Download Lower crustal extension across the Northern

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
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114 ø
CRUSTAL
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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
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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
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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-
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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
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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
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ul
E
•
I
I
i
c
I
I
I
!
I
I
I
i
i
!
i
•
i
(tuq) qldacI
(tu) tlldac!
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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).
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Southern
EXTENSION
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kilometers
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kilometers
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240.0
i
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i
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,
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
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-
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Phase
,,
.•m.
•4•-
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_•
,,,
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.
We also thank the Australian Geological Survey Organization
(AGSO), GECO-PRAKLA, and TGS Geophysicalfor grantingus
accessto the seismicreflectionand exploratorywell data from the
northern Carnarvon basin and surroundingregion. This work was
supportedby the Minerals and EnergyResearchInstituteof Western
Australia (MERIWA) grant M260 and a researchgrant from Ampolex,BHP Petroleum,MOBIL, Santos,Texaco,WAPET, WMC Petroleum, and Woodside Offshore Petroleum.Additional funding was
providedby the PalisadesGeophysicalInstitute and the Office of
Naval Research.Woods Hole OceanographicInstitution publication
9575. Lamont-Doherty Earth Observatorycontribution5713.
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