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The Mojave Extensional Belt of Southern California

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The Mojave Extensional Belt of Southern California TECTONICS,VOL. 8, NO. 2, PAGES363-390, APRIL 1989 THE MOJAVE EXTENSIONAL BELT OF SOUTHERN CALIFORNIA 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 the middleto 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 the centralMojave Desert[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 the Mojave 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 the NewberryMountains 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
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