Exploration Geophysrcs(1991)22, 123-128
The Roleof IntraplateStressin Tertiary(and Mesozoic)
Deformation
of the Australian
Continentand lts Margins:
A Key Factorin Petroleum
TrapFormation
M. E t h e r i d g ê ,H . McQu e e nand K. Lambeck
Research School of Earth Sciences
Australian Nation al Un iversitv
G.PO. Box 4
Canberra,A.C.T.2601
Abstract
The horizontalstressfield within plateinteriorsis largelythe
r e s u l t o f i n t e r a c t i o n sa t p l a t e b o u n d a r i e s .T h e r e i s
geologicalevidenceand theoretical
considerable
supportfor
the hypothesisthat largehorizontalstressescan propagate
for thousandsof km into plate interiors,and that changesin
plategeometryor boundaryconfigurations
thereforeleadto
significantvariationsin stressin plateinteriors.Becausethere
is a global interdependence
of all plate mot¡ons,a major
change¡n the natureof one plate boundary(e.9.,the IndiaAsiacollision)may haveglobalimplicationsand observable
geologicalconsequencesmany thousandsof km from the
source.There are two importantconsequencesof large
horizontalintraplate stresses and stress variations for
petroleumexploration.First,flexuraldistortionsof the plate
will be localizedby variationsin strengthand/orthicknessof
the plate,such as at sedimentarybasin boundaries.These
distortionsmay give rise to transgressions/regressions
and
unconformities
that may be confusedwith eustaticsea-level
effects.Second,an increasein stressor change in stress
orientationmay be relievedby reactivationof a pre-existing
structurein the plate interior.Reactivationof basin-forming
faultsis a particularly
widespreadconsequenceof intraplate
stresses.Analysis of the structural petroleum traps in
Australia'smain producing basins shows that a large
proporl¡onof the lraps were generated by reactivationof
underlying,usuallybasin-forming
faults.We discussexamples
fromthe Carnarvon,Bonaparteand GippslandBasins,and
relatethem to the globaland regionalplatetectonichistory
in the Tertiaryand Mesozoic.
Key words: trap formation,intraplatestress,flexure,fault react¡vation
Transmission
of Stressin the Lithosphere
Reconstructions
platemotionsshowevidence
of Phanerozoic
of repeatedepisodesof platecollisionand riftingon a variety
of scales.The largercollisions,such as the Himalayan,lead
to substantiallyincreased compressionalstress in the
surrounding
lithosphere
and may haveglobalrepercussions,
includingreorganization
of the wholepatternof platemotions.
Riftingand shearingof continentalblocks,on the otherhand,
representmechanismsfor releasingstresswhich has built
up to criticallevels.One of the less visibleconsequences
of
theseeventsis a significantchangein the horizontalstress
fieldin the lithosphere
overa largepartof the earth'ssurface,
and it is now becomingincreasingly
evidentthatthesestress
field fluctuationsleavetheir imprintin the stratigraphy
and
structureof sedimentarybasinsin severalways.
Geologicaland geophysicalestimatesof the levelsof stress
in the deepcrustand uppermantleare not wellconstrained,
but avaìlablefigures(Table1) suggestthat valuesof tens of
MPa are commonand stressesup to severalhundredMPa
may occur.The largestvaluesare probablyreachedonly at
depths of severaltens of km in platesexperiencingactive
orogenesisand are not expectedto be the generalrule.
However,force balanceconsiderationsdictatethat stresses
largeand widespread
enoughto causecompleteriftingof the
lithosphere
and to build5-10 km mountainrangescannotbe
isolatedto the narrowbeltswhere lithosphericdeformation
is most evident,but must be supportedby similarlevelsof
stressthroughoutthe adjacentlithosphericplate.Horizontal
transmissionof stressis enhancedby the greaterstrength
of the cooler upper lithosphererelativeto the underlying
mantle.This strengthlayering,a vital part of platetecton¡cs,
permitstransmiss¡on
of stressfor thousandsof km across
platesenablingthem to move as coherentunits.
