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