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10-1996 Late Cenozoic Structure and Tectonics of the Northern Mojave Desert Elizabeth R. Schermer [email protected]

B. P. Luyendyk

S. Cisowski

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Recommended Citation Schermer, Elizabeth R.; Luyendyk, B. P.; and Cisowski, S., "Late Cenozoic Structure and Tectonics of the Northern Mojave Desert" (1996). Geology Faculty Publications. 8. https://cedar.wwu.edu/geology_facpubs/8

This Article is brought to you for free and open access by the Geology at Western CEDAR. It has been accepted for inclusion in Geology Faculty Publications by an authorized administrator of Western CEDAR. For more information, please contact [email protected]. TECTONICS, VOL. 15, NO. 5, PAGES 905-932, OCTOBER 1996

Late Cenozoic structure and tectonics of the northern Mojave Desert

E.R. Schermer GeologyDepartment, Western Washington University, Bellingham

B.P. Luyendyk and S. Cisowski Institutefor CrustalStudies and Department of GeologicalSciences, University of California,Santa Barbara

Abstract. In the Fort Irwin region of the northernMojave desert, Introduction late Cenozoic east striking sinistral faults predominate over northweststriking dextral faults of the same age. Kinematic The Cenozoic tectonic history of the Mojave Desert block indicatorsand offset marker units indicate dominantlysinistral (Figure 1) has been the subject of much recent study, in part strike slip on the east strikingportions of the faults and sinistral- becauseof its relation to the San Andreasfault systemand thrustslip on northweststriking, moderately dipping segments at southernCalifornia seismicity. Deformationin the Mojave the east ends of the blocks. Crustal blocks -7-10 km wide by Desert providesa link betweenthe San Andreassystem, the -50 km long are boundedby complex fault zones up to 2 km Garlockfault, and the SouthernDeath Valley fault zone, and wide at the edgesand endsof eachblock. Faultinginitiated after transfersplate margin deformationto the Basin and Range ~11 Ma, and Quaternary depositsare faulted and fc•lded. We Province [Atwater, 1970, 1989; Dokka and Travis, 1990b]. document a minimum of 13 km cumulative sinistral offset in a Dokka and Travis [1990a,b] namedthis apparentlyinterrelated north-southtransect from southof the Bicycle Lake fault to north region of dextral shear from the San Andreas fault to the of the Drinkwater Lake fault. Paleomagneticresults from 50 SouthernDeath Valley Fault zone the EasternCalifornia Shear sitesreveal two direction groupsin early and middle Miocene Zone and argued that it accountsfor -15% of the total shear rocks. The north-to-northwestdeclinations of the first groupare alongthe Pacific-NorthAmerica plate boundary. Quantitative close to the middle Miocene reference pole. However, rock dataon deformationacross the entirewidth of theplate boundary magnetic studies suggestthat both primary and remagnetized zone provide constraintson the mechanicsand dynamicsof directionsare presentin this group. The northeastdeclinations of transformmargin processes;thus it is critical to determinethe the secondgroup are interpretedas primary and 63.5ø +_7.6 ø kinematics of deformation in the Mojave block as a whole. clockwise from the referencepole and suggestnet post middle Geodetic studies indicate 8-10 mm/yr of right-lateral shear Miocene clockwiserotation of severalof the easttrending blocks resolvedon northweststriking faults [Sauberet al., 1986, 1994; in the northeastMojave domain. The JurassicIndependence Savage et al., 1990] compared with -35 mm/yr on the San Dike Swarm in Fort Irwin may be rotated 25-80 ø clockwise Andreasfault, andgeologic and plate tectonic studies suggest that relative to the swarm north of the Garlock fault, thus supporting faultsof theMojave block have been accommodating a portion of the inferenceof clockwiserotation. Using a simple-shearmodel therelative plate motion for at leastthe last -10 m.y. [Dokkaand that combinessinistral slip and clockwiserotation of elongate Travis, 1990a]. crustal blocks, we predict -23 ø clockwise rotation using the Questionsand controversy exist regarding the amount,timing, observed fault slip, or one-third that inferred from the anddistribution of strainon faultsand folds in theMojave block paleomagneticresults. The discrepancybetween slip androtation and regardingthe importanceand distributionof vertical-axis may reflect clockwisebending at the endsof fault blocks,where rotationsof fault blocks[e.g., Gaffunkel,1974; Luyendyk et al., most of our paleomagneticsites are located. However, at least 1980; Bartley et al., 1990; Dokka and Travis, 1990a;Luyendyk, 25o-40 ø of clockwise tectonic rotation is consistent with the 1991]. In the northernMojave Desert region boundedby the observedslip on faults within the domain plus possible"rigid- Garlock fault, the Avawatz Mountains, the GoldstoneLake fault, body"rotation of the region evidencedby clockwisebending of andthe Cadyfault (Figure1; hereincalled the Northeast Mojave northweststriking domain-boundingfaults. Our estimatesof Domain)the major faults appear on publishedmaps [Jennings et sinistralshear and clockwiserotation suggestthat approximately al., 1962; Jennings,1992] as eaststriking as opposedto north- half of the 65 km of dextral shear in the Eastern California Shear west striking as they are elsewherein the Mojave block. The Zone over the last 10 m.y. occurredwithin the northeastMojave geometricsimilarity of the NortheastMojave Domain to the Domain. The remainder must be accommodatedin adjacent TransverseRanges has led to predictionsof left slip and either structuraldomains, e.g., eastof the Avawatz Mountainsand west litfie rotation[Gaffunkel, 1974] or clockwiserotation [Luyendyk of the Goldstone Lake fault. et al., 1980; Carter et al., 1987] of elongatefault blocksof the NortheastMojave Domain in conjunctionwith right slip and minor counterclockwiserotation of blocksbounded by northwest Copyright1996 by theAmerican Geophysical Union. strikingfaults. In contrast,Dokka and Travis [1990a] and Dokka [1992] proposed that deformation in the Northeast Mojave Papernumber 96TC00131. 0278-7407/96/96TC-00131 $12.00 Domainoccurred mainly by large amountsof right slip on north-

905 906 SCHERMERET AL.: CENOZOIC STRUCTUREAND TECTONICS,MOJAVE DESERT

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Figure 1. Generalizedfault map of Mojave Desert [afterJennings, 1992]. Box outlinesmap in Figure 2; shaded area indicatesNortheast Mojave Domain as describedin text. Abbreviationsare AVM, Avawatz Mountains; BLF, Bicycle Lake fault; CLF, CoyoteLake fault; CM, Cady Mountains;DLF, DrinkwaterLake fault; FIF, Fort Irwin fault; GLF, GoldstoneLake fault; GSF, Garlic Spring fault; MF, Manix fault; SDVF, SouthernDeath Valley fault; TMF, Tiefort Mountain fault. Paleomagnetic studies are indicated by the large dots: M, MacConnell et al. [1994]; V, Valentine et al. [1993]; AM, Alvord Mountains studiesof Ross et al. [1989] and R.E.Wells and J.W. Hillhouse (personalcommunication 1994); P, Pluhar et al. [1991]; CM, Cady Mountains studyof MacFadden et al. [1990]. Inset is regionalmap of the Mojave Desert and adjacenttectonic provinces. GF, Garlock fault; SAF, San Andreas fault, PMF, Pinto Mountain Fault; ETR, WTR, Eastern and Western TransverseRanges, respectively.

west strikingfaults with minor left slip andlittle rotationon east Previous work in the Fort Irwin region of the Northeast strikingfaults. In this paperwe describegeologic mapping and Mojave Domain consistsof reconnaissancemapping for the structuraland paleomagnetic analysis in the Fort Irwin National Trona 1:250,000sheet [Jennings et al., 1962], togetherwith more Training Center (Figure 1). This studywas designedto address detailedrecent work related to the presentstudy [Miller et al., the questionof how the plate motionis partitionedwithin and 1994; Yount et al., 1994] and studiesin areasbordering Fort acrossthis part of the plateboundary zone, specifically to deter- Irwin [Byers, 1960; Brady, 1984a,b; Spencer, 1990a,b; mine the geometry,kinematics, and timing of faulting and the MacConnell et al., 1994; Sabin et al., 1994]. Our new mapping role and distribution of vertical-axis rotations. We also consider and geochronology documents the following pre-Tertiary the questionof the relation betweenfault slip and rotationof geologic history of the Fort Irwin region. The oldest rocks blocksand the size,shape, and rigidity of crustalblocks, as well consistof Precambrianbasement (-1.4 Ga) and probableLate asimplications of the datafor modelsof theMojave region. Precambrianand Paleozoicmiogeoclinal metasedimentary rocks that occur as screensin Jurassicand Cretaceousplutonic rocks. Plutonic and volcanic rocks were deformed in Middle Jurassic Geologic Background time, cut by the 148 Ma Independencedike swarm,and deformed Geologicaland geophysical studies of theMojave desert block again at -105 Ma [Stephenset al., 1993; Schermeret al., 1994; haverecognized three important types of Cenozoicdeformation. Stephens,1994]. The Mesozoic eventsleft a pervasivemylonitic During early Miocene time, the central Mojave region foliation and lineation in pre-Late Cretaceousrocks in much of experiencedlarge-scale, northeast directed extension [e.g., Dokka the Fort Irwin region. Deformed rocks were intrudedby Late et al., 1988; Glazner et al., 1988; Dokka, 1989; Glazner et al., Cretaceous(-80 Ma) granitoids [Miller and Sutter, 1982]. A 1989; Walker et al., 1990], locally accompaniedby clockwise period of uplift and erosionoccurred following Late Cretaceous rotation [Rosset al., 1989; Ross, 1995]. Later strike-slipfaulting plutonism and prior to deposition and eruption of Miocene along northweststriking dextral faults apparentlybegan during sedimentaryand volcanicrocks. late Miocene time [Dibblee, 1961, 1967; Dokka, 1983; Dokka Tertiary volcanicrocks in Fort Irwin and surroundingregions and Travis, 1990a]. In addition,north-south shortening has been range in age from -21 Ma to 5 Ma and consistof silicic to mafic recentlyrecognized as playing an importantrole in the Miocene volcanic rocks, including several vent complexes [Spencer, and youngerdeformation of the Mojave region [Bartley et al., 1990b; Sabin et al., 1993, 1994; Keith et al., 1994; Sabin et al., 1990; Glazner and Bartley, 1994]. 1994; Schermer,1994]. Most of the units dip gentlyand range in SCHERMER ET AL.: CENOZOIC STRUCTURE AND TECTONICS, MOJAVE DESERT 907

