TECTONICS,VOL. 8, NO. 2, PAGES363-390, APRIL 1989

THE MOJAVE EXTENSIONAL BELT OF SOUTHERN

Roy K. Dokka

Departmentof Geologyand Geophysics, LouisianaState University, Baton Rouge

Abstract.It is proposedthat the regional pattern normalfaults. At leastone generation of high-angle of early Miocenelow- andhigh-angle normal faulting faultspostdates the formationof the detachments. anddike swarmemplacement in the westernand Differential extension between the domains was centralMojave Desertis theresult of regional accommodatedby strike-slipfaults (transfer zones). extensiondeveloped within a roughlyeast trending Similarly,the lack of earlyMiocene extensional zone,named here the "MojaveExtensional Belt". elementsnorth of the westernsegment of the Garlock This zoneof extension,although now disruptedby fault suggeststhat the Garlockmay havebeen active late Cenozoicright-slip faulting, can be tracedfrom in the early Miocene and servedas the northern the intersection of the San Andreas and Garlock transferboundary of the Mojave ExtensionalBelt. faultsto the easternMojave Desertnear Bristol Lake The Mojave ExtensionalBelt initiallybegan to open and the Granite Mountains fault. Restoration of about22 m.y. ago, with the major phaseof post-Oligocenemovement along the SanAndreas extensionoccurring between --22 and20 Ma. Local, fault systemplaces the now buriedwestern part of high-angle,normal faults cut older detachmentsand theMojave Extensional Belt oppositea similarage- were active between --20 and 17 Ma. Kinematic extended area of west central California. Surface indicatorssuggest that the westernpart of the Mojave mappingand limited subsurface data suggest that the ExtensionalBelt openedin a NE-SW direction, dominanttectonic process responsible for whereasthe easternportion extended in an ENE- developmentof the Mojave ExtensionalBelt was WSW direction(present-day reference frame). low-angle,normal sense, simple shear (detachment Rotationof the regionabout vertical axes as revealed faulting). Strainwithin the Mojave ExtensionalBelt by paleomagneticstudies suggests that the direction is partitionedbetween four domains(Edwards, of extensionwithin the Mojave ExtensionalBelt was Waterman,Daggett, and Bullion terranes)that each originallyoriented-N-S andwas rotated as muchas consistof oneor morehalf-grabens. Each half- 50ø (clockwise)between •-20 and 18 Ma (Early grabenis composedof tilted,normal fault-bounded Miocene). blocksthat lie abovea rooted,low-angle, brittle- ductilenormal sense shear zone. Tilting and extensionof the upperplate resulted from the INTRODUCTION superpositionof severalgenerations of originally steeplydipping (-80 ø)planar to slightlycurviplanar The boundariesof thewedge-shaped Mojave Desertblock of southernCalifornia are defined by the Garlock fault zone to the north, the San Andreas Copyright1989 and Pinto Mountain fault zonesto the south,and the by the AmericanGeophysical Union Granite Mountainsfault to the east [Dokka, 1983](Figure1). Until recently,most tectonic Papernumber 88TC03966. synthesesof the westernUnited Stateshave 0278-7407/89/88TC-03966510.00 consideredthe Mojave to be devoidof themiddle to 36t1 Dokka:The MojaveExtensional Belt of SouthemCalifomia

125 ø 120 ø 115 ø 110 ø late Cenozoic extensional elements that characterize

•/(, tX \ o•. x N. Great Basin I the adjacentBasin and Range province. This misconceptionprobably stems from the absencein theMojave of therelatively young high-angle normal faults(i.e., basin-rangefaults) that are responsible for thepresent-day physiography of theBasin and -3s' ;.+_• MDB', EMb " +- RangeProvince. Tectonic models generally consider theMojave to be largelythe product of late Cenozoic fight-slipfaulting associated with the SanAndreas transformsystem [e.g., Dibblee, 1961; Garfunkel, 1974]. N '• • ',•o,.'.-- In a previouspaper I havedescribed the structure andtiming relations of earlyMiocene extension of thecentral [Dokka, 1986]. Thispaper I extendsthat synthesis to includethe western Mojave Desertand proposes a tectonic model to accountfor Fig. 1. Indexmap of theMojave Desert region. regionalrelationships. The major thesis of themodel MDB, MojaveDesert block; EMD, EasternMojave is thatearly Miocene extensional tectonism in the Desert;GMF, GraniteMountains fault; SAF, San MojaveDesert was concentrated in an regional E-W Andreasfault; GF, Garlock fault; PMF, Pinto belt,termed here the "MojaveExtensional Belt". The Mountainfault; TR, TransverseRanges; SJF, San structureand geologic history of theMojave Jacinto fault. ExtensionalBelt hasimportant implications for

BASIN & RANGE PROVINCE

SIERRA NEVADA

N ,\ \ \ Waterman

AM

ee e

ße • OM

I 20miles I 04,•,,•,e ,,•.'"• 20 km I i

o Los Angeles

Fig.2. Locationof structuraldomains of theMojave Extensional Belt of southemCalifomia. Localities andfeatures mentioned in text:AM, AlvordMountains; AB, AntelopeButtes; BM, BullionMountains; BrM, BristolMountains; BL, BristolLake; CM, CadyMountains; DR, DaggettRidge; FP, FremontPeak; GH, GravelHills; HL, HarperLake; HH, HinkleyHills; KH, KramerHills; MR, MitchellRange; MH, Mud Hills;NM, NewberryMountains; OM, OrdMountains; RM, RandMountains; RdM, Rodman Mountains;RgL, Rodgers Dry Lake;RH, RosamondHills; RL, RosamondDry Lake;WH, Waterman Hills. KSF,Kane Springs fault; LMF, LaneMountain fault; BWF, BaxterWash fault. Dokka:The Mojave Extensional Belt of SouthernCalifornia 365 models of the tectonic evolution of the westem translatedthe Mojave ExtensionalBelt into its present United Statesas well as the structuraldevelopment of configuration[Dokka, 1983, 1987; R. K. Dokka and rifted continentalmargins. C. J. Travis, manuscriptin preparation,1988].

Internal Structureof the ExtensionalDomains ARCHITECTURE OF THE MOJAVE EXTENSIONAL BELT The overall structure of an individual extensional domainof the Mojave ExtensionalBelt is thatof a General Relations singleor a seriesof half-grabens(usage after Bally [1981]), in whichan upperplate of multiply Plate 1 is a generalizedgeologic map of the deformed, tilted, normal fault-bounded blocks lie Mojave regionhighlighting early Miocene abovea rooted,low-angle detachment fault (usage extensionalelements as well as syntectonicand after Davis et al. [1980] and Dokka [1980]). Seismic posttectonicsedimentary rocks. This map also datafrom the westernMojave Desertsuggest that the showsthe locationof detailedgeologic maps and westernpart of the Edwardsterrane is composedof crosssections that are presentedbelow. The inferred severalNE dippingdetachments (Cheadle et al. locationof the Mojave ExtensionalBelt is shownin [1986], Spongbergand Henyey [1987], Lawson Figure 2. [ 1987], Serpaand Dokka [ 1988], and this paper). Data presentedhere andelsewhere [Dokka, 1986] This imbricatecharacter is also suspectedbut not yet suggestthat the uppercrust and at leastportions of documentedfor the Daggettterrane. Plate 3 is a the middlecrust of the Mojave ExtensionalBelt were seriesof geologicand seismic sections that illustrate extendedin earlyMiocene time andthat extension the structureof the domainsof theMojave Extensional Belt. was accomplishedby low-anglenormal faulting, Edwards terrane. The current surface of the high-anglenormal faulting, and extension fracturing accompaniedby intermediateto siliciccomposition Edwardsterrane is composedof tilted upperplate intrusion. At its presentlevel of exposure,most of fault blocksthat are surroundedby Quaternary the Mojave ExtensionalBelt is composedof sediments(Plate 1). In all but the southernmost moderatelytilted fault blocksthat lie in an upperplate portionof the terranenear the SanAndreas fault positionrelative to detachmentfaults that underlie the (e.g., AntelopeButtes), upper plate rocks are tilted extendedregion. Thesefault blockstypically consist 30o-50 ø to the southwest in association with NE of Oligocene-lowerMiocene volcanic and/or dipping,moderate angle, normal faults. Strikes sedimentaryrocks that rest unconformably on pre- changefrom-WNW to-N-S on approachto the San Tertiaryigneous and metamorphic rocks (Plate 1). Andreas(Dibblee, 1967b),probably reflecting Exposuresof detachmentfaults, associated brittle postextensionstrain. Evidence that detachment faults floor the tilted and/or ductile fault rocks, and subdetachmentrocks normal fault-bounded blocks of the Edwards terrane are limitedto theNewberry Mountains and to a series of low mountains from near the hamlet of Lockhart is providedby seismicreflection surveys (Cheadle et to the Mitchell Range(Plate 1). Brittle structuresand al. [1986] (Plate 3a), this paper(Plate 3b), fault rocksof the NewberryMountains detachment Spongbergand Henyey [ 1987], Lawson[ 1987], occuron the perimeterof a large,east-trending arch Serpaand Dokka [1988]) andsurface mapping in the westernNewberry Mountains (Plate 2). In [Dokka et al., 1988] in the HarperLake area(Plate contrast,early Miocene age mylonites and mylonitic 3c and 3d). Plate 3c is a schematiccross section that gneissescrop out in the footwallsof detachment illustratesthe generalgeometric relationships faultsin the Mitchell Range,Waterman Hills, between extended and tilted normal fault-bounded Hinkley Hills, andwestern Harper Lake area(Plate blocks of the Kramer Hills and the brittle-ductile, 1, Figure 3). Theserocks are uppergreenschist to normal senseshear zone exposedwest of Harper amphibolitegrade metamorphic rocks and represent Lake. Apparently,the Edwardsterrane ends the deepestlevel exhumedby extensionalprocesses. abruptlyagainst the westernpart of the Garlockfault The overallstructural and petrologic relationships to the northwest and the San Andreas fault to the exposedin theseareas are similarto themetamorphic west (Figure 2). core complexesof the westernUnited States[c.f. Watermanterrane. Althoughthe Waterman Davis and Coney, 1979; Crittendenet al., 1980]. terraneis the smallestof the domainsof the Mojave The Mojave ExtensionalBelt canbe subdivided ExtensionalBelt (Figure2), geologicrelations into four domains on the basis of differences in fault exposedthere provide important information on the geometryand associated tilt patternof lower Miocene sequenceof eventsand on theprocesses involved in strata; from northwest to southeast,the terranes are extension. The surface of the domain consists of namedEdwards, Waterman, Daggett, and Bullion variablySW dippingMiocene sedimentary and (Figure2). Post-13Ma (perhaps5 Ma) fight-slip volcanicrocks that rest unconformablyon upper faultingalong NW- strikingshears has cut and platepre-Tertiary crystalline complex (Plate 1). 366 Dokka: The Mojave ExtensionalBelt of SouthernCalifornia

ß ß

Fig.3. Photographsofmylonitic rocks of the Waterman Gneiss. (a) gneissic foliation, northern Mitchell Range;(b) stretchinglineation and mylonitic foliation, northern Mitchell Range; (c) augenin marble tectonite,northern Mitchell Range.

Theserocks lie in low-anglefault contact with al., 1988]. Along the eastflank of the northern myloniticrocks that are exposedin thenorthern Mitchell Range,both brittle and ductile fabric Mitchell Range(Figure 4) andWaterman Hills elementsof theMitchell detachmentare cut by a [Dokkaand Woodburne, 1986; this paper; Glazner et postdetachment,NW striking,down-to-the-east, Dokka:The Mojave ExtensionalBelt of SouthernCalifornia 36?

