Download The state of stress within the Australian continent

6
The stateof stresswithin the Australiancontinent
KURT LAMBECK (t), H.W. S. McQUEEN ('), R. A. STEPHENSON('), D. DENHAM (t)
(L) ResearchSchool of Earth Sciences,Australian National University, Canberra ACT, Australia
(2) GeologicalSurvey of Canada, Calgary, Alberta, Cqnada
(3) Bureau of Mineral Resources,Geology and Geophysics,Canberra ACT, Australia
Received15103184,accepted02107184.
pointto a predominantly
compresseismicity
and<in situ> stress
measurements
ABSTRACT.Faultplanesolutions,
sive state ofthe Australian continent. This stress is being relieved by brittle failure in the upper and middle regions
of the crust only. The orientation of the axes of maximum compression varies conside¡ably across the continent.
Most Australian earthquakes occur in tectonic provinces that range in age from Palaeozoìc to A¡chaean. There
is no evidence that these regions have been actively rejuvenated in more ¡ecent times. In eastern Australia much of the
seismicity is associated with the Lachlan Fold Belt which was cratonized by Late Palaeozoic time. Earthquakes
occur down to about 20 km depth but no recent surface faulting has been observed. Deep events fail predominantly
by thrust faulting and shallow events by strike-slip faulting. The seismicity is consistent with the superposition of a
local stress field caused by the erosion and rebound ofthe highlands, on a predominantly southeast-northwest regional
field Seismicity in South Australia is confined to the Cambrian Adelaide Geosyncline and its vicinity. Earthquakes
occur here at generally shallower depths than in eastern Australia. The predominant direction of the axis of compression is northeast-southwest. In western Australia the earthquake activity is diffuse and not obviously related with
major tectonic features. Seismicity here is well documented only in the Archaean Yilgarn Block where much of the
activity is very shallow and associated with surface faulting. This activity may also be the consequence of a local stress
freld. related to the nearby Perth Basin and Darling faull superimposed on a regional east-west field. That seismicity
occu¡s in these old structures reflects the relaxation of deviatoric stress associated with past tectonic upheavals.
As relaxation takes place at depth, the effective elastic thickness of the iithosphere is reduced and deviatoric stress
in the upper crust increases until failure occurs, possibly triggered by the imposition of the regional stress field. Where
non-hydrostatic stressesare greatest, as in the centre of the continent, levels of seismicity are exceedingly low. This
suggeststhat such regions are held in mechanical equilibrium by loading buoyancy, elastic and viscoelastic forces and
by horizontal compression. In the case of central Australia the required compression is approximately north-south.
The continent-wide stress f,reld could be associated with the plate tectonic movement of the Aust¡alian Plate or it
could be the result of temperature and density differences between continental and oceanic lithosphere. The observations and models suggest that this regional force is variable in azimuth with a magnitude of the order 100-200 MPa.
Key words: tectonicstress,seismicity,
continentallithosphere,Australia.
AnnalesGeophysicae,7984,2, 6, 723-742.
INTRODUCTION
The studyofpresentand paststressstatesofa continent
is of relevanceto the understandingof a number of
tectonic problems that have been much discussedin
the recent geologicaland geophysicalliterature. These
problems include : (i) the interpretation of < in situ >>
stress measurements and intraplate seismicity, (ii)
the study of tectonic processeswithin the lithospheric
plate and along plate boundaries,(iii) the quantitative
evaluationof continental lithosphericrheology,and (iv)
the evaluationof driving mechanismsof plate tectonics.
Evidencefor present and past stressstatesis of three
andgeological
main types: (i) that basedon geophysical
geòphysical
(ii) that basedon
modelfield observations,
ling of tectonic processesin the lithosphereor of convectionin the mantle, and (iii) that basedon laboratory
experimentsof the rheologicalbehaviourof lithospheric
and mantle rocks. The volumes of papers edited by
tilyss (1977)and Hanks and Raleigh (1980)contain
0755 0685l84l06723 lg $ 3,gOO
AnnalesGeophysicae,
EGS-Gauthier-Villars
good examplesof both the geophysicalrelevanceand
observationsof stressin the lithosphere.
Geological evidenceincludes the f,reldobservationsof
folding and faulting on both the micro- and macroscales.
The former includes induced microstructures,such as
changesin dislocation densities and grain sizês and
the recrystallization of grains along faults and in
kimberlite intrusions.Thç interpretation of thesepalaeostress indicators remains uncertain. For examplg
stressdifferences
of i00-200MPa (1-2kbars)havebeen
determinedfrom dislocationdensitiesalong fault zones,
but these yalues are þenerally 2-3 times larger than
estimatesdeducedfrom recrystallizedgrain sizeswithin
the samerock (e.g.Christieand Or4 1980).
Geophysical observational evidence includes < in
situ )) stressmeasurements,present and past evidence
of seismicity, the seismologicalevidencefor faulting
and lateral variations in crustal structure,and the analysis of topography and gravity. The major difliculty
with the < in situ > measurementsis that these are
K U R T L A M B E C KH
. . W . S M C O U E E NR, A - S T E P H E N S O D
N, DENHAN/
made near the surfaceso that they may not be very
representativeof the state of stress in the crust, or
lithosphere,asa whole.Near surface< in situ ))measurements usually result in inferred stressdifferencesthat
are of the order of a few tensof MPa but it is expected
that deeper in the crust these may increase several
fold (McGarr and Gay, 1978).
Seismicityis indicativeof where stresses
have concentrated so asto exceedthe brittle failure limit of the crust.
However,becausethe strengthof the crustis unknown,
earthquakes
cannotbe usedto estimatethe magnitudes
ofthe regionalstress.Fault planesolutionscan be used
to estimatethe directions of the principal stressesbu!
becausefailure usually takes place along pre-existing
zones of weakness,the estimatesof principal stress
directions from individual solutions may have errors
of up to 90o (the principal compressionalstressneed
only be in the samequadrant as the principal compressional axis of the focal mechanismsolution; see,for
example,McKenzie,1969; Raleighet al.,1972).Estimates of seismicstress-dropsinferred from crustal earthquakesindicatethe changein the stress-state
that occurs
at the time of the earthquake rather than the actual
value of stress(e.g.Hanks, 1977).
Analysis of gravity data gives information on stress
but only if specificmechanicalmodels,such as a statement on the nature of isostatic compensation, are
introduced.Even then, the deducedmagnitudesof the
stressesare uncertain and rheology dependent(e.g.
Lambeckand Nakiboglq 1980).Estimatesof minimum
stress-difference
are obtained if the assumptionof local
isostasyis made (Jeffreys,1970)but if compensation
is regional the stress-differences
may be severaltimes
greater than these minima. Topography may better
represent past and present stress states - because
the load is more clearly defined- but the stressestimates remain model dependent : on the mechanism
.bywhichthe topographyformed,on the stateof isostasy
attained,on the crustal rheologyand on past erosion
(e.g.Stephensonand Lambeck, 1984).Lateralvariations
in crustal structure as observedby seismicrefraction
and reflection surveys or from the analysisof traveltime anomaliesare also indicative of a non-hydrostatic
stress state. But even when such observations are
combinedwith gravity and topographicdata it remains
diffrcult to separatethe assumptionsmade about rheology from any statementson the stressstateand tectonic
processitself.
