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have undergone the strain pattern dictated by the rolling hinge. In this paper, the structural development of the footwall of the central Mojave metamorphic core ...
TECTONICS,

VOL.

9,

NO.

3,

PAGES

521-534,

JUNE

1990

TERTIARY EXTENSION AND CONTRACTION OF LOWER-PLATE ROCKS IN THE CENTRAL

MOJAVE METAMORPHIC SOUTHERN

CORE COMPLEX,

CALIFORNIA

JohnM. Bartley and JohnM. Fletcher Departmentof Geology and Geophysics Universityof Utah Salt Lake City Allen F. Glazner

Departmentof Geology Universityof North Carolina ChapelHill

Abstract. Severalauthorsrecentlyhave proposedthe "rolling-hinge"model for the formation of extensionaldetachmentfaultsin metamorphiccore complexes.In this model,isostaticuplift of the denudedfootwall of a majornormalfault causesthe near-surfacepart of the fault to rotatefrom a higher to a lower dip. For the rolling-hingemodelto be acceptable,the footwallsof detachmentfaultsmust haveundergonethe strainpatterndictatedby the rolling hinge. In thispaper,the structural developmentof the footwallof the centralMojave metamorphiccorecomplexis usedasa field test. Tertiarystructures in the lowerplateof the core complexrecordNE-directedductileextensionalshear followedby cataclasticNE-sideup shearwith a significantcomponentof contractionalongthe previousextensiondirection.This strainhistory agreeswith the rolling-hingemodel,andparticularly with a versionof the modelin which footwall uplift occursby a combinationof elasticflexure and flexurallydrivenupper-crustal failure. Similarlatestagelayer-parallelcontractionappearsto be present in someothertectonicallydenudedterranesof the BasinandRange,suggesting thatthe flexural-failure rolling-hingemodelfor extensionalterranesmaybe widely applicable. INTRODUCTION Much recent interest in continental extensional tectonics has focused on the existence and nature of

Copyright 1990 by the AmericanGeophysicalUnion.

low-angledetachments.Controversyhascentered on the questionof whethergentlydippingnormal faults (detachmentfaults) were active with their presentlow dips [e.g., Davis et al., 1980; Wernicke, 1981; Davis and Lister, 1988], or were active with

relativelyhigh dipsandlaterpassivelyrotatedto low dips [e.g., Proffett, 1977; Miller et al., 1983]. Becauseof this still unresolvedcontroversy,in this paperwe usethe term "detachment fault" asa nongeneticdescriptiveterm for the gentlydipping normalfaultscharacteristic of metamorphiccore complexes,regardlessof their initial dip. This usage differs from that of Davis and Lister [ 1988], who

includeda low initial dip aspart of theirdefinition. A new model for the manner in which

detachmentfaultsmay havebeenseismicallyactive with steepdips,but reachedthe surfacewith gentle dips,hasbeenproposedrecentlyby severalauthors [Wernicke and Axen, 1988; Buck, 1988; Hamilton,

1988]. In this "rolling-hinge"model (Figure 1), the low dipsof detachmentfaultsresultfrom archingof a moderate-to high-anglenormalfault in responseto isostaticforcescausedby tectonicdenudation of the footwall. The hingethat separates the shallow rotatedanddeeperunrotatedfault segments"rolls" throughthe footwall as it migrateswith the hanging wall. The potentialimportanceof isostasyin formingthe archedfaultsandfabricsof Cordilleran metamorphiccorecomplexeswas notedpreviously by Spencer[1984] in the contextof a low initial dip for the detachmentfault. The rolling-hingemodelis uniquelysuccessful in explainingthe following observations:(1) the lack of recordednormal-slip earthquakes with gentlydippingnodalplanesin areas of modem continentalextension [Jackson,1987]; (2)

Papernumber89TC03485. 0278-7407/90/89TC-03485

$10.00

the lack of evidencefor deeperlargenormalfaults beneathsomeupwarpedbreakawayzonesand

522

Bartleyet al.: Lower-PlateStrainin Mojave Core Complex

compensation.Variouselasticflexuralmodels[e.g., Buck, 1988] corroboratethisby showingthata very small effective elasticthickness(of the orderof 1

km) is requiredin orderto interpretthe short wavelengthsof the observedgeometriesasflexures. Sucha smalleffectiveflexuralthicknessappearsto conflictwith the observationthatfocal depthsof major extensionalearthquakes are commonlyat 10 to 15 km [Jackson,1987], implyingan elasticlayerat least several kilometers thick.