Fig. 3. (continued)

normalfault zone(Plate 3e, andFigure 4). The fault the WatermanGneiss by Bowen [ 1954]. These continues to the northwest between the Waterman rockscan be classifiedas type I and II S-C Hills andMud Hills andlikely containsmore than 1 mylonites(nomenclature after Lister and Snoke km of syntectoniclower MiocenePickhandle [1984]) and typicallypossess a well-developed,SW- Formationand the lowerpart of the Barstow NE stretchinglineation; structural data from these Formationin its hangingwall. The Pickhandle rocksare summarizedin Figure 5. The finite strain Formationof McCulloh [1952] and Dibblee [1968] ellipsoidis prolatewith 3.• oriented S45ø+10øW consistsof tuffs andgranitic and tuffaceous landslide [Weiss, 1954; Bartley and Glazner, 1987; F. Sierra brecciasshed from unextendedhighlands lying to the and C. L. McCabe, written communication,1987]. north and northeast[Dibblee, 1968; C. J. Travis, Senseof shearwas determined on myloniticrocks of personalcommunication, 1988]. Theserocks are the WatermanGneiss using the methodsoutlined in overlainin theMud Hills by gravelsand tuffs of the Simpsonand Schmid [1983] andindicate that upper lowerpart of the Barstow Formation. An4øAr-39Ar levelswere transportednortheast relative to lower single-crystallaser fusion date (biotite)of 19.3_+0.02 levels[Dokka and Woodbume, 1986; this paper]. Ma has been obtained from a tuff located near the Protolithsof theWaterman Gneiss were probably base of the Barstow Formation in the Mud Hills Upper Paleozoicmarine clastic and carbonaterocks [MacFaddenet al., 1989a]. At least1 km of mainly [Burchfielet al., 1980]which were intruded by lacustrinedeposits belonging to the posttectonic, Jurassicand/or Cretaceous granitoids [Kistler, middleMiocene portionof the BarstowFormation 1974]. Burial andsubsequent metamorphism (to completedthe filling of thehalf-graben. temperaturesexceeding the4øAr-39Ar closure The Waterman and Edwards terranes contain the temperatureof biotite(-300øC) occurredbetween deepestexposed levels of the Mojave Extensional Jurassicand early Miocene[Dokka andBaksi, Belt. Middle crustal rocks in the footwall of the 1988]. Theseductile fabrics are overprinted by Mitchelldetachment (Waterman terrane) and Harper faultsand joints (Figure 6), aswell asby extension Lakedetachment (Edwards terrane) are now exposed fracturesfilled with silicicdikes (Figure 7). in a seriesof fault-boundeduplifts in the eastern Thermochronologicstudies suggest that Waterman partsof the Waterman(northern Mitchell Range myloniticrocks of theWaterman Hills, Mitchell (Plate3e), WatermanHills, andHinkley Hills) and Range,Harper Lake areacooled from above300øC Edwardsterranes (small hills located west of Harper to near70øC (on thebasis of thedegree of annealing Lake (Plate3c)). Theserocks include upper of fissiontracks in apatite)near 19 Ma [Dokka and greenschistto amphibolitefacies schists, gneisses, Baksi, 1988;manuscript in preparation,1988]. andmetacarbonates that were originally designated as Rapid coolingand exhumation of middlecrustal 368 Dokka: The Mojave ExtensionalBelt of SouthemCalifomia

EXPLANATION ' "..',;' ß, .;x_ ,- .. œ,.•,:'--,'_•,, •_ ,,, ,, ,, IOal Alluvium /,-i•.` -% .'..,,,_-•..•';_,.-_-:;;-,,-..',,•,-,, ...:_.';- . x/,.• •' -.'•'•:d";:.c•,;,,,;;,5,'-,"'i Older alluvium

,/,, ,s • •,'• . ,• Xø-'•II•.,,,,,-,,•. •, , UNCONFORMITY / • f o• \!l_;l•,•r•t,, _• . ? ß • / ø' "-. t," ' / ,._'::ii ß...... •• • ', o.,%X /, / _ ,:':':': UndifferentiatedTertiary ...... '•/ X , / -:'• -x x, --• ,..'.:.'.,'..f_.:.:.:.:. sedimentaryand volcanic rocks ,_ .. ,•',.-N. ø" / ,.':.:.:.:.:?..:.:.: ,1' -, •,t,, ' -N / :.:.:.:.:.:.:c,:•-':..-. UNCONFORMITY x ,' i ' • / •:.'.'.'-•"'-":'•'"" •.- q- • X • •'""" "":'"'"' ' ' ½'

• ,•. X ,, t • , n !•t I".".'.".".".".'.'.',P'.'.%...... / \X •, %'.'.'.'.'.'.'.t.'.'.'.'. ..'.'.'.'. •oa .'.'.'.'.' ß .'.':, ß ,' • .oo...... o..oo... o..o.o.,•...• "'..•. ß ...... _. ,I, '• Undifferentiatedpre-Tmxiary • , , •s , ...... ::.j•..::::::...-..•.k.. crystallinerocks I ! •..,•...... -,..• •..• ..... ,• ...... FAULT •,'• , XIoø,ø.ø.",øoø.øo :•'• .".øoøoøo"oøo".øo".:::::::::::::::::::::::::::::::::::::::::::::::::::::::: ',1, .,••- ,X. , / ":::::::::::::::::::::::::::::::::::::::::::::::::::: Diorite

s , - ß. ,--..:.:.:.:.:-:.:.:.:.;..':;•..F.•..?.*..:.: .. '.:., ;::.:.:. ß,'¾',' ,-, ',_k , /• '•':':••:...:.'.::i:'.'.'-'....-.':':'.'.•::::::::::.X.'::....,' ' ' •....1 , -,., ,; %,. / "'•-•-:"-.::•.'.':.:.•:.:i:i::.::::'"'"""'""... o,,• ":" '.....'.':'•".'..:'.:::.•::::.'.,";• I wgl I • '• IY • • g ";':':':':':':"'"'""-'"'"'•:':'.".".".".".".X:':':'."...... J '•.•,• \ o• v(...... '.'.'.'.'.'.'.'•.'.'.ooa ...'. :.'..:.:..N:.'.'.I Waterman Gneiss • /•"'_':X ' • :::::::::::::::::::::::::::::::::::::::::::::::::: • - • o, ======3O ,q•' ? p,,-/• F ,• ,,,•_-,•.•% V .... Qa- --•;.,::•,•.:.:.:.:.:.:.:.:.:.::.:'•.:'½.'.'.'.'.'.'.'.'.'.' .'.'.'.'. Contact,showing dip "' /i • r, • %l,, k • ,- •,,'1,, • ..---...-...... oO.'•. '• ,:...... ::_::Z/...:i::':i:..,•,...o...... O..o...%...,4,," ...... "o'."'%" ß .'",4/ •Z •, o,,, ,• ',z, ,_ __ o; ,'.:.3:.:.:.:.:-:.'.-.-, .:.:.:...'.:.-....::•:•!,::':'•:, ::.'.'::::?'•.: !! :. Dashedwh•re gradationalor approximately1ocat•cl /.,.+/* xt, , ' Y / 'x ' "' -, '•.'"'ß.:.:...... :.:.:.:.:.:.. ' """ß.'.'.....-.'.'.'_ ...... •_•-.,'•'.-•'C:::':':'••'&.:.:.:.: :',,'.'.. :. :. :• -iv. ß * •Pi,-, • . • ,po> F, •.:..%•, ...... :-:-:-:.:-:-:-'.,,•.-:..'.'.:•,<.,;.:.:• .:.:.:.:".)•.".':'...'"-'...:,..";;:•::'.,ø" oa' 20 • ,m, _.. ß ? ß ß ++ + ',•-• %[ , i. • • ., •iii:•,•i•:•ß.:.•...•, ,' ¾. .•.•o:. :-:':':"'Z'-. •o"....":':'•'....".'!.:',•,.':.,'..'.:...'...... •52'':.....' ...•.-o.o..O•..•.•o•oO•:. ".-'-" '.;:';!:., vL• % ":•."Iß.., . .• • ...... ,ø •o .".•."o"o".øo".ø.•oøoø•'• '.",•.'• Low-anglenomaal fault + + + , r•_ •,• .• ,,• / / ., [..-.:.: •.:.:.:.:.:.:.:.:.:.:.:.:.:.:•;..:.:.:.:.:.:.:.:.:.:.:.:.:.:• ..: Dashe•lwhtr e ind•inite, dott•l wh•re concealed; + + . + +t .,.o .• 1 t x2 j // ,¾'.•; , <.-:':':':':':':':-:':-:':-u •:': q•eried wheredoubtful; lOdeS on hangingwall; + + Y ß It'Y •,15',•'.':,.•:.-.'.x'.'.'•..:.:.....'..'ßi•l •'"'""',ß ...... ?-;i.?:i:!:i:i:i:!:i:i:i:;,, "-'.'-'-'.'.'.'.'.'.'.'.'t :.. -..:.:• t:.: arrowshows a•irnuth and plungt of Idntrnaticindicators ,& t '3 fo .:.ß::. %.....o.o•-' •."•"•o•.".•.".ø•'."o•.• ...... •' •,- v -;• •.-.•,...... ,...:.:. '.' ...... •.....-, ,%•...... •o :.• :.....-.:.':ß'• ..Oo•o.. ? ,,,j., , X /ol•4iiit'"'•'*:...... •'"...... "-' -•'•'.',,...... :.:...... • ß:'•-." :-.":' :': •' 70 ,,• _ ><,q,•!i:tt; ...... '•:.:•.•••ß.•, -..-'•, c.'.a •..:; ' •,.• -.....-../,...... -.-.•:.'.'[5:.:, D/eiii•.':? .•.'...'. • ' •.'.'.• •.'.'. v ;•, '• J• ::::.:::: .... .' ß V .... Moderate-to high-angle •'.'.il' •' • ß . .I •,'.'. ". normal fault '-•z•.'_,•.'•, '.'. ß t,".', ' '. • • '-' / ---'..f• -x',::•:* •.... .','.' .',::, '.'.'.', ,-.-.c.'-',v':'( Dashedwhert indefinitt, don•d whert concealed; x,,-•"1•',• ,.-.',. •'-!'-'-'..• •.'-,; ,,' "-"..%,.'.'. '. queri.d wh•re doub{ful; arrow shows azimuth and ?• • / :-•c _•- •s;I •';' ,'::½,¾....• •:,' plungeof Idnematicindicators

F o. !::,'o. 2O to .,,,Y• ,.:'• Strikeand dip of strata

Strikeand dip of foliations 0 1 MILE 0 1KILOMETER ,,,/S /,6 L • • • m • Treadand plunge oflineations Barb giv• sere of ahe*,r,upper l•vels rno•d in direction barb points Fig.4. Geologicmap of a portionof thenorthern Mitchell Range highlighting detachment-related mylonitization,low-angle brittle faulting, and slightly younger moderate-angle normal faulting. Location of map shownon Plate 1.

rocks is considered to have been the result of tectonic andLingrey, 1979; Davis et al., 1982;DeWitt et al., denudation[Armstrong, 1972; Dokka et al., 1986] 1986] andthe Ruby Mountains,East Humboldt associatedwith extensionof theupper crest. Similar Range,Nevada [Dokka et al., 1986]. rapidcooling of tectonicallydenuded areas has been Althoughthe surfaceof the southernMitchell observedin otherCordilleran detachment complexes Rangeis not affectedby supracrustalnormal suchas the , California [Dokka faulting,it is includedin theWaterman terrane Dokka:The Mojave ExtensionalBelt of SouthernCalifornia 369

N N N

.- o

Fig. 5. Fabricdata from mylmfitic rocks of theWaterman Gneiss. Northern Mitchell Range: (a) polesto foliationand (b) myloniticstretching lineation. Waterman Hills: (c) myloniticstretching lineation. Hinkley Hills (datafrom Weiss[ 1954]:(d) foliation,(e) myloniticstretching lineation (contours 1-7-13- 19-25%per 1% area)and (f) joints (contours1-2-3-4-5-6% per 1% area).

Fig. 6. Field photographsof the Mitchell detachmentin the northernMitchell Rangeshowing late, low- anglebrittle faults and joints superimposed on myloniticgneiss and mylonite. 370 Dokka:The Mojave ExtensionalBelt of SouthernCalifornia

ß . •.o26

ß .

Surface sediments

Older alluvium -.)