Specific examples of geophysical modelling for the
stress-state
include the analysisof passiveloading of
the lithosphere by volcanic loads, sediments,ice or
water. In these casesa history of loading is required.
Observationsthat constrainthe modelsinclude gravity,
deflectionsof the crust and the time-dependentbehaviour of deformation. Less direct examplesof geophysical modelling for estimating tectonic stressinclude
the analysisof subduction tectonicsand of sedimentary
basin formation by processesother than passiveloading. Here the resultant stressestimatesare intimately
relatedto the assumptionsmadeabout the forcesassociated with the geophysical process.
These brief introductory comments serve onlv to
724
indicate that the measurementand interpretationof
stressin the lithosphere,whethercontinentalor oceanic,
is not unique.Only by combiningthe various lines of
evidenceis a coherentpicture likely to emerge,a picture
that should relate to the structure, evolution and
motion of a continent.Thesecommentsalso serveto
illustrate that any statementsthat we make in this
paper about the stresswithin the Australiancontinent
should be consideredas preliminary and that more
questionsare likely to be raised than answersgiven.
Thereforg what we attempt t.o do is twofold. Firs!
we survey some of the geophysicalevidencefor the
presentstressstate within the continent and discuss
someof the implicationsof this evidence.Second,we
presentsome preliminary analysesof the stressstate
associated
with certainaspectsof the presentAustralian
continentaltopographyand gravity.Threetopographic
and gravity featuresin particular will be discussed;
the stressstate implied by the large gravity anomalies
in the centre of the continent, the stressstate near
the Darling Fault and Yilgarn Block in westernAustralia and the stressstate associatedwith the eastern
highlands.
OBSERVATIONAL EVIDENCE FOR THE STRESS
STATE IN THE CRUST
Seismicity
The first permanentseismograph
stationswereinstalled
in Australia early in the twentieth century and much
of the equipmentoperating from this time up to about
1960 was of relatively low gain and sensitiveto low
frequencysignalsonly. Instrumentationgraduallyimproved and from about 1960onward eventsof Richter
magnitude M > 5 occurring anywhere in Australia
would havebeenlocated(Doyleand Underwood,i965).
The network of seismographstations has continued
to improve (e.g.Cleary, 1,977),but there are still areas
of northern Queenslandwhere earthquakesof magnitude 4 cannotbe located.Thus only a very shortrecord
of Australian-wide quantitative seismicity is available
at this time. Surveyshave beengiven by Burke-Gaffney
(1951), Doyle et al. (1968); Drake (1974); and Denham et al. (1979\.From thesestudiesit becomesclear
that contrary to what is often supposed and despite
the fact that the land mass lies far from active plate
boundaries,the Australian continent is not particularly
aseismic.
The largestrecenteventwasthe 1968Meckering earthquakeof magnitudeMr: 6.8(Gordon"t97I;
Gordon and Lewis, 1980). Other large earthquakes
include an Mr:7.7
earlhquakewhich occurred off
the northwest coast in 1906 and an M, - 7.0 event
which occurred northeast of Geralton in western
Australia in 1941.
The known Australian seismicity is characterizedby
three main regionsof activity within each of which the
distribution of earthquakesis diffuse (fig. 1). The southern part of the easternregion is well covered from
about1960onwardsandmosteventsof magnitudeM > 3
would have been located after about 1965(e.g.Jaeger
and Read,1.969).
7 earthquakesof magnitude M >- 5.5
"
CONTINENT
W I T H I NT H E A U S T R A L I A N
S T A T EO F S T R E S S
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WITH MAGNITUDE
EARTHOUAKES
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Magnitude < 5.0
O Magnitude 5.O-5.9
)Magn¡tude>
1946 - Year of earthquake w¡th
magn¡tude 6 O or greater
5.9
Figure 1
Earthquakeswith magnitudes
4 or greaterrecordedin Aus*aliaîrom 1873ta 1980.The year of the eventis indicatedforeventsof
magnitude6 or greater.
16 stations,including three stations run by the Sydney
Metropolitan Water Board. It is supplemented
by stations establishedin Victoria from 1976by the Preston
Institute of Technology(PIT) (Gibson et al., l98l) and
by two stations operated by the Bureau of Mineral
Resources(BMR). The recordedseismicityfrom 1960to
September1983is illustrated in figure 2ø. Someof the
smaller earthquakeslocated in Victoria by the PIT
network only have not been included. No attempt
has beenmadehere to distinguishbetweenmainshocks
and aftershocksand some of the clusteringis a consequenceof aftershocksequences.Only reliably located
events,where a minimum of five stations were used to
determine the hypocentres,are illustrated. The locations of all events of M" ) 3 recorded from 1960 to
September1983are illustrated in figure 2b.Superimposed upon this distribution of seismicityare the station
locations and the limits beyond which eventsof magnitude less than 3 cannot, as a rulg be reliably located
from records at a minimum of 5 stations.In figure 2c
all events of M" < 3 located by the southeastern
network are illustrated.
have occurredin this region from 1959to 1982.The
central region coincides apprôximately with the Adelaide Geosynclinein South Australia and a northward
extensioninto the Simpson Desert of the Northern
Territory. This northern part is the most active area
in Australia and at least five earthquakesof M"2 6
haveoccurredthere in the last 100years.The location
of the smaller (M" < 4) earthquakesin the central
region did not becomepossibleuntil-about 1962(Sutton and White, i968). The third region includesmuch
of westernAustralia where only sinceabout 1980has
it been possibleto locate eventsof magnitude4 or
greater throughout the region. Much of the better
known seismicity in this region occurs in the southwestern regior¡ east of Perth, and is also referred to
as the southwestseismiczone (Doyle, l91l).
SoutheasternAustralia
The principal information on the seismicity of this
region comesfrom the network operatedby the Australian National University (ANU) since1958.The network has been gradually improved and now comprises
The accuracywith which the earthquakesare located
varies considerablywith their position and magnitude,
with the state of the network at the time of the evenl
725
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2a)
Figure 2
(a) Seismicity oÍ southeastern Australia from 1960-1983. (b) Earthquakes of magnituìle 3 or greater in southeastern Australia from
1960-1983. The topography of the highlands is iniJicated by the 500 anil 7500 m contours. The 1000 m contour is shown anly partly
( dotted line). This ilefines approximately the topographic low in the dividing range near 35o latitude. Elevations above 1500 m are
shaded. The 4000 m depth contour defines the lower edge of the continental shelf. Solid triangles refer to the seismograph network.
