andsubsequentshearingofjoints(Aydin,2000).Eachmechanismproducesadiffer - PDF document

andsubsequentshearingofjoints(Aydin,2000).Eachmechanismproducesadifferentsetofstructuralcomponentswithacharacteristicgeometryanddis-tributionresultingindistinctivefaultarchitectures.Deformationbandsarecharacterizedbythintabularzonesofcataclasisinsandstone(Engelder,1974;AydinandJohnson,1978;Antonellinietal.,1994)thataccommodatemillimeterstocentimetersofoffset(Fig.1a).Faultscomposedofdeformationbandsaccumulateoffsetbyadditionofnewdeformationbandstoformananastomosingzoneofdeformationbands.Eventually,aplanardiscontinuityformsalongthezoneofdeformationbandsprovidingaslipsurfacethataccommodatesthemajorityofsubsequentoffset.Astheoffsetincreases,sodothelengthandthenumberofslipsurfaces.Thus,thesefaultsarecom-posedofthreecharacteristicstructures:(1)individualdeformationbands,(2)zonesofdeformationbands,and(3)slipsurfaces.Asecondmacroscopicdeformationmechanisminsandstoneischaracterizedbyshearingofpreexistingdiscontinuitiessuchasjointsorbeddinginterfaces.Shearingofjointsresultsinlocaltensionnearthejointtipswherenewjointscalledsplayjointsmayprop-(CotterellandRice,1980;SegallandPollard,1983;Granier,1985;Cruikshanketal.,1991).Splayjointshavebeenidentifiedintheliteratureashorsetailfractures(Granier,1985),pinnatefractures(Engelder, Fig.1.Mechanismsoffaultdevelopmentinsandstone:(a)deformationbanding(DB)and(b)jointingandsubsequentshearing.N.C.Davatzesetal./Tectonophysics363(2003)1–18 1987),splayfractures(Marteletal.,1988),splay(Martel,1990),kinkfractures(Cruikshanketal.,1991),bridgecracks,andtailcracks(CruikshankandAydin,1994).Weadoptthetermsplayjointemphasizethatthesestructuresarejointsthatformunderspecificcircumstancesandresultinacharacter-isticgeometrywithrespecttotheslidingdiscontinu-ity.SegallandPollard(1983)Martel(1988)describedjointsthathadbeenshearedandcoalescedintoathrough-goingfaultbyasystemofsplayjointsingranodiorite.Myers(1999)documentedtheforma-tionofzonesoffragmentation,breccia,andfaultrockbytheformationofsplayjointsthatconnectedshearedjointsinenechelonorparallelarraysinsandstone.Asoffsetincreases,early-formedsplayjointscanbereactivatedinshearandformasecondgenerationofsplayjoints(Myers,1999).Theorientationoffaultsthatincorporatemultipleshearedjointsdependsonhowindividualshearedjointsarelinkedbysplayjoints(e.g.,thethreecasesinFig.1bFaultrockalongthistypeoffaultischaracterizedbywhitecolor,grainsizereduction,poorconsolidation,andisspatiallyassociatedwithbrecciaandfragmen-tation.Thecharacteristicstructuresof‘‘joint-basedfaulting’’include(1)joints(orotherpreexistingdis-continuities,forexampleaslipsurface),(2)shearedjoints,(3)splayjoints,(4)zonesoffragmentation,and(5)faultrock(Myers,1999)Thestructuralanalysisoffaultedterrainsiscom-monlybasedoncriteriaincludingthelength,height,offset,spacing,andorientationoffaultsandsetsoffaults(e.g.,Anderson,1951;Krantz,1988;Barnettetal.,1989;DawersandAnders,1995;Schlischeetal.,1996;Maerten,2000).Thistypeofanalysisdoesnotadequatelyaddresstherelativetimingoffaultsorpropertiesoffaultzones.Wecontendthatadetailedanalysisoffaultingmechanismsandtheresultingarchitecturecomplementsaregionalstructuralanaly-sisthatintendstoreconstructthetectonichistoryofafaultedterrain.Furthermore,weproposethatrecog-nizingfaultarchitectureastheproductofspecificdeformationmechanismsprovidesabasisforpredict-ingfaultzonecharacteristicssuchastheoccurrenceanddistributionofjoints,fragmentedorbrecciatedrock,ordeformationbandsalongafault.Faultorientations,spacing,andsliphavebeenusedtocalculatetheorientationsandratiosofallthreeprincipalstrains(Krantz,1988).Withoutdistinguish-ingphasesoffaultdevelopment,onlythefinitestateofstrainisrepresented.Similarly,distinguishingneo-formedfaultsfromfaultsreactivatedduringasubse-quentphaseoffaultactivityisintegralforaccuratestressinversion(HuangandAngelier,1989)orforfaultslipdistributionanalysis(Maerten,2000)WhetherafaultwillactasfluidconduitsorbarriersmightdependonthedevelopmentanddistributionoffaultrockaswellastheattendantstructureslikejointsanddeformationbandsinthedamagezoneandLogan,1986;Caineetal.,1996;Matthaietal.,1998;Knipeetal.,1998;Tayloretal.,1999;Aydin,2000;Flodinetal.,2001).Hence,thearchitecture,temporalevolution,andoverprintingrelationshipsoffaultsarecrucialforelucidatinghydrocarbonmigra-tionpathwaysandpotentialtraps.ThisstudyexaminedtheChimneyRockfaultsystemintheSanRafaelSwellofUtah,USAto Fig.2.Distributionoffaultsinthestudyarea.BasicfaultmapwasmodifiedfromMaerten(1999)N.C.Davatzesetal./Tectonophysics363(2003)1–18 characterizedeformationbandandjoint-basedfault-ingstylesinmultiplefaultsetsinasinglelocationandsinglerocktype.Wemappedthestructuralcompo-nentsoffaultstodistinguishdifferentmechanismsoffaultformation.Basedondetailedmappingandanal-yses,weestimatedtherelativecontributionofthestructuralproductsassociatedwitheachmechanismtofaulting.Themacroscopicdeformationmechanismsprovidedameanstodeterminevariousphasesoftheformationandevolutionofanormalfaultsystemcomposedofmultiplesetsoffaultsinsandstone.2.GeologicsettingTheChimneyRocknormalfaultsystemislocatedin-dippingstrata(Krantz,1988)straddlingthenorthernanticlinalaxisoftheSanRafaelSwellof