Intrusive dacite Mile

Limestone Tuff Quartz monzonite andaplite N

' Felsiteand quartz Granodiorite latiteporphyry o

o

Quartzdiorite o

Hornblendediorite andgabbro

Gneiss Marble QuartziteI•.ø (mylonitized inMiocene)

Fig. 7. Geologicmap of theHinkley Hills (modifiedfrom Dibblee [1967b]) showing Miocene dike swarmsthat cut ductile fabrics of mylonitesof theWaterman Gneiss. The strikeof thedikes is subperpendicularto local horizontal extension direction. Dokka: The Mojave ExtensionalBelt of SouthernCalifornia 3 71

becauseof regionalstructural considerations. The consists of Jurassic and/or Cretaceous rangeis surroundedon the westand north by the hypautomorphicgranite-quartz monzonite and Watermanterrane and on theeast and south by the diorite,along with MesozoicSidewinder Volcanic Daggettterrane (Figure 2). Mappingof the southern Series(dacite tuff and flows). A 10- to 100-m-thick Mitchell Rangeshows that it containsan intact,late zoneof chaoscomposed of microbrecciasand Oligocene-earlyMiocene age caldera complex cataclasites(Plate 3f) liesdirectly beneath the [Lambert,1987]. Tilting of the stratais only seen detachment.This chaosis probablythe result of the alongthe southeasternmargin of therange and is smallopenings created by rotationof thefault blocks consideredto be relatedto dragassociated with a above the detachment[Wemicke and Burchfiel, upperplate of theWaterman terrane. This model 1982; Grossand Hillemeyer, 1982]. Thesegaps are predictsthat the Mitchell detachmentshould lie now filled with faulted and shatteredrock, beneaththe range. An alternativehypothesis is that suggestingthat the gapswere closed by a the southernMitchell Rangewas fixed to an combinationof small-scalefaulting and cataclastic unextendedportion of thecrust as surroundingareas flow. Zircon fission track thermochronometers from extended.The southernMitchell Rangemay be thefault zoneindicate that temperatures have not similarin structuralsetting to the ButteMountains beenabove -175øC sincethe early Tertiary [Dokka, andnorthern White PineRange of eastcentral 1986;Dokka andBaksi, 1988]. Exposureof the proposedstrike-slip fault boundarybetween the shallowupper part of theNewberry Mountains Daggettand Waterman terranes [Lambert, 1987]. detachmentby syntectonicand posttectonic arching Lambertproposed that the southernMitchell Range may be dueto isostaticuplift causedby unloadingof was a large,intact block thatlay within the extended thefootwall of thedetachment, as has been proposed Nevada,considered to be large,unextended, by Spencer[ 1984]. supracrustalblocks within the northernBasin and Bullionterrane. Upperplate lower Miocene strata Rangeprovince [Gans,1987]. of theBullion terrane generally dip-30øNE in Daggett terrane. Stratified25-20 Ma rocksof the associationwith steep,SW dippinghigh-angle normal faults (Plates 1 and 3h)[Dibblee, 1967c; Daggettterrane generally dip 40øSWand sit in the Polovina, 1980; Dokka, 1986]. The Bullion terrane hangingwalls of down-to-the-NE,moderately dipping,normal faults [Dokka, 1980, 1986; Mathis, is uniquebecause the polarity of tiltingof extended 1986; and Mathis and Dokka, 1986]. The supracrustalrocks is oppositeto thatof the other southwesternedge of this terraneoccurs in the terranes.On thebasis of thelower degree of tilting Newberry Mountainsand is markedby the Stoddard theBullion terrane may be the leastextended of all Valley fault, a curviplanarnormal fault ("breakaway" the domainsof the Mojave ExtensionalBelt. in the terminologyof Howard et al. [1982])(Plates2 and3f). The breakawayfault is consideredto havea TransferZones "scoop-shaped"geometry by Dokka [1986] because of the developmentof reversedrag-related features in the hangingwall andby the curvedtrace of the fault. Althoughmost segments of the boundaries Its inferreddownward flattening geometry and betweendomains of the Mojave ExtensionalBelt are temporalcoincidence with thedetachment suggest obscuredby syntectonicand posttectonic thatthe breakaway fault may be theuppermost sedimentaryrocks, map-scale geometric and portionof the NewberryMountains detachment kinematicrelations and mapping of exposedportions [Dokka, 1986]. Internaldeformation of the upper suggestthat these boundaries are strike-slipfault platein the breakawayregion was the resultof (1) zones. These faults are similar to transfer faults of multiplegenerations of high-angle,curviplanar rifted continentalmargins described by Bally [ 1981], normal faults, and (2) the translationof the blocksto Gibbs [1984], andEtheridge et al. [1985]. Transfer lesssteeply dipping portions of the detachment zonesof the Mojave ExtensionalBelt accommodated [Dokka andGlazner, 1982]. Early, originallyhigh- the differentialextension between adjacent angle(-80ø), normalfaults have been rotated to extensional domains or between an extensional subhorizontalpositions by laterfamilies of faults domainand an unextendedregion [Dokka, 1986; [Dokka, 1986]. Threegenerations of originally Dokkaand Baksi, 1986]. Thesefault zonestypically high-anglenormal faults have been recognized in the havestraight traces and yield subhorizontalfault Newberry Mountains(Plates 2 and 3f). In contrast, planekinematic indicators that trend parallel to the only one generationof planarto slightlycurviplanar extensionaxis determined for the adjacentterranes. normal faults can be observed at the surface in the The Kane Springsfault of the Newberry,Rodman, centralpart of theterrane (central ; andBullion Mountains separates the Daggett and Plate 3g)[Mathis, 1986]. Bullion terranes(Figure 8a) andis the best The NewberryMountains detachment fault crops documentedexample of thistype of structure outnear the breakaway of theDaggett terrane (Plate 2 [Dokka, 1986; Dokka and Baksi, 1986]. The Lane and3f) andshows evidence of onlybrittle Mountainfault of theLane Mountain-Coyote Lake- deformation[Dokka, 1986]. The lowerplate CalicoMountains-Mud Hills (locallyreferred to as 372 Dokka: The Mojave ExtensionalBelt of SouthernCalifornia

116o15 ' 116o10 '

Qoa

SEDIMENTARY AND VOLCANIC ROCKS I Qa QfI Surficial sediment• Qa, alluwum Qf,fan gravel

UNCONFORMITY

Older alluvium

Older valley •edilnent Oof, olderfanglomerate and gravel 34 ø40 ø UNCONFORMITY

Balsalt of Ash Hill Qrb, basaltflows

Rhyolitic tuff

UNCONFORMITY

Tah Intrusive wdcanic rock• Tbi, basalt, •ntrus•ve Tap, andes•teporphyvy Tah, andes•teporphyry, hydrothermally leached • Contact Show• dashedwhere gradat•onal or approximately located Volcanic and sedilnentarv rock• O f•o ...... ?..... Tfa,fanglomerateof a•ules•t'•cdetritus D Fault Tfb,fanglomerate of basalt•c detwetus Showndashed where •ndefimte dottedwhere concealed Tb, basalt querr•edwhere existence •s doubtful, U, upthrown Ta, andes•te block, D, downthrown block; arrow indicates Tab, andestte brecc•a observed&p of /ault plane. It, tuff breccm Tr, rhyol•ttcJelszte _•o Strike and dip of beddine or of lava fioxx• PLUTONIC IGNEOUS ROCKS Strike and dip of fioxx-bandln,u

Granitic rock Horizontal beda

Hornblende diorite Shaft Adit Prospect Fig. 8. Geologicmaps of transferzones of theMojave Extensional Belt. (a) KaneSprings fault (modified fromDokka [ 1986]). Thissegment separates SW dippingfault-bounded blocks of theBullion terrane (areasoutheast of fault)from NE dippingfault-bounded blocks of theDaggett terrane. (b) LaneMountain fault(modified from Dibblee [1967b]). The fault separates the extended Waterman terrane to thesoutheast from theunextended region to thenorthwest. (c) BaxterWash fault (modified from Dibblee and Bassett [ 1966]). Thisfault separates the extended Daggett terrane located to thesoutheast from the unextended regionto the northwest.

the FossilBed Canyonfault) area(Figme 8b) andthe transfer zone and served to accommodate the motion BaxterWash fault of thenorthern Cady Mountains betweenthe extendingMojave ExtensionalBelt and (Figure8c) areother examples of transferfaults that the then stable Sierra Nevada-western Basin and accommodated the differential extension between the Rangeregion. This transferzone style of behavioris extendingbelt andadjacent unextended areas to the reminiscentof the behaviordisplayed by theGarlock noah. in middleand late Miocene time asproposed by The lackof correspondingearly Miocene-age Hamiltonand Myers [1966] and laterclarified and extensionalstructures in theregion north of the expandedby Davis andBurchfiel [ 1973]. In contrast MojaveDesert block (Plate 1) suggeststhat the with theearly Miocene activity, this later interval of westernsegment of the Garlockfault was alsoa motionis thoughtto haveinvolved the entireGarlock Dokka:The Mojave ExtensionalBelt of SouthemCalifomia 373

/

/ 37q Dokka:The MojaveExtensional Belt of SouthemCalifomia

ß Qa Qog? Qoa

Qa

•"Qof

Baxter Wash fault•

•Ta

o .... 1 M,ILE 0 ------1 KILOsMETER

SCALE Fig. 8. (continued)

fault and was associated with the differential deeperlevels and are the primary elements that extensionbetween the Basinand Range province to facilitateextension. The detachmentseparates an the northand the thenstable Mojave Desertblock to undeformedlower plate from an upperplate the south. consistingof a seriesof similarlyoriented tilted fault blocks. Overall Pattern of Strain Data bearingon lithosphere-scalestrain relations of the Mojave ExtensionalBelt arerestricted to the Two broadcategories of hypotheses,pure shear upperand middle crust; an understandingof the and simpleshear, have been advanced in recentyears behaviorof deeperparts of the lithosphereis lacking. to explainthe structureof continentalextension zones Becauselithosphere-scale relations are sopoorly suchas the Mojave ExtensionalBelt. Pure shear constrained,the assessmentpresented below mustbe models[e.g., Miller et al., 1983] treatthe detachment consideredspeculative. as the brittle-ductile transition. The detachment The occurrenceof rooted,low-angle normal servesto accommodatebrittle extension of the upper faults, the lack of evidence for ductile deformation plateand moreuniform stretching of the lower plate beneathsome of thedetachments, and the strong by ductileflow andintrusion of igneousbodies. asymmetryof tiltingof upperplate rocks displayed in Althoughthe structureof the upperplate is generally eachof the domainsimply that a simpleshear model depictedin mostpure sheartype modelsas is moreuseful than a pureshear model in explaining consistingof symmetricallyarranged, tilted fault geologicrelations in theMojave ExtensionalBelt. blocks,the modelexperiments of McClay andEllis Pure shearmodels cannot be rejected,however, until [1987] suggestthat a strongasymmetry of these detailedinvestigations of deeperparts of the blocksmay developif the detachmentis slightly lithosphereenable us to (1) gaugethe extentof inclined. Simple shearmodels [e.g., Wernicke, involvement of the middle and lower crust of the 1981, 1985] considerthat detachmentfaults Mojave in extensionand (2) determinethe typesof (originallyinclined at < 30ø) root in the lithosphereor Miocene-agestrains in the middleand lower crust. West East B B'

-- ...... I I J: J .-.;t [: t. I I-• I- l--,J::._: I- I• • .•..•;,i . t_-.. •:•..• .....x-t•.---.l•--'.• '•,• :t- •-• -'•:•E:.•--O •-•"•

o _.