The lines I anil 2 ilefine the region beyond which magnitu¿le3 events cannot be located from 5 or more stations. (c) Earthquakes of
magnituile less than 3. AIso indicateil are known surfacefaults anil the limits of the major basins and fold belts.
and by the state of knowledgeof the seismicvelocities
of the crust and upper mantle. Using known quarry
blasts as a calibrator, it appears that epicentres of
eventsoccurring within the perimeter of the network
are usually determined to within I 5 km, and that
these locations are not very sensitiveto the adopted
crustal model. Events outside the network perimeter
are progressivelylesswell determinedas their distance
from the seismographstations increases.Earthquakes
near the limits depictedin figure 2a have an epicentral
precision that is typically about 10 km. Accuraciesof
these locations may be less satisfactory becauseof
systematicerrors resulting from the simplicity of the
adopted twoJayer crustal model.
Depth determinations are much more problematical.
Typical uncertainties for events within the network
perimeter are 5 km with the better events having a
precision of 2-3km. Figure 3 illustrates the depth
726
W I T H I NT H E A U S T R A L I A N
CONTINENT
S T A T EO F S T R E S S
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distribution of events for which the solutions for
depth, based on P and later phases,yield precisions
that are better than 2 km. The mean depth of all events
is about 11-12km and is about the samefor all magnitude intervals. As noted by previous investigatorsof
Australian seismicity(e.g.Doyle et al.; L968)few events
occur at depthsgreaterthan about 25 km and no event
can be attributed with any degree of certainty as
occurring in the mantle. The depth distributions are
suggestive of concentrations of seismicity at two
depths, near 6 km and near 15 km, with reduced
levels of seismicity in the interval 8-12km. This may
reflect the presenceof a low velocity layer in the crust
(e.g.Finlaysonet a1.,1979),in which stressis released
partly by creep and partly by brittle failure. It may
727
also indicate that stressdifferencesin this part of the
crust are reduced, as would be the case if flexural
stressesare significant and the middle plane of the
effective elastic or viscoelastic lithosphere is near
and Lambecþ 1984).It must be
10 km (e.g.Stephenson
remembered,however,that becauseof the uncertainty
of the depth determinations,and becauseof the dependence of these depths on the chosen crustal model,
this distribution may be a consequenceof the method
of analysisand this needsto be examinedmore closely
before either of the above interpretations can be
accepted. The horizontal distribution of seismicity
does not exhibit any dependenceon depth : deep and
shallow events have essentially the same geographic
distributions.
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152
The seismicityof Victoria has beenreviewedby Underwood (1972)and Gibson et at. (1981.).
ievels of seismicity are greatest in the eastern part of the state
wherg apart from a concentration of activity in the
westernpart of the Gippsland Basin (frg.2a) seismicity
remains diffuse.Any event of M" < 3 that occurs in
the westernGippsland Basin would not be recorded
on the ANU network but Gibson et al.'s results leave
no doubt that such eventsoccur frequently both here
and in the region immediately to the north and east
of Melbourne.
Seismicityin the New England Fold Belt to the north
is only poorly documentedand the paucityof recorded
eventsin this regionis probablymore a consequence
of
the lack of seismographstations than of low levels of
seismicity.The Lachlan Fold Belt was shapedduring
the Palaeozoic by intense folding and deformatior¡
by major igneousintrusions, and by volcanic activity,
and cratonized by Early Permian time. The New
England Fold Belt stabilized later, by Early Mesozoic
time. Subsequenttectonic activity in both fold belts
appearsto have been minor.
Present-day seismicity does not exhibit any clear
relation with either the continental topography (hg.2b)
or gravity. Nor is there a clear correlation with geology
The ANU network is capable of locating M,4 3
eventsfor much of the Lachlan Fold Belt (f,rg.2c) and
activity is relatively diffuse throughout the region.
728
W I T H I NT H E A U S T R A L I A N
CONTINENT
S T A T EO F S T R E S S
approximately with regions where the density of
known surface faults is also greater (fig. 2c). Cleary
(1967)suggestedthat much of the activity occurs on
or at the edgesof the abundant granite intrusions in
the Lachlan Fold Belt and many of the events do
appear to occur near granite outcrops. However, the
precisionof their location is inadequateto determine
the nature of this association.What is curious is that
this relation is limited to the S-type granites; very
little seismicity is associatedwith the I-type granites
of southernNew South Wales and easternVictoria.
( ) The off-shoreseismicityoccurslargelywithin about
70 km of the shoreline,below the narrow continental
shelf and slope (see figs. 2b and 2e). The depths of
theseeventsare typically in the range 1.6-20kmbelow
sealevel.It has not beenpossibleto obtain fault plane
solutions for these events.
AII Magnitud.es
M 2,5-2.9
237 euents
depth
depth
Figure 3
Depthdistributionof earthquakesin southeastern
Australiafrom 7960
to 7983whoseapparentdepthsare locatedto within 2 km.
South Australia
or tectonicstructure(fig. 2c).Nevertheless,
somegeneral
commentscan be made.
(1) The M"2 3 events are distributed quite uniformly through much of the Lachlan Fold Belt. There
is a decreasein recordedactivity towardsthe northwest
and offshore beyond the continental shelf. These
decreasesdo not appear to be a network effect since
the smallerevents(M < 3) are observedout to about
the same distances(compare figs. 2b and 2c). There
is alsoa seismicallyquietzoneto theeastof the southern
part of the Dividing Range (below Eucumbene in
frg. 2a). This was previously noted by Drake (1.974)
and again,is not a network effect.
(2) The M,"< 3 events (frg. 2c) have a very similar
geographic distribution to the larger earthquakes,
with similar areaswhere activity is lower than average.
Small eventsbecomeproportionally less frequent in
the northern part of the SydneyBasinand in the Murrây
Basinandthis is probablya consequence
ofthe decreasing radius of detectability at smaller magnitudes.In
several localities the levels of seismicity are much
greater than average.Some of thesereflect aftershocks
of larger events(e.g.Picton and Robertson).Others
reflectinducedseismicitysuch as that associated\/ith
the Talbingo Dam (Muirhea{ 1981) and possibly
the EucumbeneDam (Cleary,1977;Bock and Denham,
1983).Other areas of high levels of seismicactivity
include the Dalton-Gunning,Crookwell and Young
areas which coincide approximately with the topographic low in the southeasternhighlands (fig. 2b,
seealso frg. 9 below).This region hashad a consistent
record of seismicity since the first settlers arrived
(Burke-Gaffney,l95l; Cleary,1.967),butno successful
explanationhas yet been proposed.
(3) Some of the seismicity appears to be associated
with known faultg particularly those'occurring in the
Snowy Mountains (Cleary et al-, 1964; Bock and
Denharr¡ 1983). For most other seismic regions,
notably the Dalton-Gunning seismiczone,no obvious
correlation with faults has been found" other than
that regionsof higher than averageseismicitycoincides
729
Recent seismicity in southern Australia has been
discussedby Sutton and White (1968),Stewart et al.
(1973),Sutton et al. (1977),and McCue and Sutton
(1979).Much of the seismicityis conhnedto the Adelaide Geosynclineand the adjacentgulf grabenregions
(frg. 4). This geosyncline,formed during a prolonged
orogeny in Late Proterozoic-Cambriantimg consists
of a thick sequenceof sedimentsthat have been folded
tectonic
down to depthsof 5 km or more. Subsequent
activity appearsto have been minimal. Much of the
seismicity has occurred in the Mt. Lofty-Flinders
Rangespart of the syncline,but this may be a consequenceof the restrictednature of the network prior
to about 1969.In that" year three additional stations
were set up in the northern part of the synclineand
a
,l <M<5
O Àt>5
Fig. 4
Seismicityfrom 1960-1983in South Australia.