Fig.3.TimingofeventsintheChimneyRockarea.Jaggedlinesduringnondepositionofunitsareunconformities.ThetwocurvesrepresentmaximumandminimumestimatesofoverburdenthicknessabovethetopoftheNavajoFormationestimatedbycombiningdatafromGilluly(1928).UnitsyoungerthanLateCretaceousareextrapolatedfromsurroundingareasbecausetheyareerodedfromtheSanRafaelSwell.N.C.Davatzesetal./Tectonophysics363(2003)1–18 centralUtah,USA(Fig.2).Theswellisadome-likestructurecomposedofaPhanerozoicsedimentarysequence(Kelley,1955;DickinsonandSnyder,.ThenormalfaultsystemoffsetsJurassicNavajosandstoneandoverlyingthinlybeddedlimestone,silt-stone,andmudstoneofthebasalCarmelFormation.Inthestudyarea,theeolianNavajosandstoneisawell-sorted,cross-stratified,quartzarenitewith13–25%porosity(ShiptonandCowie,2001).NavajoFormationthicknessvariesfrom140–170m(Gilluly,1928;GillulyandReeside,1928).Thefootwallsoffaultsupto6kmlongareexposedasscarpsofresistantNavajosandstone.Polishedandstriatedfaultsurfacesalongthesescarpsdip55–90(Krantz,1989).Stria-tionsrakefrom90–40EandW(Maerten,2000)Maximumverticaloffsetacrossasinglefaultintheentirestudyareais38m(Maerten,2000;Maertenetal.,.BeddingplaneexposureofNavajosandstonealongfaultsisprimarilylimitedtothefootwallandfaultzonescrossedbywashes(Fig.2)PreviousauthorssuggestedthatthetimingoffaultformationcoincidedwithupliftoftheSanRafaelSwellduringtheLaramideorogenybetween66.4and37Ma(Fig.3)(Krantz,1988;Maerten,2000;ShiptonandCowie,2001).ThetimingofupliftoftheSanRafaelSwellisconstrainedbychangingalluvialarchitectureandthinningofthePriceRiverFormationintheearlyCampanian(83.5–71.3Ma)(GuiseppeandHeller,1998).Duringthisperiod,theminimumburialdepthoftheNavajoFormationwas2–4km(Fig.3).ThelackofintactunitsyoungerthanLateCretaceousintheareadoesnotallowreconstructionofadefinitivemaximumburialdepth.PreviousresearchattheChimneyRockfaultarrayinUtahhasfocusedongeometric,kinematic,andmechanicalcharacteristicsofthefaultarray(Krantz,1988;CowieandShipton,1998;Maerten,2000;ShiptonandCowie,2001)butnotontheoccurrenceanddistribu-tionofmultiplefaultingmechanisms.3.ResultsAllexposedfaultswereexaminedfortheoccur-rence,distribution,andrelativetimingofdeformationmechanisms.TheNorthandGlassfaults(Fig.2)wereselectedfordetailedmappingtodocumentandinterpretdeformationmechanismsbecauseofexcellentexpo-sures.Locationsisolatedfromfaultintersectionswerechosentominimizetheimpactofdifferentfaultsetsontheoccurrence,distribution,andorientationsofstruc- Fig.4.ComponentsofWNW-strikingfaults.(a)Splayjointsresultingfromsliponjoints.Offset:1cm.(b)Well-developedbrecciaandfaultrockadjacenttofaultsurfacealongGlassfault.Verticaloffset22m.FaultrockalongWNW-strikingfaultsischaracterizedbywhitecolor,grainsizereduction,poorconsol-idation,andisspatiallyassociatedwithbrecciaandfragmentation.(c)FragmentationandbrecciazonealongLaSalfault.Verticaloffset13m.Severalexamplesofeachstructuralcomponentareindicatedinthephotograph.FigurelocationsareindicatedinFig.2N.C.Davatzesetal./Tectonophysics363(2003)1–18 tures.TheGlassfault(Fig.2)isaWNW-strikingandSSW-dippingnormalfault,withamaximumverticaloffsetofabout38m.TheNorthfault(Fig.2)isanENE-strikingandNNW-dippingnormalfaultwithamax-imumverticaloffsetofabout28m.Detailedmapswereproducedinthefieldonphotographsobtainedfromacamerasuspendedbelowaheliumweatherballoonapproximately100mabovetheoutcrop(see,forinformationaboutthetechnique).Thesephotographsprovidesufficientresolutiontorecordtheoccurrenceandtypesofstructuresthatcomposethefaultanddamagezone.Wemappeddeformationbands,zonesofdeformationbandsandassociatedslipsurfaces,jointeddeformationbands,joints,shearedjoints,fragmentationandbrecciazones,andfaultrock.Structurefrequencyandorientationweremeasuredalongscanlinesnormaltothefaults.Wepresenttheleastcomplicatedfaultsfirst,beginningwiththeGlassandLaSalfaults,andthenmoveontothemorecomplicatedexamplesoftheNorthandCottonwoodfaults(Fig.2)3.1.StructurescomposingWNW-strikingfaultsAlongtheGlassfault(Fig.2),jointsarethedominantstructuralcomponents.Thetraceofthefaultischaracterizedbyazoneofhighjointdensity(Figs.4and5).Theupperandlowertipsofjoints Fig.5.DetailedmapofportionoftheGlassfault,offsetinthislocationisapproximately36m.Thefaultisprincipallycomposedofjoints.Onlythreedeformationbandsweremappedatthenorthernextremityofthemap.FigurelocationisindicatedinFig.2N.C.Davatzesetal./Tectonophysics363(2003)1–18 