MOJAVE

f 3:- I ,2

LINE6 I I KmI

Plate3. Architectureofextensional domains ofthe Mojave Extensional Belt. Locations forsections are•ven on Plate 1, except where noted below. (a)Block diagram ofthe western Mojave Desert (Edwards terrane) based on Consortium forContinental Reflection Proffiing seismic reflection dan [Cheadleetal., 1986]. Note reflection B and the inferred high-angle normal faults that lie above it. ReflectionB is interpreted tobe a detachment tault (b)Migrated industry seismic line (Texaco) and interpretation ofa portion ofthe Edwards terrane inAntelope Valley illustrating character ofmodeme anglenormal faults, associated tilted upper plate fault blocks, and low-angle normal faults. Mg, Mesozoic granitoid (probably mylonific atdepth); Fmu, undifferenfiatedMiocene swam; Tins, Miocene syntectonic sedimentary rocks; Trap, Miocene posttectonic sedimentary rocks; QT, undifferenfiated Quaternaryand/or Tertiary sedimentary rocks and sediments; (c)Schematic structure section from the Kramer Hills to hills west of Harper Lake (Edwardsterrane), based onmaps presented inDibblee [1960a, b,1968] and Dokka etal. [1988]. wg, Waterman Gneiss; Mg, Mesozoic granitoid qm, quartzmonzonite; gd,granodiorite ofFremont Peak; Tmfl, Tropico Group (lower part); Tmrb, Red Buttes dacite; Tmtu, Tropico Group (upper pan); Trak,formation ofKramer Hills; Qal, Quaternary alluvium. (d) Schematic slructure section from Harper Lake to the Rand Mountains (Edwards terrane)[SerpaandDoldca, 1988]. wg, Waterman Gneiss; Mg, Mesozoic gnmitoid; Tmu, undifferentiated Miocene swam. (e) Geologic section across thenorthern Mitchell Range (Waterman terrane), based on relations shown in Figure4. Notetruncation of brittle and ductile sn-uctures of the Mite[ell detachmentbylate normal fault (f) Geologic section (balanced) across thenorthern Newberry Mountains (Daggett terrane), based onmap relations presentedinPlate 2. (g)Geologic section ofthe central Cady Mountains (Daggett terrane)[after Mathis, 1986]. Mg, Mesozoic granitoicl; Tvu, unclifferentiatedlowerMiocene volcanic rocks; Qal, Quaternary alluvium. (h)Geologic section across theBullion Mountains (Bullion termne)(mo•fied fromDibblee [1967c]. Mg, Mesozoic granitoid; Tt,lower Miocene tuff; Tb, lower Miocene basalt; Ta,lower Miocene andesite-dacite; Tap,lower Mioceneandesite-dacite intrusives; Qf,Quaternary fanglomerate; Qoal,Quaternary older alluvium; Qai, Quaternary alluvium. C BreakawayZone • CoreComplex C

SSW •• '-c • -• NNE -• f : _• LOCK.ART Z'e • K• • & • •r Lake • •ME FRE• P•K • I• ' •ER HILLS ß K•-Mc• • -'•. • • • •E

' ßß •. , • .... .:,-• . '/ ,[•:'f•-,'-•:,,; . '-'--,•:• •-;.:: lllt• • va, •.; v.• •h • • • ...... :: : / -.•.. .:..... ?•:.•-• :'.-:...... :-"•:- --•,..,,;-:• •wn •-- -

Transfer Zane CoreComplex E E' D õ ; LOCKHARTD' NW • .•- DOME ESE a•,•raximr.locationofMilchel• detochm_ent WesternGarlock RandThrust • • HarperLake Fault -,• 2700-• • • '• '• • • / • / /• • • •'• pTu

- S• ,•,• .... ••••' RAND MOUNTAINS- •o•ecte

F

ENE

• •o,,: DAGGEI"[ •<•. RIDGE• • •• .... , •• • •-• • 1stgeneraban •- • NEWBERRY 4000- '•o.f,• Oa OaOa Tmdrß • • . •:: .• :.... ,• o I •1 • , • • • • • 1stgenerati• SPRINGS & /•••,•-• ...... :::•:::::•.• •' I •• - • - . • • • - • • Tmn• •'•-'•"••••'• ß-' ...... :: ...... •1•.•...... •.•chaos-- • ••"• ' .: . ' ß• ' '••• '••'• 3000- •,•.• --,.._c•.:.-*....:;:*:....=.• ' .'• .' '. ' '•''. '••••'. , •2000- ß ._.':• , -' -'- ßß '-'- --'- -" -- '" • . "•'•'-••••:"-•::;•.• •"::::::::::::":...... •h[I• ...... ::-:":::::::::::::::::::::::'••••"•••-•-••••.%X.•• :::::::::::: ...... Mg...... ; 1000- • ...... : :• :::: •.• 0 X "::::::.::::::.:: ...... • x •"'"' '•' '••' '• ••• :• -•:•

G G'

Plate3. (continued) + + + + + EXPLANATI O N Generalized Geologic Map + + ++:++ ++ of the ++ + Rocksof the Mojave Desert Rocksof the Mojave ExtensionalBelt Mojave Desert, California and SurroundingRegions Posttectonicsedimentary with Emphasison the MojaveExtensional Belt Alluvium andigneous rocks UNCONFORMITY UNCONFORMITY ,( Roy K. Dokka Syntectonicstratified rocks Playaclay (megabreccia,conglomerate, sandstone, 1989 and volcanic rocks) UNCONFORMITY UNCONFORMITY (modified after Dibblee, 1960 ':'.':::• and Jennings,1977) Basalt UndifferentiatedTertiary UNCONFORMITY sedimentaryandvolcanic rocks UNCONFORMITY Nonmarinesedimentary and volcanic rocks Undifferent,atedpre-Tertiary ß MoJave crystallinerocks UNCONFORMITY FAUL1

Marine sedimentaryrocks and metasedimentaryrocks RgL Fig. Granite-QuartzMonzonite Edw,'ards,," (brittlelydeformedalong detachment)

Barstow Rosamonc• ß Daggett • • ". Plutonicand hypabyssalrocks WatermanGneiss (myloniticwithbrittle overprint)

Ludlow ++++++:+I ß Low-anglenormal fault Contact ß Lancaster N Dashedwhere gradatJonal or approximatelylocated quened where doubtful, nodes on hangingwall II II .... ?.... Moderate-to high-angle normal fault Palmdale Strikeand dip of strata 5 I 0 15 20 25 30 35 40 45 50Miles ,) '-... Dashed where mdefinffe, dotted where concealed; ßAmboy queriedwhere doublful; t•ck n•arks on hangingwall 5 0 5 10 15 20 25 30 35 40 45 50 Kilometers Littlerock

Hate1. Regionalgeologic map of theMojave Desert block and surrounding regions. Locations of geologic maps and cross sections presented in the reportare shown. Localities and features mentioned in text: AM, Alvord Mountains; AB, Antelope Buttes; BM, ; BrM, ;BL, Bristol Lake; CM, Cady Mountains; DR, Daggett Ridge; FP, ; GH, Gravel Hills; HL, HarperLake; I-IH, Hinldey Hills; KH, KramerHills; MR, MitchellRange; MH, MudHills; NM, NewberryMountains; OM, ; RM, RandMountains; RdM, Rodman Mountains;RgL, Rodgers Dry Lake; RH, Rosamond Hills; RL, Rosamond Dry Lake; WH, Waterman Hills. KSF, Kane Springs fault; LMF, Lane Mountain fault:,BWF, Baxter Wash fault. ;ø% .• I ß '.:i:i:::i::'::::::,' ...... ""'":':':':' / " •,''½ '•'•' ::::::::::::::::::::::E...... ::••_'":'::::::::::':'. • •0 - 3rdgeneration.?ß ,•,; •. ,..... •' :• ',•..¾•;.•• ...... •.•--••., •'•%-• • x: •, :-:::::::::.:::::'' ' •:::u .:_• .• ,,,, ,=., -- '...::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::: . • .. ..:.... sE:•:•:E:E:E:•:E::.:.:.:.' . . '" :t. ß ======1St •e•eration' '• '• ::: : :::: ' ..-.-- :'.:' ß ,ss•• 3rd-"-'-'-'-'-'--.'generation: --...... --.' :::::::::::::::::::::::::::::::::::::::::::::::::::::•]]•E]]• :':':::-- '"• ':'":'"'"':'"'"'"'"'"'""......

o=• .... -•' ...•:•:•:•: ßß-:-:-:-:-:-:-:-:':':':'• " • •? • • -.:.::...•:•:•:: '"'::::::::::::::...... •.•?:::;:::::•:::: • •:: ..:.•:•' ::•:;•:?.•:.•::?'.. st le ...... • •" %% • - --, -• -' -'•-.;½ '•7'•• • ,'...... •;':::¾;)?•?.::::::• s:•::.::::.::,:,---.-.-.-.-.-...-.---- ...... ::.'""':%'. ::::•:::-:'::M:E:•...::?•:' ...... • '%' * ß• • % ':::::¾:::::':::::..... •aULT" ):..ss':.?:;:':.:: -: ssssss:'...... :.:-':::-:::':-:"

•' • QUATERNARYDEPOSITS :•.- ..:•?: .. e'•-]-::'..- .. ':':':':'...... • BARSTOWFORMATION •,, • . •. % •;;• finsSYN- of SlashANDX Ra•ch.POST-TECTONIC StCdard Valley. DaggeH DEPOSITS:R•dge '.'.%".".'.".'.'--'-"-"-"-'-"-2-= .., • -,...... ;.:.:.;.., ,.-,.-:,....< :...... -.- ß • -.-.• ..';:'•UPPERMinn•la P•TERidge ROCKS: f•s.oJNewber• S•rmgs, .-:=:-:":?"., ":?•'•½.• .-...-.-.- ...... •,...... ,.-.-.-.-.,,..,,,...... - ;.• UPPERPLATE CRYSTALLINE BASEMENT: ...... =."."r;•-' • •-'-'- . .-...... '•'-'-'-'-'-'-'-'-'-""•-'-" • - . •,, k;.:½ •:-;-: :'-j- ß. , LOWERPLATE ROCKS: MesozOic granite ½. monz - • with S•dewinderVolcanic Series, gneiss,ma•le

• NON-EXTENDEDBASEMENT Low-angle• reverse• fault Low-angle• normal fau• •erale- and NMDF= Newber•Mountains h•gh-anglenormal faults detachment faull

Plate2. Generalizedgeologic map of the Newberry Mountains highlighting thelocations ofthe Newberry Mountains detachment fault (NMDF), Kane Springstransfer fault, upper plate normal faults, and the Stoddard Valley breakaway fault. Section F-F shown onPlate 3f. Dokka:The Mojave ExtensionalBelt of SouthemCalifomia 379

EXPLANATION

SEDIMENTARY AND VOLCANIC ROCKS PLUTONIC, HYPABYSSALAND METAVOLCANIC IGNEOUS ROCKS

OaQf Diorite dikes Surficial sediments Qrs, rlver sand Qrg, riverterrace gravel Qs, wtndblownsand Granite Aplite dikes Qc, day Qa, alluwutn Qf, alluwalfan gravel Merevolcanic rocks UNCONFOFIMITY

Older surficial sediments

Qoa, older alluwum 7O Oog,older gravel •_--.•--- UNCONFORMITY Contact,showing dip Dashedwhere gradational or approxz- matelylocated, queried where doubtful Older fanglomerateand gravel 30 UNCONFORMITY /• ll•l I I Fault, showingdip Dashedwhere tndefintte,dotted where concealed;queried where doubtful. Arrow tnchcatesobserved chp of fault plane Coarse.sedimentary rocks T•, volcaniclasticfanglomerate

Taf, andesiticfanglomerate lO Tbf, basalticfanglomerate Tgf, graniticfanglomerate Slxikeand dip of bedsand lava flows UNCONFORMITY

Finer sedimentaryrocks Ts, sandstoneand claystone N Tsb, basalt TaiTap Tab Intrusive volcanic rocks Tai, andesiticrock, intrusive Tap, andesiticporphyry, intrusive Tah, andesiticporphyry, hydrothermally leached

Volcanicand sedimentaryrocks Tb, basalt Tbb, basaltbreccta Tfb, fanglomerateof basalticdetritus Ta, andesite Tab, andesiticbreccia Tt, tuffbreccia Tfa, fanglomerateof andesiticroc• Tg, cobblegravel T$$, sandstone TI, limestone

UNCONFORMITY

Fig. 8. (continued)