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til Quaternary dry lakes 0 Jurassic, Cretaceous plutonic rocks 20 km II] Quaternary and Tertiary E3A Mesozoic metavolcanic rocks '\ '\. Mesozoic tectonites volcanic rocks lliJ Early Quaternary, [IT] pre- MesozQic metamorphic rocks \ Cenozoic faults Tertiary sedimentary rocks

Figure 2. Generalized geologic map of the Fort Irwin region, modified from Jennings et al., [1962] and Spencer, [1990a]. Fort Irwin National Training Center isoutlined by dashed box; China Lake Naval Weapons Center is area west of Fort Irwin. Abbreviations not defined in Figure 1 are AV, Alpine Valley; ASF, Arrastre Spring fault; BL, Bicycle Lake; BS, Bitter Spring; CCF, Coyote Canyon fault; CR, Coyote Ridge; DD, Dacite Dome; DKSF, Desert King Spring fault; FI, Fort Irwin town; GL, Goldstone Lake; GSF, Garlic Spring fault; LL, Leach Lake; LWL, Langford Well lake; ML, McLean Lake; MLF, McLean Lake fault; MSF, Mule Spring branch of Garlock fault NL, Nelson Lake; NLF, Nelson Lake fault; NWR, Northwest Ridge; OMF, Old Mormon Spring fault; PC, Pink Canyon; RPF, Red Pass fault; RPL, Red Pass lake; SR, Stone Ridge; SWR, Southwest Ridge. Locations of Figures 3b-9 are outlined by boxes.

age from 19 to 16 Ma [Sabin et ai., 1994; Schermer, 1994]; locally intercalated within sedimentary sequences that lie however, locally younger (-12 Ma) rocks occur at China Lake to unconformably above older rocks. the west [Sabin et al., 1994] imd Alvord Mountain to the south Miocene sedimentary rocks are sparse and dominantly crop (A.F. Glazner, written communication, 1995) (Figures 1 and 2). out in eastern Fort Irwin and the Avawatz Mountains (Figure 2) The typical Tertiary sequence consists of thin silicic tuff and tuff where Spencer [1990a, b] documented early to middle Miocene breccia overlying basement, followed by thick rhyolite lavas then extension along high-angle normal faults along with basin basalt. The thicknesses of units are highly variable owing to formation and filling. Tertiary deposits in Fort Irwin include erosional paleotopography on the pre-Tertiary basement and gently to moderately dipping medial to distal alluvial fan and paleotopography created by silicic flows and domes. Mafic fluvial deposits of middle Miocene age and alluvial and playa (basalt and basaltic andesite) and silicic (rhyolite and dacite) deposits of Pliocene to Quaternary age, with local intercalations magmatism are coeval; however, basalts predominate at the top of silicic air fall tuff [Sobieraj, 1994; Sobieraj and Sc hermer, of the section [Keith et £11.,1994; Schermer, 1994]. Volcanism 1994; Yountetai., 1994]. largely ceased in this region by -12 Ma [Sabin et al., 1994]; The presence of subhorizontal bedding (except where locally however, small-volume basalt lavas of latest Miocene age (5.6 affected by strike-slip faults) and the relative abundance of Ma [Schermer, 1994]) and silicic air fall tuffs of Pliocene age volcanic rocks and sparseness of sedimentary rocks suggest that (-3.5 Ma (D. M. Miller written communication, 1995» occur little extension occurred in most of the study area. This is in 908 SCHERMERET AL.' CENOZOICSTRUCTURE AND TECTONICS, MOJAVE DESERT

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u• 910 SCHERMER ET AL.: CENOZOIC STRUCTURE AND TECTONICS, MOJAVE DESERT contrastto the large-magnitudeextension in the centralMojave Detailed Structural Geology of Fault Zones desert [Dokka, 1989; Glazner et al., 1989] and in the Avawatz Becauseof the complexityof many of the fault zonesand the Mountains [Spencer, 1990b]. Extension in the Avawatz importanceof distributeddeformation within many of the fault Mountains was followed by late Cenozoic east vergent reverse blocks, we describe here in some detail the deformationalong faultingand folding that may be relatedto theeastern termination two of the major east strikingfault zones,the CoyoteCanyon- of the Garlock fault, or to right shearalong northweststriking Tiefort Mountain fault system,and the Fort Irwin fault zone, and faultspossibly connected to the southernDeath Valley fault zone two of the northwest striking fault zones, the GoldstoneLake (Figures 1 and 2) [Brady, 1984b;Brady and Verosub,1984; fault and Desert King Spring fault, which exemplify the most Brady and Dokka, 1989; Spencer,1990a, b]. importantfeatures. Detailed mapsof all the faults (Figures3-9) anda summaryof the age andnature of displacedunits (Table 1) Geometry and Kinematics of Faulting is supplementedby descriptionsof deformationalong the other faults in Appendix 1. The distributeddeformation appears to The Mojave block haslong beenrecognized to containat least play an importantrole in producinglarge rotationsdetermined two different Late Cenozoic structuraldomains, one containing from paleomagneticanalysis relative to the amountof fault slip northweststriking dextral faults and the other containingeast observed, as discussed below. We use subdivisions of strikingsinistral faults [Garfunkel,1974; Luyendyk et al., 1980, Quaternary•nits following the criteria described by Yountet al., 1985; Carter et al., 1987]. More complexmodels of the domain [1994] and Miller et al., [1994]; locationsof the detailedmaps structurehave beenproposed [Dokka and Travis, 1990a;Dokka, are shown in Figure 2, and brief descriptionsof widespread 1992],but in general,northwest or eaststriking strike-slip faults lithologicunits are providedin Figure 3a. predominatein all models.Fort Irwin is locatedin a domainof Coyote Canyon Fault. The Coyote Canyon fault strikes eaststriking faults bounded to the eastand westby north-north- approximatelyeast and extendsfrom the Pink Canyonarea in the west and northweststriking faults (Figure2), but detailedstruc- west,across Coyote Ridge, to a likely connectionwith theTiefort turalstudies have not beenpreviously conducted in the region. Mountain fault to the east (Figures2 and 4). A distinctMiocene A summaryof observationsof faultingin Fort Irwin is shown volcanicsequence of intercalatedtuffs andbasalts is offsetacross in Table 1. Detailed mapsand structuraldata are in Figures3-9, the fault zone. Cumulativestrike separation on a contactbetween and detailed fault zone descriptionsare given in electronic basaltand tuff near the baseof the sequenceis 4.1 km (Table 1 supplementAppendix 1•. In general,east striking faults are and Figure 4). Deformation along the CoyoteCanyon fault is typically subvertical to steeply south dipping, relatively distributedin a zone -1.5 km wide that includesmost of Coyote continuous across Fort Irwin, and curve to northwest strikes at ridge and CoyoteCanyon and includesnorthwest plunging folds their east and west ends. Left-lateral strike-slip with a small and reversefaults (Figure 4). reversecomponent occurs on eaststriking segments, and thrust or The western terminationof the Coyote Canyonfault occurs left-obliqueslip occurson the northweststriking end segments. whereit is truncatedby the GoldstoneLake fault (Figure9). In The major east strikingfault strands(Drinkwater Lake fault to this region the several fault strands curve into a more McLean Lake fault; Fort Irwin fault; Tiefort Mountain fault to northwesterlyorientation, suggesting dextral drag related to the CoyoteCanyon fault; and Bicycle lake fault; Figure 2) define GoldstoneLake fault [MacConnell et al., 1994]. At its east end, fourrelatively coherent elongate crustal blocks. The eaststriking the fault is buried beneath Holocene alluvium. faults do not follow preexisting structural weaknesses(e.g., Tiefort Mountain fault (north and south). The Tiefort comparethe trendsof Mesozoiccontacts and fabricswith faults Mountain fault strikes east from the northernmargin of Tiefort on Figures3, 4, and5 ). Relativelycontinuous northwest striking Mountain and bifurcates into northern and southern strands at faults are most importantin westernFort Irwin and possiblyat easternTiefort Mountain (Figures 2 and 5). Abundantsteeply the easternboundary of the domain, where they are less well dipping brittle fault planes with subhorizontalslickensides exposed(Figure 1). A dextral componentof slip is observed demonstratedominantly strike slip (Figure 5). East trending along northwest striking faults in northern and western Fort folds are common adjacent to the northern strand, and there Irwin, including the GoldstoneLake, Desert King Spring, and appearsto be a significantcomponent of south-sideup reverse Garlic Springfaults, but a dip-slipcomponent is alsopresent. faulting near the easternend (Table 1). To the west,the senseof The relativequality of featuresused to estimatefault slip and verticalseparation appears to changeto south-sidedown normal separationis shownin Table 1, togetherwith descriptionof offset faulting. The changefrom reverseto normaldip-slip component features. Few linear features are available to provide true may be related to the presenceof a releasingbend in the areaof piercing points; however, for planar features, we combine information from slickenlines and kinematic indicators on brittle North Tiefort Ridge, where the main strandof the fault changes strike-10 ø (Figure 5 and Table 1). Offset markers include fault planestogether with measurementof separationon planesto irregularpre-Tertiary intrusive contacts and a verticalTertiary(?) assessthe strike-slipand dip-slipcomponents. If the slickenlines rhyolite dike (solid triangle pattern on Figure 5), indicating do notreflect the long-term fault history,this interpretation could be in error. sinistralslip of >3.4 km on the major northand southstrands, but the northernmoststrand on the northflank of North Tiefort Ridge may accommodateadditional slip (Table 1 and Figure 5). An offset Quaternary alluvial fan deposit with distinctive •SupportingAppendices 1 and 2 areavailable on diskette or via metamorphic clasts forms a shutter ridge along the northern Anonymous FTP from kosmos.agu.org,directory APEND (Username-- anonymous,Password -- guest).Diskette may be orderedfrom American strandthat indicates750 m sinistralslip since early Pleistocene GeophysicalUnion, 2000 Florida Avenue,N.W,, Washington,DC 20009 time (Figure 5). This evidence, togetherwith the strongtopo- or by phoneat 800-966-2481; $15.00. Paymentmust accompany order. graphicexpression of the north strand,suggests that the northern SCHERMER ET AL.: CENOZOIC STRUCTURE AND TECTONICS, MOJAVE DESERT 91 1