K U R TL A M B E C KH. W . S M C O U E E NR,. A S T E P H E N S O D
N, DENHAN¡
these indicated that there is considerableseismicity
in this region as well (Stewart and Mount, 1972).
To the south west the seismicityoccurspredominantly
to the west of the Lincoln Fault on the Eyre Peninsula.
Seismicity within the syncline is generally shallow,
with more than 501 of zl located eventsbeing at
depthsof lessthan 5 km (Stewartand Mount 1972).
These are also of small magnitude (M" < 3) and
appearto be confined mainly to the thick sedimentary
folded into the syncline.They may therefore
sequences
reflect more a local responseto a regional stressfield
than the regional stressstate itself. Events of M" > 3
occur down to greaterdepths but few eventsoccurring
within the perimeter of the network of stations that
recorded the event are located at depths greater than
about 20 km. McCue and Sutton (1979)estimatean
averagedepth ofabout 10 km for sucheventsrecorded
in the two year interval of 1976-19'77.
Persistent seismicity of large magnitude (MrÞ 6)
has been recordedsince 1900 in the SimpsonDesert
in the south east corner of the Northern Territory
(Burke-Gaffney,1951) (lig. 1). However, only a few
precise locations and two fault plane solutions are
available (Stewart and Denham, 1974; Everingham
and Smith, 1979). Ãny relation with deep crustal
structure is maskedby recentsedimentsand the origin
of this activity remainsan enigma.
Western Australia
The seismicityof westernAustralia has been discussed
by Everingham(1966)and summarizedin Denhamet al.
(1979). This area experiencesthe highest levels of
earthquake activity throughout the continent (fig. 1).
There does not appear to be a clear relation between
seismicityand the major tectonicfeatures.For examplg
there is no known seismicity on the nearly 1000km
long Darling Fault (total vertical throw - 10 km)
separating the Perth Basin from the Yilgarn Block,
even though movementson it have occurred continuously sincethe Permian (fig. 5). The offshoreseismicity
extendsfarther out to seathan it doesalong the eastern
and southernmargins but the shelf and slope are also
much wider here and as elsewhere,much of the seismicity appearsto lie below the shelfand sloperegion;
betweenthe coastline and the 5000m isobath.
tI
.
calinsir¡
t",SYti
' '.ÌX-'
Fig. s
Seismicity in the southwest seismic zone of
western Australia. Also illustrated are the
surJaceJaulting ( light lines) and the directions
of the compressive axes as determined for the
Cadoux, Calingiri and Meckering earthquakes (heavy lines) andfrom < in situ ) stress
measurements (meilium lines). The maximum
< in situ > sfress is 23 MPa near Cadoux.
730
CONTINENT
W I T H I NT H E A U S T R A L I A N
S T A T EO F S T R E S S
Depth control on the earthquakesis very poor but the
surface faulting associatedwith the larger events in
the south-westseismiczone. as well as the isoseismal
patterns, indicates that most if not all of the earth
quakes here take place near the surface and in the
upper crust (e.g.Everingham et al., 1982).
Fault plane solutions
A number of reliable fault plane solutions have been
obtained for the larger earthquakesthat have occurred
in Australia since about 1960. Solutions have also
been possible for some of the smaller events that
occurred within the regional networks. Figure 6
and table 1 summarize the results. In general, these
solutions point to compressiveforces throughout the
continent (see,for example,Cleary, 1967; Fitch et al.,
1973; Mills and Fitch, 1,977
; Denham et al., 1979)
but considerable variation in the directions of the
compressive
axesdoes occur.
Reliable fault plane solutions for earthquakes in
New South Wales have been obtained by Mills and
Fitch (1977),Everingham and Smith (1979),Denham
et al. (1979,1981,
1982)and Bock and Denham(1983).
All solutions yield compressionaxes that are nearly
horizontal but whose azimtthal orientations vary
to souththrough about 90o,from northeast-southwest
east-northwest(fig. 6a). The different azimuths may
be the result of faulting being controlled by the geometry of pre-existingcrustal faults or zonesof weakness.
Most deeper events, where the depth (D) is greater
than about 15 kr& are associatedwith thrust-faulting
mechanisms and most shallow events (D I 5 km)
indicate failure by predominantly strike-slip faulting
(table 1). The two intermediate depth earthquakes
near Bowning are indicative of strike-slip faulting
with a major thrusting component.
Earlier, Underwood (1967) had suggestedthat some
of the Dalton-Gunning earthquakeswere the result
of tensional stressbut his solutions were poorly constrained and the more recent work has not confirmed
this interpretation.
Fault plane solutions for eventsin Victoria have been
discussedby Underwood (1972) and Denham et aI.
(1981).The 1977Balliang event about 50 km to the
west of Melbourne (f,rg.6b), is indicative of thrust
faulting at a depth of 21 km but the depth is poorly
constrained. Two nearby events in the Victorian
highlands yield quite different fault plane solutions.
A re-examinationby Denham et al. (1984)of the 1966
Mt. Hotham event suggeststhat failure is by normal
tensionaxis(fig.6a).
faulting with a northeast-southwest
This represents the only well documented case of
tensionalstressin the continent. The nearby Wonnangatta event of 1982indicated failure by thrusting with
a northwest-southeastorientation of the compressive
axis. Depths of the events are poorly determinedbut
both earthquakesare believedto have occurredin the
middle crust.
Table 1
Summary offautt plane solutions.All events except the Mt. Hotham are compressional.'l denotes thrust fault, SS strike-slip fa¡¿l¿,SS-T strike-slip with
a significant thrust component, N normal fault.
Year
Earthquakelocation
Magnitude
ML
Snowy Mountains New South Wales
59
7l
76
81
Berridale
Middlingbank
Pilot
SugganBuggan
Dalton Gunnins Seismic Zone New South Wales
7l
Dalton
73
Dalton
74a
Dalton
74b
Dalton
77a
Bowning
77b
Bowning
5.2
4.0
3.8
3.7
Depth
km
T
] J
SS
T?
SS
5
6
6
Appin
Appin
Victoria
Mt. Hotham
Wonnangatta
Balliang
SouthAustralia
SimpsonDesert
SimpsonDesert
Quorn
Blinman
Melton
WestAustralia
Meckering
Calingiri
Lake Mackay
Halls Creek
SS
SS
SS-T?
SS
SS-T
SS-T
Compression
(or
teniion) axis
'(trend
plúnge)
Denham et al., l98l
Bock and Denhar4 1983
Bock and Denharn,1983
Bock and Denharn,1983
262,12
77,24
320,5
271,t0
Denham et al., 198I
Denham et al., I98l
Denhamet al.,798t
Denhamet a1.,7981
Denham et al., l98l
Denham et al., 1981.