TIMING RELATIONS dikes. The final phaseof exhumationand cooling of the formerlydeeply buried (and ductile) footwall of The chronologyof eventsassociated with the the Mitchell detachmentoccurred during this interval developmentof the Mojave ExtensionalBelt is as the Mitchell Range,Waterman Hills, Hinkley constrainedby datedcross cutting relations, the age Hills, and the hills westof HarperLake were uplifted of syntectonicsedimentary rocks, and extension- alonghigh-angle normal faults. relateddike swarms.Timing relationsare summarizedin Figure9 andindicate that extension begannear 22 Ma (earliestMiocene) and terminated KINEMATICS before17 Ma (lateEarly Miocene). Openingof the Mojave ExtensionalBelt occurred Analysisof kinematicand strainindicators in two stages.The first stageis bracketedas 22-20 (fibrousmineral growths,slickensides striations, Ma andwas dominatedby regionallydeveloped myloniticstretching lineations, and transferzones) detachmentfaulting (low-angle, brittle-ductile within the Mojave ExtensionalBelt showsthat the faultingand rotational upper plate high-angle normal horizontalextension axis is generallyoriented SW- faulting). All of the extensionaldomains of the belt NE (Figure 10). There is, however,an apparent were establishedduring this stage. Thesestructures systematicchange in orientationof the axisfrom east were then cut anddisrupted between 20 and 17 Ma to west. In the easternportion of the Daggettterrane by locallydeveloped, high-angle normal faults and andthe adjacentBullion terrane (eastern Mojave) the 380 Dokka:The Mojave ExtensionalBelt of SouthernCalifornia

Post-extension Barstow Formation sedimentation,Newberry Mountains 1 Slipon Kane Springs transfer fault 2

Extensionfracturing and dike 3 emplacement,Rodman Mountains Depositionof syntectonicHector Formation,Cady Mountains 4 Daggett Terrane Normal faultingand tilting, CadyMountains 5 Depositionof monolithologicmega- breccias,Newberry Mountains 6 Lategeneration normal faulting •nd tilting,Newberry Mountains Early generationnormal faulting andtilting, Newberry Mountains 8 Slip on NewberryMountains detachment fault 9

Post-extensionportion of Barstow Formationsedimentation, MudHills 1ø Late generationnormal faulting, MitchellRange 11 Waterman Depositionof syntectonicPickhandle Terrane Formationand lowerpart of Barstow Formation,north of Barstow12 Normal faultingand tilting,Kramer Hills district13

...... Rapid coolingof Mitchelldetachment ...... (>300o-100øC)14

14 18 22 26

TIME (Ma) Fig. 9. Summaryof ageconstraints on thetiming of extensionof theMojave Extensional Belt. Stippled regioncovers time interval of majorextension (22-20 Ma). Theconstraints are numbered from 1 to 14. Constraint1, flat-lying17-13 Ma BarstowFormation overlying all extension-relatedstructures [Dokka, 1986;Dokka andWoodburne, 1986]. Constraint2, youngestdated rocks truncated by KaneSprings fault (23.1+_2.0Ma, K-Ar wholerock on alkaliolivine basalt [Nason et al., 1979])and flat-lying tuff of Kane Springsthat unconformably overlies the fault (21.05-_1.0 Ma, fissiontrack zircon [Dokka, 1986]). Constraint3, NW strikingandesite dike swarm (21.1_-+0.3 Ma [Dokkaand Baksi, 1988]). Fieldlocation of datedsample is 34ø40'50"N,116ø30'05"W. Constraint 4, tuff locatednear base of unitdated as 22.9_+0.4Ma [Miller, 1980]. Fossilcamel (N16 Ma) locatedat topof unit[Miller, 1980];tuff neartop datedas 17.950.3Ma [Miller, 1980]. Constraint5, moderatelytilted volcanic strata (basal tuff datedas 24.0+_2.6Ma [Dokka,!986]) overlainby nearlyflat-lying Hector Formation (intercalated tuff datedas 21.6+1.1Ma by Woodburneet al. [ 1974;recalculated according to Dalrymple,1979]). Constraint6, intercalatedtrachyte flow locatednear top of formationof SlashX Ranchdated as 20.2+0.3 Ma (K-Ar wholerock; R. K. Dokka andA. K. Baksi,manuscript in preparation,1988). Field locationof dated sampleis 34ø50'17"N,116ø58'21"W. Age of baseof formationis poorlyconstrained. Constraint 7, lowerlimit definedby theyoungest rock cut by faults(the trachyte cited in Constraint6). Faultsare overlainby stratacorrelated with theBarstow Formation [Dokka, 1986] and are considered to be about16 Ma old [M. Woodburne,written communication, 1988]. Constraint8, sameas thoselisted under Constraint2. Constraint9, lowerlimit definedby theyoungest rock fully displacedalong fault (basalt citedunder Constraint 2). Upperconstraint is theage of theoldest posttectonic sedimentary rock depositedon thedetachment surface (Barstow Formation; see Constraint 7). Constraint10, lower age limit definedby underlying19.3_+0.2 Ma tuff nearbase of BarstowFormation [MacFadden et al., 1989a]. Upperlimit basedon 13.4_+0.2Ma ageof LapilliTuff fromnear top of BarstowFormation [MacFadden et al., 1989a]. Constraint11, lowerage limit constrainedby coolingages of crosscut Waterman Gneiss (see Constraint14 below). Upperlimit poorlyconstrained. Constraint 12, lower timing limit poorly constrained;minimum lower limit definedby 19.3+0.2Ma tuff lyingwithin syntectonic gravels of lower Dokka:The MojaveExtensional Belt of SouthemCalifomia 381

N N N

N N N N

ø o o

o

(d) e Fig.10. Summaryof kinematicindicators from regions of theMojave Extensional Belt. (a)Newberry Mountains[Dokka, 1986] and (b) CentralCady Mountains [Mathis, 1986] show the orientations of fibrousmineral growths and slickensides striations on low-angle and high-angle normal faults. (c) Plotof orientationsof fibrous mineral growths and slickensides striations from the Kane Springs fault of the Newberryand Rodman Mountains. Data shown in (d),(e), (f), and(g) indicate orientations of stretching lineationsfrom mylonitic rocks of theWaterman Gneiss in thecentral and southern Hinkley Hills, northernMitchell Range, Waterman Hills, and northern Hinkley Hills, respectively; the sense of shearwas determinedfor each lineation using S-C criterion of Simpsonand Schmid [ 1983]. All samplesshowed evidencethat upper levels moved to the northeast relative to lower levels. Several localities were sampled forlaboratory microstructural analysis; all thin section specimens confkmed outcrop interpretations.

extensionaxis is oriented-•N70øE, whereasin the Ma, wasrestricted to thecentral part of theprovince. westernportion of theDaggett terrane (Newberry Rosset al. [ 1987, 1988] reportthat 25-21 Ma rocks Mountains) the axis trends.•N50øE. The extension of theDaggett terrane as well as the adjoining axis is oriented .•N40øE in the Waterman terrane unextendedarea (Alvord Mountains) have been (Dokkaand Woodburne [1986] and this paper). rotated-*50ø (clockwise).Data presented in The regionalvariation of kinematicindicators is Golombekand Brown [1988] indicatethat 21-20 Ma apparentlynot primary but is likelythe result of post- rocksof theeastern part of theEdwards terrane have 20 Ma rotations about vertical axes associatedwith been rotated-•25 ø in a clockwise sense. MacFadden fight shearalong the SanAndreas transform zone. et al. [ 1989a],working in thenearby Mud Hills, Paleomagneticdata suggest that at leasttwo intervals provideevidence that the entiresection of the of rotationhave occurred in theMojave since20 Ma. overlyingand regionally extensive -•19-13 Ma The early event,20-18 Ma, affectedthe westernand BarstowFormation is eithernot rotated or only centralMojave, whereasthe later event,circa 5-0 slightlyrotated in a counterclockwisesense (<5ø).

portionof BarstowFormation [MacFadden et al., 1989a].Burke et al. [ 1982]have reported a K-Ar date of 18.9-k-_1.3Ma on the intercalated Opal Mountain basalt member of theupper part of thePickhandle Formation.Upper limit constrained by overlyingmiddle Miocene part of BarstowFormation. Constraint 13,lower constraint defined by ageof moderatelytilted volcanic strata (Red Buttes andesite, 21.2_+0.5 Ma, K-Ar wholerock; R. K. Dokkaand A. K. Baksi,manuscript in preparation, 1988). Field location of datedsample is 34ø53'54.5"N,117ø30'50"W. These rocks are overlain by -•flat-lying 20.3+0.7 Ma SaddlebackBasalt (Armstrong and Higgins [ 1973]; recalculated according toDalrymple [1979]). Constraint14,defined by4øAr-39Ar plateau ages onbiotite and fission-track agesand track length distributionsfor apatite[Dokka and Baksi, 1988]. 382 Dokka:The Mojave ExtensionalBelt of SouthernCalifomia

N e metamorphic core complex --,.-trend & vergence ofextension •,/MZ • front of fold & •hrust belt ..•.•,,majorLaramide block-thrust i• structu res • major faults with senseof offset

ß Fig. 11. Cenozoicextensional terranes of thesouthwestem United States (modified from Wust [ 1986]. Arrowsindicate extension direction. MEB, MojaveExtensional Belt (extensiondirection restored to early Mioceneorientation); DV, DeathValley turtlebacks;HM, HomerMountains; B, Buckskin,Rawhide, WhippleMountains; WH, WhiteTank Mountains, Harquahalas, Harcuvars; MW, MidwayMountains; SM, SouthMountains; MH, /CopperMountains; J, JacksonMountains; SB, Sierra Blanco,Comobabi, Kupk Hills; SC, Picacho,Tortolita, Santa Catalina-Rincon; P1, Pinaleno; C, Coyote, Alvarez(north); PV, PozoVerde; MZ, Magdalena,Madera, Mazatan (southwest).

Hillhouseand Wells [ 1986]report that the regionally DISCUSSION distributed--18.6 Ma PeachSprings tuff in the south centralMojave Desert is notrotated. Combined, Datapresented above strongly support the notion thesedata suggest that the first regional rotation thatthe westernand central Mojave Desertwas occurred over a short interval of time, 20-18 Ma extendedin earlyMiocene time. In additionto [Golombekand Brown, 1988], duringopening of helpingus to betterunderstand the tectonic evolution theMojave Extensional Belt but afterthe major phase of theMojave Desert, recognition of theMojave of extension.Paleomagnetic studies in the Cady ExtensionalBelt hasimportant implications for the Mountainsby MacFaddenet al. [ 1989b]indicate that reconstruction of southern California as well as for the areawas rotated26 ø (clockwise)after 16 Ma. the late Cenozoicextensional history of the This is in line with the timing and senseof rotations southwesternUnited States. In this section, we will predictedby Carteret al. [ 1987]for theMojave specificallyexplore (1) therelationship between the Desert Block on the basis of rotations detected in the MojaveExtensional Belt andthe extended areas adjacenteastern Transverse Ranges. Carter et al. located to the east in southeastern California and [ 1987] alsopredict that the rest of theprovince is westernArizona (2) thepossible tectonic connection rotated <15 ø in a counterclockwise sense. These later with anomalousearly Miocene extensional structures rotationsare likely relatedto theonset of fight shear located west of the San Andreas fault in central associatedwith the developmentof the SanAndreas California,and (3) the role of extensionaltectonics in fault beginningafter--13 Ma [R. K. Dokka andC. J. thepaleogeographic evolution of theMojave Desert. Travis, manuscriptin preparation,1988]. In summary,paleomagnetic data suggest that the Relationshipof theMojave ExtensionalBelt to Other westemand central Mojave were rotated 25ø-50 ø in a clockwise manner--20-18 Ma. This was followed ExtendedAreas of the SouthwesternUnited States --13-0 Ma by 0ø-15ø counterclockwiserotation of the westernand southern Mojave and26 ø of clockwise The Mojave ExtensionalBelt sharesmany rotationof the centralMojave. Together,kinematic similarities with other extended areas of the indicatorsand paleomagnetic data suggest that the southwestas well as severalimportant differences. Mojave ExtensionalBelt originallyopened in an Thesesimilarities include the timingof development -*N-S direction. andthe architectureof upperand middle crustal Dokka:The Mojave ExtensionalBelt of SouthernCalifornia 383