EXPLANATION OF MAP SYMBOLS -1'•5(• Strikeand dip, horizontal bedding '•5 Strikeand dip of volcanic flow layering '•5 Strikeand dip of tectonic foliation ...... Depositionalor intrusivecontact: dashed where approximate, dotted where inferred or covered ...... Fault contact: dashedwhere approximate,dotted where inferredor covered Fault plane attitude;ball showsdip, arrow shows trend and plungeof striae

Anticline

Syncline

Paleomagneticsites Piercingpoint or separalJonmarker; white and shaded trianglesused for other markers along same fault

EXPLANATION OF MAP UNITS

Holocenewash and alluvialfan deposits(Qya) and playa deposits (Qp)

Pleistocenealluvial deposits showing desert pavementand rockvarnish development,moderately developed soils. Qia/Qoa indicatesyounger fan surface developedon older deposits. Age probably20-180ka (Yountet al., 1994)

Pleistocenealluvial fan depositsshowing well developedor eroded soils,moderately incisedand eroded fan morphology.Age probably>250ka (Yountet al., 1994).

Pleistoceneand Pliocene(?)alluvial fan deposits,moderately indurated, with stronglyeroded soils and highlydissected fan morphology;locally probablyolder than Tyb. Age >500 ka (Yountet al, 1994).

Youngerbasalt: Latest Miocene(5.6 Ma) andesiticbasalt

Tertiary(Miocene) sedimentaryrocks: Fanglomerate (Tc), sandstoneand siltstone(Ts), and lacustrinedeposits including shale, siltstone,evaporites, and limestone,with local silicictuffs dated at 11.7 Ma (TI), Tertiaryvolcanic rocks: Miocene basalt and andesiticbasalt (Tb/,andesite (Ta), dacite and rhyodacite ('I'd), rhyolite (Tr) an(] silicictuffs (Tt). Dates in local area from 19-16 Ma.

Tertiary(?)aphanitic rhyolite dikes Cretaceous graniticdikes; granitoids,including granite (Kg) and quartz monzonite(Kqm)

Jurassicor Cretaceousgranitoids, including granite (JKg) and quartz monzonite(JKqm); used where mapping and geochronologyare insufficient to determine probable age Late JurassicIndependence dikes: mafic and felsic

Jurassicgranitoids, generally unfoliated (Jqm); locallyfine-grained, strongly foliated(Jfg)

Jurassicintermediate to mafic plutonicrocks, generally foliated; includesgranodiorite to quartz monzodiorite(Ji), dark-colored granodioriteand monzodiorite(Jm), diorite(Jdi), and gabbro(Jgb)

Triassic or Jurassic metavolcanicrocks, includingrhyolite ignimbrite and andesite,with locallyintercalated volcaniclastic sedimentary rocks metamorphosedsedimentary rocks (ms), typicallymica schist, marble with local schistintercalations (mr)

Figure 3a. Explanationof mapunits and symbols used in Figures3b-9. Only lithologicunits that are present on morethan one map are shown here; units of restrictedimportance are identified in individualfigures. Ages based on Ar/Ar datafrom Schermer[1994, alsounpublished data, 1994]; subdivisionof Quaternaryunits after Yountet al. [1994]. ._+_

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.".".:".".• .":.".'! SCHERMERET AL.: CENOZOIC STRUCTURE AND TECTONICS, MOJAVE DESERT 915 916 SCHERMER ET AL.: CENOZOIC STRUCTURE AND TECTONICS, MOJAVE DESERT strandis younger than the southernstrand. Additional evidence thus constrainingthe initiationof faultingto post -11 Ma consistentwith this interpretationis the steep topographic [Schermer,1994; Sobieraj, 1994]. Strand5 cutsQuaternary gradient and -750 m of relief on the north flank of Tiefort alluvial and playa deposits,dips steeplysouth, and has a Mountain, suggestingthe presence of an older south strand componentof reverseas well as left slip, indicatedby both beneath the uplifted and dissectedfans (Figure 5). Further topographyand slickenlinedata (Figure 6). Severalstrands of evidence of a reverse fault boundingTiefort Mountain comes the Fort Irwin fault bend to a northwest strike at its eastern end from a well drilled throughalluvial fans on the northwestflank, andaccommodate a component of northeastvergent thrusting and which is interpretedto showgranific rocks of Tiefort Mountain folding(Figure 6). However,whether the easternmost segments thrust northward above alluvium (R. Quinones, personal mergewith or are cut by northweststriking faults farther east is communication,1992). uncertainbecause northwest striking faults are not well exposed. A complex breccia zone of northweststriking faults at East Along the westernextension of the Fort Irwin fault in the Nelson Tiefort Ridge containsboth northeast and southwestdipping dip- Lake area,there appears to be left separationof Tertiaryvolcanic slip and strike-slipfaults (Figure 5, right inset) that are overlain unitsacross several linear, east striking ridges of upliftedalluvial unconformablyby early Quaternaryor late Tertiary fan deposits and volcanicdeposits (Table 1, Figure 2, and Appendix1). (QTf) and are cut by the Tiefort Mountain fault (N). This Pyroxenedacite that cropsout northwestof McLeanLake may suggeststhat either the northwest trending breccia zone is an have been derived from a dome/vent complex of similar older, unrelated feature or that earlier strands of the Tiefort pyroxenedacite southeastof Nelson Lake (Figures2 and 7a), Mountain fault may once have curved to northweststrikes but whichwould allow for >7.5 km left separationacross the Nelson were later crosscutby eaststriking strands. Coarse fanglomerate Lake and McLean Lake faults. and brecciadated at <19 Ma [Sobieraj, 1994] and megabreccia Goldstone Lake and associated faults. Faults at the western sheetsinterpreted as landslide depositsoverlie -10 Ma fluvial boundaryof the NortheastMojave Domain mapped in thisstudy and lacustrinedeposits south and east of the Tiefort Mountain include the Main Gate fault, the Old Stable Fault, and the Rifle fault (D. Miller, writtencommication 1995) (unit Tcg, Figure5). Rangefault, togetherwith reconnaissanceobservations along the Thesedeposits were derivedfrom East Tiefort Ridge andeastern GoldstoneLake fault (Figure 9) [alsosee Dokka, 1992;Miller et Tiefort Mountain [Sobieraj, 1994] and suggestthe presenceof al., 1994;Yount et al., 1994]. The Rifle Rangefault, Main Gate major topographic relief bordering a basin east of Tiefort fault, and Old Stable Fault are likely related.strands of the Mountain. Becausefaults are not exposedat eithermargin of the GoldstoneLake (east)fault of Dokka [1992], but connectionsand basin,it is difficult to determinewhether these landslide deposits relationshave yet to be establishedby directmapping, and the arerelated to normal,thrust, or strike-slipfaulting (see Appendix southeastern extent of all the faults is uncertain. The Main Gate 1, "Red Pass faults" for further description of the Tertiary faultis markedby a subverticalnorthwest striking breccia zone in deposits). Jurassicand Cretaceousplutonic rocks. Two setsof fractures Fort Irwin fault. The Fort Irwin fault extendsfrom just west with subhorizontalslickensides occur along one segmentof the of the Avawatz Mountains to west of Nelson Lake (Figure 2). fault (Figure9), onethat strikes approximately east with dextral Details of the western extent in the Nelson Lake area are kinematic indicators and the other that strikes northwest with uncertainbecause the fault is coveredby alluvium and land has poorlydeveloped sinistral kinematic indicators. Net slip on the been extensivelymodified (due to military activity), so the area fault is poorly constraineddue to lack of distinctivemarker units describedherein includesonly the segmenteast of the Granite andpresence of similar Cretaceousgranite on bothsides of the Mountains. The fault was first recognized[Jennings et al., 1962] fault. A crude estimateof -4 km of dextralseparation is to have -2 km left separationof a contactbetween a Tertiary? providedby the offset of the gently dippingcontact between basaltplug andJurassic or Cretaceousgranite. Our more detailed Tertiary volcanicrocks and Cretaceousgranite, but this is not a studiesdocument the existenceof six fault strands(numbered on unique contact relation. A contact between Cretaceous Figure 6) with subhorizontalslickenlines indicating cumulative rfiuscovite-garnetgranite and Jurassic quartz diorite mapped west left-slipon the exposedintrusive contact of >3.7 km [Sobieraj, of the areaof Figure9 by Miller andSutter [1982] may be the 1994; Sobieraj and Schermer,1994]. Contactsbetween different offsetequivalent of the contacteast of the Main Gatefault, which units within the <11.7 Ma Miocene fan depositsare offset would suggest-3 km dextral separation(Figure 9), but the (strands2 and 3), andthe availableevidence suggests that these plutonicrocks may not be equivalent,as Yountet al., [1994] Miocenedeposits are offset as much as the older rocks (Table 1), suggestedthe Cretaceousgranite is offset in a sinistralsense.

