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JOURNAL OF GEOPHYSICAL Lower crustal extension RESEARCH, VOL. 103, NO. B3, PAGES 4975-4991, MARCH across the Northern 10, 1998 Carnarvon basin, Australia: Evidence for an eastward dipping detachment Neal W. Driscoll Departmentof Geologyand Geophysics, WoodsHole OceanographicInstitution,WoodsHole, Massachusetts Garry D. Karner Lamont-DohertyEarth Observatoryof ColumbiaUniversity,Palisades,New York Abstract. Asymmetryand strainpartitioningalongconjugatemarginsare often explained in terms of detachmentfaults. Nevertheless,cogentevidencefor their existenceremains limited. Furthermore,evenwhen inferred detachmentfaults are imagedseismically,it is difficultto demonstratethat differentialdisplacementhas occurredacrossthesesurfaces. We presenttectonicand stratigraphicevidencefrom the Northern Carnarvonbasin, northwestAustralia,that documentsthe existenceof an intracrustaleastwarddipping detachment.By integratingsequencestratigraphywith kinematicand isostaticmodelsof basindevelopment,we concludethat the Northern Carnarvonbasinwas formed as a consequence of four extensionevents:(1) a broadlydistributedlate Permianevent,(2) a predominantlylocalizedRhaetian event responsiblefor the inceptionof the Barrow and Dampier subbasins, (3) a localizedCallovianfault reactivationwithin the Barrow-Dampier subbasins, and (4) a Tithonian-Valanginianeventthat generatedlarge post-Valanginian regionalsubsidence acrossthe Northern Carnarvonbasinwith only minor accompanying brittle deformationand erosionaltruncation.The regionaldistributionand amplitudeof the post-Valanginiansubsidenceare not consistentwith the minor amountsof TithonianValanginianbrittle upper crustalextensionobservedon the margin.Large portionsof the platformwere emergentor at very shallowwater depthsprior to the TithonianValanginianextension.To matchthe distributionand magnitudeof the post-Valanginian "thermal-type"subsidence requiressignificantlower crustaland mantle extensionacross the Northern Carnarvonbasin.Sucha distributionof extensionimpliesthe existenceof an eastwarddipping,intracrustaldetachmenthavinga ramp-flat-rampgeometrythat effectivelythinnedthe lower crustand lithosphericmantle.The detachmentbreachedthe surfacecloseto the positionof the continent-ocean boundary,west of the Exmouth Plateau.The flat componentof the detachmentoccurredat midcrustaldepths(---15km) acrossthe plateauand rampedbeneaththe Australiancontinent.Lower crustalductile extensionprovidesa viable mechanismto generatelarge regionalsubsidence with little attendantbrittle deformation,which may explainthe paradoxthat both sidesof many conjugatemarginsappearto be the "upperplate." 1. Introduction Many conceptualand theoreticalmodelsfor extensionaldeformation of the lithosphereinvoke shallowdipping crustal shearzonesand/or low-anglenormal detachmentsto explain the architectureand subsidencepatternsobservedalong conjugatecontinentalmarginsand basins[RoydenandKeen,1980; Tankard and Welsink,1987;Lister and Davis, 1989;Mutter et al., 1989; Boillot and Winterer, 1988; Weisseland Karner, 1989; Lister et al., 1991; Abers, 1991; Wernicke, 1992; Boillot et al., 1995;Driscolland Karner,1995;Driscollet al., 1995].The existenceof low-anglenormal detachments,however,remains controversial[Jackson,1987;Andersand Christie-Blick,1994]. Besidesthe theoreticalconsiderations,the controversyover low-angledetachments is compoundedby the fact that there is a paucityof criticalobservations. We presentkey tectonicand stratigraphicevidencefrom the Northern Carnarvonbasinthat atteststo the existenceof a low-angleeastwarddippingdetachment. The Northern Carnarvonbasin encompasses the Barrow, Dampier, and Exmouth subbasinsas well as the Peedamullahshelf,Alpha Arch, and the ExmouthPlateau (Figure 1 [Copp, 1994]). Previousseismicreflection and refraction studiesin the regionproposedthat a detachmentfault exists acrossthe Exmouth Plateau at the base of the sedimentary succession that dips toward the west and has a "duplex"geometry [Mutteret al., 1989].However,the interpretationof an intrasedimentarydetachmentis not consistentnor supported by the stratalrelationships,subsidence patterns,or the spatial and temporaldistributionof the deformationobservedacross the Northern Copyright1998 by the American GeophysicalUnion. Carnarvon basin. Paper number 97JB03295. The northwesternAustralianregionconsistsof severalgeologicterranes:Archeanand earlyProterozoiccratons,Protero- 0148-0227/98/97JB-03295509.00 zoic basins, Paleozoic basins, and a series of Mesozoic and 4975 4976 DRISCOLL AND 112 ø .18 ø KARNER: LOWER 114 ø CRUSTAL EXTENSION 116 ø 118 ø _18 ø rust. Abyssal Plain Northern Carnarvon Basin Tectonic Alpha Elements Arch .20 ø .20 ø Cuvier .22 ø Australia Abyssal .22 ø Plain 112 ø 114 ø 116 ø 118 ø Figure 1. Locationmap of the northwesternAustraliancontinentalmargin illustratingthe spatialrelationship among the Gascoyneand Cuvier Abyssal Plains and the northern Carnarvon basin. The Barrow, Exmouth,and Dampier subbasins haveno bathymetricexpressionand are locatedbeneaththe present-day continentalshelf. The Australian GeologicalSurveyOrganization(AGSO) 101 and 110, GECO-Prakla GPCT93, and Digicon 93BA seriesseismicreflectiondata as well as industryand Ocean Drilling Program (ODP) wellsusedin thisstudyare shown.LinesAGSO 110-12and 93BA-38shownin Plate 1 and the depth profile shownin Figure 5 are denotedby heavylines and are labeled accordingly. Cenozoicbasins.The onsetof seafloorspreadingpropagated from north to southalongthe northwestAustralianmargin;in the Argo basin,seafloorspreadingbeganduringCalloviantime (marine magneticanomalyM25), whereasin the Gascoyne, Cuvier, and Perth basins,seafloor spreadingbegan in Valanginiantime (anomalyM10 [Exonet al., 1992;BooteandKirk, 1989]). The ExmouthPlateau, a thinned and subsidedcontinental blockwith a crustalthicknessof -20 km, formspart of the Northern Carnarvonbasin (Figure 1). Previousseismic reflection studies across the Exmouth of an eastwarddippingdetachmenton margin development. Toward this goal, we apply an integratedapproachthat combines sequencestratigraphyand kinematicbasin modelingto determine (1) the amount of spaceavailablefor sediment accumulationacrossand alongthe Northern Carnarvonbasin asa functionof time, (2) the subsidence historyof the margin, (3) the flexural responseof the lithosphereto loading and unloading,and (4) the historyof rift flank erosion. Plateau indicate that the preservedsedimentarysuccession consistsof -5000 m of Paleozoic rocks,an averageof -3000 m of Triassicsediments, and --•1000m of youngerCretaceousand Cenozoicsediments [WillcoxandExon, 1976].The purposeof thispaperis to define the tectonic significanceof the sedimentarypackagespre- 2. Methodology Our integratedapproachto basinanalysisincorporatesboth sequencestratigraphyand kinematicbasinmodelingto examine the first-orderprocesses responsible for both the deformation of the lithosphereand the transportand depositionof served on the Northern Carnarvon basin in terms of the exsedimentswithin the evolvingbasin. Iterating between the tensionaleventsresponsiblefor the developmentof the mar- modeledtime line stratigraphyand the observedstratal patgin. In particular,we will focuson the Tithonian-Valanginian ternsallowsus to establisha quantitativerelationshipbetween extensionaleventto demonstratethe existenceand importance the tectonicdeformationof the lithosphereand the formation DRISCOLL AND KARNER: LOWER CRUSTAL EXTENSION of stratigraphicsequencesand their boundingunconformities. A. Initial Conditions Consequently, this combinationprovidesa very powerfultool for predictingthe structuraland stratigraphicdevelopmentof sedimentarybasinsand continentalmarginsin responseto compressionaland extensionalforces. Sequencestratigraphy is the studyof sedimentsand sedimentary rocksin termsof repetitivelyarrangedfaciesand associated Lower plate stratalgeometryand makesuseof the fact that sedimentary suc(footwallblock) cessions arepervadedbyphysical discontinuities [Sloss, 1988;Vail, ,Asthenos 1987; Van Wagoneret al., 1990; Christie-Blick,1991]. These physicaldiscontinuities, whichoccurat a greatrangeof scales, are generatedby a number