WemickeandAxen [1988] proposeda qualitative solutionto theparadoxby suggesting thatthe geometryof isostaticuplift of thefootwallmaynot be controlledby elasticflexureof the crustbut rather by failure of the crust. However,a quantitative mechanicalmodelfor therollinghingeproposed by Buck [ 1988] explainsthe sameobservations in the context of crustal flexure.

Fig. 1. Successivecrosssections,illustratingthe rolling-hingemodelfor formationof upturned breakawayzones(B) andmetamorphic core complexes(MC), after Wemicke and Axen [1988], Buck [1988], andHamilton [1988]; alsoseeSpencer [1984]. D 1, D2, andD3 showsuccessive positions at whichwe infer thedeformationphasesrecognized in thecentralMojavecorecomplexoccurred. metamorphiccorecomplexes[e.g.,Wemicke and Axen, 1988], which would be requiredif reorientation of detachmentfaultswere accomplished by domino-stylerigid-blockrotation[cf., Proffett, 1977;Miller et al., 1983]; (3) the largeshear displacements alongat leastsomedetachment faults thatareindicatedby geologicaldata[Bartleyand Wemicke, 1984; Reynoldsand Spencer,1985;Davis et al., 1986;Glazneret al., 1989] andby thermal considerations[Buck, 1988]; and (4) thermochronologicalevidencefor steepinitialdipsof at leastsome detachments[e.g.,Fosteret al., 1988]. While very successful in theseregards,the rolling-hingemodel doesnot addressdirectlytheprocesses or kinematics of coeval lower-crustalextension,and indeedmost of the variouspublishedmodelsfor lower-crustal extensioncouldbe adaptedto work with a rolling

hingein theuppercrust. Thispaperdoesnot addressthe importantissueof lower-crustal extensionalprocesses. Spencer[ 1984] andWernickeandAxen [ 1988] pointedout that observeddetachmentfault

geometries in corecomplexesandbreakawayzones canbe fit closelyby assumingpointwiseisostatic

This model is based on

the fact thatcalculatedfiber stresses predictedfor a stronglyflexedelasticplateexceedthe strengthof muchof theupperandmiddlecrust. As a result,in stronglyflexedregionsonly therocksof the midcrustalstrengthmaximum[BraceandKohlstedt, 1980] canbe expectedto contributeto flexural rigidity [ChappleandForsyth,1979;Buck, 1988]. In particular,compressional fiber stresses in the upperpart of the concave-upward footwall flexure leadto compressional cataclastic failurein theupper crust,which sharplydecreasesthe radiusof curvature of the flexure and the effective elastic

thicknessof the plate (Figure2). In this way, the effectiveelasticthicknessfor isostaticflexuremay be lessthanthe thicknessof the elasticlayerdefinedby normal-faultearthquakes.In thisflexural-failure model, most of the footwall deformation is not

elastic,but the elasticresponseof the crust nonetheless controlsthe strainpatternobserved. Specifically,theflexural-failuremodelpredictsthat the upperpart of the footwall shouldundergolate

Deviatoric

stress

dt. pth Fig. 2. Flexural-failuremodelfor the rollinghinge proposedby Buck [1988]. Stress-depth curvefor flexed beam is modified from Buck [1988]; the

shadedarearepresents the predictedstress distribution in the crust as a result of combined

elasticflexureandflexurallydrivenfailure. Notethat upper-crustal cataclastic failureis predictedto be in horizontalcompression in theconcaveupward flexure in the footwall

of the normal fault.

Barfieyet al.: Lower-PlateStrainin Mojave CoreComplex contractionalongthe previousextensiondirection (Figure 2). In thispaper,we comparepredictionsof the rolling-hingemodelto observations of footwall strain in a Cordilleranmetamorphiccorecomplex.In particular,we comparepredictionsof Buck's[1988] flexural-failure CENTRAL COMPLEX

model with our field data.

MOJAVE

METAMORPHIC

CORE

Evidencefor significantmid-Tertiarycrustal extensionin the centralMojave Deserthasbeen recognizedfor severalyears[DokkaandGlazner, 1982; Dokka, 1986, 1989; Dokka et al., 1988; Glazner et al., 1988a, 1989; Walker et al., 1990a]. Recent work has shown that the Waterman Hills,