Figure 6. Map of easternFort Irwin fault zone,location on Figure2. Subdivisionsof Tertiaryunits modified from$obieraj [1994]. Insetshows equal-area stereonet projection of faultdata: main fault planes and slickenline orientationsshown with solid lines and solid symbols; conjugate and subsidiary faults and slickenline orientations shownby dashedlines with open symbols. For clarity,not all foldsare shown by fold symbols,but can be identifiedfrom bedding attitudes. Bold numbers indicate different strands of faultreferred to in text.Tertiary lacustrinedeposits (unit T1) northof strand4 datedat 11.7+0.1Ma[Sobieraj, 1994]. Unitsnot identified in Figure3a areQop, older(?) Quaternary playa deposits, incised by modern wash; Tcs, Tcg, interfingering finer (sandstoneto cobble conglomerate) andcoarser (pebble to boulder conglomerate) Miocene alluvial fan deposits; Tof, conglomeratewith differentclast compositions from Tcs, Tcg [see$obieraj, 1994]; Tib, hypabyssal intrusive basaltic andesite. SCHERMERET AL.' CENOZOIC STRUCTURE AND TECTONICS, MOJAVE DESERT 917

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Dokka [1992] proposed-2.5 km dextralslip on the Goldstone 1 reveals that deformation is distributed over several kilometers Lake fault but did not indicate the offset units. The Old Stable adjacentto each fault zone. The five major east strikingfaults, Fault strikesnorth-northwest and curvesto more northerlystrikes the Garlock, Drinkwater Lake-McLean Lake, Fort Irwin, Tiefort in severalsplays at its southernend (Figure9). Yountet al., Mountain, and Bicycle Lake faults, divide the region into blocks [1994] interpretthe splaysas either a horsetailsplay at the ~7-10 km wide by 40-50 km long, which we term "crustal terminationof a dextral fault or curved due to the margin of a blocks"(Figure 2). Typically, ~0.5-1 km of the northand south ]:hyoliteplug southwest of thefault. TheRifle Rangefault edgeof eachblock is occupiedby complexlydeformed rocks in consists of several strands with minor lateral and dip-slip the bounding fault zones, but the width of deformed rock displacement(<100 m [Miller et al., 1994; Yountet al., 1994]). increasesto 2-3 km adjacentto fault jogs and bends. The faults Farther to the northwest,the GoldstoneLake (east) fault appears tend to have multiple strands,and rocks are folded and sheared to cut the CoyoteCanyon fault. MacConnellet al., [1994] report within the fault zone and adjacentto each strand;these define ~28 ø of clockwise rotation of Miocene volcanic rocks north of "local"blocks. Local blocksare alsodefined by the intersection theCoyote Canyon fault andeast of theGoldstone Lake fault and of subsidiarynorthwest and northeaststriking faults with the attributethe rotationto dextralbending along the GoldstoneLake main fault zones (e.g., Figures 3, 4, and 6). The east striking fault, consistentwith our observationsof the changein strike of faults typically have cumulativesinistral slip of 3-5 km that is volcanicrocks (Figures 4 and9). Strandsof the GoldstoneLake distributed between the several fault strands in addition to a fault that cut GoldstoneMesa (Figures2 and 9) showevidence significantcomponent of reverseslip (Table 1). Thus, in our for ~0.5 km of right separationand ~100-150 m vertical considerationof the regional significanceof the deformationin separation(northeast side down) of the gently north dipping the northeastMojave domain, a simple plane strain model of contactbetween the basaltand Cretaceousgranitic rocks. crustal blocks boundedby discrete, single, strike-slip faults is Desert King Spring fault. The DesertKing Springfault clearly too simple. (Figure7b) was interpretedto be a right-lateralfault by Dokka While it is evident from this study that the area of deformed and Travis, [ 1990a] andDokka, [1992], thoughno evidencewas rock is significant relative to the block size, there are still cited for amountor senseof displacement.The fault is marked undeformed areas ~5-10 km wide between each fault zone. by a breccia zone that cuts throughJurassic and Cretaceous Although a singlemarker unit is rarely presentin more than one rocks, but no markers exist in the Quaternaryunits cut by the crustal block, nearly all the blocks (with the exception of the fault (Figure 7b and Table 1). Slickenlinesand small-scale block between the Tiefort Mountain fault and the Fort Irwin kinematic indicators are sparse. Because the fault zone fault) have geologicfeatures that can be tracedthe entirewidth of juxtaposesidentical granitic rocks cut by theIndependence dike the block, suggestingthat no significantslip is accommodatedon swarmon eitherside and does not appearto significantlydisplace unrecognizedfaults within the blocks. Furthermore,most of the the southern,intruded boundary of thedike swarm,dextral slip is markers used to measure slip have been followed acrossthe likely <5 km (Figure7b). Moreover,the DesertKing Spring deformed block edges;thus the slip values reflect cumulative fault is clearlycut by severalsmall-displacement splays of the displacementacross the deformed zones. Disappearanceof AlpineValley fault (Figure 7b), whichhas <2 km of cumulative marker units from one fault block to another, however, leaves left-lateralslip on it. Thusif theDesert King Springfault is a openthe possibility that unrecognizedor buriedfaults exist at the long,continuous fault, its offsetcontinuation south of theAlpine edges of the blocks that could accommodatemore slip. We Valley fault shouldappear farther southeastin the Granite considerthis unlikely, however,due to the overall continuityof Mountains.Although no detailedmapping has been done in that such features as the belt of Mesozoic metavolcanic rocks, the partof the GraniteMountains, no evidencefor thefault canbe Independencedike swarm, and the belt of Jurassicmylonitic seen on aerial photos, on Landsat TM images, or in rocks (Figures2 and 10). In someareas, however, the width of reconnaissancemapping. A northweststriking fault with undeformedblocks appear to be <5 km, for example, in the subhorizontalslickensides occurs just west of the Desert King Nelson Lake area and north of Red Pass Lake (Figure 2). Springfault and has apparent left separationof a contactbetween Elsewhere,where Quaternaryunits cover significantportions of Jurassicand Cretaceousgranite (Figure 7b); however, the the fault block (e.g., north of Tiefort Mountains), the only kinematicsand timingof this fault andits relationto the Desert constraint on true block size is for the faults that have been active King Springfault areuncertain. Thus the 24 km dextralslip on duringQuaternary time (all of thosein Table 1), andit is possible theDesert King Springfault predicted by Dokka,[1992] remains therewas a morecomplex pattern of faultingduring earlier times. unverified. Part of the basis for the large slip value was the Fault intersections. Deformation at the ends of fault blocks purportedexistence of a largesphenochasm basin northeast of the occurs at the intersectionsof northwestand east striking faults. fault. However,we havemapped outcrops of basementrocks in Unfortunately,many of theseintersections in Fort Irwin occurin smallhills andgullies within the alluvialfan complexnortheast areasof low hills of sedimentaryrocks that are poorly exposed. of thefault (Figure 2; alsoshown by Jennings et al., [1962]), and Fairly simple crosscutting relations are exhibited at the limitedgravity data [Nilsenand Chapman,1971; Saltus and intersection of the Goldstone Lake and Coyote Canyon faults Jachens,1995] showno evidencefor a low. Theseobservations (Figure 9) where the Coyote Canyon fault curves to a more suggestthat the areanortheast of the DesertKing Springfault northwesterlystrike androcks adjacent to the fault are interpreted containsa thin veneer of alluvium or pedimentover shallow to be folded due to dextral drag on the Goldstone Lake fault granitebasement rather than a largedeep basin. [MacConnell et al., 1994]. Fault intersectionsfarther north along the Goldstone Lake fault are not well exposed,but both the Discussionof Geometric Characteristics of Faulting Nelson Lake fault and McLean Lake fault appearto be crosscut Size of fault blocks. The detailedmapping of the fault zones by the GoldstoneLake fault in areasof uplifted Quaternaryand andareas between major faults described above and in Appendix Tertiary sedimentaryrocks. The intersectionof the Bicycle Lake 922 SCHERMERET AL.: CENOZOICSTRUCTURE AND TECTONICS,MOJAVE DESERT

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•:...... •volcanicLateMiocene rocks Site declination: primary, remagnetized, low inclination .•..•TertiaryEarly Quaternary, sedimentary rocks Cenozoic faults *j::•Early-Middle Miocene volcanic rocks XX•:!•il!•ii' Independence dikes; •-• Pre-Tertiaryrocks,undivided iii interpreted rotation Figure 10. Map of the Fort Irwin region showingthe locationof palcomagneticsampling locations, mean declinationdirection at eachlocation (Table 2), and outcropsof the IndependenceDike Swarm. Independence dike trendsare usedto interpretrotation of Fort Irwin outcrops(arrows) relative to dike outcropsnorth of the Garlock fault (see text). Site location abbreviationsare BL, Bicycle Lake; BS, Bitter Spring; CC, Coyote Canyon; DD, Dacite Dome; FINW, Fort Irwin Northwest (Northwest Ridge); FISW, Fort Irwin Southwest (SouthwestRidge); GM, GoldstoneMesa; GO, Gary Owen; PC, Pink Canyonlocation of MacConnell et al. [1994]; SR, StoneRidge. Other abbreviationsas in Figure 2. Declinationarrow shownfor BL and BS is an averagefor theselocalities (see text).

and Garlic Springfault is extremelycomplex and is manifested intensebrecciation and complexfaulting (Figures2 and 5). It is by a zone of thrusting,folding, and both sinistraland dextral unclearwhether the complexityof deformationin this regionis faulting that makes up most of Beacon Hill (Appendix 1 and related to the intersectionof broadly coeval faults. It is also Figure 3) [Miller et al., 1994; Yount et al., 1994]. At the possible that the northwesttrending fabrics are related to an intersectionof the DesertKing Springfault andthe AlpineValley earlier deformationassociated with the sheddingof megabreccias strandof the McLean Lake fault, mappedfault tracesindicate into the adjacent basin. The eastern ends of the other east that the McLean Lake fault cutsthe Desert King Springfault and strikingfaults bend to northweststrikes and developa significant splaysout into many strandswith small (tens to hundredsof thrust or reverse componentbefore they are cut by northwest meters)displacement (Figure 7b). strikingfaults (Figures6 and 8). It is unclearwhether any large At the easternmargin of the domain,the northweststriking northwest striking dextral faults exist within or east of the faults eastof Tiefort Mountain, includingthe southernstrand of Avawatz Mountainsat the easternboundary (Appendix 1). the Tiefort Mountain fault and the Red Passfaults (Figure 2) are In general,the fault intersectionsin the study area appearto cut by the northstrand of the Tiefort Mountainfault in a zoneof result in transpressionand positive relief, as opposedto the SCHERMERET AL.:CENOZOIC STRUCTURE AND TECTONICS,MOJAVE DESERT 923 formationof extensionalbasins such as the sphenochasmsthat appearto largely postdatethe Avawatz formationand its havebeen inferred to mark the endsof blocks[e.g., Luyendyket associatedextensional event [Sobieraj, 1994; Spencer, 1990b]. al., 1980;Carter et al., 1987;Dokka, 1992]. In easternFort Timing of Faulting Irwin, wherelow-relief areas are presentat the endsof the blocks,any basin formed there may be related to overthrusting of The majorfaults in the northeastMojave domain were active unitsto theeast. Modelingof gravitydata [Saltus and Jachens, duringpost-middle Miocene through Quaternary time. Early 1995]suggests that basins exist at theeast ends of thefault Miocene volcanicrocks (18-16 Ma) [Schermer,1994] appearto blocks,and the abundance of Mioceneand Pliocene(?) deposits be displacedas much as Paleozoic and Mesozoic rocks, in this area (Figure2) may indicatethat long-livedbasins suggestingfaulting began after -16 Ma. Alongthe Fort Irwin developedatthe ends of theblocks. Whether these late Neogene fault, sediments<11.7 Ma appearto havebeen offset as much as basinsare due to extensionor shorteningis unknown,but they olderrocks [Sobieraj, 1994]. Thesedata are consistent with the