of different processes[Christie- B. Kinematic Extension Blick and Driscoll, 1995]. Common to virtually all of these o E • I•-discontinuities, independentof scaleand origin,is that to a first approximationthey separate older depositsfrom younger ones. The recognition of these discontinuitiesin the stratigraphic record is important becausethey allow the stratigraphic succession to be divided into geometricalunits that have time-stratigraphicand geneticsignificance. Defining the stratal architecture,faciesdistributionand their spatial and temporalvariabilityacrosscontinentalmarginsis a necessary ', :" • l// first step for determiningwhere and when accommodationis createdor destroyedalongand acrossan evolvingmargin.The sequencestratigraphicapproachdoesnot actuallyrequire any C. IsostaticResponse assumptions abouteustasy[Christie-Blick andDriscoll,1995];it is simplya tool to determine,layer by layer,how sedimentary o E 4977 k••imple Shear Upper plate (hangingwall block) Lithosphere • Pure Shear L I Lithospher; • iii::i!11i • . L I 0 successionsare formed, from their smallest elements to the largest. Most importantly, we have establishedcriteria that allowus to identifyunconformities in the stratigraphicrecord that havea tectoniccomponent[Driscollet al., 1995]. The preservedstratal geometryand associatedfaciesdistributionin activelydeformingregionscanbe usedto distinguish betweentimes of renewed tectonicactivity and eustaticsea levelchanges alone[DriscollandHogg,1995].For example,the formationof unconformitiesin tectonicallyactivebasins,such as half graben and foreland basins,recordssubsidencein one portionof the basinthat may be accompaniedby contemporaneoustectonicuplift elsewhere[Christie-Blick and Driscoll, 1995;Driscollet al., 1995].As a result,unconformities canpass laterallyfrom localsubaerialunconformities into marineonlap surfaces.Deformationaleventsfollowedby periodsof tectonic quiescence are recordedin the stratigraphicrecordby marine onlappingpackagesthat exhibit an overall shoalingupward trend as sediment infills the available space. However, the occurrenceof tectonicdeformationdoesnot precludethe existence of contemporaneouseustatic sea level fluctuations. Consequently, sequencestratigraphicanalysisallowsus to ascertainthe link betweentectonicprocesses and the preserved stratigraphy.The Australian GeologicalSurveyOrganization (AGSO) 101 and 110, GECO-Prakla GPCT93, and Digicon 93BA seriesseismicreflection data as well as industryand OceanDrillingProgram(ODP) wellswereusedin thisstudyto determine the tectonic and stratigraphicevolution of the NorthernCarnarvonbasin(Figure1). In recent years, there have been considerableadvancesin our understandingof lithosphericrifting and passivemargin development[Braunand Beaumont,1989; Weisseland Karner, 1989;Karneret al., 1998].Theseconceptualmodels,following McKenzie[1978],alsohelp to definethe relationshipbetween basinsubsidence and heat input inducedby lithosphericthinning. The recognitionof low-anglenormal faults by surface mapping has modified our conceptionof how extensionis accommodatedwithin the crust. The brittle collapseof the t• Molho- Lithospher :e ,,A?,he, n?s,p,he, r,e . Figure 2. (a) Schematicrepresentation of lithosphericextensionby simpleshearin the upper crustand balancingplastic (ductile) deformationin the lower crust/lithospheric mantle used to model the developmentof rift basinsand flanking topographyis shown.Slip along a border fault producesa topographicdepressionwhich is controlledby the heave E acrossthe fault and the shapeof the fault (here drawnlistric solingintothe baseof the crust).In thisexample,the balancing zone of lithosphericmantle extensionis spatiallyoffset and broader than the zone of crustalextension.The degree of lithosphericmantle extensioncriticallycontrolsthe distribution and amountof heat addedto the lithosphereduringrifting.(b) The extension factor8(x) canalsobe parameterized in terms of the amount of upper plate brittle deformationwhen the fault soles into a lower crustal weak zone with the lower crust and lithosphericmantle ductile deformationbeing describedby/3(x). (c) The flexuraladjustmentof the lithosphere to crustalunloadingand the input of heat to the baseof the lithosphereis shown.The resultantshapeof the rift basinis givenby the summationof the kinematicdepressionand the total flexuralreboundof the lithosphere. uppercrustin responseto extensionresultsin thinningof the crust that producesspatially discreterift basins.Within the lower crust and lithosphericmantle, extensionis accommodated by plastic(ductile) deformation.Referringto Figure 2, we candescribethe responseof the lithosphereto extensionby consideringthe distributionand magnitudeof the variouski- 4978 DRISCOLL AND KARNER: LOWER nematicallyproducedloads [e.g., Weisseland Karner, 1989]. Startingwith a prerift lithosphereof thicknessa and crustal thicknesstc, slip along the border fault producesa surficial hole by collapseof the hangingwall block (Figure 2). The heaveacrossthe fault,or equivalently the amountof extension, is E. After extension,the crustaland lithosphericthicknesses are t'canda', respectively. Thinningof the lithospheremantle inducesa thermal load representedby the passiverise of the lithosphere-asthenosphere boundary(Figure 2). The passive rise of thisboundaryintroducesheat to the baseof the thinned lithosphere.For convenience,we parameterizethe degreeof lithosphericthinningin termsof •5(x), the ratio of the prerift/ postriftcrustalthickness,and/3(x) the ratio of the changein lithospheremantle thickness.Note that while the cumulative extensionrepresentedby •5(x) and /3(x) must be balanced, their spatialdistributionsneednot be relatedin a "one-to-one" fashion(the "offset"variablein Figure2). The shapeof the fault and where the fault solesare constrainedby the seismic reflection and refraction data. In Figure 2 we are explicitlyassumingthat the Moho is actingas a horizontaldetachment.When this is not the case and the border fault soles into a weak zone within the lower crust,•5(x) isthe extension factorfor the upperplateand/3(x) is the extensionfactor for the lower plate. In sucha scenario, /3(x) describes the extensionwithin the lithosphericcrustand mantle.Thesegeneraldefinitions of/5(x) and/3(x) allowusto explorethe isostaticeffectsof intracrustaldetachments. In this paper, we use the followingdefinitionfor detachment:a diffusezone that separates(detaches)the brittle deformationin the uppercrustfrom the ductiledeformationin the lowercrust and lithosphericmantle (e.g., strainpartitioning[Driscolland Karner,1995]).We are not adoptingthe more restrictivedefinition of detachmentsproposedby Wernicke[1981] and subsequentlymodifiedandrefinedbyListerandDavies[1989]that CRUSTAL EXTENSION tation will tend to amplify any basin subsidencebecauseit representsa positiveload being addedto the lithosphere. The computercodeusedto investigatemarginevolutionis a two-dimensional (2-D) kinematicforwardmodelingprogram whichtracksthe thermaland mechanicalresponseof the lithosphereboth during and after rifting and thusthe historyof rift andpostriftbasinsubsidence. Previousbasinmodelshavebeen limited to predictingthe time line stratigraphyof the postrift sectionacrosspassivecontinentalmargins[Roydenand Keen, 1980; Wattset al., 1982;Kuszniret al., 1987]. This restriction was imposed because of the original assumption in the McKenzie[1978] lithosphericextensionmodel in which rifting is treatedas an instantaneous event(i.e., a thermallyadiabatic process).Numerousfinite riftingmodelshavealsobeendeveloped to examinethe structuraland thermal evolutionof rift basinsduring extensionaldeformation [e.g., Cochran, 1983; Buck, 1988;Wanget al., 1989].Nevertheless, in theseapplications the synrift stratigraphicarchitecturewas neither predictedby or usedas an input to the models.Our finite rift and rerift modelingprocedureallowsus to predictthe architecture and stratigraphicdevelopmentof rift basinsas a function of spaceand time duringextension.It wasnecessary to investigate thisaspectbecauseof the needto modelthe stratigraphyof the synriftformationsand the thermalimplicationsassociated with multiple episodesof extensionaldeformationand migrationof