3
2
4
5
l2
13
5.6
5.5
19
21
T
T
81
Aftershock
4.6
15
12
T
T
) 1 <?
280,5
66
82
77
5.5
5.4
4.4
15
N
l t
T
T
39,2
11 3 , 1
72
78
77
77
77
6.2
4.7
3.8
4.0
7
?
5
10
15
68
70
70
78
6.9
5.9
6.7
6.2
5
1
15
l't
À 1
ol
IJ
Á 1
2l
731
SS-T
SS
SS
SS
ss
T
T
T
T
Reference
327,tO
136,r4
326,8
95,1
0
2.5
3.0
3.8
Ã')
4.8
SydneyBasin New South Wales
Robertson
Picton
r'åulr
Prane
117 ?5
154,0
46,34
646
DenhanL1980
Mills and Fitch, 1977; Denham et al.,1980
Denham et al., 1982
Denham et al., 1982
)q) )5
Denhamet a1.,7984
Denham et aI., L984
Denham et al., I98l
181,1
125,O
)\a r1
41,0
60,25
Stewa¡tand Denham, 1974
Everinghamand Smith, 1979
McCue and Sutto4 1974
McCue and Suttorl 1974
McCue and Suttoq 1974
)71 17
Denham et aI., 1980
Denham et al., 1980
Denham et al., 1980
Everinghamand Smith, 1979
282,t6
218,24
7)\ A
K U R TL A M B E C KH, . W S N / U O U E E R
N ., A . S T E P H E N S O D
N ., D E N H A M
Figure 6
(a) Fault plane solutions
for some southeast Australian earthquakes.(b) Fault
plane solutions for same
earthquakes
in
other
regions of Australia. Inward pointing arrows indicate inferred direction of
principal
compressional
stress in the horizontal
plane.
6a)
Fault plane solutions for South Australia have been
givenby McCue and Sutton (1979)for 3 eventsin the
Adelaide Geosyncline,and by Stewart and Denham
(L974) and Everingham and Smith (1979) for two
eventsin the SimpsonDesert.All solutionsare indicative of failure by strike-slip faulting irrespectiveof the
focal depth (Fig. 6b). The AdelaideGeosynclinesolutions are for eventsthat arelocatedbelow the sediments
and the three solutions give very similar northeastsouthwest azimuths for the axis of compression.
The Simpson Desert earthquake of 1972 has a fault
plane solution that is indicative of north-south compressionwith failure along a steeplydipping strike-slip
fault (Stewart and Denham, 1974). A nearby event,
while not producing a good focal mechanism, did
yield a well constrainedhorizontalaxisof compression
but with an azimuth that differs by nearly 600from the
1972 event (Everinghamand Smith, 1979).
Fault plane solutions for western Australia have been
publishedby Fitch et ø1.(L973),Everinghamand Smith
(1979) and Denham et al. (1980).All events indicate
that failure occurred by thrust faulting irrespectiveof
the focal depth of the earthquake (table 1). Azimuths
of the compressiveaxes are again quite variable. The
two events in the Canning Basin in the northwest of
Australia are separated by only about 275 km and
yield compressiveaxes that differ by 90o in azimuth.
The solutions for the two events in the south-west
seismic zong the Meckering (1968) and Calingiri
(1.97L) earthquakes, are very consistent with each
other and with the evidencefor surfacefaulting from
theseand other events(e.g.Lewis et al., 1,98I;Gordon
and Lewis, 1980).
This brief summary of the fault plane mechanisms
732
confirms that the predominant stress state within
the Australiancontinentis one of compression.
There
are however,regional variations in the failure mechanisms and the directionsof the principal stressaxes.
In south-easternAustralia" the near-surfacefaulting
is predominantly by strike-slip motion while the
apparently deeperearthquakesreflect failure by thrust
faulting. In South Australi4 failure is by strike-slip
faulting even for the deeper events. In contrast all
solutions for western Australia indicate failure by
thrust faulting from surfaceeventsto depthsof 17 km.
The axesof compressionlie along all possibleazimuths
and this suggeststhat failure is partly controlled by
past geological structures. The different azimuths
obtained for the Hall's Creek and Lake Mackay earthquakesin the northern part of the Canning Basin may,
for example,be a consequenceof a local stressfield
associatedwith the basin and basin-margin superimposedupon a more regional stressfield. Likewise,
the different azimuths obtained in eastern Australia
may be controlled by older faults or it may be the
result of a local, topographically related stress field
superimposedupon a regional stressfield. The different azimuthsfor the two SimpsonDesert earthquakes
are less readily associatedwith local structure, even
in the qualitativemanner usedfor the other solutions.
One trend that does occur is that the compressive
axesare approximatelyperpendicularto the continental
margins for those events within a few hundred kilometersof the margin. The compressiveaxesin southeast
Australia are predominantly east-westor northwestsoutheast, in South Australia they are northeastsouthwest,in the southwest they are again east-west,
and the Hall's Creek earthquake has a northwestsoutheast axis of compression.
CONTINENT
W I T H I NT H E A U S T R A L I A N
S T A T EO F S T R E S S
tï'ü.tl
' Picronl9Z3
1977ht
Appin l98l
Berridole
t959
Roberlson l9ól
lDolton-Gunn¡ng lgzl
l n r t 9 7 3
x
t
1971o
I
\
r
n
197Ä6
Mt Hothom 19óó
Wonnongotto 1982
6b)
<(In situ > stressmeasurements
Since 1957 over 1500 measurementsof < in situ >
stress have been made in Australi4 predominantly
using flat jacks and, since 1963,bore-holeovercoring
methods(Worotnicki and Denham, 1.976).Geophysically useful and reliable results are, however,few and
thesehave been summarizedby Denham et al. (1979,
1980).Thesemeasurementsare in generalagreement
with the results from the fault plane solutions in that
(i) the predominant stressstate is one of compressior¡
and (ii) there is considerableregional variation in the
direction of the compressive axis. In southeastern
Australia, the overcoring methods indicate that the
maximum principal stress is approximately east-west
in the westernpart of the Lachland Fold Belt and
more nearly north-south in the easternpart (seealso
Gray, 1982).Local geologicalconditionsand nearby
topography may strongly influence the total stress
field so that significant changesin stressmay occur
from one locality to another.Analysisof stressinduced
by topography of small slope (McTigue and Mei,
1981)indicatesthat surfacehorizontal stressvariations
of up to 5-10MPa may be expectedin the neighbourmay therehood of 1 km topography.Closeagreement
fore not occur between the < in situ r> measurements
taken at the surlâceand the principal stressorientations
deducedfrom the fault plane solutions for earthquakes
occurring at considerable depths below the surface.
< In situ )) stress measurementsof up to 23 MPa
made near the Meckerine and Cadoux earthquakes
733
Bolliong
1977
in westernAustralia, a region of particularly low relief
show a much more consistent relation to the faultplane solutions,with both sets of resultspointing to
east-westcompression(fig. 5).
