TECTONICS, VOL. 8, NO. 3, PAGES453-467, JUNE 1989

EVOLUTION OF EXTENSIONAL BASINS AND BASIN AND RANGE TOPOGRAPHY WEST OF DEATH VALLEY, CALIFORNIA K.V. Hodges,L.W. McKenna,J. Stock •, J. Knapp, L. Page,K. Sternlof, D. Silverberg,G. Wrist2, and J. D. Walker 3

Department of Earth, Atmospheric,and Planetary Sciences,Massachusetts Institute of Technology, Cambridge

Abstract. Neogeneextension in the Death Valley system remained active during development of region, SE California, has produceda variety of PanamintValley and, thus, during developmentof sedimentarybasins. Diachronousmovements on an modernBasin and Rangetopography as well. These array of strike-slip and normal systemshave results indicate that large-scale extension in the resultedin the uplift andpreservation of olderbasins Death Valley region,accommodated by movementon in modernranges. One of the bestexposed of these low- to moderate-angle normal fault systemsand is the Nova basin on the western flank of the high-angle strike-slipfault systems,is a continuing Panamint Mountains. The Nova basin includes over process. Basin and Range topography in the 2000 m of sedimentaryand volcanicrocks deposited PanamintValley- Death Valley areawas generatedat during denudation of the Panamint Mountains least in part by displacements on low-angle metamorphiccore complex in late Miocene(?)- early detachmentsrather than high-angle normal faults. Pliocenetime. The principalgrowth structure for the basinwas the Emigrant detachment,which initiated andmoved at a low angle. Modern PanamintValley, INTRODUCTION west of the range, developedas a consequenceof Late Pliocene- Recent,kinematically linked move- The fact that topographyin the Basin and Range ment on the right-slip, high-angleHunter Mountain Province of the western United States is controlled fault zone and the low-angle Panamint Valley by normal faulting has been recognizedfor over a detachment. Detailed mapping of the intersection century [e.g., Gilbert, 1875]. Surprisingly,there is between the Emigrant and Panamint Valley still no consensuson the geometric behavior of detachmentsdemonstrates that segmentsof the earlier observedrange-bounding faults as they dip beneath the adjacentbasins. For many decadesthe bulk of the geological community believed that the vast majorityof thesestructures are planar and dip steeply 1Now at Department of Earthand Planetary (see reviews by Thompson, [1967], and Stewart, Sciences,Harvard University, Cambridge [1971, 1978]), even thoughdetailed studiesin areas 2 Nowat GeologyInstitute, Swiss Federal of high relief and good exposure(like the southern Instituteof Technology,Z'tirich, Switzerland Basin and Range) have shownthat at least someof 3 Nowat Department of Geology,University of the structuresthat dip steeplyat the surfaceare listtic, Kansas, Lawrence. flattening abruptly beneath the adjacent basin [Longwell, 1945; Hamblin, 1965; Anderson, 1971; Copyright1989 Wright and Troxel, 1973]. by the AmericanGeophysical Union With the recognition that large-scale extension occurredin the Basin and Range in early to middle Papernumber 890TC00015. Tertiary time [Hamiltonand Myers, 1966; Hamilton, 0278-7407/89/89TC-00015510.00 1969; Proffett, 1977] and that much of this extension 454 Hodgeset al.: Evolutionof ExtensionalBasins West of DeathValley

Fig. 1. Tertiaryfault systems of theDeath Valley region. Ranges are stippled. Normal and strike-slip faults are shownby heavylines; hatchures are on the hangingwall sideof normalfaults, and arrowsindicate relative motionon strike-slipfaults. HM, HunterMountain; HMFZ, HunterMountain fault zone;SV, SalineValley. The areashown in the tectonicmap (Plate 1) is outlined.

may have beenaccommodated by movementon low- GEOLOGIC SETTING angle normal faults of regional extent [Armstrong, 1972; Wernicke, 1981, 1985; Allmendinger et al., The Death Valley area of southeasternCalifornia 1983], some workers postulated that the late has been the site of large-scaleextension since at Cenozoic development of Basin and Range least middle Miocene time [Cemen et al., 1982]. topographywas a fundamentallydifferent process This extension was accommodatedby listric and from earlier extension on low-angle faults [e.g., planar normal faults and by NW-trending, right- Zoback et al., 1981; Eaton, 1982]. This view was lateral strike-slip faults which served as transfer challengedby Hamilton [1982], who suggestedthat structures between normal fault systems. The Neogene- Recent Basin and Range topography interplay between normal and strike-slip fault formed concurrently with movement on large systems has led to the development of two detachmentsystems; by Dickinsonet al. [ 1987], who spectacularexamples of "pull-apart"[Burchfiel and argued that characteristic Basin and Range Stewart, 1966] or rhombhedral basins: Panamint topographyof mid-Tertiary age was controlledby Valley andcentral Death Valley itself. Thesevalleys low-angle faulting in the Galiuro Mountains, are separated by the Panamint Mountains, a southeasternArizona; and by Burchfiel et al. [1987], metamorphiccore complexwhich evolvedover the who demonstratedthat the modemfault boundingthe middle Miocene- Recentinterval (Figure 1). Panamint Mountains, southeastern California, must It is convenient to think of the northern and central haveinitiated at an angleof lessthan 15ø . PanamintMountains in termsof four tectonicplates The Panamint Mountains (Figure 1) constitutean distinguishedby metamorphicgrade and structural especially interesting case for the study of history(Figure 2). The structurallylowest plate (or extensionalbasin evolution becausethey contain an "Parautochthon")consists of Precambriancrystalline uplifted, upper Miocene(?) - Pliocene basin, basementintruded by Miocene (?) graniteporphyry developed during the extensional evolution of a dikes [Hunt and Mabey, 1966; Stern et al., 1966]. "metamorphiccore complex,"that lies immediately The "Lower Allochthon"is composedof middle and adjacent to a modern Basin and Range basin: lower Proterozoic crystalline basementand upper PanamintValley. In this paper we discussthe age Proterozoic strata, which were metamorphosedat and structuralevolution of eachbasin and arguethat greenschistto lower amphibolitefacies conditions they formed as part of a continuousextensional between middle Mesozoic and early Tertiary time process in which low-angle faulting played a [Labotka et al., 1985] and have been intruded by dominant role. Cretaceousgranites. Several phasesof ductile to Hodgeset al.' Evolutionof ExtensionalBasins West of DeathValley 455