the deformationthrough time acrossand along the northwest Australianmargin.Relevantinput parametersto the kinematic and isostatic summarized model in Table for the Northern 1 and modeled Carnarvon basin are time lines and their tec- whichpassthroughthe entire crustand/orlithosphereand are recordedby "myloniticdetachmentterranes"(or metamorphic core complexes). The kinematicallycreated loadsrepresentedby the crustal "hole" and the input of heat to the base of the lithosphere (Figure 2) are readjustedby isostasy.For example,during rifting, the partial removalof the hangingwall block unloads the lithosphere,inducing an isostaticrebound. If flexural strengthis maintainedduringextension,then this reboundwill be regionallydistributedwith respectto the kinematicallyproducedloadsresultingin the formationof rift flanks.The same principlesapplyfor transtensional systems (i.e.,wrenchtectonics)with the kinematicallycreatedloadsrecordingthe amount tonicsignificance are summarizedin Table 2. Initial crustaland lithosphericthicknesses were assumedto be 34 and 125 km, respectively.Thesevalueswere selectedso that prior to extensionthe topographywasisostatically balancedabovesealevel. While there is no direct evidenceto supportthis assumption, the fluvialprerift sedimentshelp to definea minimumcrustal thickness.Furthermore, low-relief topographyand a lithospheric thicknessapproachingequilibrium(i.e., 125 km) are consistent with the largetime spansincethe lastorogenicevent to affect the region (340 Ma, CarboniferousAlice Springs orogeny[Warris,1993]).Porositycharacteristics are input as a function of position acrossthe margin, which are based on industryand ODP well log data (Figure 1). The processof erosionis alsotreated as an exponentialfunctionthat defines the destructionof topographyin terms of a "half-life." Subaerial erosionis regardedas a functionof height(i.e., highest topographyis eroded quickest).The assumptions dictating subaerialerosion serve only to approximateprocess.We are concernedwith the longevityof the rift flank topographyrather of extension than the details of how it is eroded. defines detachments as narrow zones of relative movement across the fault associated with the transtensional deformation.Unless the strike-slipcomponentof the deformation causesa changein lithosphericthickness,then there is no additionalkinematicallycreated load with this component 3. of the deformation. The combination of the crustal hole with Tectonic and Stratigraphic Evolution In this section,we presentstratigraphicevidencedocumentthe isostaticrebounddefinesthe generalarchitecture of the rift ing the timing and spatial distributionof where and when was createdor destroyedacrossthe Northern basin(Figure2). Followingextension,the heat introducedto accommodation the baseof the lithosphereduring rifting dissipates,allowing Carnarvonbasin.These observations,taken togetherwith the the lithosphereto cool, contract,and subside.Thus the rapid resultsfrom the kinematic and isostaticbasinmodeling,allow of the preservedstrasubsidencethat characterizesthe rift phase is replacedby a us to determinethe tectonicsignificance slower form of subsidence associated with the conductive cooltigraphy acrossand along the basin. Seismiclines AGSO ing of the rifted lithosphere.Consequently, basinformationin 110-12andDigicon93BA-93are an east-westprofileacrossthe an extensionalenvironmentrequirescrustalthinning,eitherby PeedamullahShelf, Barrow subbasin,Alpha Arch, Exmouth extensionor erosion.Lithosphericmantle thinningalone is not subbasin,and ExmouthPlateau (Figure 1 and Plate 1). The sufficientto generatea basin.Finally,rift and postriftsedimen- Exmouthsubbasinis delineatedby the Alpha Arch to the east DRISCOLL AND KARNER: LOWER CRUSTAL EXTENSION 4979 Table 1. Modeling Parametersfor Northern CarnarvonBasin Parameter Description Value X• Z horizontal t time sincerifting/reriftingevent finite rifting interval upper plate extensionfactor lower plate thinningfactor upperplate compression (inversion)factor lowerplate compression (inversion)factor lithospherictemperaturestructure asthenospheretemperature thermal expansioncoefficient thermal diffusivity specificheat backgroundheat flow prerift crustalthickness postriff crustalthickness intracrustaldetachmentgeometry asymptoticlimit of crustaldetachment equilibriumlithosphericthickness postriff lithosphericthickness densityof asthenosphere at 0øC densityof asthenosphere at Tm densityof crustat OøC sedimentdensity sedimentgrain density bulk sedimentporosity surfaceporosity porositydecayconstant lithosphericrigidity ETe(x, t)/12(1 - v2) effective elastic thickness Poisson's ratio 0.25 Atrift a(•) t3(x) a•(x) t3,(x) r(x, z, t) Tm qo tc tc ! ta(x) ! tc ! Pa Pc p•(z) P• ,(z) •o •(x, t) re(X, t) E and vertical coordinates h(x, t) ah(x, t) Young'smodulus gravitationalacceleration controllingisothermfor Te topographicrelief topographicload removedby erosion k• • erosional time constant Ze 5-20 Myr 1333øC 3.28 x 10-s K -1 8 x 10-7 m2 s-i 1.05J 42 mW m -2 34 km 125 km 3330kg m-3 3179kg m-3 2800kg m-3 2650kg m-3 • oe- kp z 60% 2.5 km 6.5 x 10lø Pa 9.82 m s-2 450øC crust 82 Myr 10-40 Myr sediment maximum ASz• first-order eustatic sea level variation 200 m Table 2. Modeled Time Lines and Their TectonicSignificance Time Line Geological Age, Ma Time SinceBasin Initiation, Myr 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 253 233 228 215 210 209 200 194 188 181 175 169 155 150 144 134 125 91 0 20 25 38 43 44 53 59 65 72 78 84 98 103 109 119 128 162 19 2O 21 86 15 0 167 238 253 GeologicalFormation* Locker Shale Process rifling 1 Mungaroo sands lower Dingo clay stone (Athol Formation) postriff 1 rifling 2 upper Dingo clay stone rifling 3 Dupuy sands Barrow Group postrift 3 rifling 4 Muderongshale upper Gearle siltstone upper Gearle siltstone postriff 4 Trealla limestone (Cape Range Group) *Time linesdefinethe approximatebaseof the geologicalformations. inversion postriff 4 postriff 4 4980 DRISCOLL AND KARNER: LOWER CRUSTAL EXTENSION ul E • I I i c I I I ! I I I i i ! i • i (tuq) qldacI (tu) tlldac! DRISCOLL AND KARNER: LOWER CRUSTAL EXTENSION and the Exmouth Plateau 4981 to the west. The northern extent of the Exmouth subbasinoverlapswith the southernportion of the Barrow subbasin, and the termination of these basins is controlledby major accommodationzones.The southernBarrow subbasinis boundedto the eastby the PeedamullahShelf and to the westby the Alpha Arch and its southernequivalents (Figure 1). 3.1. Late Permian Extension The basal member of the Triassic, the Locker Shale, is a transgressive marine depositrestingunconformablyon Paleozoic sediments[Kopsenand McGann, 1985] (Figure 3). The Locker shale passesupward into the Mungaroo formation, whichis a deltaicsystemthat gradesverticallyinto a thick unit of stackedfluvial channeldeposits[Barber,1982;Exon et al., 1992] (Figure 3). The overallthicknessof the Triassicsection systematically increasestoward the west with the depocenter occurringalong the westernedge of the Northern Carnarvon basin [Cockbain,1989]. Minor lateral thicknessvariationsof the Locker and Mungarooformationsare observedacrossthe Rankin Platform and the PeedamullahShelf (Plate 1). The Mungarooformationis cappedby a thin transgressive successionof nearshoreand shelffine-grainedclasticsand carbonates of Hettangian-Sinemurianage. This marine transgression occurredin a generalsoutheasterly direction.The greatthickness of fluvio-deltaicdeposits(-3000 m) indicatesthat basinsubsidenceacrossthe region was occurringduring most of the middle 3.2. to late Triassic. Model Predictions for Late Permian Extension Prior to examiningthe tectonic and stratigraphicdevelopment for any giventime interval,it is necessary first to model the entire stratigraphicsuccession of the Northern Carnarvon basinbecausethe preservedstratigraphyrecordsthe total history of margindevelopment.By forwardmodelingthe thickness, stratal geometry, and paleowater depths acrossthe NorthernCarnarvonbasin(Plates1 and2), we are then ableto use the model predictionsto examinethe tectonicand stratigraphicdevelopmentof the marginfor anygiventime interval (Figure4). Note the closecorrelationbetweenthe depthsections