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
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• BARSTOW
sedimentary :• .."........-..Rhyol ite,
Tertiary Tertiary(?)
rocks
Dikes
Waterman Hills detachment fault
(dotted where inferred or concealed)
Isotropic
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Waterman
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Late Cenozoic
Mesozoic,
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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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