typicallyabutagainstcross-beddingordune-bound-ingsurfaces.Somejointshavepreservedplumosesurfacemorphology(PollardandAydin,1988).Otherdiscontinuitiesofthesameorientationhavenormaldisplacementsof0.5–5cm,implyingthatjointsformedandweresubsequentlysheared.Youngerjointstypicallyabuttheseshearedjointsinageom-etryandsenseofshearconsistentwithsplayjoints(Figs.1band4a)Nearthefaultsurface,somejointswithsplayjointgeometryaccommodateshearoffset.Theseshearedsplayjointsalsohaveabuttingjointsinsplayjointgeometry.Faultsurfacesthataccommodateoffsetgreaterthan1mareconsistentlyborderedbywhitefaultrockthatissurroundedbybrecciaandfrag-mentedrock(Fig.4b).Brecciablocksareboundedbyshearedjointsandjoints.Thedensityofjointsisloweratthemarginsofthebrecciazonewheresomeofthesejointsareidentifiableassplayjointsfromtheagerelationship,senseofshearacrosstheshearedjoint,andgeometry.SeveraldiscontinuousandsubparallelfaultsurfacesborderedbybrecciaandzonesoffaultrockaretypicallypresentalongtheGlassfault(e.g.,Fig.5).Asingle,continuousfaultsurfaceisexposedfor10malongthewallofamineshaft(locationinFig.2).Thisfaultsurfaceisborderedbyacontinuouszoneoffaultrock.ThefaultjuxtaposesNavajosand-stoneagainstCarmelFormationshaleindicatingoffsetgreaterthan20m.TheLaSalfaultisoneoftheWNW-strikingfaults,butincontrasttotheGlassfault,itdipstotheNNE(Fig.2).Thefaultzoneiscomposedofjoints(Fig.4bandc),shearedjoints,andassociatedsplayjointssimilartothosedescribedalongtheGlassfault.OtherfaultsintheWNW-strikingpairofsetssharethisarchitectureregardlessofdipdirection(Fig.2).Inallcases,jointdensityishighestaroundtheWNW-strikingfaultsconsistentwiththeinferreddeforma-tionmechanism(Fig.6).DeformationbandsparalleltotheWNW-strikingfaultsetsarerare.Whendefor-mationbandsarepresent,theyoccurattheback-grounddensity,arenotadjacenttotheprimaryfaultsurface,andarenotcontinuousalongthefault.Wheredeformationbandsarepresent,theyarealwayscutbyjointsandshearedjointsindicatingthatjointsareyounger. Fig.6.Frequencyofdeformationbands(DBs)andallstructuresformedasjoints(e.g.,joints,shearedjoints,andsplayjoints)alongtransectsnormaltothetrendoftheGlass(Fig.5),LaSal,andNorth(Fig.8)faults.TransectsareindicatedinFigs.5and8.Graybinsincludeslickensidedfaults.N.C.Davatzesetal./Tectonophysics363(2003)1–18 3.2.RegionaljointsetsJointsalsooccurthroughoutthefieldarea.Threedistinctregionaljointsets(Fig.7a)arerecognizableatadistancefromfaultsinthefieldarea.Allofthejointsetsdipapproximately90.ThesejointsareprimarilyexposedinpavementsurfacesofthinlimestonebedswithintheoverlyingCarmelformation.Thethreejoint Fig.7.StrikeofdeformationbandsandjointsinNavajosandstonefor(a)jointsdistantfromfaults,(b)Glassfault,(c)Northfault,and(d)allmeasurementsinthefieldareaincludingmeasurementsdistantfromfaults.AlowdensityofWNW-andENE-strikingdeformationbandsisfoundthroughoutthefieldarea. Fig.8.ComponentsofENE-strikingfaults:(a)Well-developedzoneofdeformationbands(DBs)(lightercoloredbands)withadjacentslipsurfacealongtheNorthfault;verticaloffsetisabout20minthislocation.(b)ZoneofdeformationbandswithslipsurfaceisoverprintedbysplayjointsontheCottonwoodnormalfault;verticaloffsetisabout6minthislocation.Notethatjointsareobliquetotheoutcropsurface;thusintersectionanglesappearslargerthantheyare.Severalexamplesofeachstructuralcomponentareindicatedinthephotographs.FigurelocationsareindicatedinFig.2N.C.Davatzesetal./Tectonophysics363(2003)1–18 setsalsoappeartoexistinlimitedexposuresoftheNavajosandstonedistantfromthefaults.Consistentrelativeagesbetweenthejointsetsareevidentfromabuttingrelationships.Themostprominentsetofjointsstrikes110–140(Fig.7a)paralleltotheWNW-strikingfaults(Fig.7b)andisintermediateinage.Theoldestandyoungestjointsetsorientedfromto180(Fig.7a)andfrom050to080respectively,arelessprominent.Jointsintheoldesttwosetsweresubjectedtoasmalldegreeofshearingevidentfromoffsetmarkers.ThepresenceofWNW-strikingregionaljointsisconsistentwithdevelopmentofWNW-strikingfaultsbyshearingalongthejoints.3.3.StructurescomposingENE-strikingfaultsBothdeformationbandsandspatiallyassociatedjointsaredistributedalongtheentirelengthoftheNorthfault(Fig.2).Deformationbands,zonesofdeformationbands,andslipsurfacesadjacenttosomeofthedeformationbandzones(Fig.8a)constitutethemajorityofstructuresalongtheNorthfault(Figs.6and8).Thesestructureswereproducedbythedefor-mationbandingmechanism(Fig.1a)JointsalongtheNorthfaulttypicallyoccuradja-centtodeformationbands(Fig.9);someretainplumosestructureconfirmingtheyformedasjoints.Jointsaretypicallystraighterthandeformationbandsandcutadjacentsubparallelbutwavybandsindicat-ingthattheyareyounger.Somejointsadjacenttodeformationbandshaveseveralcentimetersofoffsetacrossthefracturesurfaceindicatingreactivationinshear.Singledeformationbandsintheareaconsis-tentlyhave0.5–5mmnormaloffset.Theadditionaloffsetacrossjointeddeformationbandsresultsfromshearingacrossthejointplane.Somejointsareatanglesof20–40totheprimaryslipsurfaceoftheNorthfault.Thesejointscharacter-isticallyabutagainsttheslipsurfaceandarearrangedinageometryconsistentwithsplayjointsfortheobservedsenseofslip.Wherethedensityofthesejointsishigh,theydefinerockfragmentsrangingin Fig.9.DetailedmapofportionoftheNorthfault,verticaloffsetinthislocationisapproximately22m.Thefaultiscomposedofdeformationbandscutbyjointsandsplayjointsconsistentwithboththedeformationbandingandjoint-basedmechanisms.FigurelocationisindicatedinFig.2N.C.Davatzesetal./Tectonophysics363(2003)1–18 