strain. The differencesbetween the regionsare mainlykinematic in natureand may giveus insights i:;;;•!iT•!i;.:k•.7060Sr87/86 LINE '..• NEVA•)A•.... ' , - into the dynamicsof late Cenozoicextension of the southwestern United States. iiiiilili!i:ii:::.... CALIFOR•41A " The samearray of earlyMiocene-age extensional • ,/•/•/•'-,,.• : structures(brittle-ductile detachment faults, multiple ...... •••.• •• •/J•l>•2, generationsof high-anglenom•al faults, and ...... ••"""•"•':••:••':•:•••••'. ;•t_____..____• extensionfractures and associateddike swanks) .... '""...... '"'"".... -• ...... • •ziiil•1111• foundin theMojave ExtensionalBelt havebeen MEXICO• ß describedalong the ColoradoRiver (e.g.,Whipple Mountains [Davis et al. 1980; Davis, 1988], Homer Mountains[Spencer, 1985], and Chemehuevi CR•IONIC S•NIg LBCI•-OROCOPI• Mountains[John, 1987]) and westernArizona (e.g., NORTH •ERIC• •LLOCHIHON S•N G•BRIEL,OROCOPI•, Buckskin-RawhideMountains [Reynolds and A S•R-OBISPO S•LINI•N CliOCOLlIE•O•NI•INS I I i • • • • • • •BLE •OBNI•INS Spencer,1985]). However, the Mojave Extensional IHRBSI. IHRBSI' Belt andthe extendedterranes lying to the eastdiffer UPPERCRETACEOUS AND•' markedlyin termsof theorientation of their .LEOCE.ESTT ' ' horizontal extension directions and in the relative amountof exposedexhumed middle crustal rocks. Figure 11 showsthe locationof coincident • • •.; / extensional terranes of the southwest, with arrows indicatingthe orientation of the local horizontal extensionaxis. The Mojave ExtensionalBelt opened FRANCISCAN FUTURESITE in an ~N-S direction, whereas the terranesof the ColoradoRiver andof westernArizona apparently extendedin a ENE-WSW direction [Davis et al., Fig. 12. Locationmap andschematic cross section 1980;Spencer and Reynolds, 1986]. The ~50ø of the SantaLucia-Orocopia terrane of southern difference of trend seems to reflect fundamental California[after Vedder et al., 1983]. Emplacement differencesin thekinematics of development. of a continentalcrustal fragment of suchdimensions Somehave recently advocated that late Cenozoic on top of North Americaprobably resulted in a extension of the western United States was the result regionthat was gravitationally unstable because of of failure of an overthickened crustal welt created in overthickening(i.e., a crustalwelt). The Mojave Mesozoictime by the stackingof thrustsheets and by ExtensionalBelt aswell asother extended regions of intrusion[e.g., Coney and Hanns, 1984; Wust, southernCalifornia may havedeveloped as a 1986; Wemicke et al., 1987]. The kinematicsof consequenceof the collapseof thiswelt. failuremay havebeen controlled by the trendof the welt; the direction of extensionin southeastern region,(2) the occurrenceof several,closely spaced, California and western Arizona as well the rest of the localareas of highstrain (perhaps associated multiple southernBasin and Range Province was nearly detachmentfaults), (3) lateralchanges in the perpendicularto the strikeof the Mesozoic distributionof supracrustalstrain [Spencer, 1984; Cordilleran thrustbelt [Wust, 1986]. In contrast, Spencerand Reynolds, 1986], and (4) structurally openingof theMojave ExtensionalBelt wasnearly andmagrnatically induced changes in thedensity' parallelto the thrustbelt. Eitherthe kinematicsof distributionof the lithosphere.In contrast,core failureof thiswelt aremore complex than originally complexuplifts of theMojave ExtensionalBelt seem thoughtor anotherhypothesis is in order. An to be relativelysmall features that are likely governed alternateexplanation is thatextension of theMojave by local,short wavelength causes. For example,the ExtensionalBelt is relatedto thecollapse of a final upliftof thenorthern Mitchell Range was separatewelt centeredin southernCalifornia (Figure probablyprecipitated by latenormal faulting. The 12). Emplacementof the SantaLucia-Orocopia structuralsetting of thiscore complex may be similar allochthon,a largecontinental fragment that collided to that of the Rincon-Catalina Mountains described with North Americaduring Late Cretaceoustime by Holt et al. [1986] wheretectonic denudation of [Vedder et al., 1983], could have created such a thelower plate accomplished by earlyMiocene welt. Clearly,much more studyis neededin orderto detachmentfaulting was followed by middleMiocene developan adequatedynamic model for theorigin of high-anglenormal faulting. It is likely, however, the Mojave ExtensionalBelt. that upliftswithin extendedterranes are a Anothersignificant difference between the areasis consequenceof the combinationof both short-and that the -•Vestern Arizona terranes long-wavelengthcauses. Again, thisproblem will exhibita greaterareal exposure of exhumedmiddle not likely be resolveduntil the structureof the middle crustalrocks. This may reflectwidespread uplift due andlower crustbeneath the Mojave Desertis better to (1) greateramounts of extensionacross the entire understood. 384 Dokka:The Mojave ExtensionalBelt of SouthemCalifornia

The Mojave ExtensionalBelt and MioceneMonterey Formation [Dibblee, 1971, the San Andreas Fault 1975]. In a regionalstudy, Stanley [1987] documentedan episodeof volcanismassociated with The abruptwestern termination of the Edwards extensionalfaulting throughout west central terraneagainst the SanAndreas fault (Figure2) Californiaduring the interval 24-22 Ma. Perhapsthe suggeststhe intriguingpossibility that the extensionalfaults and dikes described by Dibblee westernmostportion of the Mojave ExtensionalBelt [ 1971, 1975] and Stanley[ 1987] are structures was truncatedand displaced northward. Assuming relatedto the Mojave ExtensionalBelt. Furtherwork 314 km of fight slip alongthe segmentof the San is neededto testthe validity of thishypothesis. The Andreasfault northof the TransverseRanges MojaveExtensional Belt mayprove to be a useful [Matthews, 1976; Fox et al., 1985; Stanley,1987], regionalmarker for the reconstructionof southern the displacedportion of theMojave ExtensionalBelt California. should be located in the Salinian block of west centralCalifornia (Figure 13). Rocksand structures reminiscentof thoseof theMojave ExtensionalBelt LandscapeEvolution and SedimentationPatterns have been described west of the San Andreas fault in of the Mojave Desert westcentral California. For example,moderately westdipping equivalents of theNeenach Volcanics In early Cenozoictime theMojave was a highland of the westernMojave unconformablyoverlie that shedsediments to the surroundingareas Mesozoicigneous rocks in the vicinityof the [Hewett, 1954;Nilsen and Clarke, 1975]. Opening Pinnacles National Monument. The volcanic rocks of theMojave Extensional Belt alteredthis pattern of are mainly 23.5 Ma [Turner,1968] rhyoliteflows, externaldrainage to oneof internaldrainage where breccias,and intrusions that are assignedto the extendedareas began to receivesediments from PinnaclesVolcanic Formation [Matthews, 1976]. adjacentunextended areas. Paleocurrentindicators Theserocks sit in the hangingwall of the east from sedimentaryrocks deposited along the northern dipping,down-to-the-east Pinnacles fault [Dibblee, andsouthern margins of the belt duringextension 1971, 1975]. Dibblee alsoreported a >3-km-wide indicatethat regional drainage and sediment transport swarmof NNW strikingrhyolitic dikes of lower was directed toward the the interior of the belt. The Mioceneage in the samearea. Tilted rocksof the thick,basinward tapering wedges of coarseclastic areaare overlain in angularunconformity by middle debrisfoun•l along the margins ofthe belt attest to

Neenach area

• 0•00 50KILOMETERS MILES "• ßLos Angeles Fig. 13. The Mojave ExtensionalBelt in west-centralCalifornia. (a) Map showingposition of the Mojave ExtensionalBelt andpossible equivalent in the Pinnaclesarea to the westof the SanAndreas fault (modifiedfrom Jennings[ 1977]). (b) Geologicmap of thePinnacles National Monument area (modified from Dibblee [1971, 1975], andLudington and Gray [1987]. Seetext for discussion Dokka:The MojaveExtensional Belt of SouthemCalifornia 385

121ø15 ' 121o10 ' Qa GLORIA i

/

,Tsg

Tsg

36o30 '

Tsg

/30

,• Tpr Q•p!

ß

ß NORTHCHALONE ' •-•, Qa Alluvium(Quaternary) PEAK QTp PasoRobles Formation (Quaternary and Tertiary) Tvs Volcaniclasticsediments (Miocene) /•3269 ( Tm MontereyFormation (Miocene) ( •,SOUTHCHALONE PEAK ( Tsg Sandstoneand conglomerate (Miocene) Tpb Brecciaof PinnaclesFormation (Miocene) Jpv"•/ •__•.,,.• •....• Rhyolitedikes (Miocene) Tpv Ventbreccia (Miocene) QTp Tpr Rhyoliteof PinnaclesFormation (Miocene) 36025 ' ß Ki Intrusiverocks (Cretaceous)

Contact Fault--Dotted where concealed 1 2 Miles I I Strikeand dip of beds Drainage / Fig. 13. (continued) thesharp division between it and the surrounding The westernportion of thebelt was divided into unextendedhighlands [Byers, 1960; Dokka, 1986; twoseparate basins as a resultof theuplift of the Dokkaand Baksi, 1986; C. J. Travis,personal metamorphiccore complex ranges near Barstow communication,1988]. duringthe interval -•20-17 Ma. Theeconomically Localhigh relief formed within the belt judging by significantBoron and Barstow Basins developed in locallyderived clastic aprons surrounding tilt blocks thesetroughs [Dokka and Woodburne, 1986]. andsyntectonic uplifts. This is well documented by Theseuplifts have persisted aspositive topographic Woodburneet al. [1974]at the type section of the featuresthroughout the Cenozoic [Dibblee, 1967b]. lower Miocene Hector Formationin the westcentral CadyMountains. Conglomerates derived from the emergingcore of theCady Mountains were shed CONCLUSIONS west(present-day reference frame) and were depositedwith angular unconformity on topof Thefollowing conclusions were reached during moderatelytilted-•24 Ma dacitesand tuffs of the thisstudy: Daggettterrane [Dokka, 1986; Mathis, 1986]. 1. Theregional pattern of low-to high-angle Severalhundred meters of similarcoarse deposits brittle-ductilenormal faulting, extension fracturing alsooccur on theopposite side of thecore on the withassociated dike emplacement, andlandscape easternside of the uplift in sections3, 4, T.9N., and disruptionin the Mojave Desert during the early R.7E. (SanBernardino Base and Meridian)(R. K. Miocenewas concentrated in an east trending Dokka,unpublished mapping, 1982). regionalextensional belt, named here, the Mojave 386 Dokka:The MojaveExtensional Belt of SouthemCalifomia