In the upperfew km of the earthwherern slfumeasurements
are possible,stresslevelsare lowerbecauseof the reduced
hydrostatic
containment,
and theirorientationis complicated
by localtopographiceffects.The best documentedarea for
is NorthAmericawherethe regionalstress
stressorientation
field away from major topographic influences has a
compressional
axis roughlyperpendicular
to the mid-Atlantic
spreadingridgewhich is expectedto be a majorcontributor
to reg¡onalscalestressIn that plate.In Europeand Asiathe
stressfieldseemsto be dominatedby and radiateawayfrom
the recentAlpine and Himalayancollisionalzones.In the
Australianplate,earthquakefocalmechanismstudiesand a
small amount of well-bore breakout data indicate N-S
TABLE1
Someestimatesof horizontalstressdifferences.
Eanhquakesressdrops
lff)-250 MPa (max.)
In-situnreîsurements
20-40 MPa ar 5 km depth (McGm, 1980)
Geologicalindicators
20-?ff) MPa mylonitezones (Christie& Ord, 1980)
80 MPa
lowercrusr (Mercier,1980)
20-30 MPa uppernrantle(Kirby, 1977)
124
E T H E R I D G EM, c Q U E E NA N D L A M B E C K
compress¡onin the continentalinteriorand compression
roughlyperpendicular
to the coastlinein thosecoastalareas
wheredata exists(Lambecket a\.,1984).The coastaleffect
may be due to local topographicfactors but the N-S
orientationin the interiorprobablyreflectsa relativelyrecent
increasein N-S compressionassociatedwith the contactof
northernAustraliancontinentalcrust with the Indonesian
Archipelago.
There is widespreadevidence of intraplatedeformation
throughout
the Australian
continentduringthe Cainozoic(e.9.,
displacements
of the LateCretaceous
erosional/deoositional
surface in northernAustralia,local fault-boundedTertiary
basins,Tertiaryupliftof basementinlierssuch as Mountlsa),
indicatingthat high stresslevelsand possiblychangesin the
b)
stressfieldhavetypifiedthe recenthistoryof our supposedly
'dead'
continent.Perhapsthe mostgraphicevidericefor these
high stresslevelshas beenthe high levelof intraplateseismic
activitywithinthe past few decades.Australiahas had over
500/oof the world's major (M>6) intraplatecontinental
earthquakeswithin the past thiñy years.
lithosphcrc
Two Consequences
of LargeIntraplate
Stressesand StressFluctuations
Thereare two lmportantconsequences
of high stresslevels
and especiallyof rapidand substantial
changesin the stress
level in the continental lithosphere away f rom plate
boundaries;
1) low amplitude,long wavelengthflexuralwarping of the
wholelithosphere,
amplifiedby variationsin thicknessand
s t r u c t u r e s u c h a s c o n t i n e n t a lm a r g i n s a n d y o u n g
sedimentarybasins,
2) low levelpermanentdeformationof the lithosphere,usually
localizedalong pre-existingzones of weakness (e.9.,
reactivationof major faults).
Lithospheric Flexure,Unconformitiesand Sea-level
A couple of simple analytical calculations suffice to
demonstratethe main principlesinvolvedin lithospheric
flexure (Figure 1). The result of applying a horizontal
compressionto the idealizedstructureof Figure 1a is to
generatea twistingmomentaboutthe zoneof thickening.In
the earth,isostaticforceswill act to translatethis into adjacent
upliftand subsidencewith.a wavelengthdeterminedby the
flexuralstrengthof the lithosphere.
The sl¡ghtlylessidealized
passivemarginbasinstructurein Figure1bwouldexperience
a magnification
of its verticaldeflectionas a resultof increased
inplanecompression.
These effectsare both due simplyto the effectof changing
the stressregimeactingon a body of complexshape,and
tensionalstress would produce the similar results in an
oppos¡tesense.lt is importantto emphasizethat it is a change
in the stress regime, specifically the non-hydrostatic
component,towardsgreateror lessercompressionthat is
importanthere, not the absolutelevelof stress.
ln order to quantifythese effects,Cloetinghet a/. (1985)
developeda numericalmodel based on a finite element
algorithmto predictthe detailedresponseof the lithosphere
to inplanestressfluctuations.