Table 2. Mean Palcomagneticdirections for Fort Irwin

Location Site Age,• Ma I, D,deg Polarityb O•95 k R

Dacite Dome 84-1, 2; 17.68 _+0.24 69.1,332.7 N 12.2 40.5 4.901 91-13, 15, 16 (90-37a) uncorrected 15.8_+0.4 (90-101) Fort Irwin SW 89-1,3,4, 7 17.83 _+0.2 46.4, 3.3 N 15.5 36.1 3.917 (89-2) correctedc CoyoteCanyon 90-5, 90-6 45.3,334.6 N NW declinations correctedd CoyoteCanyon 89-5,90-10 17.9 _+1.0 2 54.7,83.8 ½ R NE declinations (89-5f) correctedg 17.1 _+ 0.4 (89-6) BicycleLake andBitter 5.57 _+0.26 5 8.7, 163.1 T? 14.2 30.1 4.867 Springs,combined (89-11) correctedh 5.5 _+0.2 (91-16p) Goldstone Mesa 89-12 to 89-18 17.4 _+0.4 7 43.6,357.8 N 5.7 112.0 6.946 (89-14) correctedi 18.4_+0.2 j StoneRidge 90-11to 90-16 18.6 _+0.5 6 57.9, 59.5 N 5.4 153.0 5.967 (90-14); uncorrected 21.0 _+ 1.1 (90-14f) Fort Irwin NW 90-19 to 90-23 18.0 _+0.2 5 59.0, 339.6 N 12.9 36.0 4.889 (90-23) uncorrected GaryOwen 91-9pto 91-12p 15.85 _+0.11 4 46.7,50.0 ½ Rk 14.0 44.2 3.932 (91-9p) corrected1

N isnumber ofsites comprising 3 to6 sampleseach. Lavas at Dacite Dome, Stone Ridge, and Fort Irwin NW are believed tobe flat-lying andno structural corrections were applied. At Fort Irwin SW, the average ofsite mean directions corrected forfold plunge and bedding dip yieldsa95 = 15.5øand k= 36.1(n = 4); the average ofuncorrected insirasite mean directions yields o•95 = 25.4 øand k = 14.0.The mean directionsforthe northeast declination group and for the north-to-northwest declination group include both structurally corrected and in situ sitemean directions. The same group means computed with all uncorrected sitemean directions have much higher values of o•95 and lower valuesofk; o•95 = 42.0 øand k =2.0 for northeast declination sites,and a95 = 8.4øand k = 14.1for the north- tonorthwest-declination sites. Theseanalyses suggest a successful modified fold test indicating magnetization wasacquired before structural disturbance. •E. Schermerand P.B. Gans (unpublished data, 1994), except where otherwise indicated. Sample number inparentheses. bPolarity:N, normal; R, reversed;T?,transitional? CStmcturalattitudes 221ø/22 øNW, 89-1; 190ø/19øNW, 89-3,4340ø/25 øNE, 89-7;-1, -3, -4 plunge16 ø at 354ø dStmcturalattitudes 126ø/60 øSW, plunge 22 ø at 290 ø ½calculatedwith reversed directions projected through the origin fK-Arby Geochron Labs. gStmcturalattitudes 111ø/53 ø S, 89-5, plunge22 ø at 290ø hStmcturalattitudes 240ø/11 øNW, BikeLake; 349ø/6 øE, BitterSprings iStmcturalattitudes 290ø/10 øN, 89-12;302ø/18 øNE, 89-14,15;182ø/16 øW, 89-16,17, 18 Jcited by MacConnell etal. [1994] kExcept91-12 is N. 1Structuralattitudes 126ø/37 øSW, plunge 27 ø at 163ø 924 SCHERMER ET AL.: CENOZOIC STRUCTURE AND TECTONICS, MOJAVE DESERT

Table 3. Fort Irwin Mean Directions,Virtual GeomagneticPoles, and Discordance

SiteGroup a N I, D,deg 0•95 k R VGP Discordanceb deg Rotation Translation

Northeast 12 54.2, 59.7 6.6 44.4ø 11.750 41.9ø N, 315.8ø E 63.5ø + 7.6ø CW 2.6ø + 6.3 øsouth

North and NW 23 53.6, 349.8 6.3 24.4 22.099 81.5ø N, 148.5ø E 6.5ø+ 7.4 ø CCW 2.1 ø + 6.1 ø south North 11 44.6, 359.7 5.5 68.7 10.854 80.9ø N, 65.1ø E 3.5ø + 6.3ø CW 5.9 ø + 5.6 ø north NW, Remagnetized? 12 61.0, 336.2 8.0 30.2 11.636 70.3ø N, 180.6ø E 20.0ø + 9.5ø CCW 10.0ø + 7.2ø south

aBitterSprings and Bicycle Lake excluded; n=5 sites. Two dominant directions are selected; northeast or clockwisedeflected and northwest or counterclockwisedeflected. Northeast sites are Coyote Canyon 89-5, 90-10;Stone Ridge 90-11 to 90-16;and Gary Owen 91-9 to 91-12. North andNW sitesare all goodsites minus the NE sitesabove. North sitesare Fort Irwin SW 89-1, 3, 4, 7; GoldstoneMesa 89-12 to 89-18. NW, remagnetized?sites are Dacite Dome 84-1, 84-2, 91-13, 91-15 and 91-16; CoyoteCanyon 90-5, 90-6; andFort Irwin NW 90-19 to 90-23. bCW,clockwise; CCW, counterclockwise. Discordance isrelative toearly and middle Miocene reference pole of Calderone etal., [1990] at 85.5ø N, 108.9ø E, cz95= 4.4ø:I = 51.4ø;D = 356.2ø at Fort Irwin. Calculatedwith formulaeof Butler [1992] and [Demarest,1983]. øIfthe means of thelocations with NE declinations(Table 2) areaveraged together (n-3) themean direction result has k = 46.0compared to 44.4. Both of thesevalues are too high for properaveraging of secularvariation [Butler, 1992].

suggestionof Dokka and Travis [1990a,b] that faulting in the aboveand modifiedfold testsindicating that magnetizationwas Mojave block began at 6-10 Ma. Rocks as young as middle acquiredbefore structuraldisturbance (Table 2). Pleistocene, and locally Holocene [Miller et al., 1994], are The stable characteristicpaleomagnetic directions do not deformed along many of the faults; however, no quantitative appearto bear a simple relation to the elongatecrustal blocks estimateof Quaternaryslip is availableexcept along the Tiefort suchas is seenfor the CaliforniaTransverse Ranges [e.g., Carter Mountain fault (Table 1). Seismicity in the region is sparse, et al., 1987; Luyendyk, 1991]. The combined north and althoughit has increasedsince the Landersearthquake (Southern northwestdeclinations are closeto the expecteddirection for a California SeismographicNetwork, unpublisheddata, 1992- middleMiocene North Americanpaleomagnetic pole determined 1994). Modern drainagesand fan surfacesare not cut by faults, for the southeastCalifornia-western Arizona region [Calderone except along the Garlock fault. If the faults are active, long et al., 1990] (Table 3 and Figure 11). However, the mean recurrence intervals may be likely, given the subdued topographic expressionof the faults and lack of offset of the youngestQuaternary units. Longrecurrence intervals (103-104 All Sites from Fort Irwin years)have also been proposed for northweststriking faults in the n=35 Landersrupture area and the centralpart of the EasternCalifornia shear zone [Wallace, 1984; Lindvall and Rockwell, 1993; Sauber et al., 1994; Rockwell et al., 1995; C.M. Rubin and K. Sieh, unpublishedmanuscript, 1995]. There is no systematic ß Calderone et al. crosscuttingrelationship between the northwestand east striking faults, and both sets cut similar age Quaternarydeposits, suggestingboth setsare approximatelycoeval.