EVIDENCE
GRAPHY
FROM
GRAVITY
AND
TOPO-
Central Australian gravity anomalies
In the absenceof expressionsof active or recent tectonism, seismicitymay be anticipated in regions where
stress differencesdepart significantly from a hydrostatic state, that is, in regions of large variations in
topography or gravity. The largest gravity anomalies
within continental Australia occur in the centre,where
changesof 200 mgals occur in a distanceof lessthan
100km (BMR, 1976).Theseanomaliesare associated
with Late Proterozoic and PalaeozoicBasins and
with intervening uplifted arches (fi5. 7). Negative
gravity anomaliesoccur over the basins and positive
anomaliesover the arches.The elevation is relatively
constant over the region as a whole and thiq plus the
density contrasts between the basins and arches,
contributes only a small amount to the observed
anomalies.Arrival times'of P-wavesfrom teleseismic
sourcesare late at stations situated in the basins and
early at stations on the arches(Lambeck and Penney,
1984).This, plus the gravity observations,points to
sienificant low densitv and low velocitv material
N ., D E N H A M
K U R TL A M B E C KH, W S M C O U E E NR, A . S T E P H E N S O D
Musgrove
Amocleus
Arunto
tlgolio
E
¿
Figure 7
Predícted crass-section of the
crust in central Australiafrom
the OIJìcer Basin in the south
to the northern Arunta Block
in the narth (from Lambeck,
1984). A hypothetical upperlower crust boundary, originally at a depth of 15 km, is
also ind.icated.
below the basinsand to high density and high velocity
material beneath the arches. The simplest interpretation is in terms of an undulating Moho (Lambecþ
1983a,b ; Lambeckand Penney,1984)and undulations
of about 20 km are predicted. The mechanism by
which this stateevolved is believedto be one of crustal
folding through horizontal compressionof an inhomogeneousviscoelasticlithosphere.The process is
facilitated by the erosion of the uplifting areasand the
deposition of sediments into the down-warps. As
bending stressesreach critical levels, major thrust
faults developnear someof the basinmarginsand these
are observedat several localities.
To developand maintain this structure,the predominant
stressstate must have been one of horizontal compressionwith an approximately north-south azimuth.
The magnitude of the requisite stressto support the
present structure is difficult to evaluate since it is
dependent on the assumed rheology and effective
thicknessof the lithosphere but a minimum value of
about 100-200MPa appearsto benecessary
to maintain
a balancebetweenthe various forces operating (Lambecþ 1983ø).
The presentbasin structuresevolvedfrom Late Proterozoic to Late Palaeozoicwith only relatively minor
evolution having occurred since then. A regional
compressive stress must, therefore, have been in
existencesince at least Carboniferous time in order
for the structureto be maintainedagainstthe buoyancy
forces. In the absence of such a compressior¡ the
basinswould be uplifted and the archeswould subside
until isostatic equilibrium was achieved. It appears
from the near surface geology and geomorphology
that there may have been a period of rebound of the
basins and this implies reduced overall compressior¡
subsidenceof the arches and possibly a transport of
some sediments from the basins onto the blocks.
Evidencefor this is seenin the outcropsof Mesozoic
and Tertiary erosionsurfacesabovethe presentQuaternary surfaceof the basins and in the down cutting of
rivers through some of the hills within the Amadeus
Basin.It would appeaqhowevet that the compression
was again effectivein Cainozoic time since rhany of
the Carboniferous and older faults appear to have
been reactivatedover large distancesand since many
Cainozoic structural features appear to be a conse734
(Bureket a1.,I978).
quenceof a north-southcompression
can be expectedto occur
The largeststress-differences
near the basin margins where the gravity gradientsare
greatest.Yet despitethe nearby location of two highly
sensitive seismometerarrays (at Alice Springs and
Tennant Creek), the recorded seismicity for the past
15 or 20 yearsis limited to a few small eventspossibly
located on some of the old faults to the north of the
Ngalia Basin. There seemsto be no history of recent
faulting on the margins of the Amadeus Basin. In a
region wherestress-differences
are probably the greatest
that occur anvwhere in the continental Australian
lithosphereseismicity is insignificant.
The SimpsonDesert earthquakeslie about 300 km to
the eastof the AmadeusBasinand the directionof the
compressiveaxisfor the L972earthquakeis in agreement
with what is implied by the structure to the west. Yet
there is no obviousreasonwhy the earthquakesoccur
here and not to the west where the inferred stressdifferencesare much greater.
Darling Fault, West Australia
As first noted by Vening Meinesz(1948),substantial
gravity anomaliesoccur to the east of Perth in West
Australia. These coincide with the ancient Darling
Fault bounding the Archaean Yilgarn Block and the
Perth Basin. If this fault definesa near-verticalboundary betweencrustal blocks of different thickness,then
the gravity anomaly Âg acrossthe fault has an approximate amplitude of
LS - 2nG(p^ - p")LH
(1)
where G is the gravitational constant p* is the density
of the mantlg p" the density of the crustal and LH
the differencein crustal thickness of the two blocks.
The associated maximum stress difference ø*u* is
(Jeffreys,1932).
o-u* !
þ* - p) S AH
(2)
and with (1)
6 ^ u *j
Ls' slz"O :'jLs
pR
(3a)
S T A T EO F S T R E S S
W I T H I NT H E A U S T R A L I A N
CONTINENT
where þ is the mean density and R the mean radius of
the Earth (seealso Lambeck, 1980).For Âg in mgals
6^^*/ 0.23A,g(mgal)MPa .
Indion
Oceon
Shelf
Perth
Bosin
o)
(3b)
The Darling Fault separatesthe Yilgarn Block from
the Late Palaeozoic-Mesozoic
Perth Basin.The change
in gravityacrossthis fault is of the orderof 100-120mgal
with. the positive anomaly being on the Yilgarn side
of the fault. Then, with (p^- p") - 0.3 I cffi-3,
LH - 8-9 km accordingto (1) and this is similar to
what is observedon the fault (Jones,1976).With equation (3) o*", ! 25-30MPa.
In the absenQeof forces other than buoyancy, there
shouid be a tendencyfor the basin to be uplifting with
rnovement occurring on the fault, but nearly all of
the seismicactivity lies some50-100km inland (Doyle
et al., 1.968;Denham et al., 1980)(fig. 5).
I[ through the passageof time, movementon the fault
is no longer permitted becauseof some annealing
process,the effect of the buoyancy force is to create
a more regional uplift involving the westernlimit of the
Yilgarn Block. Maximum compressive forces will
develop to the east of the Darling fault but more
important in magnitude will be the near-surfacetensional forces in the vicinity of this fault. The absence
ofevidencefor the latter suggests
that thereis a regional
compressiveforce which annuls the local tensional
force in the basin but magnifiesthe local compressional
force on the Yilgarn Block (fig. 8). It may be fortuitous,
but where this maximum is predicted to occur by the
model is actually where compressive seismicity is
observed.
Thetensionalforceattainsabout100-200
MPa
and the compressiveforce to the eastof the fault does
not exceed10 MPa. To preventfailureon the fault, the
regional compressionalforce must also be near 100200MPa. Failureto the eastthenimpliesthat the stressstate within the crust is quite close to the limits at
which brittle failure occurs sincethe seismicityappears
to occur in an area where stressesexceedsurrounding
valuesby lessthan 10 MPa.