ExtensionalBelt. The now disruptedsurface LeCorgne,and K. Stewart,for makingavailable expressionof thisextended zone can be tracedwest seismicand related geophysical data of the western from nearthe western edge of theextended terranes Mojave. R. L. Parrartof SantaFe Mining, Inc. of the Colorado River to the San Andreas fault. The graciouslymade available copies of unpublished SanAndreas fault truncatedthe western part of the geologicmaps of the MojaveDesert. The figures MojaveExtensional Belt andtransported it to its andplates were carefullyprepared by M. Eggart,C. presentposition in westcentral California. Duplechin,and J. Kennedy. C. J. Travis andthe 2. Strainwithin the Mojave ExtensionalBelt was authorelectronically drafted the geologic map partitionedinto at leastfour extensional domains. depictedon Figure4 usinga microcomputer, The domainsare separated by strike-slipfaults scanner,and laser printer. Promptand professional (transferzones) that accommodatedthe differencesin editorialand production assistance was provided by extensionbetween the domainsand surrounding P. Hanbackand J. Ring. Finally, my deepestthanks unextendedareas. The lack of earlyMiocene go out to B. Page,Editor-in-chief, for his good extensionalelements north of the westernsegment of cheer,clear advice, and encouragement throughout the Gatlockfault suggeststhat the Garlockmay have the course of this effort. beenactive during early Miocene time andserved as This work wasmade possible by grantsfrom the the northerntransfer zone boundary of the Mojave National ScienceFoundation (EAR-8107524, EAR- Extensional Belt. 8407136, andEAR-8721022). Additionalsupport 3. The Mojave ExtensionalBelt wasactive was providedby an Arco JuniorFaculty award. between-22 and 17 Ma, with the major phaseof openingoccurring between -22 and20 Ma. This REFERENCES majorphase was dominated by regionaldetachment faulting,whereas later extension was concentrated in Armstrong,R. L., Low-angle(denudation) faults, local areasand was markedby high-anglenormal hinterlandof the Sevierorogenic belt, eastern faultingand by the emplacementof silicicdike Nevada and western Utah, Geol. Soc. Am. Bull., swarms. 83, 1729-1754, 1972. 4. The architecture of an individual extensional Armstrong,R. L., andR. Higgins,K-Ar datingof domainis thatof a singleor family of half-graben. thebeginning of Tertiaryvolcanism in theMojave The upperpart of a half-grabenconsists of Desert, California, Geol. Soc. Am. Bull., 84, moderatelytilted fault blocksthat are bounded by 1095-1100, 1973. planarto slightlycurviplanar normal faults. Multiple Bally, A. W., Atlantic-typemargins, in Geologyof generationsof originallyhigh-angle normal faults are PassiveContinental Margins, History, Structure apparentlyresponsible for extensionand tilting of the and SedimentoIogicRecord, Education Course upperplate. Eachhalf-graben is flooredby a low- Note Series#19, pp. 1-48,American Association angle,regional, brittle-ductile, simple shear zone of PetroleumGeologists, Tulsa, Okla., 1981. (detachmentfault). First-orderextension of the Bartley,J. M., andA. F. Glazner,Tertiary upperand middle crust of the regionwas dominantly deformation beneath the Waterman detachment, accomplishedby large-scalesimple shear. The centralMojave Desert,California, Geol. Soc. Am. natureof the overallstrain of the Mojave Extensional Abstr. with Programs,19, 582, 1987. Belt mustremain speculative until thedegree of Bowen,O. E., Jr., Geologyand mineral deposits of extensionof the subjacentlower crustand mantle is the Barstowquadrangle, San Bernardino County, determined. California,Bull. Calif., Div. Mines, 165, 1-185, 1954. Acknowledgements.Gratitude is expressedto G. Burchfiel, B.C., C. S. Cameron, P. L. Guth, J. E. Couples,E. L. Miller, B.C. Burchfiel,S. M. Jones, Spencer,and T. A. McCulloh,A Triassicoverlap M. O. Woodbume, and S. Wust for their careful and assemblagein NorthernMojave/Death Valley constructive reviews. Discussions with R. B lom, J. region,California, An interpretation,Geol. Soc. Brem, B. Cox, G. A. Davis, G. H. Davis, J. Ford, Am. Abstr.with Programs,12,395, 1980. M. McCurry, E.G. Frost, J, Lambert,R. Lawson, Burke, D. B., J. W. Hillhouse, E. H. McKee, S. T. B. Luyendyk,R. H. Merriam, R. Mathis, B. Miller, and J. L. Morton, Cenozoicrocks in the McFaddon,N. Opdyke,T. Ross,L. Serpa,and C. Barstow Basin area of southern California- J. Travis were invaluable. I also thank P. Frost, V. Stratigraphicrelations, radiometric ages, and Iliff, C. Johnson,M. Kolb, M. Mahaffie, and the paleomagnetism,U.S. Geol.Surv. Bull., 1529- lateJ. Meystayerfor theirable assistance in thefield E, 1-16,1982. andin the laboratory.Very little couldhave been Byers,F. M., Jr., Geologyof the AlvordMountain accomplishedwithout the effortsof co-workerson quadrangle,San Berdardino County, California, the Mojave Desertteam, A. K. Baksi, D. Henry,J. U.S. Geol. Surv. Bull., 1089-A, 71 pp., 1960. Lambert, C. McCabe, S. Jones,R. Mathis, J. Carter, J. N., B. P. Luyendyk,and R. R. Terres, Nunn, L. Serpa,and C. J. Travis.I alsothank Neogeneclockwise rotation of theeastern TexacoUSA, especiallyM. Cole, J. Bihm, D. TransverseRanges, California, suggested by Dokka:The MojaveExtensional Belt of SouthernCalifornia 387

paleomagneticvectors, Geol. Soc. Am. Bull. 9& Dibblee, T. W., Jr., Evidenceof strike-slipfaulting 199-206, 1987. alongnorthwest-trending faults in the Mojave Cheadle,M. J., B. L. Czuchra,T. Byrne, C. A. Desert, U.S. Geol. Surv. Prof. Pap., 424-B, Ando, J. E. Oliver, L. D. Brown, S. Kaufman, B 197-B 199, 1961. P. E. Malin, andR. A. Phinney,The deepcrustal Dibblee, T. W., Jr., Geologicmap of the Broadwell structureof theMojave Desert, California, from Lake quadrangle,San BerdardinoCounty, COCORP seismic reflection data, Tectonics, 5, California, scale 1:62,500, U.S. Geol. Surv. 293-320, 1986. Misc. Geol. Map Invest., 1-478, 1967a. Coney,P. J., andT. A. Harms,Cordilleran Dibblee, T. W., Jr., Areal geologyof the western metamorphiccore complexes, Cenozoic Mojave Desert, California, U.S. Geol. Surv. extensionalrelics of Mesozoiccompression, Prof. Pap. 522, 1-153, 1967b. Geology,12, 550-554, 1984. Dibblee,T. W., Jr., Geologicmap of the Ludlow Crittenden,M., P. Coney, and G. H. Davis, quadrangle,San BernardinoCounty, California, Cordilleranmetamorphic core complexes, scale 1:62,500, U.S. Geol. Surv. Misc. Geol. Mem. Geol. Soc. Am. 153, 1980. Map Invest., 1-477, 1967c. Dalrymple,G. B., Criticaltables for conversionof Dibblee, T. W., Jr., Geologyof the FremontPeak K-Ar agesfrom old to new constants,Geology, andOpal Mountainquadrangles, California, Bull. 7, 558-560, 1979. Calif. Div. Mines Geol., 188, 1-64, 1968. Davis, G. A., Rapidupward transport of mid-crustal Dibblee, T. W., Jr., Geologicmap of the Greenfield myloniticgneisses in thefootwall of a Miocene quadrangle,California, scale 1:62,500,U. S. detachmentfault, WhippleMountains, Geol. Surv. Open File Rept., 71-38, 1971. southeasternCalifornia, Geol. Rundsch., 77, Dibblee,T. W., Jr., Geologicmap of the SanBenito 191-209, 1988. quadrangle,California, scale 1:62,500, U.S. Davis, G. A., and B.C. Burchfiel, Garlock fault, an Geol. Surv. Open File Rept., 75-394, 1975. intracontinentaltransform structure,southern Dibblee, T. W., Jr., and A. Bassett,Geologic map California, Geol. Soc. Am. Bull., 84, 1407-1422, of the CadyMountains quadrangle, San 1973. BerdardinoCounty, California, scale 1:62,500, Davis, G. A., J. L. Anderson, E.G. Frost, and T. U.S. Geol. Surv. Misc. Geol. Map Invest., 1- J. Shackelford,Mylonitization and detachment 467, 1966. faultingin theWhipple-Buckskin-Rawhide Dokka, R. K., Late Cenozoic tectonicsof the central Mountains terrane,southeastern California and Mojave Desert,California, Ph.D. dissertation, westernArizona, Cordilleran Metamorphic Core 220 pp., Univ. South.Calif., Los Angeles,1980. Complexes,edited by M.D. Crittenden,P. J. Dokka, R. K., Displacementson late Cenozoic Coney,and G. H. Davis,Mem. Geol. Soc.Am. strike-slipfaults of thecentral Mojave Desert, 153, 79-130, 1980. California, Geology,11,305-308, 1983. Davis, G. A., D. L. Martin, D. Krummenacher,E. Dokka, R. K., Patternsand modes of early Miocene G. Frost, and R. L. Armstrong,Geologic and extensionof the centralMojave Desert,California, geochronologicrelations in thelower plate of the ContinentalExtension Processes, edited by L. Whippledetachment fault, Whipple Mountains, Mayer, Spec.Pap. Geol. Soc.Am. 208, 75-95, California,a progressreport, in Mesozoic- 1986. CenozoicTectonic Evolution of the Colorado Dokka, R. K., New perspectiveson late Cenozoic RiverRegion, California, Arizona, and Nevada, strike-slipfaults of theMojave Desert and their editedby E.G. Frost,and D. L. Martin, pp. 408- relationshipto the Garlockfault, Geol. Soc.Am. 432, CordilleranPublishers, San Diego, Calif., 1982. Abstr. with Programs,19, 645-646, 1987. Dokka, R. K., and A. K. Baksi, Structureand Davis, G. H., and P. J. Coney, Geologic developmentof Cordilleranmetamorphic core sedimentationalong the lateral boundaries of a complexes,Geology, 7, 120-124, 1979. detachmentfault terrane,Central Mojave DeWitt, E., J. F. Sutter, G. A. Davis, and J. L. ExtensionalComplex, California, Geol. Soc.Am. Abstr. with Programs,18, 102, 1986. Anderson,4øAr-39Ar age-spectrum datingof Dokka, R. K., and A. K. Baksi, Thermochronology Miocenemylonitic rocks, Whipple Mountains, andgeochronology detachment faults of the southeastern California, Geol. Soc. Am. Abstr. Mojave ExtensionalBelt, California:A progress with Programs, 18, 584, 1986. report, Geol. Soc. Am. Abstr. with Programs, Dibblee, T. W., Jr., Geologicmap of the Hawes 20, A16, 1988. quadrangle,California, scale 1:62,500, U.S. Dokka, R. K., and A. Glazner, Aspectsof early Geol. Surv. Miner. Invest.Field Stud.,Map MF- Mioceneextension of thecentral Mojave Desert, in 226, 1960a. Geologicexcursions in the CaliforniaDesert, Dibblee, T. W., Jr., Geologyof the RodgersLake compiledby J. Cooper,pp. 31-45, Cordilleran andKramer quadrangles,California, U.S. Geol. Section,Geological Society of America,Anaheim, Surv. Bull. 1089-B, 1-139, 1960b. Calif., 1982. 388 Dokka: The Mojave ExtensionalBelt of SouthemCalifomia