Hinkley Hills, andMitchel Range(Figure3) constitutea Cordilleran-typemetamorphic core complex[Davis and Coney, 1979;Davis andLister, 1988], to which we refer asthe centralMojave metamorphic corecomplex(CMMCC). A low-angle normal fault, the Waterman Hills detachmentfault,

placesa hangingwall of faultedandbrecciated TertiaryandPaleozoicsedimentary andvolcanic rocksupona footwallof mylonitizedgranodiorite andmetamorphicrocks. Thereis a complete lithologicmismatchbetweenfootwallandhangingwall rocks. Footwall

rocks record ductile shear that

is kinematicallycoordinatedwith brittleextension abovethe detachment.At leastpartof the ductile shearingis demonstrablyof Tertiaryagebecause mylonitizedrocksincludean earlyMioceneintrusion

523

sillimanite-Kfeldspargradein the Hinkley Hills [Kiser, 1981]. Outcroprelationsin the Waterman Hills indicatethatthismetamorphism andassociated deformation occurred before intrusion of the

hornblende-biotite granodiorite[Glazneret al., 1988a]. The Tertiaryductileandbrittledeformations describedbelow are superimposed uponthis Mesozoic(?)high-grademetamorphism and plutonism. The followingsections detailthe Tertiary structuraldevelopmentof lower-platerocksin the WatermanHills andMitchel Range,andthen considertheseobservations in the light of the rollinghingemodel,particularlythe flexural-failuremodel of Buck [ 1988], for footwalluplift in metamorphic corecomplexes. TERTIARY FOOTWALL

STRUCTURAL ROCKS

GEOLOGY

OF

Footwallrocksof the CMMCC recordTertiary strainthatevolvedprogressively from ductileto brittle. Threephasesof Tertiarydeformationcanbe distinguishedin footwall rocks(Figure 4). These are: (1) D 1 - formationof greenschist-facies S-C protomyloniteandphyllonitethatcontaina NEtrendinglineation,(2) D2 - foldingof themylonitic fabric aboutNE-trendingaxes,(3) andD3 formationof a NW-trendingarch,andof steeply dipping,NW-striking cataclasticzones,crenulation cleavage,andreverse-sense tension-gash arrays. D2 foldingmustat leastpartlypostdateD 1 becauseit is the S1 foliation that is folded. However, it is not yet

[Walker et al., 1990a]. Near the detachmentthe

clear to what extent formation of these folds could

myloniticfabric is overprintedby cataclasisand chloriticalteration,andhanging-wallrocksshow intensepotassiummetasomatism.Eachof these featuresis typicalof metamorphiccorecomplexesof

haveoverlappedin time with D 1 andD3. Because therelationsof D2 arelessimportantto thepurpose of the presentpaper,we do not considerthemin

the southwestern United States. The footwall of the CMMCC

consists of

WatermanGneissintrudedby hornblende-biotite granodiorite(Figure3). The nameWatermanGneiss hasbeenappliedto a variedcollectionof metasedimentary andmetaigneous rocksexposedin the centralMojave Desert[Bowen,1954;Dibblee, 1968, 1970]. The metasedimentary portionof the gneissin the Mitchel RangeandHinkley Hills consistsof probableupperPrecambrianand Paleozoicmiogeoclinalstrata[Kiser, 1981; J. M. Fletcher,unpublishedmapping,1989]. The plutonic portionof the WatermanGneissmay consistmainly of Mesozoic intrusions[Glazner et al., 1988b]. The

hornblende-biotite granodiorite bodythatcutsthe WatermanGneissis undated,butit is probably Jurassicor Cretaceousbasedon agesof similar plutonsin the region[BurchfielandDavis, 1981; Miller and Sutter, 1982; J. D. Walker et al., 1990b].

Fieldrelationsandpetrography of theWaterman Gneiss show it to have been deformed and

metamorphosed in the amphibolitefacies,to

detail here.

Brittle slipalongtheWatermanHills detachment itself postdatesF2 becausethe detachmenttruncates F2 foldsin the westernMitchel Range(Figure5 and 6). D3 cataclasficzonesmergeupwardwith the broad zone of cataclasis beneath the detachment.

Therefore,we interpretD3 in the footwallto correspond in time to the final brittlephaseof movementalongthe WatermanHills detachment. Di: Mylonitization

The Tertiarymyloniticfabricis mostclearly displayedin the granodioriteof the WatermanHills becauseit is the only tectonicfabricpresentin that rock. The granodioritegradesfrom essentially isotropicnearthe northernendof therangeto protomyloniticandthenphylloniticadjacentto the WatermanHills detachmentfault,whichis exposed in the southern partof therange(Figure3). The effectsof mylonitizationuponthe variousminerals thatcomposethe granodiorite areeasiestto distinguishin weaklyprotomyloniticsamples

524

Bartleyet al.: Lower-Plate Strainin MojaveCoreComplex

GE" 1•7o-'%_

B B•

35øN +

ß.... 117øW

•5•"

i

II

ß

ß

ß

Waterman Hills

0

0

1

2

3 km

'

"'"• ,• •:i:!:i:.' ß',:::: :5::: :5::::: :.Figure5 '":':': :':': :::::::::::::::::::::::::::::::: •i!?•!•i•:ii ?:':" :':•.... '"... :... ".•?• ':'ii?•i i•'"'"::"'":• _ HinkleyHills Mitchel

¸

ß...