The modeluseda lithospheric
F I G U R E1
Cross sect¡ons of two idealized crustal structures and their
response to increas¡ng ¡nplane compressive stress. (a) A zone
where the thickness of the high strength load-bearinglayer
increases, e.g. at a continent-ocean boundary, experienôes an
asymmetric uplift and subsidence in responseto an induced
moment. (b) A pre-existingvertical displacement,such as a
sedimentarybasin, experiencesa magn¡ficationof the flexural
signal in addition to a distributedresponseof type (a) due to
permanent inelasticdeformation.
strengthdistributionbasedon estimatesof its temperature
structure and rheologicalproperties,and predictedthe
produced
distribution
and magnitudeof upliftand subsidence
by varyinglevelsof stressfluctuations.
Figure2 showsthe
maximumdeflectionexperiencedacrossthe edgeof a passìve
margin sedimentarybasin as a functionof the age of the
marginand the changein the net forceappliedacrossthe
margin (expressedas the stress fluctuationtimes the
thicknessof the stresstransmittinglayer).For example,a
change of 200 MPa carried in a 30 km thick layer would
produceof the order of 40 m of verticalmovement(middle
curve).
Plausiblelevelsof inplanestressfluctuatìoncan therefore
producesignificantverticalcrustal movementsat pass¡ve
marginsand in intracratonic
basinsand consequently
relative
sealevel movements which should be recorded in
u n c o n f o r m i t i e s s, e d i m e n t a r y f a c i e s a n d / o r s e q u e n c e
distributions
at such localities.Seismicstratigraphy
studies
in theseregionsare now beginningto show evidenceof the
majortectonicrearrangements
affectingthe Australianregion
sincethe Jurassicrecordedin basinmarginfacies(Vailef a/.,
1990).ïhis broadbasementwarpingforceschangesin the
intrabasinalstressfield which in turn drive localizedfault
reactivation.
125
INTRAPLATE
STRESSAND TRAP FORMATION
EET (km)
100
40
30
20
recognizedin Australia'smajor producing basins are outlined
in Table 2, together with their styles and the nature of the
primary structures. The details of some of these are provided
in the following section.
80
TABLE 2
Summaryof Mesozoicand Tertiarystructural reactivationevents,
plate tectonic framework and resultant hydrocarbon-producing
structures.
¡<
.9
P
60
c)
(.)
q-r
,q
HC-producing
Tecton¡c
Structu¡es
nt(s)
40
Mid Mi@ne
20
(roRæcnt)
q
l 0 - 1 5M a
Ausùalia-
presr)
Papum
ErøÐgâ
Bmapæ
of Nonl
OI¡gæene
20
40
60
80
100
FIGURE
2
nearthe edgeof a passivemargin
Verticaldeflectionexperienced
basinin responseto a changein the inplaneforce
sedimentary
actingacroésit (i.e.the inplanestresstimesthe thicknessof the
transmittinglayer)as a functionof the age (or effectiveelast¡c
thickness,EET)of the adjacentoceaniccrust.
90-lmMa
Tsmd Sea,Coral
150- 170Ma
@ctivat¡on
Pcmim
Otway
Mid þ hE
older sncturcs
Bonapæ
ritting,
Cretrc@us
brca.kup
wrench
røcuvauon
oil shå.ls
Mìd b Lá@
Age (Ma)
ed
cxtcnsioml faul6
Pra@
120
Pcm¡ù
CMafron
shcNrcs
r, Bodslla
Ausml¡a
thclurhS
K-úgfish,Múlin,
Halibur
Saladin. Barow,
Àngcl
N\¡/ Shclf
- NW compression
Lewis Trough,
M¡dclcincTrcnd
Nonì Atlútic riÍring
Jorass¡c
Hcdinìa
Tr.Ju sNctucs
Pâpùan
collision
Mid-Eæne
md wrench
Dcvonim.