Southeast Australian highlands
The major topography in Australia is the eastern
highlands which coincides with the more intensely
folded parts of the Lachlan Fold Belt of Palaeozoic
age.Many geologistsview the landscapeas one that is
considerablyyounger than this and have argued that
a peneplainexistedin Late Mesozoicor Cainozoictime
which was subsequentlyuplifted to form the present
topography (e.g. Ollier, 1978; Wellmar¡ 1979). Stephensonand Lambeck(1984)on the other hand,argue
that the highlandsarean erosionalresidueof the Palaeozoic orogeny and that the geomorphologicalevidence
for uplift is not so much indicative of a constructive
phaseof mountain building as of a destructivephase;
that the uplift of the baseof the highlandsis occurring
in response
to theunloadingof thehighlandsby erosion.
gravity
data indicate that the highlandsare nearly
The
in isostatic equilibrium. The maximum stress differencesthat will occur in the underlyingcrubt and upper
mantle will thereforebe of the order p"gh or greater.
735
Yilgorn
L
200m
Block
o
ô
I
I
| 32km
<+
Figure I
Schematic model of the crust in western Australio across the Perth
Basin and Yilgarn Block. The buoyancy force results in uplíft of the
basin and in tensional stress near the basin surface and compression
stress in the Yilgarn Block.
Here p. is the density of the topography and å is the
elevation. For h - 1500 m, o^u*t 50 MPa. This
estimatecorrespondsto the state of local isostasyand
representsa lower bound, but if compensationis regional then the stressdifferencesmay be severaltimeslarger
than this. Most authors assumethat it is the presentday topography that controls the isostatic state of
continents (e.g. Lewis and Dormar¡ 1970; Banks
et al., 1977),but in the erosion-reboundmodel of
and Lambeck(1984)it is the long term and
Stephenson
large scale erosion of topography that controls the
stress state and flexural deformations of old continental lithosphere (see also Stephensor¡1984). A
quantitative evaluation of this model is possible. In
particular, it is possibleto predict the associatedstress
state. In the model adopted here, and discussedat
length in Stephensonand Lambeck (1984),the lithosphereis taken asa linear viscoelasticsolid whoseelastic
and viscoelasticproperties are characterizedby an
effectiveflexural rigidity D and a relaxation time constant r. The topography created during the orogeny
is assumedto be in local isostaticequilibrium at the
time that the fold belt stabilized.Erosion of this topography at any time I is assumedto be proportional to
the remaining topographic height at this time. From
a knowledgeof the presenttopography and an assumed erosion time constant the palaeotopographycan
be computed and, starting with this and now extrapolating forwards to the present,it becomespossible
to compute the stressstate arising from the erosional
unloading the viscoelasticresponseof the lithosphere
to this unloading and the change in, buoyancy force.
Figure 9 illustrates the predicted horizontal surface
stressbased on an assumption of continuous erosion
sincethe stabilizationof the fold belt about 180Ma ago.
The effectiveparametersusedareD : 1023Nm (equivaIent plate thickness: 30 km), r - 25 Ma (equivalent
viscosity- I02s Pa s or 10'u p) and an erosiontime
constantof 150 Ma. Surfacestressesbeneaththe uplifting areasare tensional but, due to the regional nature
of the rebound, become compressionalat the periphery. Stresseswithin the lower half of the plate will
K U R TL A M B E C KH. W . S M C O U E E NR,. A S T E P H E N S O D
N. DENHAIV
3ó
20OMPo
u6
t48
t50
152
Figure 9
resultingfrom the erosíonal
Present-day
horizontalprincipalstresses
reboundmadelof Stephenson
and Lambeck(1984).No regionalstress
Shadedareasrcpresentthepredictednormal
Jìeldhasbeensuperimposed.
750m elevatíons.
faulting regimes.Thecantoursrepresent
be compressivebeneath the elevatedterrain. Stress
magnitudesare difÏicult to quantify with precision but
maximum values of 200 MPa are predictedfor the
Victorian part of the highlands.
The stressmodel illustrated in hgure 9 is not wholly
satisfactorybecauseit neglectsthe stressstateassociated with the continental margin. In particular, it does
not take into account the offshore deposition of sediments which load the ocean crust at the same time
that the continent is being unloaded by erosion. This
aspectof the erosion-reboundmodel will be discussed
elsewhere.The flexural stressesare a function of the
effective flexural rigidity and of the erosional and
viscoustime constantsas well as of the time at which
the erosion and rebound is assumedto becomethe
dominant landform shapingprocess.The generalstress
pattern remains unchanged except that the position
of the line separating tensional from compressional
stressmay shift inwards oÍ outwards from the topographicaxis.
The erosion-rebound model explains adequately the
overall geomorphologicaland geologicalevidencefor
the uplift that has occurred since at least early Cainozoic time, yet it does not explain the present-day
seismicity.Areas where the predicted stressdifferences
are greatest do not correspond with the regions of
greatestseismicactivity (comparefigs.2 and 9). Furthermore, the predictedtensional failure in the upper crust
is not seenin the few availablefault plane solutions
or < in situ > stressmeasurements.This discrepancy
can be overcome if a strong compressionalforce is
superimposedon the local stressfield. This will, for
example,increasecompressionalstressin the southern
part of the SydneyBasin. It will also reduceor annul
736
the tensionalstresses
in the highlandsimmediatelyto
compression
the westof the basinand resultin increased
further to the west.Much of the seismicityin the upper
half of the plate would thereforebe associated
with the
flanks of the elevated region. This pattern should
reversewithin the lower half of the plate where the
flexural stressesshould be reducedin those flanking
areasand increasedunder the highland.Much of the
seismicitydoes occur on the flanks of the Dividing
Range $9. 2) but there is no clear differencebetween
the patterns of seismicity below and above about
10 km. This is possibly a consequence
of the uncertainty of the depthdeterminationof many of the events.
Of somepossiblesignificanceis that the depth distribution of seismicityrevealsa marginally reducedlevel
of activity near 10-12km corresponding
approximately
to the middle-planeof the plate.
I[ as we believe,the presenttopographyis essentially
an erosional residue of the Palaeozoicorogeny, then
is it inescapablethat the pattern of flexural stresses
illustrated in hgure 9 occurs.The magnitude of the
stressesare less certain, being model and rheology
dependent.The absenceof evidencefor tensionalfailure then requiresthe imposition of a regional stress
field, either one that is nearly uniform over the area
as a wholeif it is associated
with plate tectonicsor one
that is variable if it is associatedwith different mechanical and thermal propertiesof oceanicand continental
lithosphere.To remove the predictedtensionalstresses
west of Sydneyrequiresan almost east-westcompression in centralNew South Wales,and a nearly northwest-southeast
compressionin Victoria. In both instances this requisite force appears to be normal to
the coastline.