Range

Figure 9

• BARSTOW

sedimentary :• .."........-..Rhyol ite,

Tertiary Tertiary(?)

rocks

Dikes

Waterman Hills detachment fault

(dotted where inferred or concealed)

Isotropic

Mesozoic { granodiorite

High-strainzone within

Waterman

Lineated

Gneiss

granodiorite

Late Cenozoic

Mesozoic,

Paleozoic, Waterman Late Proterozoic( ?) Gneiss

high-anglefault (dotted where inferred or concealed)

Paleozoic I PzILimestone Fig. 3. Generalized geologyof thecentralMojavecorecomplex,fromDibblee[ 1960], Kiser[ 1981],Walkeret al. [1990],andJ. M. Fletcher(unpublished mapping,1989). Dot on insetshowslocationof studyarea. GF, Garlockfault;SAF, SanAndreasfault; M, Mojave; B, Barstow;Ba, Baker; V, Victorville.

collectedfromthetransition fromisotropic granodiorite to mylonite(Figure7a). Theseeffects aretypicalof low-grademylonitization nearthe brittle-ductile transition: quartzisdynamically recrystallized; feldspar is microcracked andvariably replaced by sericite,withunaltered relicsforming

crackedporphyroclasts; andbiotiteandhornblende arevariablyreplacedby chloriteandepidote,with

unreplaced biotitefragments streaked outintoarrays of "micafish" [Lister andSnoke,1984].

In theMitchelRangeandHinkleyHills, spatial variations in theintensityof mylonitization in the

525

Bartleyet al.: Lower-Plate Strainin MojaveCoreComplex

D-1

D-2

Fig.4. Schematic blockdiagrams illustrating structures formedbyprogressive Tertiary footwalldeformation in theCMMCC. Thegeometry of theD3 antiformmayrecorda morecomplex historythanshownhereif theflexural-failure modelis appliedto the

CMMCC. In the flexural-failuremodel,thefootwallanddetachment first arebentinto an antiformat thecrustallevel whereelasticflexureoccurs,andthenpartiallyunbentwhen

thesamerockvolumeisdisplaced upwardtothelevelof cataclastic compression (Figures 1 and2) [Buck,1988;writtencommunication, 1989]. Suchcomplexity is permissible but notdemandedby presentstructural data. WatermanGneissaremorecomplexthanin the

granodiorite. Kilometer-scale lensesof weakly mylonitized rocksarebounded by high-strain zones of penetratively deformed rock(Figure3). Early isoclinalfoldsof compositional layeringareobserved in theWatermanGneiss,andmightrepresentF1

folding;however,thesealsomaybepre-Tertiary foldsthatweretransposed by myloniticshearing. Texturalrelationsalsoaremorecomplexin the WatermanGneissthanin thegranodiorite because themyloniticfabricis superimposed on anolder metamorphic fabric(Figure7b), buttheretrograde mylonitization resembles themylonitization of the granodiorite. The myloniticlineation,L1, in both protomylonite andphylloniteis definedby quartz ribbonsand streaksof phyllosilicates.L1 consistently trendsnortheast, with domainmean

azimuthsof 040øto 060ø (Figure6). The D3 structural overprint,discussed below,is probably responsible for theconsiderable scatterin plunge. This scatteris particularlypronounced in a zone

several hundred meters thick beneath the detachment

in the WatermanHills, within which L1 changes

erraticallyfromoutcropto outcropbutoveralldefines a completegreat-circle girdle(Figure6b). The granodiorite in thiszoneis intenselyfractured.We interpretthegirdleof L1 orientation datato reflect small-scale rigid-blockrotations duringdistributed D3 cataclasis beneath the detachment.

In granodiorite thatcontains well-developed L1

lineation,S1 foliationcommonlyis soweakasto be unmeasurable in the field, despitethe diffuse

preferredorientation of biotitegrainsthatis present in manysamples.Wherefoliationis presentin the granodiorite, thelineationis roughlydownthedip.

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