Stovepipe_•-• highest"Upper Allochthon"includes the following: Wells (1) somesmall (< 1 km2) structuralslices of Lower Allochthon and Middle Allochthon lithologiesat its easternmost exposures; (2) a thick section of BP Paleozoic rocks of Middle Allochthon affinity at PanamintButte (Figure 2); and (3) synextensional, 10 km Pliocenesedimentary and volcanic rocks of the Nova Formation [Hopper, 1947; Hall, 1971; Walker and Coleman, 1987]. The contactsbetween the four platesin the central Panamint Mountains are temporally distinct, consistentlytop-to-the-northwest, generally low- anglenormal faults. The structuresthat separate the Lower Allochthonfrom the Parautochthonare part of the Eastern Panamint fault system [Hodges et al., 1989]. This fault systemis exposednear the eastern foot of the Panamint Mountains and presentlydips shallowlywestward. Palinspastic reconstruction of a sequenceof Upper Miocenevolcanic rocks erupted synchronouslywith developmentof the fault system indicatesthat its originaldip musthave been 45ø-60 ø westward[McKenna and Hodges, 1989]. The family of structuresthat separatesthe Lower Allochthonfrom structurallyhigher plates is defined as the Tucki Mountain detachment system. The oldestfault in this system(> 10.7 Ma; Hodgeset al., [1989]) is the Harrisburg detachment, which presently dips eastward beneath the Middle Allochthon due to Pliocenedoming of the range but is inferredto have initially dippedwestward [Hodges Parautochthon MiddleAllochthon et al., 1987]. The youngestfault in the systemis the west-dippingEmigrant detachment, which servedas LowerAllochthon I i i i [ Upper Allochthon the principal growth fault for the Nova Basin.

THE NOVA BASIN EmigrantSubsystem • PanamintFault Zone Valley TownePass Fault Hopper [1947] proposed the name "Nova Formation"for a thick sectionof alluvial fan deposits Fig. 2. Location and fault map of the Panamint ("fanglomerates")and intercalatedvolcanic rocks Range. Key indicatespattern and fault ornamen- exposed along the west flank of the northern tation; fault barbs are on the hanging wall side of PanamintRange. Hall [1971] abandoneduse of the low-angle structures. BP, Black Point; EF, term "Nova Formation" becauseHopper had been Emigrant fault; HF, Harrisburg Flats; HCP, Hall vague about its definition; instead,he preferredto Canyon Pluton; LCS, Little Chief Stock; PB, distinguisha variety of upperMiocene (?) - Recent Panamint Butte; PP, Pinto Peak; PVFZ, Panamint alluvial packages, based on their degree of Valley fault Zone; S, Skidoo townsite;SG, Skidoo induration, clast composition,and position in the Granite; TC, Tuber Canyon; TM, Tucki Mountain; sequencerelative to volcanichorizons. We believe TP TelescopePeak; TPF, Towne Pass fault; WC, that the "Nova Formation" nomenclature serves a Wildrose Canyon. The locations of Plate 1 and usefulpurpose by distinguishingthe more indurated Figure 5 are shownby the labeledboxes. upperMiocene (?) - Pliocenedeposits from thepoor- ly consolidatedupper Pliocene - Recentalluvium as. sociatedwith the developmentof PanamintValley. brittle-ductile deformation affected this plate in Throughoutthis paper we will refer to the ancient Mesozoic-Cenozoictime [Hodgeset al., 1987]. sedimentary basin represented by the Nova The "Middle Allochthon" contains lower Formation as the "Nova Basin." greenschist facies to unmetamorphosed,upper Precambrian to Permian strata, which are unconformablyoverlain by upper Tertiary alluvial basindeposits and intercalatedvolcanic flows in the The Nova Formation includes more than 3 km of northeasternPanamint Mountains. The structurally sedimentaryand volcanicrocks. It can be divided 456 Hodgeset al.: Evolutionof ExtensionalBasins West of DeathValley