sizefromcentimeterstoabout1mformingaweaklydefinedbreccia,similartothosealongtheGlassfault.Somejointsparalleltothefaultmayalsohaveformedaspartoftheyoungestregionaljointset,whichhassimilarstrike(Fig.7a)TheCottonwoodfault,anotherENE-strikingnor-malfaultinthestudyarea,(Fig.2)providesoneofthebestexamplesofoverprintedfaultingmechanisms(Fig.8b).Thefaultstrikes253anddips70–75withapproximately6mofverticaloffsetatthelocationofFig.8.TheCottonwoodfaultiscomposedofawell-developedzoneofdeformationbandsandassociatedslipsurface.Thezoneofdeformationbandsiscutbyjointsthatstrikeparalleltothefaultbutdipbetween50and90inadirectionoppositetothatofthefault(Fig.8b).Jointsofthisorientationaremoreabundantclosetothefault(e.g.,Fig.6)andabutagainstthefaultslipsurfaces.Thegeometryofthesejointsandsenseofoffsetthattheyindicateareconsistentwithformationbysplayjointing.Attheintersectionofthejointswiththedeformationbandslipsurface,increasedfracturedensityfragmentedtherockformingaweaklydefinedbreccia(Fig.8b)Similarly,jointssubparalleltodeformationbandsdemonstrateslip(Fig.8btotherightofthedeforma-tionbandzone)andhaveassociatedsplayjoints.OpeningofsplayjointsthattruncateagainstthefaultsurfacerequiresasmallamountofoffsetacrosstheCottonwoodfaultsurfaceaccompanyingjointformation.Becausejointscutdeformationbands,thisslippostdateddeformationbandformation.Similarly,theoccurrenceofbrecciaoverprintingdeformationbandsindicatesamagnitudeofsliplargeenoughtoleadtofragmentationandbrecciaformationbysplayjointingandshearing(Fig.1b).Ameterormoreofoffsetisestimatedtobeconsistentwiththebrecciaformation.Thus,wepartitiontheslipintotwophases:5mofthetotaloffsetwasaccommodatedduringdeformationbandformationand1mofoffsetasso-ciatedwithadditionalslipacrosstheoriginalslipsurfacethatproducedsplayjointsandbreccia.TheremaybeasmallcontributiontotheslipbudgetbyshearingofafewregionaljointsfromtheENE-strikingset.Deformationbanddensityisgreatestneartheprimaryfaultslipsurfaceandquicklydropstoabackgrounddensityoflessthanoneband/4mabout45mawayfromthefault(Fig.6).Inallcasesobserved,jointscutdeformationbandsindicatingthatjointsandjoint-relatedstructuresareyounger.3.4.Quantitativeanalysisoffaultstructuralcompo-TheoccurrenceofstructuralcomponentsandfaultarchitecturedescribedalongtheNorth,Cottonwood,Glass,andLaSalfaultswerecomparedtoallothermajorfaultsexposedintheregion(Fig.2).ENE-strikingfaultsareconsistentlydominatedbydeforma-tionbandsandassociatedslipsurfaces.InContrast,WNW-strikingfaultsareconsistentlydominatedbyjoints,shearedjoints,zonesoffragmentedrock,breccia,andfaultrock.Inaddition,thereexistregionaljointssetsandbackgrounddeformationbandsthatstrikesubparalleltofaultsthroughouttheChimneyRockarea(Fig.7).DeformationbandsthatstrikeENEareconsistentlycutbyasmallnumberofdeformationbandsthatstrikeWNW(ShiptonandCowie,2001).Thedeformationbandingandjoint- Fig.10.Verticaloffsetversus(a)numberofjointsand(b)numberofdeformationbandswithin2moffaultslipsurface.Datawerecollectedfromdifferentlocationsalongsinglefaultsaswellasseveraldifferentfaults.Associatedstructuressuchasfragmentation,deformationbandslipsurfaces,andfaultrockrelatedtojoint-basedfaultingarealsoindicated.N.C.Davatzesetal./Tectonophysics363(2003)1–18 basedfaultingmechanismseachoccurinfaultsdip-pingnorthandsouth.Faultdevelopmentwasinvestigatedbymeasuringthetotalnumberofjointsanddeformationbandswithin2moffaultslipsurfacesatlocationswithdifferentoffsets.Thismethodprovidedameanstoevaluatehowindividualstructuralcomponentsareaddedasafunctionoffaultslip.Thewindowofobservationwaschosenasapracticalmeansofgatheringdatafromawiderangeoffaultsandfaultoffsetdespitelimitedexposure.Theshortdistancealsoallowedustoavoidfaultintersections,wherethe Fig.11.Photomicrographsinplane-polarized,transmittedlightofNavajoformationsandstoneshowing(a)quartzovergrowthonsandgrainsinadeformationbandaswellas(b)quartzovergrowthinthehostrocknearadeformationband.Thetight,angulargeometryofovergrowthsonintactwell-roundedsandgrainstypicalofeoliandepositsindicatetheyarediageneticratherthaninherited.Porositywasdigitallyenhancedforclarity.N.C.Davatzesetal./Tectonophysics363(2003)1–18 relationshipbetweenoffsetandfaultstructureismorecomplicated.A2-mintervaliswideenoughtochar-acterizechangingdeformationintensitywithinandimmediatelyadjacenttothefaultzonebasedonobservationsfromlongerscanlines.Finally,thismeasurementisconsistentwiththehypothesisthatbothzonesofdeformationbandsandjointsassplayjointsformadjacenttothefaultduetocontinuedslip(AydinandJohnson,1978;Myers,1999)WesampledfrommultiplelocationsalongtheNorthandGlassfaults,aswellasseveralfaultswithsmallmaximumoffsetstoassemblearepresentativedatabaseoffaultsandtheirarchitecture.Thedensityofjoints(includingsplayjoints)onWNW-strikingfaultsincreaseswithincreasingoffset(Fig.10a).Incontrast,ENE-strikingfaultshaveaconsistentdensityofjointsoveroffsetsrangingfrom45cmto28m.WhiledeformationbanddensityincreaseswithincreasingoffsetonENE-strikingfaults,WNW-strik-ingfaultshavelowornulldeformationbanddensity.FaultrockandbrecciaaredevelopedalongWNW-strikingfaultsatalloffsetsplottedinFig.10;faultrockcharacteristicofjoint-basedfaultingisabsentalongENE-strikingfaults.Conversely,slipsurfacesassociatedwithdeformationbandfaultsoccuralongallENE-strikingfaultsinFig.10beginningatoffsetassmallas45cm.ThetypeofslipsurfaceassociatedwithdeformationbandzonesisnotobservedalongWNW-strikingfaultsatanyoffset.Thereare,ofcourse,faultplanesalongWNW-strikingfaultsbuttheseformedbyshearingofjointsandtherelated3.5.FaultformationandsilicacementationSandgrainsintheNavajoFormationarecementedbyquartzovergrowths(Fig.11).Theseovergrowthsareincorporatedintodeformationbands(righthandsideofFig.11)suggestingsilicaprecipitationbeganbeforeorwhilethedeformationbandsformed.Areasofintenselysilica-cementedsandstonearelocalizedalongsomeportionsoftheNorthandCottonwoodfaultsimplyingthesefaultswerepartiallyestablishedpriortoandinfluencedsilicacementation.Incontrast,jointsassociatedwiththesefaultsandattheintersec-tionbetweentheCottonwoodandGlassfaultsareopenandfreeofsilicaprecipitationandthereforepostdatesilicacementationofthesandstone.4.Discussion4.1.ThefaultingmechanismsThestructuralcomponentsobservedintheWNW-strikingGlassandLaSalfaults,theirarrangement(Figs.4–6),therelationshipbetweenoffsetandjointdensity(Fig.10),andtheWNW-strikingregionaljointsetareconsistentwithformationbythejoint-basedfaultingmechanism(Fig.1b).Incontrast,thepreva-lenceofdeformationbandsalongtheENE-strikingNorthandCottonwoodfaults(Figs.6and8)andtherelationshipbetweendeformationbanddensityand(Fig.10)indicatethatthesefaultsformedbydeformationbanding(Fig.1a).Jointsareconsistentlyyoungerthandeformationbandsonthesetwofaults.Inaddition,theoccurrenceofshearedjoints,splayjoints,andbrecciacharacteristicofjoint-basedfault-ingindicatethattheNorthandCottonwoodfaultswerelaterreactivatedandtheadditionaldeformationwasaccommodatedbythismechanism(Fig.1b).Theuniformdensityofjointsalongthesefaultsisconsis-tentwithasmall,comparablecontributiontoslipbytheshearedjoint-basedfaultingmechanismresultinginsplayjoints(Fig.10).Thesejointsaresuperim-posedonearlierdeformationbandfaultingthataccommodatesthemajorityofslip.OffsetacrosstheENE-strikingNorthfaultresultsfromthecombinedcontributionsofbothdeformationmechanisms.Theoccurrenceofbrecciabutlackofappreciablefaultrocksuggeststhattheoverallcontri-butiontoslipbyshearingacrossjointsisprobablynomorethan1–2m(Fig.1).Rockfragmentsareisolatedfromthesurroundingrockbyjoints,butarenotyetrotatedorentrainedandtransportedsignificantlyalongthefault.Theminimumoffsetassociatedwithfrag-mentationisthereforerelatedtotheapertureanddensityofjointsinthefragmentationzone(Fig.1b)Brecciationrequireslargerrotationandtransportofrock,andthusitisassociatedwithlargeroffset.Totaloffsetinthemappedlocationis22m;thissuggeststhatatleast20mofoffsetwasachievedthroughformationofdeformationbandsandoffsetacrosstheassociatedslipsurfaces.ThestructuralcomponentsandtheirarrangementalongtheNorthfaultrequire(1)formationofdeformationbands,(2)formationofjointsadjacenttoapreexistingdeformationband,(3)formationofjointsassplayjointsassociatedwithsliponfaultN.C.Davatzesetal./Tectonophysics363(2003)1–18 paralleljointsandonslipsurfacesthatformedearlierbythedeformationbandingmechanism.4.2.DevelopmentoftheChimneyRockfaultarrayTherelativeagesofstructuresproducedbyeachmechanismimplytwophasesoffaultingcharacterizedbydifferentdeformationmechanisms.Thefirstphaseoffaultingwascharacterizedbydeformationbands.Subsequently,thesecondphaseoffaultingwaspre-dominantlycharacterizedbyjoints,shearedjoints,andrelatedsplayjoints.EarlydeformationbandformationproducedprincipallyENE-strikingmap-scalefaults.Afterthefirstphaseoffaulting,theformationofregionaljointsetsmarkedachangeindeformationmechanism.Furthermore,thesetofWNW-strikingjointsprovideddiscontinuitiesthatlocalizedshearingleadingtotheformationofWNW-strikingfaults.TheoverprintingofENE-strikingfaultsismanifestedasshearingofdeformationbandslipsurfacesproducingsplayjointsandrelatedproducts.Additionalover-printingmightresultfromshearingofENE-strikingjointsformedalongdeformationbandsorbyshearingoftheregionalENE-strikingjointset.Eachdeformationmechanismduringeachrespec-tivephaseofdevelopmentcausedtheformationoffaultsupto6kmlongwith30–40mmaximumoffset.Therelativetimingandorientationofstructurescomposingthefaultssuggestthattheincrementalstrainhistoryofthefaultarraywasnon-coaxial.ThefirstphaseaccommodateddominantlyNNWexten-sionnormaltothetwoearlysetsofENE-strikingdeformationbandfaults(Fig.12).ThiswasfollowedbythesecondphaseoffaultinginwhichtheextensiondirectionwasdominantlyNNE,normaltothesetofWNW-strikingjointsandsubsequentshearedjointbasedfaults.Sometimeduringthissecondphase,ENE-strikingfaultswerereactivated.Severallinesofevidence,thoughnotconclusive,areconsistentwiththehypothesisthatallsetsoffaultsslippedsimultaneouslyforsomeportionneartheendofthesecondphaseofdeformation.First,joints,shearedjoints,andfragmentationsimilartoWNW-strikingfaultsoverprintdeformationbandsonENE-strikingfaultsalongtheslipsurface.Second,therakeof