?NW ne8in
ljþ
Tria$ic
2t0 -220 Ma
wrcnch rcúvatìon
b Erly
nco-Tethys,Papum
Jurasic
Bâsin
Pcmo-Cùb Ðd
Dcvonie
Rank¡n,Goodwin
(?) Cmning
ex@ns¡onal
Reactivation of Pre-existing Structures
stress
A secondconsequenceof changesin the l¡thospheric
fieldis the reactivation
of pre-existing
basinaland/orbasement
have
The mechanicalprinciplesof faultreactivation
structures.
been outlined by Donath and Cranwell(1981),Etheridge
(1986),Sibson(1985)andWh¡teef a/. (1986).Faultreactivation
occurs whe¡l the resolvedshear stress in the fault plane
exceedsthe shear resistanceto movement,providedthat the
strengthof the intactrock surroundingthe fault zone is not
exceeded.
Fault reactivation is of direct relevance to hydrocarbon
explorationand exploitationbecausevirtuallyall of Australia's
petroleum resources occur in structural or
traps formed by reactivationof basinstructural/stratigraphic
all of the oil and
In particula6
formingor basementstructures.
gas fields of the Carnarvon,Bonaparte,Gippsland and
EromangaBasins,and manyof thosein the Cooper,Canning
and SuratBasinsowe theirexislenceto structuralreactlvatlon.
In most cases, the reactivatedstructureswere lhose that
and the
formedduringthe intialstagesof basindevelopment,
style,locationand structureof the trapscan be relateddirectly
fault array.
to the geometryof the basin-forming
generallytook place duringdiscrete,
Structuralreactivation
relativelyshort-livedevents,althoughin some areas semicontinuousactivitylastedfor severalto a few tens of Ma. The
discretestructuringevent (or the onset of semi-continuous
activity)commonlycorrelatesacrosswhole basinsand, in
some cases, between basins over distancesexceeding
1000km, reflectingthe large scale of the causes of the
variationsin lithosphericin-planestresses.The principal
Mesozoicand Tertiary react¡vat¡onevents that have been
of
Etheridge(1986)treatedthe specificcase of reactivation
He pointedout thatthe
structures.
extensionalbasin-forming
l i n k e d n o r m a l f a u l t / t r a n s f e rf a u l t / d e t a c h m e n ts y s t e m
of extensional
orogenyprovidesopportunit¡es
characteristic
for variousstylesof reactivation
undera wide rangeof stress
of the normalfaults
Simolereversereactivation
orientations.
is widespreadin extensionalbasins(e.9.,
as thrusts(inversion)
variousbasinsin the northernEuropeanforelandof the Alp¡ne
orgeny,PapuanBasin,Alberta Basin).However,the more
generalcase involvesstressesobliqueto the pre-existing
structures,givingrise to wrenchor oblique-slipreact¡vat¡on
of one or moreof the faultsets(Figure3). In particular,steeply
dipping strike-slipfaults and relatedstructurescommonly
develop above wrench-reactivatedtransfer and/or normal
structureswhichdominatethe traps
faults.lt is thesestrike-slip
in the Carnarvon,Bonaparteand GippslandBasinsand which
contain the overwhelmingproportion of Australia's
hydrocarbonresources.
A particulargeometrythat seemsto be common,especially
throughoutthe NorthwestShelfand Timor Sea,is illustrated
faults
in generalform in Figure4. The principalreactivated
to have been generated
trend northeastand are interpreted
by strike-slipreactivationof Permian normal faults, as
in Figure4a. The specificstyle
indicateddiagrammatically
structuredepends,among other factors,
of the reactivation
on small offsetsand/orsubtlevariationsin the strikeof the
underlyingstructure.These offsetsor swings in strikeare
inferredto principallyarise from small transferfaultsthat
displacethe primarynormalfaultsin eithersenseas shown
in Figure4b. Duringstrike-slipreactivationof the Permian
126
E T H E R I D G EM, c Q U E E NA N D L A M B E c K
(á)
pl"n
FIGURE3
ptansandsections
¡ilusrrarins
(a)a
*j:llj:tt",:lgggrqr:,
ryptcatnormaUtransfer
faultgeometry
underlyinqan extenéiòÁal
reaaivarionõf rheseiau rts ¿úiiñg compression
k:l:^ql{-t!|
oDilque
to the primaryextension
dilect¡on.SeeFiqure4 fòr details
or stnKe-st¡p
geometryaboveprimarynormalfãult.