Stratigraphy of the Upper Miocene (?) - Lower beddedconglomerates with localizedmonolithologic Pliocene Nova Formation and polylithologic megabrecciahorizons which we interpret as landslide deposits. Volcanic flows, 3.7 __ 0.2 M a ranging in compositionfrom calc-alkalinebasalt to (K-Ar, wr) rhyolite,occur sporadically within the middlepart of the unit and becomemore abundantnear the top. Upper Nova Formation Major and traceelement geochemistry, as well as Sr and Pb isotopicsignatures, indicate that theseflows are part of the thick Darwin Plateausequence to the west (Figure 1), which was adjacentto the northern Panamint Mountains prior to Plio-Pleistocene opening of Panamint Valley [Coleman et al., 1987; •ooooo( Walker and Coleman, 1987]. An apparent .•ooooo<: •>ooooo< 1 unroofing sequencefor the Panamint Mountains is 300 m •ooooo< recorded in the lower Nova Formation; oooooo< 000000,: unmetamorphosedPaleozoic units of the Middle >00000< Allochthon characterized the source area for clasts •00000< •00000< and megabrecciaswithin the basalNova Formation, •00000< •>00000< but metamorphic rocks of the Lower Allochthon Middle Nova Formation •ooooo• become abundant in conglomerates and breccia •ooooo< •ooooo< sheetsnear the top of the lower Nova Formation ,•ooooo< [Walker and Coleman, 1987]. •ooooo< •ooooo< The middle Nova Formationrests unconformably •>00000( on the lower Nova andis exposedcontinuously from the east side of Pinto Peak northward to Black Point (Figure 2). Conglomeratesin the middle Nova are similar in appearanceto thosein the lower Nova, but the clastsreflect a mixed provenanceof bothMiddle

4.2__ 0.2 Ma Allochthon unmetamorphicand Lower Allochthon (K-Ar, wr } metamorphic bedrock. Megabreccias are less

...... common in the middle Nova than in the lower Nova, andvolcanic flows and tuffs are onlylocally present. •>00000• I-I I I I The upper Nova Formationlies entirely within a restricted subbasinthat is exposedwest of Pinto 5.4 ñ 0.4 Ma •00800< (K-Ar, wr) Peak. It rests with angular unconformityon both Lower Nova Formation lower and middle Nova Formation rocks near the eastern edge of Panamint Valley, but it is fault boundedelsewhere. The upperNova is composed primarily of conglomeratederived by reworkingof •00000( other parts of the Nova Formation, and it is •00000< •00000( characteristicallywell beddedcompared to the lower •00000< •00000< and middle parts of the section. No volcanicunits .•00000( havebeen identified in the upperNova. •>00000( : o•••.• Depositionalenvironments for the Nova Formation i ! i i i [ i i i ! [ range from alluvial fans to playas. All variations i [ i i i l Paleozoic i i i i i ! i i [ i i from proximal to distal alluvial fan facieshave been i i i [ i i undifferentiated observed. We do not yet know whether this wide range of facies represents fan switching and Fig. 3. Stratigraphyof the upper Miocene to lower progradationor responsesto changesin the rate and Pliocene Nova Formation, with radiometric age style of syndepositionalfaulting [cf., Bilodeauand constraintsas indicated. Other consistentK-Ar ages Blair, 1986; Blair, 1987]. We are workingpresently are available for the sequence(e.g., W. Hildreth, with the notion that faulting variations were unpublisheddata), but the stratigraphicpositions of important becauseof the active nature of the basin the dated units are less certain than those noted here. and because of the presence of abundant unconformities in the section which do not have regionalsignificance. into lower, middle, and upper parts, based on lithologyand contact relations (Figure 3). The lower Structural Evolution part of the Nova Formation attains a thickness of over 1000 m. It consistsof the initial depositsin the We are fortunate in the Panamint Mountains to Nova Basin and is characterizedby coarse,poorly have excellent three-dimensionalexposures of the Tectonic Cross Section of the Nova Basin

A ll A' HarrisburgFault

PanamintValley Fault TownePass/ Fault •[ •-":'•'---..nnV ...... --..•. - 2000_ System _•,,,,'""•ro ,,,,,,- I •,.,.•,,•,, .,•,,,,,-' •• 2000

1000- . •o.--'•. •.--; / ..-:'".7 / / • 1000

0 -

Plate I. (continued) lectonic Map ol tile Nova Basin

and Surroundings, Inyo County, California

20' 15' 117ø10 ' I

usGs 15' Quadrangle Maps 25' 54- Tnl Tnl Panamint Emigrant Butte Canyon

TIb . Qoa ; $ ßt, rnl

Tn Tnl 36 ø I 5'

Qal Telescope Peak Tb Tnl' Tnu .17 5.3Ma (K/Ar) Qal// TIb I I 7 ø I 5'

Tnl

Tb 190 .' Tnl Tn(1 A ¾,, TOWNE PASS FAULT

20 Tn ß •Oal

37' 3=am? , 0

o t/ o Tb L., Tnl 24. ß ,' Tnl Tnm%

QTI• _

N.PANAMINT FAULT ZONE

I OlderAlluvium I Tnm,, o1-1 FAULT

(• •,, y Nova Fanglomerate upper

Middle 36o15 '

Lower

Gouge associatedwith the Emigrant Fault Zone N Zpa

PahrumpGroup, undifferentiated

.'.97' 4O

r 4000

Qal Zpu Depositionalcontact: Fault with normal displacement. 0 1 2 4 Definite,approximate, Definite,approximate, concealed km concealed Symbolon down-thrownside. Fault Symbols XeoStrike anddip of Zpu (• bedding: N. Panamint Fault Zone inclined, horizontal Towne Pass Fault Zone