acrossthe Barrow and Exmouth subbasin,the Alpha Arch, and the ExmouthPlateau and the modeleddevelopment of the region(Plate 1). The distributionand thicknessof the synriftTriassicLocker Shaleand the postriftMungarooformationacrossthe margin are a directconsequence of the distributionand magnitudeof late Permian rifting (Figure 4). To model the thicknessand distributionof the Locker formation requiresbroadlydistributed,depth-independent extension(i.e., $(x) = /3(x)) across the regionat thistime (Plate 2 andFigure4). A consequence of depth-independent extensionis that accommodation is generated at the time of rifting. Conversely,depth-dependentextensioncan result in negligibleaccommodationduring rifting with large amountsof subsidencein the postrift phase.The Permian extension increased toward the west, with a maximum value of 1.5 (Figure 4). Matchingthe present-daydistribution and thicknessof the Permo-Triassicsequencesacrossthe PeedamullahShelf constrainsthe spatialextentand magnitudeof the late Permian extension.For example, if we expand the lateral distribution of Permian extension farther toward the east, then substantialthicknessesof Permo-Triassic sediments are predictedto occuracrossthe regionof the high-standing PeedamullahShelf,contraryto observations (Plates1 and 2). 4982 DRISCOLL AND KARNER: Carnarvon A. Basin- LOWER CRUSTAL Southern EXTENSION Transect F. kilometers kilometers 240.0 280.O 4(X?.O 440.0 480.0 520.0 I i i l, , 320.0 i i , 360.0 I , i i i , i i I i i 240.0 287.0 320,O 3(•),O 4(X).O 440.0 480.O 520.O O,O-i .2{RX).O • .6(XX).O - IfXXX).O pre-late Triassic/early Jurassic Valanginian G. a. kilometers kilometers 240 0 t..... 280.0 .J I 32(kO ,,i ..... 1,,, 360,0 ! I 4(X),½) ! • 440.0 ! • 480,0 [ t . t . $20,O 240.0 i i z 320.0 I . .l... 360.d) t I 4(X) d') I 4RO.() 440.0 ! I i 520.0 . .t .. 0.()-•,• 0.0 1 ~000.0 .000,01 E -6(XX).O "•8(XX) o-post-late Triassic/early Jurassic C, -,,,,•.o pre-late Cretaceous Ho kilometers 240,0 ! 2R0.0 t 280.0 i 320.0 ......... l ............ 1. z, 36•).0 • • 4(X).O ! z 440,0 ! • 480,0 z . i • kilometers 240.0 520.0 I ...... i 2RO.O J ........ ! 320.0 t i 360.0 & . t ........ 400,0 J.... 440.0 i i I ...... 480.0 I .. .$2o.o i o,o .2000.O-t • 4(xx).o E -6{XX).O-8(XRLO- ~i(XXX),O .m•x.•.o ß pre-Callovian post-late Cretaceous D, kilometers kilometers 240.0 2•R).O 320.0 3,'•).0 41X).O 440.0 480.0 520.0 240.0 280.0 320.0 36i0.0 4(X).O 440.0 $20.0 i ,, i i t I i t, ! • t . .•...... 481}.½) 1.. i i o,o I DOst-Callovian kilometers 240.0 i i .... 280.0 320.0 360.0 i ! ............ !.......... i • 4(F.LO I • 440.0 i t 480.0 ß i 520.0 s i t , Locker Shale upper Dingo Fm Ladinian-Carnian •-• Barrow Group Mungaroo Sands•-• upper &lower Gearle lower Dingo Fm •-• Paleogene Sequences upper Dingo Fm • Neogene Sequences DRISCOLL AND KARNER: LOWER The subsidencehistoryacrossthe Exmouth Plateau also constrainsthe magnitudeof the late Permianextension.Increasing the amount of Permian extensionacrossthe region increases the amplitudeof the thermal subsidence.If we assumetoo large a value for the Permian extension,then significantaccommodationis generatedacrossthe margin during Jurassic time. The observedlocalizedJurassicdepocenterssuggestthat the accommodation wasgeneratedpredominantlyby restricted brittle (i.e., fault induced)deformationratherthanby regional thermal subsidence associated with the late Permian exten- sionalevent (Plate 2). 3.3. Late Triassic Extension CRUSTAL EXTENSION 4983 Dingo formation.Along the structuralhighs,well data corroborate our interpretation(Plates 1 and 2). The late Triassic east in both the to Callovian Exmouth and interval southern thickens Barrow toward the subbasins (Plate 2). The observedthicknessvariationssuggestthat the basin boundingfault for the Exmouth subbasinwas located alongthe westernflank of the Alpha Arch andfor the southern Barrow subbasinthe border fault was locatedalong the western edgeof the PeedamullahShelf (Plate 1). The consequent footwalluplift of the Alpha Arch and PeedamullahShelf duringthe secondphaseof deformation,whichcommenced in late Triassicto earlyJurassictime (Figure 3) affectedthe drainage networks across the shelf and thus limited the access of fluvial Continentalsiliciclastics at the end of the Triassicsupercycle systemsinto the rift basinproper. (e.g.,orogeniccollapse)contrastwith the marineand margin- 3.4. Model Predictions for Late Triassic Extension al-marine siliciclastics of the lower and upper JurassicsuperThe renewed episode of extension in the late Norian/ cycles(i.e., rift phases2 and 3; Figure3). The rapidformation of accommodation recordedby the abrupttransitionfrom pro- Rhaetianaffectedthe northwestAustralianmargin,generating gradationalfluvial-deltaicclasticsof the Mungarooformation an extensivealong-strikenetwork of basins(Figure 1). The to the openmarinesiltsandclaysof the lowerDingo claystone within the Barrow and Dampier subbasinmarksthe onsetof renewedextensionbeginningin Rhaetiantime (Figure3) [KopsenandMcGann, 1985;Veenstra,1985;BooteandKirk, 1989]. The divergenceand rotation of seismicreflectorsabovethe Norian unconformityin the Northern Carnarvon basin are indicativeof differentialsubsidence and block rotation (Plate 3). The depocenter for the lowerandmiddleDingoclaystones is located in the Barrow, Dampier, and Exmouth subbasins (Figure 1) [Booteand Kirk, 1989].We have developeda defendable criterion to define the boundarybetween the Mungaroo formation and the overlyinglower Dingo clay stones within the depocentersby recognizingthat the Locker Shale and Mungaroo formation are regionallydevelopedwith only minor lateral thicknessvariationsand the lower Dingo clay stonesare locallydevelopedwith large lateral thicknessvariations (Plates1 and 2). The first packageof seismicreflectors that diverge and onlap onto the underlying parallel and roughlyconcordantreflectorsrecordsthe boundarybetween the top of the Mungarooformationand the baseof the lower extension waslimitedto a narrowportionof the marginproducingthe Barrow, Dampier, and Exmouth subbasins and the Gascoyneand Cuvier rift basins(Figure 1). Even thoughthe basins and subbasinswere narrow and localized, their location alongthe edgeof the PeedamullahShelf and the westernedge of the Exmouth Plateau made them efficient traps for clastic sediment derived from the Australian continent to the east and the Greater Indian continentto the west. Consequently,thick synriftdepositionalwedgeswere confinedwithin the structurally controlled topographyof these evolvingbasins,thereby starvingthe Exmouth Plateau and Alpha Arch of clasticsediment (Plate 2) [Exonet al., 1992;Haq et al., 1992].In addition to the extensionaldeformationbeing spatiallylimited, model resultsindicatethat the brittle and ductileextensionwas depth- independent witha maximum valueof 1.46(i.e.,/5=/3, Figure4). The Flindersfault zone, located along the westernedge of the PeedamullahShelf,was the basin-boundingfault systemto Rhaetian extensionwith the southernBarrow subbasinbeing the hangingwall blockand the Peedamullah Shelfbeingthe footwallblock(Plates1 and 2). Rift-inducedflexuraluplift of the PeedamullahShelf and its consequenterosionremoveda large sectionof the underlyingMungarooformation(Plate 2). At Dillson-1 well, the Mungaroo formation and Locker Shale were not erodedbecausethe well is locatedon a downdropped Plate 2. (opposite)Predictedtime line stratigraphyand ba- block that effectivelycompetedwith the rift-inducedfootwall sin architectureacrossthe Northern Carnarvonbasin (Ex- uplift (Plate 2). Our modelingresultssuggestthat the Alpha mouth and southernBarrow subbasins) from the Permianto Arch, which delineatesthe westernboundaryof the southern Present based on the model. The horizontal scale for the modBarrow subbasinand the eastern boundary of the Exmouth eled basincrosssectionsis relative,althoughthe origin (x = 0 subbasin(Figure1), represents boththe collapsedhangingwall km) represents the positionof the ocean-continent boundat)'. to the Flinders Fault and the uplifted footwall to the faults The westernpositioncorresponds to the ExmouthPlateau,and the easternpositioncorrespondsto the PeedamullahShelf. immediatelywest of the Alpha Arch at