast,Schwans WorthingtonMountains and the southernGram Range but differ andCampion [ 1997]used sequence stratigraphy of forelandbasin from thosein the Mount Irish-GoldenGate thrust plate and the strataand overlap assemblages to suggest that Sevier thrusting did QuinnCanyon Range above the Sawmillthrust. notbegin until the Albian(late EarlyCretaceous). The continuationof the Rimrock-Freiberg-Lincolnthrust farther Schwansand Campion[1997] thereforeinferred that Sevier to the south is less clear. Possible correlative structures include thrustingbegan in Utahvery near the end of thrustingin southern thrustsexposed on the Nevada Test Site and the Pintwater anticline Nevada. Combinedwith our revisedthrust correlations, a logical [cf. Barnesand Poole, 1968; Cole and Cashman,1999]. The conclusion would be that the southern Nevada thrust belt is not Pintwateranticline is a regional-scaleanticline above the Gass Peak continuouswith thetype Sevier belt in Utah,but instead continues thrustthat is inferredto be the majorpre-Tertiary structure in the northwardwhere it is representedby thecentral Nevada thrust belt. Sheep,Desert, Pintwater, and Spotted Ranges of southernNevada. Sucha reinterpretationwould have dramatic implications for the The anticlinewas disruptedby the late Tertiary SheepRange time-spacepattern of Mesozoicorogenesis in the westernUnited detachmentsystem [Wernicke et al., 1984;Guth et al., 1988;Guth, States. However, we do not prefer this alternativebecause 1990], and its reconstructedpre-Tertiary geometry is uncertain. substantialevidence favors the older inception of thrustingin Utah Guth[1990] suggested that the Pintwater anticline is a fault-bend proposedby Lawtonet al. [ 1997]. foldformed above a footwallramp of theGass Peak thrust. Caskey We thereforeinterpret the Garden Valley thrust system and, more and Schweickert[1992] interpretedit to be cored by a blind generally,the centralNevada thrust belt to representan internal antiformalduplex to whichthe Gass Peak was the floor thrust. If so, branch of the Sevier thrust belt. Continuation of the Mesozoic the blind duplexrepresents imbrication on a scalean orderof regionalthrust system into centralNevada significantly impacts magnitudelarger than the Lincolnduplex. Although the Lincoln interpretationof foreland-hinterlandrelations in the Sevierorogenic thrustand duplex may link to a blind duplexunder the Pintwater beltas well asregional paleogeographic reconstructions [cf., Levy anticline,no observationsspecifically favor this interpretation. and Christie-Blick,1989]. Armstrong[1968, 1982], followedby 1140 TAYLOR ET AL.: RELATION BETWEEN FORELAND AND HINTERLAND SHORTENING many others[e.g., Miller et al., 1983], interpretedeast central from the stock and contact rocks varied (Figure 7). The Nevada and adjacentUtah to have undergone,at most, minor magnetizationin contact-metamorphosedcarbonate rocks is carded Mesozoic contraction and to lie in the hinterland of the Sevier thrust principallyby pyrrhotite;this interpretationis basedon the belt. This conclusionrequires Paleozoic strata exposed in the demagnetizationbehavior of theserocks. Pyrrhotite is reputedto be Cenozoicmetamorphic core complexes of the Sevierhinterland to the dominant remanencecarrier in contact-metamorphosed haveacquired their Mesozoic metamorphic mineral assemblages in carbonaterocks [e.g., Gillett et al., 1982]. Alternatingfield (AF) the absenceof significanttectonic burial [e.g., Armstrong,1982; demagnetizationwas ineffective in isolatinga stablemagnetization, Miller et al., 1983]. However,central Nevada belt thrust sheets may yet thermal demagnetizationresulted in essentiallycomplete contributeto tectonicburial. Recognitionof the centralNevada unblockingof themagnetization by -• 330øC(Figures 7a and7c). thrustbelt as a significanttectonic element linked to the Sevier The principalremanent magnetic carder in the Lincoln stockis thrustbelt refutesArmstrong's [ 1968] interpretationof negligible magnetiteof relativelylow coercivity.About half of the samples crustal contraction in the hinterland of central and eastern Nevada. fromthe stock yielded interpretable demagnetization results (Figures In addition,it suggeststhat the