normalfaultsthe transferoffsetsact as either releasingor
restrainingbends (Figure 4b), localizingextensionalor
compressionalstructuresrespectlvely(Figure4c). Awayfrom
the underlyingtransferfaults,the strike-slip
reactivation
may
havea simplegeometry(Figure4c),producinglittleobvioui
structurein the Mesozoicto Tertiarysectionand lesstendencv
to form petroleumtraps.
Examplesof FaultReactivation
TrapsWithin
the AustralianPlate
CarnarvonBasin
Faulttraps dominatethroughoutthe CarnarvonBasin,and
the controllingfaults have traditionallybeen interpretedas
extensionalstructuresassociatedwith riftingand break_up
or the simplenormalreactivationof such structures.However,
the styles of the fault arraysare inconsistentwith simple
extension,and they are dominatedin detail by strike_slip
geometries.
Crustalextensionthroughoutthe CarnarvonBasintookplace
primarily in the (?Early) permian (yeates et at., 1987:
Etheridge,in prep),and was followedby relativelysmooth
thermal subsidence throughout the Triassic.Northeast_
trendingnormaland detachmentfaultsand northwesÈtrendino
transferfaults accomplishedthe permian extensionanã
determinedthe structuralgrainof the NorthwestShelffor the
remainderof its history.Althoughthere are posfpermian
normal faults,they are generallysteeply to very steeply
dipping,occurin cornplex,sub-vertical
faultzonesano were
responsible
for only very limltedextension.Thesefaultsare.
however,
predominantly
northeasftrending,
or at leastoccur
in northeast-trending
zones,and we concludethatthev have
been localizedby reactivationof the underlyingpármian
extensionalstructures.
wrench in cover
monocline
in cover
rtwrsc in cover
Jurassicfaultingis widespread,but is best disptayed
along
the RankinTrend,whereit was responsible
for the formation
of the enormous gas/condensatetraps. This faulting
is
traditionallyinterpretedto be extensional(althougn
iee
Woodsidepetroleum,19BB)and is ascribeà to
J major
continentalextensioneventthat heraldedJurassicseafloor
spreadingoff northwestAustralia.However,fault geometry
alongthe Rankinliend is typifiedby steepfaults,unsystematic
faultblockrotationsand faultdiscontinuity
in pian(Veenstra,
1985)- all characteristic
of strike_stip
tautting,possiblywith
a smallcomponentof extension.LateTriassicto farty..turassic
faullingis seenalongvirtuallyeverymajorfaulttrend
in the
CarnarvonBasin,and is typifiedby wrénchstyles.Limited
k i n e m a t i ca n a l y s i s s u g g e s t s t h a t t h e w r e n c h
was
predominantlyleft-lateral
on northeast_trending
fault systems.
Whentakenwith the right-lateral
react¡vation
oithe northwesf
trendingfault systemsin the CanningBasin,this indicates
north-southcompressionduring this event.
Thereis some evidencefor Middlelo LateJurassicfaulting
in the CarnarvonBasin, but the next important eventl
particularlyfrom the petroleumentrapmentviewpoint,took
pface in the Middle to Late Cretaceous.Once again, this
faultingis largelyrestrictedto northeast-trending,
sub_vert¡cal
zones with complex internal structure (including both
compressionaland extensionalfeatures in section)and
evidence of repeatedmotlon. This event is particularly
importantfor trapformationalongthe moreoil_rich
trendfrom
Saladin to Talisman,and fault geometryagain suggests
dominantlyleft-lateral
wrench motion.