'•54 surfaceDipoffault EmigrantFault Zone

MultipleStages of movementindicated by Unitconnector multiplesymbols Hodgeset al.: Evolution of Extensionall•asins West of DeathValley 459 faults that controlled the developmentof the Nova ED Basin. Those which servedas the growth faults for the lower and middle Nova Formationbelong to the ":::'4x•i•.*.,<,g?.?.i:i:::i:!:>.i:i:i::•::.:.:::i::::::::::•!...... !•i:•...... •...... : youngestfamily of structuresin the Tucki Mountain •"•-:'""'"------"-•'•.)•.-•'i: :::::::::::::::::::::::::::: ...... •,, detachment system, the Emigrant subsystem [Wernicke et al., 1986; Hodges et al., 1989]. In collaboration with Brian Wernicke at Harvard University we have mappedthe exposedtrace of the Emigrant fault zone from the northernfoot of Tucki Mountain to the mouthof Wildrose Canyon(Figure Do 2), a distanceof over 30 km, at a scale of 1:10,000. Between Tucki Mountain and Skidoo townsite the fault zoneis characterizedby fault gougeand breccia up to 50-m thick. Lithologies within the zone are predominantlyupper Precambrianmetasedimentary rocks and upper Cretaceousgranitic intrusive rocks Go of the Lower Allochthon. The zoneconsistently dips less than 200NW. South of Skidoo the zone gradually widens to becomean extensionalduplex TPF [Gibbs, 1984]. The sole fault of the duplex climbs near the crest of the PanamintRange and its dip • ..:.:::::::::::.:....-======..,...... ,...... •..:.:.:.. steepensto greaterthan 70øNW, while the roof fault do maintains a shallow northwestdip. Much of the 1 km Harrisburg Flats area (Figure 2), between the roof and solefaults, consistsof east-tiltedfault blocks(or "riders"in the terminologyof Gibbs, [1984]). Harding [1987] and Hodges et al. [1989] I I UpperNova Formation concludedthat the steepsegment of the solefault at the rangecrest marks the near-surface"breakaway" MiddleNova Formation zoneof the Emigrantsubsystem. Field relationships '"":':•:•Lower Nova Formation in the Harrisburg Flats area clearly demonstrate continuity between shallowly dipping western q•'"• Bedrock exposuresof the sole fault and steeply dipping easternexposures. Palinspastic reconstruction of the extensional riders in this area indicates that the sole Fig. 4 . Schematic cross sections illustrating fault dips less than 15øNW beneath the Nova evolution of the Nova basin. (a): Initiation of the Formation[Hodges et al., 1989]. Detailedmapping Emigrant detachment (ED) and lower Nova of the structurallylowest portions of the Nova Basin Formation deposition, (b): eruption of the upper southof PanamintButte (Figure 2; K.V. Hodgesand Miocene- lower Pliocene(?) volcanicsequence that others, unpublisheddata, 1989) suggeststhat a marks the top of the Lower Nova Formation, (c): complexlandslide of Lower Allochthonand Middle deposition of the middle Nova Formation, (d): Allochthon lithologies, shed partially off the initiation of the Towne Pass fault (TPF) and Emigrantscarp, may have markedthe earlieststage depositionof the upperNova Formation. of basin evolution. The upper Nova Formationwas depositedin a subbasingenerated by movementon the Towne Pass Although the synextensionalNova stratapresently fault (Figure 2). This structurewas describedby dip eastwardat low to moderateangles, there is no Hall [1971] as a NNW-trending,45ø-80øW-dipping evidence to suggestthat the Emigrant detachment normal fault with a minimum of 2400 m itself initiated at high angle and was subsequently displacement.We have mappedthis fault alongits rotated to a shallower dip. Reconstructionof the entire trace at a scale of 1:24,000 (Plate 1). From Precambrianstrata in the Emigrantriders to theirpre- north to souththe fault orientationchanges from a Emigrant configuration as part of the Lower NNW trend with steep to moderatedip to a NW Allochthon indicates stratal rotations of about 45 ø trendwith moderateto shallowdip. Along muchof associated with the curvature of the sole fault at the its length the immediate footwall of the fault is an breakaway [Hodges et al., 1989]. This rotation is immenselandslide deposit of upperPaleozoic strata sufficient to restore observeddips in middle and that lies within the lower Nova Formation. South of lower Nova strata, deposited near the breakaway this landslide the Towne Pass footwall consists of zone,to horizontal. Thus we infer that the Emigrant lower and middle Nova units. detachmentinitiated and moved at a low angle. Ourinterpretation of thestructural and stratigraphic There is similarly no evidencethat the Towne Pass evolutionof theNova Basinis illustratedin Figure4. fault hasbeen rotated substantially since initiation. t[60 Hodgeset al.- Evolutionof ExtensionalBasins West of Death Valley

I QalI Alluvium ,26 Tnb 8O I QoaI Older Alluvium Qal 32

Zke '"••r•'""''':' Basalt

• BrecciaDolomite ofNoonday

Zke / 37 Zkmp _/ Zk. I I KgI SkidooGranite I •Kg / Qoa •?/ "/ Qal Kinoston Peak Formation / Nc Zkmp I Middle Park Member E SourdoughLimestone Surprise Diamictite

460 meters

Fig. 5. Detailedgeologic map of WhiteSage Flat area,SW HarrisburgHats (seePlate 1 for location). Topography(in feet)is representedby thinsolid lines. The f'filedcircle indicates the collection location for geochronologysamples (Table 1).