Paleomagnetism [_ n.orthwest Sampling and Results t o • sitesnortheast We obtained oriented drill core samplesfrom 50 sites in Miocenebasalts, andesites, and dacites in the region(Figure 10, Table 2, andelectronic supplement Appendix 2). All specimens were treated by step-wise alternating field demagnetization (AFDM) andtheir stabledirections were selectedby line fitting to orthogonalvector end point diagrams. Two directiongroups defining northeastand north-to-northwestwarddeclinations were found in 35 sitesthat were of acceptablequality (Table 2 and Figure 11). Of thesesites, five are reverselypolarized and are Figure 11. Equal-areadiagram of paleomagneticsite mean deflectedsouthwest approximately antipodal to the northeast directions for the 35 units studied. Also shown are the mean deflectedsites, suggesting a successfulreversal test. directions for northeast declination and north-to-northwest At any given location the number of direction resultsis too declinationsites (see text) and the Miocenereference direction few to average secular variation (Table 2). Successfulfield from Calderone et al. [1990]. Solid circles indicate normal stability testsinclude the approximatereversal test mentioned polarity;open circles indicate reversed polarity directions. SCHERMER ET AL.: CENOZOIC STRUCTURE AND TECTONICS, MOJAVE DESERT 925

AF demagnetization of 90-15.2a AF demagnetizationof 90-22.1 from Stone Ridge from Ft. Irwin NW W, Up South W,Up I i i i i i i North

d iv = 2.0 E-4

d iv = 5.0 E-4

5oa lOOO 1..oo

South.... • ' ' North Jo = 1.9 E-3

Figure 12. Progressivealternating field demagnetizationdata for representativesamples from (a) StoneRidge (90-15.2) and (b) NorthwestRidge (90-22.1). Thesesamples demonstrate the two classesof demagnetization behavior: hard with a stable characteristicremanent magnetization (Stone Ridge), and soft with streaked directionsfound at northwestdeclination sites (Fort Irwin NW).

direction from all the northwestdeclination sites alone has good progressivedemagnetization, demagnetization ratio experiments precisionstatistics and is discordantcounterclockwise from the (below) supportthis hypothesis.Dacite Dome apparentlyis in Miocenereference pole (Table 3). The northeastdirections are the samefault-bounded crustal block as StoneRidge (Figure10). 63.50+7.6ø clockwise from the referencepole. Dacite Dome samplesshow soft demagnetization,poor direction statistics, and northwest declinations. Stone Ridge has hard ProgressiveDemagnetization Studies demagnetization, clear primary magnetization and clearly ProgressiveAFDM experimentsreveal two classesof northeastdeflected declinations. Unless there is an unrecognized behavior:(1) a largeamplitude soft component removed by 15 fault between these locations, Dacite Dome directions are a mT or less(150 Oe); and (2) resistanceto AFDM with a median remagnetizationdirection acquired after rotation. destructivefield greaterthat severaltens of milliteslas(Figure Coyote Canyon samplesshow both stylesof demagnetization 12). Most samplesfrom mostlocations are of type 1. These behavior and both northeast and northwest declinations. Here we softersamples have streaked directions on equal-areaplots. Soft sampledthree lavas in stratigraphicorder: b3, b4, b5 (Figure 4). samplesare from a varietyof rocktypes and ages and often show Hard remanenceat 89-5 and 90-10 (flow sequenceb3 and b4) is multipleoverprint directions. Many harder samples also show a southwestdeflected with upwardinclination and is interpretedas reversed overprint direction on their natural remanent a reversednortheast direction; soft remanenceat 90-5 and 6 (b5) magnetization(NRM) that is removedby 10 roT. The AFDM is northwest deflected. Both directions are not likely to be showsthe mostresistant samples are from GoldstoneMesa and primary, as this inter-pretationwould require that b4 andb3 were StoneRidge where stable characteristic reinanent magnetization reversely magnetized and that post-b4, they both rotated >90 ø (ChRM) directionsare easily obtained. Moderately resistant sites clockwise,then b5 was deposited,normally magnetized, and then are Fort Irwin SW 89-1, -3, -7 and CoyoteCanyon 89-5 and 90- all flows were rotatedslightly counterclockwise. This requiresa 10. The softestsites are Coyote Canyon 89-6, 90-5 and90-6, and largerotation in a shorttime period;probably <<1 m.y. (Table 2). all sitesat Fort Irwin NW. Samplesfrom Gary Owen have soft Since the northwest direction is in younger units than the behaviorand are moderatelystable; Dacite Dome samplesare northeastdirection, there are at least two other possibilitiesif soft and less stable. some of the rocks were remagnetized: (1) b5 was normally The AFDM behavior suggeststhat the northeastand north magnetized;post-b5 the locationrotated-90 ø counterclockwise; declinationdirections are primary becausethese locations have the lower units b4 and b3 were remagnetized in a south samplesthat are most resistant. Northwest declination directions (reversed) direction; then the location rotated-90 ø clockwise, or are closelyassociated with soft AFDM behaviorwhich makes (2) post-b5, all units rotated -90 ø clockwise, then b5 was themsuspect. Although firm remagnetizationevidence is lacking, remagnetizednormal and there was no further rotation or slight e.g., in the form of consistentdirection overprints removed by counterclockwiserotation. Explanation2 is the leastcomplex. 926 SCHERMERET AL.: CENOZOICSTRUCTURE AND TECTONICS,MOJAVE DESERT

DEMAGNETIZATION OF TRM and NRM versus IRM IE-01 primary • TRMFISW IE-02re m a g ne t J Ze d f'+ TRMGM • TRMDD ...... •] ...... NRMGM

...... • ...... NRMDD m TRM $R

[• IE-03 • TRMFINW ...... •1 ...... NRMSR ...... • ...... tl• I TRMGO

Ft. Irwin NW IE-04 / DaciteDorne

0.1 0.01 0.001

IE-05 IE-04 IE-03 1 E- 02 IE-01 IE+00

SIRM Figure13. Demagnetizationratios for TRM (heatedto 625øCthen cooled in 0.045mT field) and NRM versus SIRMof FortIrwin samples. Demagnetizations weredone at 10,20, 30, 50, and 70 mT. Remagnetizedand demagnetizedsamples such as those shown from Dacite Dome and Fort Irwin NW areindicated by significant differencesbetween their TRM/IRM andNRM/IRM demagnetizationratio curves. Seetext for details. AbbreviationsareDD, DaciteDome; FINW, Northwest ridge; FISW, Southwest ridge; GM, Goldstone Mesa; GO, GaryOwen; SR, StoneRidge.

Thestructurally corrected directions of the two normal polarity coarsermagnetic grains [Cisowski et al., 1990,Figure 6]. In sitesfrom CoyoteCanyon (90-5 and 90-6) are similarto contrast, highly altered (naturally demagnetizedand directionsfor sites at Fort Irwin NW andDacite Dome (Table 2). remagnetized)samples generally display lower NRM/SIRM Becausewe suspectthat samples from Fort Irwin NW andDacite ratiosin therange of 10-3, often with linear or concave upward Domeare remagnetized (see below), these Coyote Canyon sites curves[Cisowski, 1992, Figure 8]. As a testof stabilitywe arealso believed remagnetized. The CoyoteCanyon fold test inducedsome samples with a laboratory TRM and compared the indicatesfolding here followed rotation and remagnetization. TRM/SIRM demagnetizationratio curvesagainst the NRM/SIRM curves(Figure 13). Rock Magnetism Samplesfrom sites displaying north and northeast declinations havedifferent ratio curvesthan those samples from sites Rockmagnetic experiments also suggest that the northwest displayingnorthwest declination (Figure 13). The northeast and directionsare remagnetized,while the north and northeast north declination samples display concave downward directionsare primary. A comparisonof alternating magnetic NRM/SIRM demagnetizationcurves, similar to theirTRM/SIRM fielddemagnetization behavior of NRM to laboratory-inducedcurves. In contrast,samples from northwestdeclination sites saturationisothermal remanent magnetism (SIRM) canaid in (DaciteDome, Fort Irwin NW) haveuniformly low NRM/SIRM distinguishingremagnetized paleomagnetic samples from those ratioswith demagnetization curve shapes that are strongly thathave retained their primary remanence [Fuller et al., 1988]. dissimilartotheir TRM/SIRM curves (Figure 13). For this method,a log10/log10plot of NRM versusSIRM Theinference from these observations isthat samples from the intensityatidentical demagnetization levelsis employed.For- northeastand north declination sites have retained their primary finegrained igneous rocks that still retain much of their primary thermalremanence. The characterof the NRM/SIRM curves thermalremanent magnetism (TRM), less altered samples fromDacite Dome and Fort Irwin NW suggestsalteration and generallydisplay NRM/SIRM ratios of the order of 10-2 or above, remagnetization,sothat paleomagnetic evidence for or against andtheir NRM/SIRM demagnetizationcurves have a concave tectonicrotation may have been lost. These experimental results downwardshape. This characteristic demagnetization behavior suggestthat the NRM/SIRM curves may be useful in mayresult from a mixtureof abundantfine, and less abundant, distinguishingremagnetized sites that appearnonrotated or SCHERMER ET AL.: CENOZOIC STRUCTURE AND TECTONICS, MOJAVE DESERT 927 counterclockwise-rotated(Dacite Dome and Fort Irwin NW) from the basement.The nonconformitybeneath Tertiary strata is from sitesretaining primary magnetizationthat have not been exposedthroughout the Fort Irwin area,and no low-angle faults rotated since the time of extrusion and magnetic blocking which might causedifferential rotation betweenMesozoic (Goldstone Mesa and Fort Irwin SW). Further evidence for basementrocks and Tertiary coverhave beenidentified. The remagnetization was revealed in electron microscopy and secondpossibility, of localrotations, is discussedbelow. microprobeanalysis (Appendix 2). The northwest direction is eonsistentamong the Fort Irwin PreviousPaleomagnetic Studies NW, Dacite Dome, and CoyoteCanyon locations (Table 3). All MacConnellet al. [1994] studiedearly Miocenebasalts from threelocations are in different blocks. We interpretthe northwest GoldstoneMesa and Pink Canyon (Figures 9 and 10). The declination as a nonrotated remagnetized direction. The AFDM characterof theirsamples also appears to be type2 (hard) remagnetizationprocess and timing are unknownbut apparently as we found for our Goldstone,Fort Irwin SW, and StoneRidge occurredafter rotation was complete and, in Coyote Canyon, sites. They computedclockwise rotations for thesesites of beforefolding. 9.60+7.4ø (Goldstone)and 28.40+_9.0ø (Pink Canyon). These meandirections have high k (187.5 and55.1) suggestingsecular TectonicImplications of PaleomagneticResults variationwas not averaged.They computed discordance relative The north directions from Goldstone Mesa and Fort Irwin SW to theearly Miocene reference pole of Diehl et al., [1988]. The areinterpreted as primary and nonrotated; the northeastdirections Pink Canyonsites are rotated - 22.6ø clockwiseand Goldstone from StoneRidge, Gary Owen, and two from Coyote Canyon are sites are rotated -3.8 ø clockwise with respect to the pole of alsointerpreted to be primaryand to reflectnet clockwisevertical Calderone et al. [1990]. axis rotation; the northwest directions from Dacite Dome, Fort Rosset al. [1989] found early Miocene clockwisetectonic Irwin NW, and Coyote Canyon are interpretedas remagnetized rotation in a broad swath of the Mojave Desert includingthe and not usable for tectonic analysis (Table 3). The Mojaveextensional belt. Rotationin thebelt is constrainedto be paleomagneticdirections we interpret as primary indicate that before18.5 Ma by paleomagneticstudies on the PeachSprings -64 ø of net post early Miocene clockwiserotation has occurred Tuff [Wellsand Hillhouse, 1989]. Rosset al. [1989]also studied (comparedto the referencepole of Calderoneet al. [1990]). The nine early Miocene flows in the Alvord Mountains(Figure 1). fact that the Goldstone Mesa sites are nonrotatedor possibly They found a clockwise rotation anomaly of 53.20+_9.9ø with slightlyclockwise rotated [MacConnell et al., 1994]suggests that respect to the Miocene reference pole of Diehl et al., [1983] the GoldstoneLake fault is a significant tectonic boundary (-48 ø with respectto the Calderoneet al. [1990] pole). Rosset betweenclockwise rotated (east) and nonrotated (west) crust. al. were uncertain as to the age of the rotation becausethey As an independentcheck of the interpretationof rotated believedonly early Miocenerocks were sampled.However, the blocks,we observethat the JurassicIndependence Dike Swarm Peach Springs Tuff at Alvord Mountain is rotated 56.1ø+_5.6ø within the elongatefault blockscan be interpretedas rotated clockwise (R. Wells and J. Hillhouse, written communication, relative to the swarm north of the Gatlock fault an amount 1994), not statistically different from the Ross et al. result. broadlysimilar to the declinationvectors [see also Ron et al., Further, the andesite flows at Alvord Mountain have been 1995]. Dikesimmediately north of the Gatlockfault trend310 ø- recently dated at 12.8 Ma (K-Ar (A.F. Glazner, written 314ø , while in the Granite Mountains in northern Fort Irwin communication, 1995)). Therefore the rotation in the Alvord (Figures2 and10) theytrend -334 ø [Smith,1962]. Thissuggests Mountainscan be interpretedas youngerthan 12.8 Ma. a differential rotation of 200-24 ø between these locations. Dikes Immediately south of the Manix fault Pluhar et al. [1991] that havebeen dated at 148 +_14 Ma [Stephens,1994] and which found a rotation of 8o+_2.7ø clockwise over 2 m.y. for the Plio- may be part of the Independenceswarm also occurat South Pleistocene(2.5-0.9 Ma [Nagy and Murray, 1991]) Mojave Tiefort Mountain (Figure 3). Here they trend 0200-030ø River Formation in the crustal block between the Cady and suggestinga rotation of 660-80ø relativeto northof the Gatlock Manix.faults (Figure 1). MacFadden et al. [1990] sampledthe fault. Assumingthe northerndikes to be a referencetrend, we Hector Formation (23-16 Ma) within the northern Cady interpretthat the dike swarmoutcrops in the northeastMojave Mountains(Figure 1) and found a uniform clockwisedeclination domain have been rotatedclockwise 240-80 ø . This comparison of 18.6ø (I = 45.4ø, 0695= 5.7ø) andno declinationchange with betweenthe declinationsand dike trendsfurther suggeststhat the age within the section. This is interpretedas due to 20.6o+_7.6ø northeastmagnetic declinationsare primary and causedby clockwiserotation post-16 Ma relative to the pole of Irving and tectonicrotation. Both the paleomagnetic directions and the dike Irving, [1982] (- 22.4ø clockwiserelative to the Calderoneet al. trendsalso suggest that the northernareas are rotatedless than [1990] pole). The rotation rate of--4ø/m.y. implied for this the southernareas within Fort Irwin (Figure 10). crustal block by the Pluhar et al. [1991] study permits 22ø of If the dikes in the Tiefort Mountains are indeed part of the clockwiserotation over a period of 5 or 6 m.y., suggestingthat Independenceswarm, it followsthat the rotationaffected large rotation could have started at the end of Miocene time. crustal blocks that include both Tertiary volcanic and Ross [1995] found large clockwise declination anomaliesin sedimentarycover andpre-Tertiary basement. However, if the the southwestCady Mountains, south of the Cady fault. He Tiefort Mountain dikes are not Independencedikes, the large interpretsan early Miocene rotationassociated with extensionin tectonicrotation inferred from the paleomagneticdata could the Mojave at that time and a post-14Ma clockwiserotation that insteadrepresent either rotation of coverrocks detached from the is attributed to local rotation in a northwest oriented dextral shear basementor rotationof local fault blocksadjacent to and within zone. the fault zones that bound the coherent crustal blocks. We see no Valentine et al. [1993] found -15ø+_12ø counterclockwise geologicevidence, however, for detachingof the coverrocks rotation of middle Miocene sites and no rotation of Pliocene sites 928 SCHERMER ET AL.: CENOZOIC STRUCTURE AND TECTONICS, MOJAVE DESERT