The final episodeof wrench reactivationin the Carnarvon
Basin was in the Miocene,with some activitycontinuingto
the presentday. Again,wrench fault stylesdominate(see
below),but the kinematicevidencesuggestspredominanily
rightlateralmotion.
BonaparteBasin
There have been three principalepisodesof poslpermian
faultingin the CarnarvonBasin(Table2). LateTriassicto Eartv
The recenl discoveriesln the Timor Sea portion of the
BonaparteBasin (VulcanSub-basin)occur in northeast_
INTRAPLATE
STRESSAND TRAP FORMATION
c)
FIGURE
4
(a)Schematic
sectionnormalto primaryextensional
normalfault
wrenchstyles
showinghowbothsteepnormaland sub-vertical
maydevelopin the post-riftcoversequenceto producea variety
of fault and faultedant¡cl¡netraps.
(b) Block diagramillustratingpossiblerelationshipsbetween
transferfaultoflsetsof the primarynormalfaultandthe locations
of releasingand restrainingbendsin the overlyingstrike-slip
reactivationstructure,
(c)Schematic
mapandsectionsshowingsimplified,typicalfault
geometr¡es
in a strike-slip
systemw¡thstr¡kevariat¡ons
andsplay
faults.
trending structures that exhibit classical wrench fault
geometries(Nelson,1989).Two main periodsof movement
have been documented - Middle to Late Jurassic and Mid'
Miocene (to Recent). Nelson concluded that the Jurassic
and the Tertiarymovementwas
movementwas right-lateral
left-lateral.There is also some evidenceof the LateTriassicEarlyJurassicevent recognizedto the south.
As in the CarnarvonBasin, the reactivatedstructures¡n the
Vulcan Sub-basin are interpreted to root in the (?Early)
Permianextensionalfaults that accomplishedmost of the
variationsin the
crustalthinningin the region.Along-strike
style and intensityof reactivationare relatedat least in part
to the effectof small to largeoffsetsof the underlyingnormal
faultson northwesttrendingtransferfaults.Permiantransfer
faultsare inferredon all scalesfrom a few km to hundreds
of km. The principalevidencefor them comesfrom largescale
segmentationof the NorthwestShelf basin and sub-þasin
of northeast-trending
structure,smallerscale segmentat¡on
Mesozoicto Tertiarystructures, northwest-trending'corridors'
of discontinuous
structuring¡n the Mesozoicto Tertiaryand
a prominentset of magneticlinears(Wellmanand O'Brien,
this volume).
GippslandBasin
structuraleventin the Gippsland
The principaltrap-forming
Basin is Middle Eocene.The major fields rangefrom simple
anticlinesthroughmore complex fault traps to combined
trapsformedby erosionand sealing
structural/stratigraphic
ofthe Eocenestructures.Etheridgeefal. (1985)and Etheridge
(1986)interpretedthe Eocenestructuresin the Gippsland
wrench reactivationof the
Basin as due to compressional
faults.
underlyingEarlyCretaceousextensionalbasin-forming
stylesrangefrom fairlysimpleinversionof
The reactivation
underlyingnormalfaultsthroughcomplexwrenchreactlvation
of the northernboundingfault of the basin lo en echelon
foldingand faultingabovea deep-seatedtransferfault across
the centreof the basin.
Relationof ReactivationEventsto Plate
MotionHistory
Table2 summarizesthe interpretedrelationshipsbetweenthe
main Mesozoic and Tertiary reactivation events in the
Australianplateand contemporarylargerscale platetectonic
events.The earlierevents,particularlythe major LateTriassic
to Early Jurassic compressional wrench reactivation
throughoutnorthwesternAustralia,are more difficult to
confidentlycorrelatebecausethe surroundingplatehistory
is obscuredby youngerevents.