Fig.12.Twophasesoffaulting:(1)Faultingcharacterizedbyformationofdeformationbandsandslipsurfacesinprimarilytwoconjugatesets,(2)faultingcharacterizedbyformationandsubsequentshearingofjointsintwoconjugatesetsafterrotationoftheprimaryextensiondirection.Asallfaultsslip,asmallamountofextensionalsooccurstotheESE.N.C.Davatzesetal./Tectonophysics363(2003)1–18 slickensidescomposingfaultsthroughoutChimneyRockrotatesinproximitytofaultintersections,indi-catingmechanicalinteractionamongsetsoffaults(Maerten,2000)Previously,fourfaultsetswiththisgeometryhavebeeninterpretedtohaveformedsimultaneouslyinresponsetogeneralthree-dimensionalstrain1965;AydinandReches,1982;Krantz,1988)Krantz’s(1988)analysisderivedtheprincipalstrainratiosandorientationsbasedonthefaultgeometry,spacing,andoffsetdistributions.Hisanalysisassumedcoevalfaultformationanduniformsliprateonallfaultsetsresultingincoaxialstrain.Multiplephasesoffaultdevelopmentwerenotdistinguished.Thisstudyindicatesthatformationoffaultsetsofdifferentstrikewasnotcoeval.Infact,ENEfaultswerereactivatedduringthesecondphaseoffaultingthatproducedWNW-strikingfaults.Thetwophasesoffaultingdistinguishedbythefaultarchitectureinthisstudysuggestsnon-coaxialstrain.Duringthefirstphase,thearrayaccommodatedpredominantlyplanestrain.Incontrast,duringthesecondphase,thearrayaccommodatedthree-dimensionalstrainsinceallthefaultssetswereactive(Fig.12).Thisreconstructionmayonlybeconsistentwiththerakedatathatindicatesystematicchangeofslipdirectionwithrespecttothefaultintersections(Maerten,2000)andthatmostlikelyrepresentthelastphaseofsliponthefaults.Thus,thepreviousanalysesquantifythefinitestrainandthekinematicindicatorsofthelastslipeventsbutdonotaccountforsequentialfaultformationandnon-coaxialityinthestrainpath.Therelativecontributionsofeachdeformationmechanismtoaccommodationofoffsetcontrolstheoverallfaultarchitectureincludingdensity,geometry,andtypeoffaultzonecomponentssuchasjointsordeformationbands(Fig.10).Evenatlowoffset,faultsformedfromjointsdefineawidezoneofwell-con-nectedfractureswithinitiallydiscontinuousfaultrockpockets(disaggregatedwhitegouge).Jointdensityincreasesproportionallywithincreasingfaultoffsetassplayfracturesformaccompanyingslip.ThesejointsandtheirshearingoccurredpreferentiallyalongolderdeformationbandsthuscontributingtothefurtherdevelopmentofENEfaults.Alonganestab-lishedfaultsurface,theoffsetrequiredtoformsplayjointsmightdependonthesizeoftheslidingsurface,thestiffness,andfrictionalpropertiesoftherock(Cruikshanketal.,1991;WillemseandPollard,.Onceslipoccurs,manysplayjointsmayforminawideareaaroundtheslippingfault.Deformationbandfaultstendtodeveloptightlyclusteredzonesofdeformationbandsadjacenttotheprimaryfaultsur-faceatoffsetsaslowas0.5m.Asoffsetincreases,adiscreteslipsurfaceformsandsubsequentlynewdeformationbands.FossenandHesthammer(1997)ShiptonandCowie(2001)observedasimilarrelationshipbetweenfaultoffsetanddamagezonewidthinfaultscomposedofdeformationbandsandtheassociatedslipsurfacesontheColoradoPlateau.Theydocumentlargeincreasesinfaultdamagezonewidthwithverysmallincreasesinoffsetfollowedbysmallerincreasesindamagezonewidthperadditionalunitoffsetoncethefaultreachesafewmetersofoffset.Theyassociatedthedecreaseindamagezoneexpansionwithoffsetwiththedevelopmentofathrough-goingslipsurface.CowieandShipton(1998)offeranalternateinterpre-tationforasimilarrelationshipbasedonexaminationoftheBlueberryfault(Fig.2)andtheBigHolefaultsouthofChimneyRock.Theysuggestedthatthedecreaseindamagezoneexpansionresultedfromatransitionfromtheinteriorofthefaulttothefaulttipsandrelatedtofaultpropagation.Ingeneral,resultsfromthisstudyindicatethattheadditionofnewstructuralelementstothefaultisafunctionofoffsetforbothdeformationbandfaultsandjoint-basedfaults(Fig.10).However,onaplotofoffsetversusstructuredensity,theslopeisdifferentforeachmechanism.Thisimpliesthataclearrelation-shipbetweenfaultzonecharacteristicslikefaultrockthicknessordamagezonewidthmaynotexistifmorethanonemechanismcontributestofaultdevelopment.Inaddition,faultswiththesameoffsetorlengthwillhaveverydifferentfaultarchitectureiftheyformbydifferentdeformationmechanisms.4.3.PrimarycontrolsondeformationmechanismsThedeformationbandingmechanismiscontrolledbymaterialpropertiesatthegrainscaleincludingporosity,sorting,andcomposition(Dunnetal.,1973;Antonellinietal.,1994)andthestateofstress(Wongetal.,1997).Incontrast,shearingofplanardisconti-nuitiesiscontrolledbythepresenceofmacroscopicflaws(e.g.,joints,duneboundaries,pre-existingfaultN.C.Davatzesetal./Tectonophysics363(2003)1–18 surfaces)inafavorableorientationtolocalizeshearstress(BraceandBombolakis,1963;SegallandPollard,1983;Cooke,1997).Theformationofdefor-mationbandsduringearlyphasesoffaultingestab-lishesananisotropythatisexploitedbysubsequentshearingalongjoints(Cruikshanketal.,1991;Anto-nelliniandAydin,1995).ThismayexplainwhyjointsclosetoENE-strikingfaultsareparalleltothefaultratherthantheWNW-strikingset.Anotherpossibilityisthatthestressfieldperturbedbythenearbyslippingfaultcontrolledtheorientationofjoints(Peacock,2001;BourneandWillemse,2001)Jointsoccupyaprincipalplanenormaltotheleastcompressivestressduringfracturing(PollardandAydin,1988).Inorderforjointstoslip,shearstressesmustresolveonthejointplaneandtheirmagnitudemustexceedthefrictionalstrengthofthejointsurfa-ces.Shearstressesonthefractureplanecanresultfromeitherstressormaterialrotation.Dyer(1983)Cruikshanketal.