Table 4. TectonicRotations Assigned to CrustalBlocks in theNortheast Mojave Domain

Fault-BoundedBlock Site Name Study Rotation Age, Ma

EastTrending Blocks

Garlock-Drinkwater IDSa andGary Owen this paper 24ø-54ø CW _<15.8 Drinkwater-Fort Irwin none Nelson Lake/Fort Irwin-Coyote StoneRidge, Pink this paper,MacConnell et al. [ 1994] 23 ø-60ø CW _

Northwesttrending blocks

Goldstone-Blackwater Goldstone Mesa thispaper, MacConnell et al. [ 1994], and _<15 øCCWto4 ø -<13.5Ma Valentine et al, [i 993] CW Goldstone-GarlicSprings Fort Irwin SW this paper 7 ø CW -< 17.8 Ma

Rotationsrelative to the pole of Calderoneet al. [1990] •IndependenceDike Swarm;see text. bA.F.Glazner (written communication, 1995)

sampledin volcanicrocks betweenthe Blackwaterand Goldstone Rotation and Fault Slip Lake faults (Figure 1). These results are consistentwith our Field mapping indicates that the east striking faults are findingsand thoseof MacConnell et al. [1994] for the nonrotated sinistralwith typically 3-5 km of offset. Assuminga simple sitesat GoldstoneMesa. There is a possibilitythat the Miocene sites to the west of the Goldstone area are counterclockwise blockmodel wherein 10-km-wide blocks rotate during left slipof -5 km along eachfault [e.g., Luyendyket al., 1980;Ron et al., rotatedabout 10ø to 15ø with respectto the Goldstonesites; but 1984; Nur et al., 1989], clockwiserotation of-23 ø is predicted this observationis not statisticallyrobust. (Figure 14), or about one-third that inferred from the From our studiesand these prior studieswe concludethat crust paleomagneticmeasurements. This "mismatch"between slip and in northeastMojave domainbounded by eaststriking faults has rotationalso appearsto be true for southernparts of the domain rotatedclockwise in post early Miocene time but not coherently (e.g., Manix, CoyoteLake faults)since the magnitude of slip (-5 (Figure 10 and Table 4). The strainin the region has not been km [Meekand Battles, 1990]) androtation (-48-56 ø,Table 4) are homogeneousas Luyendyket al., [ 1980, 1985] and Carter et al., similar to those in Fort Irwin. In order to match the observed [1987] suggested,but the evidencefor widespreadclockwise , rotation is substantial. fault slip with observedrotation, coherentcrustal blocks would have to be <5 km wide, a value much smaller than that observed. The discrepancybetween slip and rotation suggeststhat the Discussion simpleblock model is not appropriateand/or that we haveeither Boundaries of Rotated Domain overestimatedthe rotation or underestimatedthe slip. There are several possible explanations for this discrepancy: (1) the Our paleomagneticresults suggest that the GoldstoneLake measureddeclination does not simply record rotation but also fault is the westernboundary of the rotateddomain. However, secularvariation (see above);(2) the "deficient"slip occurson becausethe Fort Irwin NW directionsappear to beremagnetized, otherfaults, for example,new faultsproduced when faults rotate the southwesternboundary of the rotateddomain is not well into an unfavorablestress orientation [e.g., Ron et al., 1984;Nur constrainedby the paleomagneticdata. Evidencefor possible et al., 1989];(3) the "excess"rotation is producedby mechanisms westwardextension of the BicycleLake fault betweenNorthwest besidesslip on parallel faults; and (4) someor all of the faults andSouthwest Ridges (Appendix 1 andFigure 9) andthe lack of rotatedpartly without slipping,either becauseyounger sinistral evidencefor major faultsbetween the CoyoteCanyon fault and faultsformed or becausethe entire domain rotated as a rigid body NorthwestRidge (Figures4 and 9) suggestthat the rotation without slip on the faults within it. We favor an explanation boundarylies just north of SouthwestRidge. Farthersouth, whereexcess rotation is producedby both ductiledeformation at significant differences in the trends of Mesozoic foliations and the block ends,and by rigid rotation of the Fort Irwin region lineationseast and west of theGarlic Spring fault (Figure 3 and withoutslip on the internalfaults (explanations 3 and4). Appendix 1) suggestrelative clockwiserotation of SouthTiefort Most of our paleomagneticsites were, by necessity,within a Mountainand that the boundary of therotated domain lies along few kilometers of the fault zones and at the western ends of the the Garlic Springfault. The easternand northernboundaries of crustalblocks near the domainboundary, whereas slip estimates the rotated domain remain undefined. are typically from the centralsegments of the faults (Figures3- SCHERMERET AL.: CENOZOIC STRUCTURE AND TECTONICS, MOJAVE DESERT 929

A: before deformation 10). However,local rotations adjacent to sinistralfault strands shouldbe counterclockwise,not clockwise,and thuswould not I k__m__m$ explainthe large clockwise declination anomalies. The complex intersectionof subsidiarynorthwest and northeast striking faults withthe main east striking fault zones produces blocks that could rotateclockwise (and counterclockwise)during north-south shortening(e.g., Figures 3 and4). Althoughwe do observe evidencefor north-southshortening in theform of east-trending folds and reversefaults rotation on the subsidiaryfaults would requirestrike-slip on those structures, and is notconsistent with thepredominant dip-slip observed. If the "deficient"slip is takenup on a younger,more favorably .!?::::"paleomagnetic orientedfault set that formed after blocksrotated -40 or 45 ø [e.g., direction Ron et al., 1984; Nur et al., 1989], we would expect that northweststriking segmentsof sinistralfaults in easternFort Irwin are older faults that have rotated. However, all of the studiednorthwest striking segments have lower dip anglesand a ..::::" '::i:..21.6 km B: after 23ø rotation,5 km slip larger componentof dip-slipthan the eaststriking faults and gray is referenceframe .,.:::::::"' many,as noted above, are clearly continuously curved from west ..,.:::::" .,::::::'"' to northweststrikes (Figures 5, 6, and 8). Throughoutthe study .,, -'-'-''-:::•::.::•!ii,,::,•,:..