On the globalscale,thereis a good correlationbetweenthe
beginningof riftingand break-upof Pangeaand the Early
128
E Ï H E R I D G EM
, c Q U E E NA N D L A M B E C K
Triassic-Early
Jurassic event. However,more locally it is
difficultto reconcilethe inferrednorth-southcompression
duringthiseventwiththe apparently
contemporaneous
north
to northeastextensionalongAustralia'snorthernmargin,now
preservedin the PapuanBas¡n(Home ef a/., 1gg0).A key
observationin northwestern
Australiais that there is very little
evidence of crustal extension along the present shelf
precedingthe LateJurassicto EarlyCretaceous
immediately
sea{loorspreadingalong the westernmarg¡n.Indeed,the
compressional
wrenching¡nterpreted
by Nelson(1989)in the
VulcanSub-basinis apparentlycontemporaneous
with the
beginningof sea-floorspreadingoff the northwestmargin.The
Middlelo LateCretaceouseventthat is of some tmponance
in the Dampierand BarrowSub-basins
can be correlated
with
majorplaterearrangements
alongthe southernand eastern
marginsof Australia,specifically
the openingof the Tasman
and Coral Seas, the abandonmentof the Bass Strait rift
system(Etheridgeet al., 1987),and the'beginningof slow
spreadingalongthe southernmargin.lt clearlypostdatesthe
beginningof sea-floorspreadingalongthe westernmargin.
The two major Tertiaryevents that are preservedin the
structuralrecordof Australia'sbasinsare more readilyrelated
to ¡mportantplatetectonicevents,and thereforeto the concept
of long-range
transmission
of stressthroughthe lithosphere.
The mostsignificantglobalplaterearrangement
in the Tertiary
took place in the middleEocene,betweenabout 40 and 45
Ma. lt is bestpreservedin the northernPacificOcean,where
a majorchangein platekinematicsis preservedin both the
magneticanomalyand hot spottrackrecords(e.9.,the sharp
bendin the Hawaii-Emperor
oceanislandchain).This event
is ascribedto a global redistribut¡on
of stressesdue to the
beginningof continental
collisionalongthe Alpine-Himalayan
mounlainbelt. The Middle Eoceneevent ¡s recordedin a
numberof Australia'smarginalbasins,particularlythosealong
the easl and south, but assumed by far the greatest
importanceto hydrocarbonaccumulationin the Gippsland
Basinwhere it was largelyresponsiblefor the formationof
all of the significanttraps. ln the local plate context, it
correspondsto the rapid increasein spreadingrate in the
SouthernOcean (Candeand Mutter,1982;Veevers,1990).
Of moreobviouslocalrelevanceis the subduction/collisional
historyalongthe northernmarginof the Australianplateas
it moved rapidly northwardsduring the latter part of the
Tertiary.The northernleadingedgeof the Australiancontinent
may have been the locus of subductionas early as the
Palaeocene
(60 Ma; Smith,1990).However,
the growthof the
SolomonSea platebetweenthe Australiancontinentand the
maincollisionalplateboundary(Melaþesian
Arc)throughout
the Eoceneand Oligoceneprobablyihsulatedthe Australian
cont¡nentfrom the plateboundarystressesthroughmuch of
the Tertiary.The locus of plateconvergenceswitchedback
to the continentalmarginin the early Miocene,with partial
obductionof the Solomon Sea plate onto the northern
Australianpassive margin. Collisionof the northwestern
Australiancontinentalmarginwith Indonesiain the middle
Miocene,and. accretionof lhe MelanesianArc onto the
Papuan Basin in the late Miocene(.'10 Ma) to form the
Papuan Thrust Belt clearly led to the propagationof
compress¡onal
stresseswell into the continent,reactivating
strucluresas far south as the Carnarvon,Eroman$aand
CooperBasins.Miocenereactivation
contributed
significantly
to petroleumtrap development
in the VulcanSub-basin,and
the Eromangaand CarnarvonBasins.
References
of the
Cande,S. C.,and Mutter,J. C.,(1982).A revised¡dentif¡cation
Australiaand
oldest seafloorspreadinganomaliesbetween
'151-160.
Lett.,58,
Antarctica'.Earth Planet. Sci.'Flow
stressfrom m¡crostruôtures
Christìe,J. M., and Ord,4., (1980).
of myfonites:examplesand current assessment'.J. Geophys.
Æes.,85, 6253-6262.
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