this time (Plate 2). Basin crosssectionsare presentedfor (a) pre-late Triassic/ Ramillies-1well, located on the crest of the Alpha Arch, reearlyJurassic (Rhaetian,-215 Ma) showingthe distributionof coveredthe upper Mungaroo formation, indicatingthat only the Locker and Mungaroo formationsat the end of late Per- minor erosion has occurred across the arch. This is consistent mian extension,(b) post-lateTriassic/earlyJurassic(Hettang- with the tectonichistoryof the Alpha Arch, that is, the hanging ian, -208 Ma), following the Rhaetian extension,(c) pre- wall to the extensional deformation in the east and the footwall Callovian (Bathonian, -175 Ma), prior to the Callovian to the deformationin the west. The easternedge of the Exextension, (d) post-Callovian (Oxfordian,-155 Ma), afterthe mouth Plateau also experienceduplift due to extensionalunCallovianextension,(e) pre-Tithonian(Kimmeridgian,-150 loadingeven thoughit was the hangingwall block to the late Ma), prior to the onset of Tithonian-Valanginianextension Triassic extension.This uplift occurredbecausethe wavelength and depositionof the Barrow group, (f) Valanginian (-134 of the basin wasnarrowwith respectto the flexuralwavelength Ma), after the Tithonian-Valanginianrifting, (g) pre-lateCretaceous(Barremian,-125 Ma), priorto the Turonianinver- of the lithosphere(X - 60-80 km) and the seriesof antithetic sion,(h) post-lateCretaceous(Turonian,-91 Ma), after Tu- faults causedminor extensionalunloading(Plate 1). Minor faultingacrossthe ExmouthPlateauaccompanied the Rhaeronian inversion,and (i) the Presentmarginconfiguration. 4984 DRISCOLL AND KARNER: AGE LOWER CRUSTAL LITHOLOGY EXTENSION STRATIGRAPHICTECTONIC South UNITS North I ½)UATERNARY PLIOCENE %!I I I I I I I I I I • I • I • I • I • I • I • 1• EVENTS •lam•o - [ MIOC•E I • •• I .... • • •oc• • • • • •AA•RIC•N -• SANTO'AN U • II • ......... ...... I I I I I I %L I -- ? I I ••_ I I I •••••,,,,,, I I I ! I [ I i I Illllllllllllllll .... •.o.• ............... - ..... - • • • 2 • 2 • • • '•• c•o•n • Girali• Fm. ••• • WU •nL•oc• 91• •O I I I I[! I I I I I IAI I Ii I OMGOC•E I I I i [ [ I I [ I I •_- ......... -- _•"'s I I • E •alCllUtlt8 • •%• Fm. • •••., .... .... •Haycock • • ••' •_9•r'• • "•;•-" .... sutston• ...... ................. :••• • • • • • • • z • - -• ••••••••••-- • Minor 0) Invemion Muderong • •U•RW•N z /A•NG•N • KI• •y• ox•o• •per 169- • ........... • BATHOrN • BAJOC•N • •o•.•. 198- m•.• Rifling ............ • •...•. 208 """ . - Phase Phase ,, .•m. •4•- Dingo _• ,,, c,• • Rifling Phase .............. ••'•••••••=•:•=•:••"• Mu• ............................................................................................................................ ............... • •,.•. •"•••••Z•_•Z•• ....................... • sc•..• ANIS•N .................... '"•="'"'""• Turbidite Clastics [• • ...• ß Siltstones and Silty Claystones • '-'"-"-•'• ß Sediments Fluvio Deltaic Locker • Orogenic Collapse Marine Claystones Limestones and Shales Marls Coals Dolomites Figure3. Generalized Mesozoic andTertiarychronostratigraphy for the northwest Australian marginthat buildsuponprevious workalongthe margin[e.g.,Kopsen andMcGann,1985;BooteandKirk, 1989;Cockbain, 1989;Kopsen,1994].The tectonostratigraphic chartillustrates the relationship betweenextensional events acrossthe marginand the stratigraphic packages. Four majorepisodes of extensional deformationand one minorinversion eventaffectedthe northwest Australian margin:(1) LatePermianextension andorogenic collapse, (2) Rhaetianextension associated withrift onsetin theArgobasin,(3) Callovian extension generatingthe onsetof seafloor spreading in the Argobasin(marinemagnetic anomalyM25), (4) Tithonianto Valanginian extension associated withriftingandtheonsetof seafloor spreading in theGascoyne andCuvier basins(marinemagnetic anomaly M10) (notethatthebaseof eachtectonic megasequences is definedbya floodingsurfaceandmarineonlapwithinthe depocenter), and (5) reorganization of the Indo-Australian platesroughly synchronous withtheTuronian(Gearlehorizon)minorreactivation of faultsystems withinthe Exmouth,Barrow,and Dampier subbasins. DRISCOLL AND KARNER: LOWER Tithonian 11.0- 9.07.05.0- 1.0 -100 0 I I I 100 200 300 -- I I 400 500 600 kilometers Callovian /Kimmeridgian 1.6 1.4 1.0 -200 , -100 _,,-, A , I 0 100 i 200 I 300 I 400 I 500 ! 600 kilometers Rhaetian 1.6 1.4 EXTENSION of northwest Australia -200 , -100 i 0 from Greater India and related continentalfragments[Booteand Kirk, 1989]. The Callovian extensionalepisode,whichcorrelateswith the onsetof seafloor spreadingin the Argo AbyssalPlain, reactivatedthe border fault systemsthat delineate the Barrow and Exmouth subbasins. Similar to the late Triassicevent, only minor faulting acrossthe Exmouth Plateau accompaniedthe Callovianreactivationof the subbasins (Plates1 and3). The upperDingo clay stone, which is preserved in the Barrow, Exmouth, and Dampier subbasins, recordsthis episodeof fault reactivation associated with the Callovianextension(Figure 3) [Veenstra, 1985;Kopsenand McGann, 1985;Booteand Kirk, 1989].The upper Dingo clay stoneis a thick succession of siltyclay stone depositedbelow wave base (water depths of approximately hundredsof metersor greater). Along the PeedamullahShelf, the initial prodelta shalesof the upper Dingo clay stone were eventuallyreplaced by the Dupuy sands,which are associated with progradingdeltasand turbidites (Figure 3). The east-to-westprogradationof the Dupuysandsimpliesthat paleo-waterdepthsexistedwithin the Barrowsubbasinadjacentto the Alpha Arch and mostlikely in the Exmouth subbasin.The amplitude of the clinoformsobserved in the seismic reflection 1.0 4985 within the Argo AbyssalPlain (marinemagneticanomalyM25, --•163Ma), and the Tithonian-Valanginianevent,whichgenerated oceaniccrustwithin the Gascoyneand Cuvier Abyssal Plains (marine magnetic anomaly M10, --•132 Ma). These eventsdocumentthe completecontinentalrupturingand separation 3.0- -200 CRUSTAL data indicates that minimum i I I 100 200 i 300 400 500 600 kilometers water depthsat this time were of the order of --•200m [Tait, 1985;Booteand Kirk, 1989]. 3.6. Model Predictions for Callovian Extension Permian 2.0- The Callovianextensionalepisode,whichcorrelateswith the onsetof seafloorspreadingin the Argo AbyssalPlain, reactivated the border fault systemsthat delineated the Exmouth 1.6and southernBarrowsubbasins with only minor accompanying 1.4deformation acrossthe Exmouth Plateau (Plates 1 and 2). Similarto the Rhaetianevent,the Callovianphaseof extension 1.2was depth-independentwith a maximum extensionfactor of 1.0 I I I I I I I 1.15(i.e., 8 =/3, Figure4). Furthermore,the activefault system -200 -100 0 100 200 300 400 500 600 for the southernBarrow subbasinremained along the eastern kilometers edge of the basin (Plate 2). The extensionalunloadingand Figure 4. Distribution, magnitude,and timing of extension consequentfootwalluplift of the PeedamullahShelf tendedto necessary to reproducethe generalbasinarchitectureand pre- divert river systemsawayfrom the southernBarrow subbasin. Within the Exmouth subbasin the dominant border fault served stratigraphyof the basinscomposingthe Carnarvon Basin. The amount of crustal thinning determinesthe distri- systemappearsto have switchedfrom along the westernedge bution of rift-phasesubsidence.In turn, crustalthinning is a of the Alpha Arch (late Triassicextension)to the easternedge combinationof the degreeof upper plate thinning,definedby of the Exmouth Plateau (middle Jurassicextension,Plates 1 8, and the degreeof lower plate thinning,definedby/3. Intraand 2). However,becausethe extensional deformationwasdiscrustaldetachmentsdeterminethe geometryof the upper and tributed along the eastern edge of the Exmouth Plateau,the uplift lower plates.The/3 function also definesthe degreeof lithodue to extensional unloading was counteracted by crustalthinning sphericmantle thinning and criticallydeterminesthe amount of thermal uplift and heat input into the lithosphereduring (Plate 2), thus explainingthe accommodationcreated at this rifting and the amplitude and distributionof postrift subsi- time. The alternatingfootwall/hangingwall block role of the dence.The distribution,magnitude,and timing of extension ExmouthPlateauis suggested by the westwardmigrationof the for the Permian,Triassic,Callovian,andTithonianriftingevents depocenterthroughtime in the Exmouthsubbasin(Plate 1). 