Age Constraints postdate its emplacement. Harding [1987] demonstratedthat the sequenceof faulting in the Despitethe occurrenceof severalvolcanic horizons Emigrantduplex progressed from eastto west,in the within the Nova Formation, very few direction of fault movement. As one of the geochronologicdata are availablefor the sequence. westernmostriders in the duplex, the fault block ReconnaissanceK-Ar agesreported by Hall [1971], cappedby the basaltshould be one of the youngest, Larsen [1979], Sternlof [1988] and W. Hildreth and we can reasonablyinfer that the bulk (if not all) [unpublisheddata] for lower Nova volcanic rocks of the movement on the Emigrant subsystem range from 5.3 + 0.8 Ma to 4.0 + 0.4 Ma (2s). occurredbefore eruption of the basalt. Whole rock Schweig[ 1984] arguedconvincingly that portions of K-Ar data for two samplesfrom this flow (Table 1) the fanglomeratesequence in the northern Argus yield ages of 3.6 + 0.3 Ma and 3.8 + 0.3 Ma (2s). Range (Figure 1) that he believed to be correlative Combined with existing ages for volcanic units with what we call the lower Nova Formationmay within the lower Nova, we interpret these data as havebeen deposited before 6.1 Ma. indicativeof a 6.1 to 3.6 Ma approximateage range The most importantconstraints on the minimum for movement on the Emigrant subsystem and ageof the Emigrantsubsystem are the map relations depositionof the lower andmiddle Nova Formation. illustrated in Figure 5. At this locality in the We infer, as did Hall [1971], that the active southwesternHarrisburg Flats area, a small, flat- depocenterfor the basin shiftedwestward after 3.6 lying exposureof olivine basaltlies in flow contact Ma to the hangingwall of the TownePass fault. The on one of the westernmost extensional riders of the youngestage of movement on the Towne Passfault Emigrantsubsystem. The basaltcovers the traceof is not constrained well. Several small lacustrine an ENE-striking, high-anglefault that cutsthe low- limestonedeposits that may mark Plio-Pleistocene anglefault at the baseof the rider andtherefore must sagponds occur along the fault trace(Plate 1). In the Hodgeset al.: Evolutionof ExtensionalBasins West of DeathValley

TABLE 1. K-Ar Data Sample 40Arrad,mol/gm 40Arrad/40ArtOtK20,wt% Age 1 7.0842x10-12 0.13269 1.344 3.6_+0.3 Ma 2 7.3636x10-12 0.14465 1.345 3.8_+0.3Ma Sampleis a fi'esh,porphrilic quartz-olivine basalt. Samplelocation is shownon Figure5 by solid circle sy•nbol,approximate location is UTM 11SML04842204018220.Sample preparationand analysisby J. Stockat the U.S. GeologicalSurvey laboratories, Menlo Park, California. Potashanalyses by flame photometry;Ar analyseswere performed by isotopedilution procedures [Dalrymple and Lanphere, 1969] with a standard60 ø sector, 15.2 cm radius, Nier-type massspectrometer. Precision of the data is the estimated analyticaluncertainty at 2s. Decay constantsare ;ke+},,e'= 5.81 x 10-11 yr-1 and ;k13= 4.962 x 10-10yl-1; atomic abundm•ce of40K = 1.167x 10-4.

Towne Passarea, slightly degradedscarps along the PanamintBasin is the NNW-trending, west-dipping fault tracealso indicate relatively recent movement. Panamint Valley fault zone [Noble, 1926] which occurs at the western foot of the Panamint Mountains THE PANAMINT BASIN (Figure 2). Panamint Valley developed as a consequenceof Roughly 8-10 km of fight-slip movementand 0-2 fight-lateral movementon the Hunter Mountain fault km of down-to-the-southdip-slip movementon the zone on the north and unnamedstructures separating Hunter Mountain fault zone can be establishedby the Slate Range and Panamint Mountains on the matchinga uniquelinear elementthat occurson both south(Figure 1; Burchfiel et a1.[1987]). We refer to sidesof the structure:the intersectionof a high-angle intrusivecontact and an overlappingCenozoic basalt. this Pliocene - Recent depocenteras the Panamint Because the Hunter Mountain fault zone served as a Basin. transfer structure [Gibbs, 1984] between the Panamint and Saline basins, Burchfiel et al. [1987] Stratigraphyand Basin Characteristics concludedthat the calculatednet slip on the Hunter The oldest exposed units of the Panamint Basin Mountain fault zone matchedthe cumulativeslip on sequenceare lacustrinedeposits described by Hall faultsof the PanamintValley fault zone. Palinspastic [1971]. These include white to tan claystones, reconstructionof the area basedon the derived slip siltstones,sandstones, and limestonesup to 65-m vectorcompletely restores correlative volcanic flows thick. The lacustrine deposits occur in isolated that occur on either side of the valley but do not outcrops at a variety of elevations up to 645 m, underliethe alluvial fill [MIT 1985 Field Geophysics roughly 200 m above the valley floor. Most Course and Biehler, 1987], indicating that the outcrops have been tilted slightly by young faults PanamintValley fault zonedips lessthat 15ø beneath related to the latest stagesof valley development the valley [Burchfiel et al., 1987; Sternlof, 1988]. [Smith, 1976]. Exposed scarpsalong the Panamint Valley fault The lacustrine deposits are overlain by poorly zonehave low to moderatedips (15ø -42 øW). Along consolidatedto unconsolidatedalluvial fan andplaya with the geologicaland geophysicalevidence for a deposits, with minor lenses and landslide very shallowdip of the detachmentbeneath the valley breccias. The fanglomerateswere derived both by thesesurface dip measurementsstrongly suggest that erosionof denudedbedrock and by reworking of the the fault zone is listric. This interpretation is Nova Formationconglomerates [Hall, 1971]. supportedby the observationthat volcanicflows on The depth of alluvial fill in the central part of the Darwin Plateau"roll over" towardthe valley, as northernPanamint Valley is constrainedby a core they shouldif transportedin the hanging wall of a hole that struck bedrock beneath the at a strongly listric normal fault [Hamblin, 1965]. depth of 118 m [Smith and Pratt, 1957]. MIT 1985 Detailedstudies of the mostrecent fault scarpswithin Field Geophysics Course and Biehler [1987] the PanamintValley fault zoneindicate that Holocene presentedgravity, seismicrefraction, resistivity, and movementswere characterizedby both dip-slip and magnetotelluricdata indicating between 100 and 200 right-slip components[Hopper, 1947; Smith, 1976]. m of fill throughout much of northern Panamint Minor strike-slipmovement of this kind is a common Valley. characteristic of pull-apart basins in which the boundingnormal and transferfaults strikeobliquely Structural Evolution to each other [Walker et al., 1986].