.. area,crosscutting relations (e.g., Figures 5 and6) indicatethat the '":i::::::•i•:::..two setsare broadlycoeval. •,:•.i"::::::" "::!ii•. It is likely that all of the rotationwas not accommodatedby fault slip,but instead some was caused by distributedor ductile shearing[e.g., Reches, 1993], particularlyat the endsof fault blocks. The observation that the ends of several of the fault blocksare curved(Figure 2) suggeststhe possibilitythat the rocksat theends of the blockmay rotateindependently and more areas of block thanthe main bodyof the block(Figure 14). bending and distributed It is alsopossible that the entire domain may haverotated as a deformation / rigidbody without slip on internalfaults (Figure 14). In sucha paleomagnetic casethe dextralfaults bounding the domainwould rotate while direction the internalfaults were locked. The paleomagneticresults from Goldstonepermit the interpretationthat the areabounding the northeastMojave domainhas rotated clockwise -10ø-15 ø with 33 5 km respectto areasfarther west [e.g., Valentineet al., 1993]. "rigidbody"rotation ....::::::.::::..::::: ...... ::i::::::!:::i.. Possibledextral faults in the Avawatz Mountains (Figures 1 and '"" "::::::::i:.. ..:;.-::" -,:::. 2 and Appendix1) boundingthe easternedge of the domain "::::"bent Garlock fault strikemore northerly(340o-345 ø) than faultsto the southeast (-325ø), anda similarrelationship is seenfor theGoldstone lake fault at thewestern boundary of therotated domain. A rigid body rotation would add to the rotation amount suggestedby fault displacementand accountfor 23ø + 15ø - 38ø of clockwise rotation. The rigid bodyrotation could have occurred either beforethe sinistralfaults formed or afterthey locked. A similar interpretationwas proposed for theeastern Transverse Ranges by Richard[1993], who notedthat the 41 ø clockwisepaleomagnefic rotationfound by Carteret al. [1987]could not be explainedby / the observedslip on sinistraleast striking faults. He proposed thatpart of the rotationoccurred during sinistral slip on faults / withinthe domain and part occurred during rotation of bounding

bent boundaryfaults

Figure14. Cartoon block model illustrating twomechanisms ofrotation toexplain discrepancy between fault slipand paleomagnetic data.(a) Before deformation, (b)5km of left slip on faults within domain resulting in23 ø rotation,22km dextral shear. Shaded areas atthe ends of blocks are regions where distributed deformation may produceadditional clockwise rotation observed inpaleomagnetic data.(c) "Rigid-body" rotation of15 øoccuring duringdextral slip and rotation along bounding faults. Total dextral shear is33 km. Rotation may not have occurred in two distinct stages. 930 SCHERMER ET AL.: CENOZOIC STRUCTURE AND TECTONICS, MOJAVE DESERT dextralfaults without slip on most of the eaststriking sinistral fault. The available geologic data suggestthat the Blackwater faults. The evidence for the "rigid" rotation is seen in the fault, with -8.5 km right slip [Dokka, 1983], is the only signifi- clockwisechange in trendof the fault boundingthe easternside cant dextral fault in that area. Of the total of 65 km of dextral of the rotatedeastern Transverse Ranges domain. shear across the entire width of the Eastern California Shear Zone There are someobservations consistent with the interpretation for the last 10 m.y. [Dokka and Travis, 1990a], approximately of small (-25ø), rather than large (60ø) crustalblock rotations. half occurswithin the NortheastMojave Domain, and half must The clockwise rotation inferred from the trends of the occuron dextralfaults outsideof or boundingthe domain. Independencedikes in the GraniteMountains (24-28 ø ) and the Gary Owen paleomagneticdata (54ø) are different, but they Conclusions appear to be in the same crustal block (barring differential rotation acrossthe Desert King Spring fault; Figure 7). The The major Cenozoicstructures in the northeastMojave domain paleomagneticdata of MacConnell et al., [1994] from Pink are northwestand eaststriking, strike-slip and oblique-slipfaults. Canyon are consistent with the interpretation of a small East striking faults typically have 55 km left slip and a clockwiserotation of the crustal block immediately east of the component of reverse movement, suggesting an overall GoldstoneLake fault (28ø; 23ø relative to the Calderoneet al. transpressionalregime. Field studiesindicate a minimum of-13 [1990] pole). However,the Pink Canyonsites appear to be in the kmcumulative left-lateral shear in theregion from sou[h of the samecrustal block as our StoneRidge sites(Figure 10 and Table Garlockfault to northof the CoyoteLake fault (Figures1 and2). 4). The differencesbetween the paleomagneticresults at these Right-lateralslip on northweststriking faults within the domain four sitescan not normally be explainedby secularvariation. It is lesswell constrainedbut appearsto be lessthan -10 km total. seemslikely that thereis an unmappedfault separatingthe Pink Eaststriking and northweststriking faults appear to be broadly Canyon and Stone Ridge locations. We concludethat at least coeval and affect late Pleistocene strata. Block dimensions 25ø, andpossibly 40 ø, of clockwisetectonic rotation is consistent establishedby mapping suggestblocks are (were) -10x50 km, with the observedslip on faults within the domainand bending of separatedby wide fault zonesof denselyspaced fault strands. domain-boundingfaults, while the remaining -25 ø is related to Wherethe eaststriking blocks intersect the northwest-trending local deformationat the endsof the easttrending fault blocks. marginsof the domain,uplift dueto foldingand reverse faulting occurs.The age of initiationof faultingis postmiddle Miocene. Kinematic Model for the Northeast Mojave Domain Up to 60 ø of clockwise vertical-axis rotation inferred from palcomagnetic declination anomalies is constrained to have A simple two-stagekinematic model is shown in Figure 14. occurredafter 12.8 Ma. No declinationanomaly is shownby We emphasizethat the rotation may not have occurredin two sites west of the Goldstone Lake fault and west of the Garlic discrete stages; the model is drawn to illustrate the two Spring fault (Figure 10). Several sites with northwesterly mechanismsof rotation. In the first stage(Figures 14a and 14b), declination directions appear to be partly or completely 23 ø of clockwise rotation occurs during 5 km sinistral slip on remagnetized and thus cannot be used to infer rotations. The each fault. Areas of additional rotation due to bending of the combinationof geologicaland palcomagneticconstraints defines block ends are shaded. In the second stage (Figures 14b and the westernboundary of the rotatedregion as the GoldstoneLake 14c), an additional 15ø clockwiserotation occurs by slip and fault and the southwesternboundary as the Garlic Springfault rotationof the domain-boundingdextral faults. Althoughslip on (Figures 9 and 10). The easternand northernboundaries remain thesefaults is not well constrained,we interpret the Goldstone unconstrained. Lake fault to have <5 km of dextral slip, and thus the rotating The mismatchbetween fault slipsdetermined from geologic blocksare "pinned"at the northwestcorner. The modelrequires data and rotations inferred from paleomagneticdeclination northeast-southwest contraction, which is consistent with the anomalies is due to the three-dimensional nature of the mappedstructures in easternFort Irwin (Figures2, 6, and 8). deformationin the domain and the apparentnonrigidity of the We can use our block model togetherwith the structuraland fault blocks. Simple plane strainrotating block modelsare not palcomagneticdata to evaluatethe amountof dextralshear across appropriate to predict fault slip from vertical-axis rotations. the northeastMojave domain (Figure 14) and its role in the However, it is also possiblethat someof the observedclockwise Eastern California Shear Zone. For 23 ø of rotation within the declinationanomaly is due to a regional-scalerigid bodyrotation domainconsistent with the observedslip on the east striking of the blockswithin the NortheastMojave Domain. faults, the dextral shear is -21.6 kin. This rotation is consistent with the observed bend of the eastern Garlock fault -20-30 ø clockwise relative to the western Garlock. The total of-38 ø of Acknowledgments.We greatlyappreciate the cooperationof the staff rotation obtained by adding in the rigid body rotationresults in at the GoldstoneDeep SpaceCommunications Center and the Fort Irwin -33.5 km of dextral shear. Slip on the GoldstoneLake fault is National Training Center including Wait Cassidyand Rene Quinones. likely <5 kin; thus the model predicts -28 km of dextral slip Field assistantsinclude Jeff Johnson,Jim Carter, Roger Haston,Mike along the eastern boundary. This value is consistentwith the Hettinga, Hillary Dervin, Marty Parris, Tim Ross, and Chris Travis. suggestionsof Brady, [1984b 1994] and Troxel, [1994] for this Marty Parris and Bob Dunn assistedwith lab work. Ar/Ar work was donein collaborationwith Phil Gansand Andy Calvertat UCSB. Dave boundary,although Davis and Burchfiel, 11993] suggest<8 km Miller andJim Yountof the USGS arethanked for financialand logistical slip (see Appendix 1). Dokka and Travis [1990a] and Dokka support,as well as beneficial discussionsin the field, and John Nakata [1992] proposed-57 km of dextral shearin the region from the and Allen Glaznerprovided additional K-Ar data. G.I. Smithprovided Blackwater fault to the Avawatz Mountains due to oroclinal aerial photos,reconnaissance maps, and helpful advice. This research bending of the Garlock fault (Figure 1). If their estimate is was supportedby National Science Foundationgrants EAR 9105047 correct,the shearmust be taken up by -24 km of dextral shearin (Luyendyk) and EAR 9104915 (Schermer). Contribution0235-61TC of the region betweenthe Blackwater fault and the GoldstoneLake the Institute for Crustal Studies. SCHERMER ET AL.: CENOZOIC STRUCTURE AND TECTONICS, MOJAVE DESERT 931

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