(1991)recognizedacomplexstresshistorycharacterizedbyrotationofthestressfieldfollowingjointformationleadingtoshearingofthejoints.Incontrast,PengandJohnson(1972)advancedaconceptualmodelforfailureofagranitespecimeninatriaxialdeformationexperimentongranite.Failurebeginswithformationofcloselyspacedenechelonjoints.Thesejointsdefineindivid-ualrock‘‘beams’’thatsubsequentlyrotateandfailbyslippingandformingconnectingsplayjoints.Thismodeldoesnotrequirerotationofthefar-fieldstress.Finally,thepresenceofsomedeformationbandsandtheobservationofjointsalongdeformationbandsassociatedwithWNW-strikingfaultssuggestthatthedeformationbandsmayhavecontributedtotheformationofsomeearlyjointsslightlyoutoflinewithrespecttotheregionalstressfield(Cruikshanketal.,1991;AntonelliniandAydin,1995)Itisunlikelythatsinglejointsspanthemorethan140mthicknessoftheNavajosandstone.Jointsofvaryingheightcontrolledbycross-bedding,dune-boundingsurfaces,andotherstructuresaremostlikelydistributedthroughoutthethickness.Asaresult,themannerinwhichjointsareshearedandlinkedbysplayjointsultimatelycontrolsthedipdirectionoftheresultingfaultanalogoustorockmechanicsexperi-(PengandJohnson,1972).Similarly,theoppor-tunitytoformanextensivenetworkofshearedjointsmaybetheprimarycontrolonthespacingofWNWfaults.Thisprocessalsoleadstotheelevatedjointdensitynearfaultsassplayjointsareformedaccom-panyingslip.Thelocallyincreasedjointdensity,fragmentation,breccia,andfaultrockareradicallydifferentthanthoseofENEfaultscharacterizedbydeformationbands.Formationofquartzovergrowthsonsandgrainsiscommonlyobservedattemperaturesgreaterthan90to(BjørlykkeandEgeberg,1993)inquartzrichsandstonessuchastheNavajoFormation.Iftemper-atureiscontrolledbyburial,anormalgeothermalgradientof30C/kmrequires3kmofoverburdentoachievewell-developedquartzovergrowth.BecausedeformationbandformationoverlapswithsilicacementationthisempiricalrelationshipsuggeststhatdeformationbandsdevelopedatornearthemaximumburialoftheNavajosandstoneachievedduringtheLaramideOrogeny(Fig.3)Thetransitionbetweendeformationmechanismsmaybeinitiatedbyachangeinrheologyordeforma-tionenvironmentorboth.Achangeinrheologymayaccompanycementation(DvorkinandNur,1996).Anincreaseincementandlossofporespacecouldrenderthedeformationbandingmechanismunfavorable(AydinandJohnson,1978;Antonellinietal.,1994)Hence,continuedsilicaprecipitationmayhavebeenakeycauseofthechangeinfaultingmechanism.Inaddition,deformationbandsgenerallyformundercompressivedifferentialstresses(Wongetal.,1997;Mairetal.,2000)whereasjointsrequireeffectivetensilestresstoform.Achangeinthedeformationenvironment,suchasthestateofstressorporefluidpressure,couldcauseatransitionfromdeformationbandingtojointing.Theinferencethatdeformationbandingdevelopsatconditionsclosetothemaximumburialdepthmightalsosuggestthatthetransitiontojointformationcoincidedwiththeonsetofexhuma-4.4.ImplicationsforfaultsandfluidflowChangingfaultarchitecturebetweendeformationphaseshasapotentialimpactontheabilityofafaulttoactasafluidconduitorabarrier.Theeffectsofafaultonsubsurfacefluidflowresultsfromthetypeofstructurescomposingthefault,theirdistribution,andconnectivity(Caineetal.,1996;Knipeetal.,1998;Myers,1999;Tayloretal.,1999;Aydin,2000).HowN.C.Davatzesetal./Tectonophysics363(2003)1–18 andwhatelementsareaddedduringfaultdevelop-mentisafunctionofdeformationmechanismandwillcontrolthepermeabilitybehaviorofthefaultthroughtime.Jointsandslightlyshearedjointsarepotentialfluidconduitsenhancingfluidflowparalleltothe(Tayloretal.,1999).Alongfaultsformedbyshearingofjointsandtheattendantproducts(theWNW-strikingfaultsinthisstudy),faultparallelpermeabilityisgreatlyincreasedwhereasfaultnormalfluidflowisreducedafterafewmetersofslipascontinuityoffaultrockdevelops(Flodinetal.,2001;Jourdeetal.,2002).Deformationbandsinhibitfaultnormalfluidflowduetotheporosityreductioninthe(Antonellinietal.,1994;Matthaietal.,1998)Furthermore,eachmechanismisdistinguishedbythenumberofstructuresaccruedwithoffset.Adensenetworkofdeformationbandswillformwithonlysmallslip,whereasjointseasilycutandbreachdeformationbands.Theoverprintingofthesetwodeformationproductswillresultinachangeofper-meabilitybehaviorovertimefromprimarilysealingtolargelyconduit.5.ConclusionsWehaveidentifiedthestructuralcomponentsofmultiplefaultsetsinChimneyRock,SanRafaelSwell,Utah,USA.Thestructuralcomponentsareorganizedintotwodistinctfaultarchitecturescharac-teristicoffaultsformedby(1)deformationbandsand(2)byshearingofjointsandassociatedsplayjointing.Thedistributionofstructuresproducedbythesedeformationmechanisms,theirrelativeintensity,andtheirtemporalevolutionallowustopartitionthedeformationintotwophasesoffaultingthatcouldnotbedistinguishedbasedonthemap-scalefaultpatternalone.TheearlierphaseinvolvesdeformationbandfaultscomprisingtwooppositelydippingsetsstrikingENE–WSWandminordevelopmentoffaultsstrikingWNW–ESE.Thelaterphaseofjoint-basedfaultingdominatesfaultdevelopmentalongWNW–ESEfaultsandmakesaminorcontributiontoENE–WSWfaults.Insummary,whatappearstobenearlyhomogeneousthree-dimensional,fault-relatedstrainfieldwasactuallyproducedbytwophasesoffaultingreflectedbyachangeindeformationmechanism.Thejuxtapositionofbothdeformationmechanismsalongtwowell-definedpairsoffaultsetswithcommonstrikedirectionsuggeststhatfaultsofdifferentarchi-tecturecandevelopwithinasinglerocktypeatthesamelocality.Wehavedemonstratedthatfaultswiththesamegeometriccharacteristicsoflengthandoffsetinasinglerocktypeandlocalitycanhaveradicallydifferentfaultarchitecture.Theearlierfaultscom-posedofdeformationbandswereprobablysealingforfluids.Incontrast,laterfaultingprobablyestab-lishedhigherpermeabilitypathwaysbecauseofjointdevelopmentandjointsintersectingdeformationbands,therebynegatingthesealingeffectoftheolderfaults.AcknowledgementsThisworkgreatlybenefitedfromsupportbyseveralindividuals.LaurentMaertensharedmaps,data,andadviceproducedduringhisownworkintheChimneyRockfaultarray.BobKrantz,MaryBethGray,andPhilResorcriticallyreviewedthemanu-scriptduringitsdevelopmentandprovidedusefulandcandidadvice.NickDavatzeswouldalsoliketoacknowledgethehelpfulenvironmentandfinancialsupportprovidedbytheRockFractureProjectthatcontributedtothecompletionandqualityofthiswork.WethankTerryEngelderandZoeShiptonfortheirconstructivereviewsthathelpedtoimprovethemanuscript.ReferencesAnderson,E.M.,1951.TheDynamicsofFaultingandDykeFor-mationwithApplicationstoBritain.Oliver&Boyd,Edinburgh.206pp.Antonellini,M.,Aydin,A.,1995.Effectoffaultingonfluidflowinporoussandstones;geometryandspatialdistribution.Am.As-soc.Pet.Geol.Bull.79,642–671.Antonellini,M.A.,Aydin,A.,Pollard,D.D.,1994.MicrostructureofdeformationbandsinporoussandstonesatArchesNationalPark,Utah.J.Struct.Geol.16,941–959.Aydin,A.,2000.Fractures,faults,andhydrocarbonentrapment,migrationandflow.Mar.Pet.Geol.17,797–814.Aydin,A.,Johnson,A.M.,1978.Developmentoffaultsaszonesofdeformationbandsandasslipsurfacesinsandstone.PureAppl.Geophys.116,931–942.Aydin,A.,Reches,Z.,1982.Numberandorientationoffaultsetsinthefieldandinexperiments.Geology10,107–112.N.C.Davatzesetal./Tectonophysics363(2003)1–18 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Overprintingfaultingmechanismsduringthedevelopmentofmultiplefaultsetsinsandstone,ChimneyRockfaultarray,Utah,USANicholasC.Davatzes,AtillaAydin,PeterEichhublDepartmentofGeologicalandEnvironmentalSciences,StanfordUniversity,Stanford,CA94305-2115,USAReceived6March2002;accepted1November2002ThedeformationmechanismsproducingtheChimneyRocknormalfaultarray(SanRafaelSwell,Utah,USA)areidentifiedfromdetailedanalysesofthestructuralcomponentsofthefaultsandtheirarchitecture.FaultsinthisareaoccurinfoursetswithoppositelydippingfaultpairsstrikingENEandWNW.TheENE-strikingfaultsinitiallydevelopedbyformationofdeformationbandsandassociatedslipsurfaces(deformationmechanism1).Afterdeformationbandformationceased,threesetsofregionaljointsdeveloped.Theoldesttwosetsoftheregionaljoints,includingthemostprominentWNW-strikingset,weresheared.LocalizeddeformationduetoshearingoftheWNW-strikingregionaljointsformedWNW-strikingmap-scalenormalfaults.Theformationmechanismofthesefaultscanbecharacterizedbytheshearingofjointsthatproducessplayjoints,breccia,andeventuallyacoreoffaultrock(deformationmechanism2).Duringthissecondphaseoffaulting,theENE-strikingfaultswerereactivatedbyshearacrosstheslipsurfacesandshearingofENE-strikingjoints,producinglocalizedsplayjointsandbreccia(similartodeformationmechanism2)superimposedontoadensezoneofdeformationbandsfromthefirstphase.Wefoundthatnewstructuralcomponentsareaddedtoafaultzoneasafunctionofincreasingoffsetforbothdeformationmechanisms.Conversely,weestimatedthemagnitudeofslippartitionedbythetwomechanismsusingthefaultarchitectureandthecomponentstructures.Ouranalysesdemonstratethatfaultsinasinglerocktypeandlocation,withsimilarlengthandoffset,butformingatdifferenttimesandunderdifferentloadingconditions,canhavefundamentallydifferentfaultarchitecture.Theimpactbyeachmechanismonpetrophysicalpropertiesofthefaultisdifferent.Deformationmechanism1producesdeformationsbandsthatcanactasfluidbaffles,whereasdeformationmechanism2resultsinnetworksofjointsandbrecciathatcanactaspreferredfluidconduits.Consequently,adetailedanalysisoffaultarchitectureisessentialforestablishinganaccuratetectonichistory,deformationpath,andhydraulicpropertiesofafaultedterrain.2002ElsevierScienceB.V.Allrightsreserved.Keywords:Deformationband;Joint;Shearedjoint;Faultingmechanism;Faultarchitecture1.IntroductionTwodistinctmechanismsoffaultformationinsandstonehavebeendescribed:(1)faultsformedbydeformationbandsand(2)faultsformedbyformation0040-1951/02/$-seefrontmatter2002ElsevierScienceB.V.Allrightsreserved.doi:10.1016/S0040-1951(02)00647-9 *Correspondingauthor.E-mailaddresses:davatzes@pangea.stanford.edu(N.C.Davatzes),aydin@pangea.stanford.edu(A.Aydin),eichhubl@pangea.stanford.edu(P.Eichhubl).www.elsevier.com/locate/tectoTectonophysics363(2003)1–18