Burchfiel et al. [1987] suggestedthat Panamint Age Constraints Valley and SalineValley to the northare pairedpull- apart basinson oppositesides of the predominantly The opening of Panamint Valley transectedthe strike-slip,NW-trending Hunter Mountain fault zone Nova Basin, such that Nova Formation lithologies (Figure 1). The principal growth fault for the are exposedin the rangeson eitherside of the valley 462 Hodgeset al.: Evolutionof ExtensionalBasins West of DeathValley

[Schweig, 1984]. Thus initiationof the valley must characteristicof the Emigrant fault zone. These postdatethe 6.1-3.6 Ma ageof the lower and middle relationships support other lines of evidence Nova Formation. This conclusion is consistent with (discussedabove), suggestinga post-Emigrantage the 5.0 Ma age obtainedby Stemlof [1988] for the for the PanamintValley fault zone. youngest basalt flow which was clearly offset by Approximately1.7 km north of Tuber Canyonthe movement on the Panamint Valley and Hunter PanamintValley structurestrikes directly into a large Mountain fault systems. Scarps of the Panamint hill of upperNova Formationfanglomerates, and is Valley system are developed on upper Nova truncatedby a NE striking, NW dipping fault that Formationsubcrop, suggesting a post-3.6Ma age of placesupper Nova lithologieson Emigrantfault zone initiation. If Saline Valley and Panamint Valley rocks (Plate 1). Northeast of the hill of developedsimultaneously, as arguedby Burchfiel et fanglomerates,for a distanceof- 1 km, this structure al. [1987], then the maximum age of initiation of actsas the range-boundingfault. Along this segment Saline Valley (3.0 Ma; Burchfiel [1969], Ross the relief at the rangefront is more subduedthan it is [1967, 1968], and Stemlof [1988]) correspondsto near Tuber Canyon (Figure 6) even though the the maximum age of Panamint Valley. An footwall lithologies are similar; linear regression abundanceof Recent fault scarpsalong the Hunter analysis of nine topographic profiles along this Mountainand PanamintValley fault zones,as well as segmentyielded an estimatedfault dip of 20 ø _+5 ø within the valley, attestto the continuingevolution of (2s), while one direct measurement of the surface the basin [Smith, 1976]. indicated a somewhat steeper dip of 28 ø. A few hundred meters south of the mouth of Wildrose Canyon, this low-angle structure climbs into the THE WILDROSE CANYON AREA Panamint Range. It is replaced as the range- boundingfault by a poorly exposed,NW striking The Emigrant fault zone, the Towne Passfault, fault which dips 78øSW and juxtaposesQuaternary and the PanamintValley fault zoneconverge near the alluvium and upper Nova Formation lithologies. mouth of Wildrose Canyon(Plate 1 andFigure 6). Unfortunately,the intersectionbetween this fault and These structureshave been partially exhumed by the more shallowlydipping structure is unexposed. erosion, providing direct evidence of their three- We have mappedthe northwardcontinuation of the dimensionalgeometry. We havemapped most of the more shallowly dipping fault for -2.5 km into the area at a scale of 1:10,000, and our results are shown range. Throughoutthis distancethe structuredips in simplifiedform asthe tectonicmap in Plate 1. shallowly (< 27 ø) westward. At 2.5 km the surface The westernlimits of this area display structures clearly bifurcates. One strandmaintains a shallow characteristicof much of the easternboundary of the dip (< 20 ø) but strikes off to the northeast and Panamint Basin. The individual range-bounding correspondsto the roof fault of the Emigrant fault faults (representingthe PanamintValley fault zone) zone in the Harrisburg Flats area (Figure 2). The are rectilinear to slightly curvilinear in map view, other strand maintains its northerly strike but have dips ranging from nearly 80øW to less than increasesits dip and correspondsto the Towne Pass 30øW, andjuxtapose Quaternary alluvium and upper fault. We stressthat there is a completelysmooth Precambrian metasedimentaryrocks. Individual transition between the three structures; there is no faultshave traces which vary in lengthfrom lessthan indicationhere that the low-anglefault approaching 1 km to roughly 4 km. Contiguousfaults overlap from the south cuts the Towne Pass fault, or that the substantially,and some segmentsclearly veer away Towne Passfault cutsthe Emigrantroof fault. from the range front and into the range as they terminate. North of the mouthof Wildrose Canyonthe range- INTERPRETATION bounding fault scarps are somewhat degraded because the footwalls predominantly consist of Field relationshipsin the Wildrose Canyon area poorly consolidatedNova fanglomerates. At the imply that the Towne Passfault is a hangingwall southernend of the area the footwall is made up of splay off the Emigrant detachment. In this crystallinerocks, and the steep,western front of the interpretationthe low-angle surface south of the range appearsto representa virtually unmodified, merger betweenthe two faults is a segmentof the exhumed surfaceof the PanamintValley fault zone. Emigrant roof fault that remained active (as a Here the lower portionsof the range front are quite segmentof the TownePass fault) afterthe rest of the planar. Linear regression analysis of eight Emigrantsubsystem had ceasedactivity. topographicprofiles and severaldirect measurements The field evidence also suggeststhat this same of the surfaceyield an estimateddip of 34ø _+5 ø (2s low-anglesegment acted as a portionof the growth uncertainty). fault for modernPanamint Valley. We infer that the In the vicinity of Tuber Canyon (Plate 1), the Nova and Panamint basins did not develop footwall of the PanamintValley fault zoneconsists of independentlyand that PanamintValley is the most variegated fault gouge and brecciated structural recent phasein a continuum. Just as the principal horses of upper Precambrian strata, both depocenterfor the Nova Basin steppedwestward 463