Dokka, R. K., and S. H. Lingrey, Fissiontrack ColoradoRiver region,California, Arizona, and evidencefor a Miocenecooling event, Whipple Nevada, editedby Frost,E.G., and Martin, D. Mountains, southeastemCalifornia, in Cenozoic L., pp. 256-266, CordilleranPublishers, San palcogeographyof the westernUnited States, Diego, Calif., 1982. editedby J. Armentrout,M. Cole, and H. Hamilton, W., andW. B. Myers, Cenozoictectonics TerBest, Jr., pp. 141-145, Pacific Section, of the westernUnited States,Rev. Geaphys.,4, Societyof EconomicPaleontologists and 509-549, 1966. Mineralogists,Bakersfield, Calif., 1979. Hewett, D. F., Generalgeology of the Mojave Dokka, R. K., M. J. Mahaffie, and A. W. Snoke, Desertregion, California, in Geologyof Southern Thermochronologicevidence of majortectonic California,edited by R. Jahns,Bull. Calif. Div. denudationassociated with detachmentfaulting, Mines, 170, 15-18, 1954. northernRuby Mountains-EastHumboldt Range, Hillhouse, J., W., and R. E. Wells, Assessmentof Nevada, Tectonics, 5, 995-1006, 1986. rotationsin the easternMojave Desert, California, Dokka, R. K., M. McCurry, M. O. Woodburne,E. andArizona, from palcomagnetismof the Peach G. Frost,and D. Okaya,Field guideto the Springstuff, EOS Trans. AGU, 67, 922, 1986. Cenozoiccrustal structure of the Mojave Desert,in Holt, W. E., C. G. Chase, and T. C. Wallace, ThisExtended Land, GeologicalJourneys in the Crustalstructure from three-dimensionalgravity SouthernBasin and Range,edited by D. L. modelingof a metamorphiccore complex: A Weide, and M. L. Faber,pp. 21-44, Cordilleran modelfor uplift, SantaCatalina-Rincon Section,Geological Society of America,Las Mountains,Arizona, Geology,14, 927-930,1986. Vegas, Nev., 1988. Howard, K., P. Stone, M. Pernokos,and R. Dokka, R. K., and Michael O. Woodburne, Mid- Marvin,Geologic and geochronologic Tertiary extensionaltectonics and sedimentation, reconnaissanceof the Turtle Mountainsarea, centralMojave Desert,California, La. StateUniv. California,West border of theWhipple Mountains Pubs. Geol. Geophys.,Tectonics Sediment., 1, detachmentterrane, in Mesozoic-Cenozoictectonic 55 pp., 1986. evolutionof theColorado River region, Etheridge,M. A., J. C. Branson,and G. Stuart- California,Arizona, and Nevada, edited by Frost, Smith,Extensional basin-forming structures in E.G., andMartin, D. L., pp. 341-354, BassStrait and their importance for hydrocarbon CordilleranPublishers, San Diego, Calif., 1982. exploration,APEA J., 25, 344-361, 1985. Jennings,C. (compiler),Geologic map of Fox, K. F., Jr., R. J. Fleck, G. H. Curtis, and C. California, scale 1:750,000, Calif. Div. Mines and E. Meyer, Implicationsof the northwestwardly Geol., Sacramento, Calif., 1977. youngerage of thevolcanic rocks of west-central John,B., Geometryand evolutionof a mid-crustal California, Geol. Sac. Am. Bull., 96, 647-654, extensionfault system,, 1985. southeasternCalifornia, Continental extensional Gans,P., An open-system,two-layer crustal tectonics,edited by M. Coward,J. Dewey, and P. stretchingmodel for the easternGreat Basin, Hancock, Spec. Publ. Geol. Sac. London, 28, Tectonics, 6, 1-12, 1987. 313-335, 1987. Garfunkel, Z., Model for the late Cenozoic tectonic Kistler, R., Phanerozoic batholiths in western North historyof the Mojave Desertrand its relationto America:A summaryof recentwork on variations adjacent areas,Geol. Soc. Am. Bull., 85, 1931- in time, space,chemistry, and isotopic 1944, 1974. compositions,Earth Planet. Sci. Annu. Rev., 2, Gibbs, A.D., Structural evolution of extensional 403-418, 1974. basinmargins, J. Geol. Soc.London, 141,609- Lambert,J. R., Middle Tertiarystructure and 620, 1984. stratigraphyof the southemMitchell Range, San Glazner, A., J., Bartley, and J. Walker, Geologyof BerdardinoCounty, Califomia, M.S. thesis,120 the Waterman Hills detachmentfault, central pp., La. StateUniv., Baton Rouge, 1987. Mojave Desert,in This ExtendedLand, Lambert, J. R., R. K. Dokka, and A. K. Baksi, GeologicalJourneys in the SouthernBasin and Lead Mountaincaldera complex, earliest Tertiary Range, editedby D. L. Weide, and M. L. Faber, magrnatismin the centralMojave Desert, pp. 225-238, CordilleranSection, Geological California, Geol. Sac. Am. Abstr. with Societyof America,Las Vegas,Nev., 1988. Programs,I9, 1987. Golombek, M., and L. Brown, Clockwise rotation Lawson, H. R., Geophysicalstudy of the western of the westernMojave Desert,Geology, 16, 126- Mojave Desert,M.S. thesis,163 pp., Univ. 130, 1988. South.Calif., Los Angeles,1987. Gross,W. W., and F. L. Hillemeyer, Geometric Lister, G. S., and A. W. Snoke, S-C mylonites,J. analysisof upperplate fault patternsin the $truct. Geol., 6, 617-638, 1984. Whipple-Bucksk/ndetachment terrane, in Ludington,S., and K. Gray, Mineral resourcesof Mesozoic-Cenozoictectonic evolution of the the PinnaclesWilderness Study area, Monterey Dokka:The Mojave ExtensionalBelt of SouthernCalifornia 389

and San Benito Counties, California, U.S. Geol. Reynolds,S., and J. Spencer,Evidence for large- Surv. Bull. 1705-C, C1-C13, 1987. scaletransport on the Bullarddetachment fault, MacFadden, B. J., N. Opdyke, M. O. Woodburne, west-centralArizona, Geology,13,353-356, Paleomagnetismin the middle Miocene Barstow 1985. Formation, Mohave Desert, southernCalifornia, Ross, T. M., B. P. Luyendyk, and R. B. Haston, Magneticpolarity stratigraphy and tectonic Neogenerotations in the Cady Mountains,Mojave rotation, EOS Trans. A GU, 68, 291, 1987. Desert, California, Geol. Soc. Am. Abstr. with MacFadden, B. J., C. C. Swisher III, N. D. Programs, 19, 824, 1987. Opdyke, and M. O. Woodbume, Ross, T. M., B. P. Luyendyk, and R. B. Haston, Paleomagnetism,geochronology, and possible Paleomagneticevidence for Neogenetectonic tectonic rotation of the middle Miocene Barstow rotationsin the CentralMojave Desert,California, Formation,Mojave Desert, southern California, Geol. Soc. Am. Abstr. with Programs,20, 226, Geol. Soc. Am. Bull., in press, 1989a. 1988. MacFadden, B. J., M. O. Woodburne, and N. D. Serpa,L., andR. K. Dokka, Reinterpretationof Opdyke,Paleomagnetism and Neogene clockwise Mojave COCORP data,Implications for the rotationof the northernCady Mountains, Mojave structureof the Mojave Rift, Geol. Soc.Am. Desert, California, J. Geophys.Res., in press, Abstr. with Programs,20, 229-230, 1988. 1989b. Mathis, R.S., Structuralgeology of a portionof the Simpson,C., and S. Schmid,Microstructural centralCady Mountains,San Berdardino County, indicators of senseof shear in shear zones, Geol. California, M.S. thesis,136 pp., La. State Univ., Soc. Am. Bull., 94, 1281-1288, 1983. Baton Rouge, 1986. Spencer,J., Role of tectonicdenudation in warping Mathis, R.S., and R. K. Dokka, Upper plate and uplift low-anglenormal faults, Geology, 12, structureof the CentralMojave Extensional 95-98, 1984. Complex,Cady Mountains, California, Geol. Spencer,J., Miocenelow-angle normal faulting and Soc.Am. Abstr.with Programs,18, 154, 1986. dike emplacement,Homer Mountainsand Matthews, V., III, Correlation of the Pinnaclesand surroundingareas, southeastern California and NeenachVolcanic Formations and their bearingon southernmost Nevada, Geol. Soc. Am. Bull., 96, the San Andreasfault problem,Am. Assoc.Pet. 1140-1155, 1985. Geol. Bull., 60, 2128-2141, 1976. Spencer,J., and S. J. Reynolds,Some aspects of McClay, K. R., and P. G. Ellis, Geometriesof middleTertiary tectonics of Arizona,Frontiers in extensionalfault systemsdeveloped in model Geologyand Ore Depositsof Arizonaand the experiments,Geology, 15, 341-344, 1987. Southwest,edited by B. Beatty, and A. K. McCulloh, T. H., Geologyof the southernhalf of Wilkinson, Ariz. Geol. Soc. Dig., 16, 102-107, the Lane Mountainquadrangle, California, Ph.D. 1986. dissertation,182 pp., Univ. Calif., Los Angeles, Spongberg,M. E., and T. L. Henyey, Crustal 1952. reflectionprofiling in the westernMojave Desert, Miller, E. L., P. B. Gans, and J. Garing, The Snake California, Geol. Soc. Am. Abstr. with Rangedecollement: An exhumedmid-Tertiary Programs, 19, 454, 1987. ductile-brittle transition,Tectonics, 2,239-263, Stanley,R. G., Implicationsof thenorthwestwardly 1983. youngerage of thevolcanic rocks of west-central Miller, S. T., Geologyand mammalian California,Alternative interpretation, Geol. Soc. biostratigraphyof a partof thenorthern Cady Am. Bull., 98, 612-614, 1987. Mountains, California, U.S. Geol. Surv. Open Turner,D. L., Potassium-argondates concerning the File Rept. 80-978, 121 pp. 1980. Nason, G. W., T. E. Davis, and R. J. Stull, TertiaryForamiferal time scale and San Andreas Cenozoicvolcanism in the NewberryMountains, fault displacement,Ph.D. Dissertation,99 p., San BernardinoCounty, California, in Cenozoic Univ. Calif., Berkeley, 1968. paleogeographyof the westernUnited States, Vedder, J. G., D. Howell, and H. McClean, editedby J. Armentrout,M. Cole, and H. Stratigraphy,sedimentation and tectonic accretion TerBest,Jr., pp. 89-96, PacificSection, Society of exotic terranes,southern Coast Ranges, of EconomicPaleontologists and Mineralogists, California,Studies in ContinentalMargin Bakersfield, Calif., 1979. Geology,edited by J. Watkins,and C. Drake, Nilsen, T. H., and S. H. Clarke, Jr., Sedimentation Mem. Am. Assoc. Pet. Geol. 34, 471-498, 1983. andtectonics in the earlyTertiary continental Weiss,L. E., A studyof tectonicstyle - structural borderland of central California, U. $. Geol. investigationof a marblequartzite complex in Surv. Prof. Pap. 925, 64 pp., 1975. southernCalifornia, Univ. Calif. Publ. Geol. Polovina,J., Geologyand mineral deposits of the Sci., 30, 1-102, 1954. BagdadChase mine andvicinity, Stedman Wemicke, B., Low-anglenormal faults in the Basin District, San BernardinoCounty, California, M.S. andRange province--Nappe tectonics in an Thesis, 125 pp., Univ. Calif., Los Angeles,1980. extendingorogen, Nature, 291,645-648, 1981. 390 Dokka: The Mojave ExtensionalBelt of SouthemCalifomia

Wernicke, B., Theory of large-scale,uniform-sense in the CentralMojave Desert, Califomia, in normalsimple shear of the continentallithosphere, Geologicexcursions in theCalifornia Desert, Can. J. Earth Sci., 22, 108-125, 1985. compiledby J. Cooper,pp. 47-64, Cordilleran Wemicke, B., and B.C. Burchfiel, Modes of Section,Geological Society of America,Anaheim, extensional tectonics,J. Struct. Geol., 4, 105- Calif., 1982. 115, 1982. Wust, S., Regionalcorrelation of extension Wemicke, B., R. L. Christiansen,P. C. England, directionsin Cordilleranmetamorphic core and L. J. Sonder,Tectonomagmatic evolution of complexes,Geology, 14, 828-830, 1986. Cenozoic extension in the North American Cordillera, Continental ExtensionalTectonics, editedby M. Coward,J. Dewey, andP. Hancock, Spec.Publ. Geol. Soc. London,28, R. K. Dokka,Department of Geologyand 203-221, 1987. Geophysics,Louisiana State University, Woodbume, M., R. Tedford, M. Stevens, and B. BatonRouge, LA 70803 Taylor,Early Miocenemammalian faunas, Mojave Desert, Califomia, J. Paleontol., 48, 6-26, 1974. (ReceivedFebruary 16, 1988, Woodburne, M., S. T. Miller, and R. Tedford, revised October 3, 1988, Stratigraphyand geochronology of Miocenestrata acceptedOctober 7, 1988.)