1.8- are shown for the Exmouth and southern Barrow subbasins. 3.7. Tithonian Extension During Tithonian-Valanginian time, the continental lithosphere along the western boundary of the Exmouth Plateau and the southwesternAustralian margin was breached and 3.5. Callovian Extension oceaniclithospherewas emplaced(marine magneticanomaly Two Mesozoic tectonic events resulted in the formation of M10 [BooteandKirk, 1989]).Minor fault reactivationoccurred oceaniccrust;the Callovianeventthat generatedoceaniccrust within the Barrow,Exmouth,and Dampier subbasins aswell as tian developmentof the Exmouth and southernBarrow subbasins(Plates1 and 3). 4986 DRISCOLL AND KARNER: LOWER CRUSTAL EXTENSION w E Plate 3. An enlargedportion of an AGSO multichannelseismicreflection profile acrossthe Exmouth Plateau, locatedjust north of line AGSO 110-12, illustratingthe minor erosionaltruncation beneath the Tithonian unconformity.The truncationassociatedwith this unconformityis observedacrossmost of the region,suggesting that largeportionsof the ExmouthPlateauwere emergentor at veryshallowwater depths prior to the depositionof the Barrowdelta.The tope of the Barrowsuccession is markedby the Valanginian unconformity. the ExmouthPlateau (Plates 1 and 2). AGSO seismicreflection profilesacrossthe ExmouthPlateauimageda Tithonian angular unconformitythat records minor truncation of the underlyingMesozoicstratigraphicsuccessions (Plate 3). This truncationis observedacrossmost of the region, suggesting that large portionsof the plateau were emergentor at very shallowwater depthsprior to the depositionof the Barrow delta. A broad shallowwater environmentacrosslarge portions of the platform is also consistentwith faciesanalysisof the Barrowdeltasediments[Erskineand Vail, 1988;Tait, 1985]. Paleo-waterdepthestimatesfor the Valanginiantogetherwith the present-daybathymetryand thicknessof the late Cretaceousand Tertiary stratigraphicsequences(Plates 1 and 2) indicatethat significantsubsidence occurredacrossthe Northern CarnarvonbasinsinceValanginiantime. In contrastto the localizedaccommodation associated with the lowerand upper Dingo clay stones,the post-Valanginianaccommodation appears to be regionallydistributedand increasestoward the west(i.e., towardthe ocean-continent boundary).There is only minor Tithonian to Valanginianbrittle deformationacrossthe ExmouthPlateau(Plates1-3). Note that the post-Valanginian accommodation has been amplifiedalongthe easternmargin by sedimentloading(Plate 2) and, taken togetherwith the long-wavelength subsidence that increasestowardthe west,has resultedin an overallarchingof the margin (Figure 5). Thus we havean interestingparadoxwhere regionalpostriftsubsi- denceis associated with only minor amountsof upper crustal brittle deformation during the Tithonian-Valanginianextensionalevent (Plate 2). Reflector geometriesobservedin the seismicdata indicate the following:(1) the accommodation generatedby brittle deformation in Tithonian time across the Northern Carnarvon basinwaslocalizedandits magnitudewasminorin comparison to the regionalsubsidence, (2) the truncationassociated with the Tithonianunconformityis minimal,and (3) the Tithonian to Valanginiansequencethins predominantlyby onlap onto structuralhighs(Plate 3). These observationsare the critical constraintsrequiringlargebroadlydistributedextensionacross the Exmouth Plateau to accommodate the Barrow delta sedi- ments, the general arching of the Exmouth Plateau, and the largepost-Valanginiansubsidence requiredto explainboth the present-daywater depths and postrift sedimentthicknesses observed 3.8. across the Northern Model Predictions Carnarvon for Tithonian basin. Extension Clearly,the regionaldistributionand amplitudeof the postValanginian subsidenceis not consistent with the minor amountsof Tithonian-Valanginianbrittle crustalextensionobservedacrossthe Northern Carnarvonbasin (Plate 2). The form, distribution,and longevityof this subsidence phaseare characteristicof the coolingand contractionof extendedlithosphericmantle. However,the magnitudeof extensionneeded DRISCOLL AND KARNER: LOWER CRUSTAL EXTENSION 4987 A. Modelprediction for Carnarvon Basin kilometers -200.0 0.0 I I -- meters 200.0 I Oceanic I 400.0 I I Exmouth Exmouth Arch Plateau crust 600.0 I I I Exmouth _ Barrow _Peedamullah subbasin subbasin Shelf 0.0- -: -10000.0 - -20000.0 - !ii .. Moh0" Ocean/continent -30000.0 boundary - "'"""• Continental '..-'...-i.-..'-i.-..'t Jurassic -• Triassic and [• and Cretaceous Tertiary crust -40000.0 - older section section B. Velocity-depth profileacrossCarnarvonBasin Exmouth Dampier _ Pilbara Plateau subbasin Craton 0 ...... •. ..... .•.•:. ................... :.•: cl 20 30 0 • 100 km • I ...... :,i Ivelocities Upper Continental 5.75 - 6.2 crust: km/s Boundary ?•• Lower Continental crust: velocities 6.4 - 6.55 km/s I I Mantle velocities: !,• Magmatic underplating: 8+ velocities 7.0 -7.2 km/s km/s Figure 5. (a) General "ramp-flat-ramp"detachmentgeometryresponsiblefor partitioningextensionbetween the upper and lower platesduringTithonian-Valanginianextension.The detachmenthas a ramp-flatramp geometrysuchthat it breached,or shoaledcloseto, the surfaceof the crust near the continent-ocean boundary.The "flat" componentof the detachmentoccursat midcrustaldepthsacrossthe plateauand ramps beneaththe Australiancontinentbeneaththe PeedamullahShelf.The post-Valanginiansubsidence increases toward the west awayfrom the ExmouthArch, which constrainsthe eastwarddip of the detachmentin this region. (b) Velocity depth profile acrossthe Northern Carnarvonbasin (modifiedfrom Staggand Colwell [1994], reprinted by permissionof the Australian GeologicalSurveyOrganisation).The crosssectionis projectedalongAGSO line 101-09shownin Figure 1, whichcrossesthe Dampier subbasin.The composite depthprofilewasderivedfrom AGSO deep-seismic data, expandingspreadprofiles[Mutteret al., 1989],and Drummond [1981]. Note that the seismicvelocitiessuggestthat lower crustalthinning occurredacrossthe ExmouthPlateauwith the boundarybetweenthe upper and lower crustoccurringat -15 km depth,consistent with the model predictions. to generatethe regional post-Valanginiansubsidence(maximum/3(x) of 2.8) precludesit from being the thermal subsidence phase to the minor and laterally restrictiveextension that occurredduring Calloviantime (maximum/3(x) of 1.15, Plate 3). In addition,it is difficultto reconcilethe long delay (-30 Myr) betweenthe Callovianextensionalevent and the onset of post-Valanginianthermal subsidence.Consequently, to model the distribution and magnitude of the post- Valanginiansubsidencerequired significantlower crustaland mantle extensionacrossthe Exmouth Plateauwith only minor upper crustal deformation (Figure 4 and Plate 3). Such a depth-dependentdistributionof extension(maximum •5(x) and/3(x) of 1.18 and 2.35, respectively)impliesthe existence of an eastwarddippingintracrustaldetachmentthat effectively thins the lower crustand lithosphericmantle, generatingthermal-typesubsidence acrossthe region (Figure 5). If/3(x) re- 4988 DRISCOLL AND KARNER: LOWER flectedonlymantlethinning(Plate3), thentherewouldbe no net subsidence after the rift-inducedheat dissipatedfrom the region becausebasin formation in an extensionalsettingis ultimatelythe consequence of crustalthinning.In contrastto thedepth-independent extension duringlatePermiantime,the Tithoniandepth-dependent extensionrequiredthat negligible CRUSTAL EXTENSION alongthe northwestAustralianmarginremainsequivocal.For example, the emplacement of isolated volcanic constructs along the northwestAustralianmargin (e.g., Joey Rise and PlatypusSpur) appearsinsufficientto explainthe distribution and timing of the observedwidespreadsubsidencebecause their emplacementpostdatesthe onset of seafloorspreading. accommodation be createdduringthe rifting eventwith large Furthermore, becausethe thermal input by a hotspot only amountsof subsidence generatedin the postriftphase(Plate delaysthe reequilibrationof previouslyextendedlithosphere, needsto