•?-!,•"'• "••;•-:.½,•.'.'.' •:",•;*...;•;:•'*..... De a t h Va lie y ,-.;.•½ .• ,:•':.... '"'

.. N ' 5'.

P'an.ami.nt:V.alley

b

Fig. 6. Obliquephotographs of a topographicmodel of the DeathValley NationalMonument at the Furnace Creek Visitors Center,which providesthe vantageof aerial photographyat a fraction of the cost. (a): Southeastwardview from aboveHunter Mountain. The Nova basin sensustricto is the diamond-shapedarea exposedbetween the Emigrant fault, the Panamint Valley faultzone, and State Route 190. PanamintValley lies in thelower fight comerof thephotograph. Merger of theEmigrant, Towne Pass, and Panamint Valley faultsis well-displayedin this photograph.(b): View lookingNNE, takenfrom aboveSurprise Canyon. Note the topographicexpression of theNova basin's growth faults, and the inflection of theEmigrant fault's trace at fault junctions.EF, Emigrantfault; PB, PanamintButte; PP, PintoPeak; PVFS, PanamintValley fault system;TM, Tucki Mountain;TP, TelescopePeak; TPF, TownePass fault; and WC, WildroseCanyon. g6•4 Hodgeset al.: Evolutionof ExtensionalBasins West of DeathValley

Figure 7. Interpretive perspectivediagram of the relationshipsbetween the Emigrant, Towne Pass, and PanamintValley faults. View is toward the south. The heaviestlines indicatethe surfacetrace of the fault, while the thinnestlines indicate the dip of the fault surface.The front, vertical surfaceis equivalentto the cross sectionin Plate 1. Note that faultsjoin at inflectionsof thefault traceand that all threefaults have the sametrace southof Tuber Canyon (here lying at the samelatitude as the northerntip of the North arrow). The Towne Passand PanamintValley faultssole into the Emigrantfault at depth,indicating that while the activedepocenters of the basinsmigrated westward with time, deformationat depthwas occurringalong the samesurface. beyondthe Towne Pass fault in Late Pliocenetime, it We do not mean to imply here that high-angle, shiftedstill furtherwestward (to PanamintValley) in range-boundingfaults do not occur in the Basin and latest Pliocene to Pleistocene time (Figure 7). In Range; dozens of studies have documented the many ways, Panamint Valley is simply the most existenceof a wide variety of geometries,from low- recent subbasinwithin the developingNova Basin. angle to high-angle and from planar to markedly Seen from a perspectivemillions of years in the listric [e.g., Andersonet al., 1983]. However, field future, the Nova Basin and PanamintValley would relationships in the Wildrose Canyon area and appearto be part of a single, complex extensional elsewherein the southwesternUnited States [e.g., basin. Dickinson et al., 1987] illustrate the existence and importance of young, low-angle, range-bounding IMPLICATIONS faultsin the Basin and Range.