areatypically called the hinterland of 7b and7d). The magnetizationhas a moderatepositive inclination the Sevierthrust belt may havebeen the early formedpart of the anda northto northwestdeclination (Figure 7e). Cordilleranfold-and-thrust belt andonly later lay in the hinterland, Samplesfrom all four sites(two contactsites and two in the assuminggenerally normal-sequence thrusting. stock)yielded well-grouped magnetizations that were isolated over a rangeof moderate-to high-peakalternating fields or laboratory 7. Conclusions unblockingtemperatures. Site meandirections from all sitesare reasonablywell defined(Figure 7e) with an a95value of < 10ø. The We expandthe definitionof the GardenValley thrustsystem characteristicmagnetizations are interpretedto reflect mid- suchthat it consistsof the Sawmill,Schofield Canyon, the Rimrock- Cretaceousthermal remanence based on interpretation ofpublished Freiberg-Lincoln,and GoldenGate-Mount Irish thrustsand the K/Ar agesas the age of plutonemplacement• This is consistentwith Lincolnduplex, exposed in theGrant, Quinn Canyon, Golden Gate, the normalfield polarityobtained at all four sites.The four in situ Timpahute,and Mount Irish Ranges and the Worthington site means are 7ø-10ø shallower in inclination but similar in Mountains.We proposethat this thrust system is directlystructurally declinationto the mid-Cretaceousexpected directions for the linkedto the Sevierorogenic belt by the correlationof the Golden localitybased on threeestimates of mid-Cretaceouspaleomagnetic Gate-Mount Irish thrust to the Gass Peak thrust of the Sevier belt in poles(Figure 7) [Irvingand Irving, 1982;Globerman and Irving, southernNevada. The correlationimplies that the GassPeak thrust 1988; Gordonet al., 1984]. eitherdoes not continuenortheastward or thatonly someof its slip The lack of a declinationanomaly between observedand is transferredor partitionedonto thrusts exposed in Utah. A direct expecteddirections suggests that the Tempiute Ridge block did not link of the central Nevada thrust belt to the Sevier belt carries two undergosignificant vertical axis rotation since the mid-Cretaceous. furtherregional tectonic implications. First, it providesrobust This conclusionis consistentwith paleomagneticresults from evidenceof significantSevier age crustalcontraction in eastern Oligoceneash flow tuffs in the Mount Irish area and the Golden Nevada,including the metamorphiccore complex belt. Second,it GateRange [Overtoom and Bartley, 1996; Overtoom, 1994]. suggeststhat easternNevada should be regardednot as an undeformedpart of the hinterlandof the Sevierorogenic belt but Acknowledgments.This work was supportedby American insteadis an integralpart of thatfold-thrust belt. ChemicalSociety/Petroleum Research Fund grant 22462-AC2 to JohnM. Bartley;grants from Mobil Explorationand Producing, Appendix 1: PaleomagneticData and Methods U.S., Inc., Chevron,and UNLV to WandaJ. Taylor;grants from MarathonOil Companyto Mark W. Martin, JoanE. Fryxell,and To determinethe timing of tilting relative to intrusion of PhillipA. Armstrong;and grantsfrom SigmaXi, the Geological Paleozoic strata exposed in Tempiute Ridge, we obtained Society of America, the American Associationof Petroleum paleomagneticdata from the mid-CretaceousLincoln stock(two Geologists,Mobil Explorationand Producing,and Universityof sites)and carbonate wall rocks(two sites)within 25 m of thecontact Utah to PhillipA. Armstrong.We appreciatethe valuablereviews (Figure2). Sampleswere collected as standard 2.5 cm coresusing providedby PeterGuth and Hugh Hurlow. Specialthanks to Jay a portabledrill. Specimenpreparation, measurement of natural Schofield for assistingwith access into the area of the remanent magnetization, progressive demagnetization,and Lincoln/EmersonMine in the TimpahuteRange and to Bonnie evaluationof the componentsof magnetizationall follow standard Schofieldfor helpingus keep in contactwith the outsideworld proceduresdescribed elsewhere [e.g., Holm et al., 1993]. duringthe field seasons.The stereonetprogram written by R. The overallresponse to progressivedemagnetization of samples Allmendingerwas used to plot structuraldata.

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