be consistentwith the cumu3). Becausethe lower crustalductile extensionand thermal this delayedsubsidence uplift causedby lithosphericmantle thinningare of a compa- lative distributionand magnitudeof earlier crustalextensional rable wavelength during the Tithonian-Valanginian rifting events.The smallmagnitudeand restrictivedistributionof the event, the thermal uplift approximatelybalancesthe rift- late Triassic and middle Jurassic crustal extension across the inducedsubsidence, which effectivelydelaysthe generationof Northern Carnarvonbasincannotexplainthe large, regional accommodationuntil after rifting (Plate 2). The subsidence post-Valanginian subsidence observed acrossthe basin(Plate3). pattern requiresthat this detachmenthave a ramp-flat-ramp Therefore the observed subsidence across the Northern Carnargeometrysothat it breached,or shoaledcloseto, the surfaceof vonbasinisbestexplainedin termsof an intracrustaldetachment. Finally, our model resultsindicatethat lower crustalplastic the crustnearthe Gascoynecontinent-ocean boundary(Figure 5). The uppercrustalextensionthat balancesthe lowercrustal extensionprovidesa viablemechanismto generatelarge"therand lithosphericmantle extensionthat occurredbeneath the mal-type"subsidence with little attendantbrittle deformation. Northern Carnarvon basin is assumed to occur where the deThe lowercrustalextensionappearsto be mostdominantdurtachment breaches the crust. This breach must occur at or west ing the late stagesof the rifting phasejust phor to continental of the continent-oceanboundary becauselarge-scalebrittle breakupbecausethe upwellingof asthenospheric heat causes deformation is not observed across the Northern Carnarvon the quartz-richlower crustto deformplastically.Tectonicand stratigraphicevidencefrom many conjugatemargins (e.g., basin(Figure5). The eastwarddip of the detachmentis constrainedby the Newfoundlandand Iberia conjugatemargins)documentsthe eastward decrease in subsidence from the continent-ocean existenceof late stageregional subsidencewith only minor boundaryto the ExmouthArch (Figure5). The magnitudeand accompanyingbrittle deformation and erosionaltruncation wavelength of this subsidenceacrossthe Exmouth Plateau [Tankardand Welsink,1987;Boillotet al., 1988;1995;Driscoll awayfrom the ExmouthArch cannotbe explainedby flexural et al., 1995].To explainthe distributionand magnitudeof the with little or no attendant couplingacrossthe continent-oceanboundary[Karneret al., "thermal-type"regionalsubsidence 1991].The "flat" componentof the detachmentoccursat mid- brittle deformationrequiressignificantlower crustaland mancrustaldepthsacrossthe plateau,and its depth is constrained tle extensionacrossand along these margins.For example, by the magnitudeof the post-Valanginiansubsidence. For ex- Tankardand Welsink[1987]proposedthat a westwarddipping ample,if the depthof the detachmentbecomesshallower,then detachmentseparatedthe Newfoundlandupperplate from the the magnitudeof the post-Valanginiansubsidence would in- Iberian lower plate (Figure 6). Likewise,Boillotand Winterer creasesimilarto what occurswestof the ExmouthArch (Fig- [1988] explainedthe discrepancybetweenthe large regional (/3 - 3.4 to 4.6) and the minimalbrittle deformaure 5). Likewise,if it waslocateddeeperwithinthe crust,then subsidence the subsidencewould diminish. Toward the east, the detach- tion (/5 - 1.6 to 1.9) on the Iberian margin by having an ment againrampsdownmergingwith Moho beneaththe Aus- eastwarddipping detachmentseparatingthe Iberian upper traliancontinent(i.e., PeedamullahShelf).Its positionis con- platefrom the Newfoundland lowerplate (Figure6). Basedon strained by the distribution of Tithonian subsidencewith the distributionand style of subsidenceobservedalong the minimal subsidenceoccurringeast of the ramp (Figure 5). conjugatemargins,both researchersinterpret their marginas Seismic refraction work across the northwest Australian marthe upper plate [Tankardand Welsink,1987; Boillot et al., gin corroboratesour interpretationof an intracrustaldetach- 1988]. Similar observationshavebeen made for the conjugate ment and suggests that lower crustalthinningoccurredacross marginsof the SouthAtlantic[Karneret al., 1997].We propose the Exmouth Plateau with the boundarybetween the upper that a diffusezone (i.e., detachment)separatesthe brittle and andlowercrustoccurringat ---15km depth(Figure5) [Exonet ductile deformationin the crustwhich shoalsin the region of al., 1992;Staggand Colwell,1994]. maximumheat input. Therefore,dependingon the locationof An alternativehypothesisfor the observedthermal subsi- athenosphericupwelling (e.g., the future ocean/continent denceis to havethe northwestAustralianmarginsituatedover boundary),the detachment will dip towardboth margins.The a hotspotduringthe early rift phases(Rhaetian and Callov- balancingbrittle deformationis focusedin a narrow region ian), and subsequently during the Valanginianto have the adjacentto the continent-oceanboundaryand solesinto the region move off the hotspot thereby allowing the thermally detachment.The deformedcontinentalcrust in this region is modifiedlithosphereto cooland subside.In effect,thisprocess highlyintrudedand overprintedby volcanismassociated with would allow thermal subsidence in the absence of crustal exrift-induced decompressionmelting. Vertically, the detachtension similar to the subsidencepattern observedat other ment separatesbrittle deformationabovefrom plasticdeformargins influenced by documented hotspot activity [e.g., mationbelow,and its depthmigratesthroughoutthe historyof Karneret al., 1991;Driscollet al., 1995]. Although petrologic the rifting in responseto the input of heat during the rifting evidencefrom ODP Legs 122 and 123 suggeststhat the tho- process.Consequently, the horizontaldisplacementacrossthe leiitesof the Argo and Gascoyneoceanicbasinsare consistent detachmentvariesspatially.In fact, alongsomeportionsof the with higherthannormalasthenospheric temperatures[Ludden detachment,there need not be significantdifferential motion and Dionne, 1992],geologicevidencesupportingthe existence acrossthe detachment,only a differencein the styleof deforof a hotspotand, more importantly,the timing of its arrival mation.To conclude,the terminologyof upperand lowerplate DRISCOLL AND KARNER: LOWER CRUSTAL EXTENSION A.Newfoundland Margin Bonavista JeanneD'Arc Galicia Margin Flemish Flemish • . .P!a.tf0rm . Basin ••$ •-3•J 4989 Pass Peridotite Cap Riddles westwarddipping • [--•Continental crust • -0 • detachment 0i Mesozoic• 200 km i Cenozoic• B.Newfoundland Margin Mantle Iberia Margin ====================================== ...... 40 Figure 6. (a) Extensionalmodel for the Newfoundland-Iberianconjugatemarginsderivedfrom seismic reflectionand well data suggesting extensionaldeformationalonga westerndippingdetachmentseparating the Newfoundlandupper plate from the Iberian lower plate (modifiedfrom Tankardand Weisink[1987], reprinted by permissionof the American Associationof PetroleumGeologists).This detachmentmodel accounts for variableamountsof extension andthe existence of late stageregionalsubsidence with onlyminor accompanyingbrittle deformation and erosional truncation. (b) Extensional model for the Iberian- Newfoundland conjugate marginssuggesting extensional deformationalongan easterndippingdetachment separatingthe Iberianupperplatefrom the Newfoundlandlowerplate (modifiedfromBoillotet al. [1988]and Boillot and Winterer[1988], reprintedby permissionof the Ocean Drilling Program).Boillot et al. [1988] explainedthe discrepancy betweenthe large regionalsubsidence (/3 - 3.4 to 4.6) and the minimal brittle deformation(3 -- 1.6to 1.9) on the Iberianmarginby havingthe Iberianmarginbe the upperplate.Likewise, Tankardand Weisink[1987]explainedthe discrepancy on the Newfoundlandmarginby havingit be the upper plate. On the basisof subsidence styleand distribution,both marginsappearto be the upperplate. is applicablewhen describingthe morphologyof the brittle deformation observed on conjugate margins. However, in terms of describingthe distributionand style of subsidence observedon conjugatemarginsthe use of upper and lower plate is misleadingbecauseboth marginsmay