Fault relationships on the western flank of the CONCLUSIONS PanamintMountains illustrate the dangersinvolved in presumingthat large-scaleextension in the Basin The Nova and Panamint basins have similar and Range is divorced from the development of structural and sedimentological histories; both modern Basin and Range topography. Many developedas a consequenceof westwardmovement workers [e.g., Davis and Burchfiel, 1975; Wemicke on low-angle detachments.The Nova Basin records et al., 1982; and Wernicke et al., 1988] present unroofing of the Panamint metamorphic core compellingevidence that large-magnitudeextension complex in late Miocene(?)- Pliocene time by occurrednorth of the Garlock fault in Neogenetime. displacement along the Emigrant detachment and The Panamint Mountains fit all of the stock related faults. The Panamint basin was formed in definitions of a "metamorphiccore complex" [e.g., late Pliocene - Recent time during simultaneous Davis and Coney, 1979; Armstrong, 1986; Davis evolution of the high-angle Hunter Mountain fault and Lister, 1988], yet one of the structures zone and low-angle Panamint Valley detachment. responsible for its unroofing (the Emigrant Becauseof the lack of evidencefor a significanttime detachment)clearly moved as recently as Pliocene break between deposition in the two basins, and time [Hodges et al., 1989]. Development of the becausesome growth fault segmentsseem to have Panamint Valley fault zone, in every sensea Basin been common to both basins, we conclude that and Range "range-boundingfault," seemsto have Panamint Valley is simply the most recent stagein been part of the same process of core complex the evolution of the Panamint metamorphic core evolution. complex. In this area thereis no indicationthat the Hodgeset al.: Evolutionof ExtensionalBasins West of DeathValley •465 development of "classic" Basin and Range Burchfiel, B.C., Geology of the Dry Mountain topography is a different process than tectonic Quadrangle,Inyo County,California. Spec.Rep. denudationby movementon low-angledetachments. Calif. Div. Mines and Geol. 99, 19 pp., 1969. Burchfiel, B.C., and J.H. Stewart, "Pull-apart" Acknowledgments. We would like to thank origin of the central segmentof Death Valley, Clark Burchfiel, Craig Jones,Peter Molnar, Zeke California, Geol. Soc. Am. Bull., 77, 439-442, Snow, Bennie Troxel, Brian Wernicke, and Lauren 1966. Wright for sharingwith us their thoughtsabout and Burchfiel, B.C., K.V. Hodges, and L.H. Royden, data from the Death Valley area. B.M. Page, E.S. Geologyof PanamintValley - SalineValley pull- Schweig, and two anonymous reviewers made apartsystem, California: palinspastic evidence for comments on an early form of this paper that low-anglegeometry of a Neogenerange-bounding drastically improved the present version. Robert fault, J. Geophys. Res., 92, 10,422-10426, Fleck of the U.S.G.S. kindly providedaccess to the 1987. geochronologylab at Menlo Parkfor K-Ar datingof Cemen, I., R. Drake, and L.A. Wright, Stratigraphy samples. The National Park Service personnelat and chronologyof the Tertiary sedimentaryand DeathValley NationalMonument and the Bureauof volcanic units at the southem end of the Funeral Land Management personnelresponsible for the Mountains, Death Valley region, California, in Panamint Valley conservationarea have been most Geologyof SelectedAreas in the WesternMojave gracioushosts during our fieldwork. This research Desert and Southern Great Basin, California, was undertakenas part of the 1986 and 1987 MIT editedby Cooperet al., pp. 77-87, Death Valley geology field camps. Financial support was PublishingCompany, Shoshone, Calif., 1982. providedby the Departmentof Earth, Atmospheric, Coleman, D.S., J.D. Walker, and M.E. B ickford, and Planetary Sciences at MIT, Chevron Oil Geochemistry of Mio-pliocene volcanic rocks Company, Shell Oil Company, and the National from northern Panamint Mountains and Darwin Science Foundation (EAR-8314161 and EAR- Plateau,Basin and Rangeprovince: implications 8512283). for extension and regional geology, Geol. Soc. Am. Abstr. Programs., 19, 623, 1987. REFERENCES Dalrymple, G.B., and M.A. Lanphere,Potassium- Argon Dating, 258 pp., Freeman Cooper, San Allmendinger, R.W., J.W. Sharp, D. Von Tish, L. Francisco, 1969. Serpa, L.Brown, S. Kaufman, J. Oliver, and Davis, G.A., and B.C. Burchfiel, Garlock fault; An R.B. 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Mio-Pliocene rocks of the northern Panamint Wright, L.A., and B.W. Troxel, Shallow-fault Mountains and Darwin Plateau: implicationsfor interpretationof Basin and Range structure,in normal-fault development and the opening of Gravity and Tectonics, edited by K.A. DeJong Panamint Valley, Geol. Soc. Am. Abstr. and R. Scholten, John Wiley, New York, pp. Programs, 19, 879, 1987. 397-407, 1973. Wernicke, B.P., Low-angle faults in the Basin and Zoback, M.L., R.E. Anderson, and G.A. RangeProvince-Nappe tectonics in an extending Thompson, Cainozoic evolution of the state of orogen,Nature, 291,645-648, 1981. stress and style of tectonism of the Basin and Wernicke, B.P., Uniform-sensenormal simple shear Range province of the western United States, of the continentallithosphere, Can. J. Earth Sci. Philos. Trans. R. Soc. London, Ser. A, 300, 407- 22, 108-125, 1985. 434, 1981. Wernicke, B.P., J.E. Spencer,B.C. Burchfiel, and P.L. Guth, Magnitudeof crustalextension in the southernGreat Basin, Geology, 10, 499-502, 1982. K.V. Hodges, J. Knapp, L.W. McKenna, L. Page,D. Silverberg,and K. Sternlof,Department of Wernicke, B.P., K.V. Hodges, and J.D. Walker, Earth, Atmospheric, and Planetary Sciences, Geological setting of the Tucki Mountain area, MassachusettsInstitute of Technology,Cambridge, Death Valley national Monument, California, in MA 02139 Mesozoic and Cenozoic Structural Evolution of J. Stock, Department of Earth and Planetary Selected Areas, East-Central California Sciences,Harvard University, Geological Museum, Guidebook, edited by G.C. Dunne, Cordilleran 24 Oxford St., Cambridge,MA 02138 Section,Geological Society of America, Boulder, J. D. Walker, Departmentof Geology,University Colo., pp. 67-80, 1986. of Kansas, Lawrence, KS, 66045 Wernicke, B.P., J.K. Snow, and J.D. Walker, G. Wast, Geology Institute, Swiss Federal Correlation of early Mesozoic thrusts in the Instituteof Technology,Ztirich, Switzerland southernGreat Basin and their possibleindication of 250-300 km of Neogene crustal extension,in This ExtendedLand, Geological Journeysin the SouthernBasin and Range,Field Trip Guidebook, edited by D.L. Weide, and M.L. Faber, (ReceivedAugust 18, 1988; Cordilleran Section, Geological Society of revised December 27, 1988; America, Las Vegas, Nev., pp. 255-267, 1988. acceptedDecember 29, 1988)