displaysubsidencepatternsthat are characteristic of the upperplate (e.g., Newfoundlandand Iberia). episodeof basin inversionin the Turonian (Figure 3). The forced fold observedin seismicline AGSO 110-12 (Plates 1 and2) suggests minoruplift alongthe northwestern edgeof the Exmouth 3.10. subbasin. Model Predictions for Late Mesozoic and Tertiary Development The shift from terrigenoussedimentsto predominantlycarbonate sedimentsalongthe Northern Carnarvonbasinresults The Barrow delta was cappedby the diachronousMardie- from a relative sea level rise inducedby the thermal reequiliBirdsongsandsandoverlyingMuderongshales(Figure3). The bration of the rifted lithospherecoupledwith a systematic Muderong shale forms a widespreadtransgressivemarine change in climatic conditionsassociatedwith the northward shaleblanketingmost of the Exmouth Plateau and northwest migrationof the Australiancontinent.Without invokingsome Australian margin. Overlyingthe Windalia sandstoneis a ra- highly contrivedstretchingfunction acrossthe margin it was diolarian-richsiltstonecontainingglauconiteand minor sand- not possibleto generate the forced fold observedin seismic stone and clay stone,which in turn is overlainby the Gearle line AGSO 110-12(Plates1 and 2). The simplestexplanation siltstone(Figure 3). As clasticinput waned during the late for thisstructureis inversionalongthe northwestern edgeof Cretaceous-Tertiary, thick and widespread carbonate- the ExmouthsubbasinduringTuronian time. The amount of dominatedsequences progradedout alongthe margin.These shorteningassociatedwith this reactivationis minor (i.e., observations suggestthat early Cretaceousdepositionwasoc- shortening-500 m). Nevertheless, it is not clearwhat tectonic curringin an open marine outer shelf environment. event was responsiblefor the inversionwhen the northwest In additionto markingthe onsetof current-controlleddep- Australian margin should only be undergoingthermal subsiosition,the Gearle siltstoneappearsto be recordinga minor dence.A possibleexplanationis the majorplate reorganization 3.9. Late Mesozoicto Present PassiveMargin Formation 4990 that DRISCOLL occurred between the Greater Indian AND KARNER: LOWER and Australian plates at this time [Candeet al., 1989]. CRUSTAL EXTENSION that significantlower crustal and mantle extensionoccurred acrossthe Northern Carnarvonbasin with only minor upper crustal deformation. Such a distribution of extension is best explainedin terms of a eastwarddippingintracrustaldetachment that effectivelythins the lower crust and upper mantle Our analysisof the tectonicand stratigraphicdevelopment generatingthermal-typesubsidence acrossthe region.Furtherof the Northern Carnarvon basin has led to the following more, the subsidencepatterns require that this detachment havea ramp-flat-rampgeometrysuchthat it breachedthe crust predictionsfor its geologicaland thermal evolution: 1. Permian basin developmentacrossthe Northern Car- close to the position of the continent-oceanboundary.The natron basin, •500 km across,is broadly distributedand rep- "flat" componentof the detachmentoccurred at midcrustal resentsthe late stagesof intracratonicbasin formation across depths(-15 km) acrossthe plateauand rampedbeneaththe the region. The distribution and thicknessof the Triassic Australian continent beneath the Pilbara block. 5. Post-Valanginianthermal subsidenceacrossthe NorthLocker Shale and Mungaroo formation acrossthe margin and, in particular,acrossthe PeedamullahShelf, are a direct con- ern Carnarvonbasinengendereda relative sealevel rise and is sequenceof the distributionand magnitudeof lithospheric recordedby the shalesoverlyingthe Barrow sands.The Mudextension.The Locker and Mungaroo sequencesthin toward erong shaleforms a widespreadtransgressive marine unit and the eastby onlap.The modeledcrosssectionfor pre-late Tria- blankets most of the Exmouth Plateau and northwest Austrassic time in Plate 2 illustrates the Permo-Triassic succession lian margin.As clasticinput wanedduringthe late Cretaceousthat representsthe synrift(Locker Shale) and postrift(Mun- Tertiary, thick and widespread carbonate-dominated segaroo formation) sequencesassociatedwith the Permian ex- quencesprogradedacrossthe openmarineshelfenvironment. 6. Tectonic and stratigraphicevidencefrom many margins tensionjust prior to the late Triassicextensionalepisodeacross the northwestAustralian margin. (e.g., northwestAustralia,Grand Banks,Iberia, West Africa, 2. A renewed episode of extensionin the late Triassic Brazil) documentsthe existenceof late stageregionalsubsiaffectedthe northwestAustralianmargin,generatingan exten- dencewith only minor accompanyingbrittle deformation and sivealong-strikenetwork of basins.The extensionwas limited erosionaltruncation.To model the distributionand magnitude to a narrow portion of the margin producing the Barrow, of the "thermal-type" regional subsidencewith little or no Exmouth, and Dampier subbasinsand the Argo, Gascoyne, attendantbrittle deformationrequiressignificantlower crustal and Cuvier rift basins.Even thoughthe rift basinsand subba- and mantle extension acrossand along these margins. The sinswere narrow and localized,their locationsalong the edge lower crustal extensionappearsto be most dominant during of the PeedamullahShelf and the westernedgeof the Exmouth the late stagesof the rifting phasejust prior to continental Plateaumade them efficienttrapsfor clasticsedimentderived breakup.Lower crustalextensionand the associatedregional from the Australian continent to the east and the Greater subsidence play an importantrole in controllingmargin archiIndian continentto the west. Consequently,the thick deposi- tectureandsubsidence patterns.Finally,constraining the magtional wedgesof the lower Dingo clay stonewere confinedto nitude of lower crustal extension associated with intracrustal these basins,thus starvingthe Exmouth Plateau and Alpha detachmentsalongmanyconjugatemarginpairsmight explain Arch of clastic sediment.The Flinders Fault Systemwas the the disparityin stretchingestimatesbasedon brittle deformabasin-boundingfault systemfor the southernBarrow subbasin. tion (3) versusinferredthermalsubsidence (/3),which,in turn, The Alpha Arch, whichdelineatesthe westernboundaryof the addressesthe paradoxthat both sidesof many conjugatemarsouthern Barrow subbasinand the eastern boundary of the ginsappear to be the "upper plate." Exmouthsubbasin,representsboth the collapsedhangingwall to the Flinders Fault Systemand the uplifted footwall to the 4. Summary and Conclusions Exmouth subbasin. 3. Callovianextensionreactivatedthe border fault system that delineates the Barrow and Dampier subbasins.Once again,the main extensionaldeformationwas restrictedto the subbasins with minor but broadly distributedextensionoccurring acrossthe Exmouth Plateau. The Flinders Fault System remained the activefault systemfor the southernBarrow subbasin. Within the Exmouth subbasin the dominant border fault systemappearsto have migratedfrom along the westernedge of the Alpha Arch (late Triassicextension)to the eastern boundaryof the ExmouthPlateau(middleJurassicextension). Consequently,the depocenterfor the upper Dingo clay stone was shiftedtowardthe westwith respectto the depocenterfor the lower Dingo clay stone. 4. During Tithonian-Valanginian time the continental lithospherealong the westernboundaryof the Exmouth Plateau and the southwesternAustralian margin was breached and oceaniclithospherewas emplaced.Minor fault reactivation occurredwithin the Barrow, Exmouth, and Dampier subbasinsand acrossthe Exmouth Plateau.The regionaldistribution and amplitudeof the post-Valanginiansubsidenceare not consistentwith the minor amountsof Tithonian-Valanginian brittle crustalextension.This pattern of subsidenceindicates Acknowledgments.We would like to dedicatethis paper posthumouslyto Larry Sloss,the pioneerof sequencestratigraphy,who provided a critical and thoroughreviewof this manuscript.We also acknowledgediscussions with Jeffrey Weissel,Nick Christie-Blick,and Howard Stagg.David Falvey and Chi-Yuen Wang critically read an earlierversionof thismanuscript,and their commentsare appreciated. 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