'We
previously noted that the near-surfacefault plane
solutions are predominantly indicative of failure by
strike-slip faulting while the deeper events failed by
thrust faulting. In the highland area all three principal
stressesdue to erosion are tensionalwith the vertical
stress ø,, being the minimum. A uniaxial regional
stressmay annulone of the horizontalerosionalstresses
leaving o"" as the intermediatestress.Fault plane solutions would, therefore, be strike-slip mechanisms.At
greaterdepth below the highlandsthe horizontal forces
are compressional while o,, remains tensional and
cannot be an intermediatestress.The fault plane solutions are thereforeconsistentwith the superpositioning
of a regionalstressfield on the erosion-reboundstress
field.
DISCUSSION : MORE QUESTIONS THAN ANSWERS
Much of the Australian seismicity is associatedwith
tectonicfeaturesof Palaeozoicor older age and there
is little evidenceto suggestthat thesestructureshave
been activated in more recent times. Some activity
occursin youngerbasins,particularly the PalaeozoicMesozoic Gippsland Basin of Victoria. This activity
appearsto be relatedmore to the underlying basement
than to the basins themselves.Possibly here old faults
are being reactivated by the superpositioningof the
CONTINENT
W I T H I NT H E A U S T R A L I A N
S T A T EO F S T R E S S
basin-generatedstressfield on a more regional stress
f,reld.One possibleinterpretation of the seismicityin the
older provinces is that it is a reflection of the release
of tectonic stressby a slow migration of non-hydrostaticstressfrom the lower part of the lithosphereinto
the middle and upper crust (e.g.Kusznir and Bott,
197'7;Lambeckand Nakiboglu, 1981).As the stresses
in the lower regions relax, the maximum stressdifferencesin the colder and elasticpart of the upper crust
increasewith time and this may lead to brittle failure
long after the introduction of the initial stress field.
Implicit in this argument is that continental crust that
has been subject to signiftcanttectonism is near a
critical stresslimit for much of its history, and that
seismicityis initiated by the superpositionof stress
fields that, in themselves,may be inadequateto produce failure. This additional stress f,reld could be
causedby sedimentationand erosion or by a change
in the global plate-tectonicstressregime.
Some support, albeit very tentativg for this concept
of stressrelaxation comes from the depth distribution
of the seismicity.As emphasizedabove,depthsfor the
Australian seismicity are generally poorly determined
but there is a suggestion that seismicity decreases
in depth acrossthe continent from eastto lvest,corresponding to increasingage of the major tectonic events.
In easternAustrali4 the depths of eventsare distributedfairly uniformly down to about 20km (flrg.3),although
there are no known eventsthat have given rise to surface faulting. In South Australia, the small magnitude
seismicity occurs predominantly at depths shallower
than about 5-10 km, but the larger events(M" > 3)
may occurmore uniformlydown to about20 km depth.
In the southwestseismiczone all the evidenceis consistentwith near surfacefailure.A closerexamination
of the data is warranted, in particular as the above
suggestion contradicts the argument presented by
Chen and Molnar (1983),basedon equallyinadequate
evidence,for increasingdepthsof continentalseismicity
with tectonic age.Chen and Molnar argue that, as the
lithosphereages,it also strengthensand becomesmore
at depth. What
supportive of greater stress-differences
we suggesthere is t}:'at,at least within the Australian
continent the present-daycontinental seismicity may
be the resultof relaxationof stresses
that wereimplanted
into the lithosphereat the time of tectonism,possibly
triggered by minor stressfields of more recent origin.
An interestingaspectof this propositionis that regions
of past tectonicactivity may remain seismicallyactive
long afterthe originaltectoniceventceased.
The upward
migration of stress may be sufficient to keep nearsurfacefaults active and so result in a distribution of
in the original
seismicitythat followszonesof weakness
structure. Such delayed stress build-up is predicted,
model of Stefor example,by the erosional-rebound
phensonand Lambeck (1984)in which the combinastress
tion of the erosionthrough time,plus viscoelastic
relaxation,resultsin maximum stressdifferencesoccurring some 100 to 200 Ma after the initiation of the
erosionalphase.Conceivablythis offersan explanation
for the widespreadbut small-volume Cainozoic volcanism of eastern Australia.
Most of the better constrainedsolutions for earth737
quake depthsindicate an absenceof seismicityin the
crustbelowabout 20 km and no earthquakes
havebeen
observedin the upper mantle. Models of tectonic
processes
within the Australiancontinentindicatethat
the effectiveflexural rigidity of the lithosphereis of the
and Lambeck,L984;
order (1-5)1023Nm (Stephenson
Lambeck, I983a) and the corresponding equivalent
thicknessof the layer is about 30-50km. Stress-difference estimateswithin this layer remain model dependent but it appearsinevitable that magnitudesof 50200 MPa occur in the lower crust and upper mantle.
That stress differencesin the lower crust cannot be
negligibly small is also indicated by the inferenceof a
variabledepth Moho in severalparts of Australia(e.g.
Drummond, 1981;Lambeckand Penney,1984)requiring stressdifferencesof the order of 50 MPa. The
implication is that thesestressesin the lower crust do
not relax by brittle failure but by a creepprocesswith
a very long time constant.This is largely in agreement
with presentconceptsof lithosphericrheology based
on laboratory results of a brittle upper crust and a
ductile lower crust (e.g.Brace and Kohlstedt, 1980).
That earthquakesdo not appear to occur in the upper
mantle beneaththe Australian continent suggeststhat
this part of the lithosphereis alsoductile,or wasductile
in the past when temperatureswere higher than today,
or that stressesthere have already relaxed by brittle
failure in theserelatively old cratons. Chen and Molnar (1983)discusssomeevidencefor mantle seismicity
in other parts of the world and suggestthat the uppermost part of the mantle is relatively strong and that
deformation by brittle failure occurs therein. Possibly
what distinguishesthe Australian continentalseismicity
by Chenand Molnar
from that in the regionsdiscussed
is that, in the latter, tectonic processesare operating
today, primarily in the form of collisional tectonics.
In contras! the absenceof deeper seismicity in the
Australian continent could mean that the shallower
activity is not predominantly the result of present-day
driving forces.
There are, however, some difhculties with this interpretation. Firs! the predominant mode of failure near
the surfacedue to the stressrelaxationand the readjustment to erosionwould be one of failure by normal faulting. Secondwherethe processis anticipatedto be most
important, such as in central Australi4 present-day
levels of seismicityare also low. Thesedifficulties can
be overcomeif a regional compressivestressfield acts
on much of the continent. Indeed.the evidencefor the
predominant state of compressionof the Australian
continent does suggestthat there may be such a field
with the consequencethat some of the local tensional
forces are cancelledand local compressiveforces are
reinforced.Examplesof where this is inferred include
the southwestseismiczone,the southeasternhighlands
and the intracratonic basins of central Australia. The
nature of this regional compressionalforce remains
obscure.Severalattemptshave been made to interpret
both the stressheld and the seismicitywithin the framework of the plate tectonicshypothesis.Sykes (1970)
speculatedthat the seismicityin westernAustralia may
be part of a nascentsubductionzone; Cleary and Simpson (1971)suggestedthat the Indian-Australianplate
K U R TL A M B E C KH, W . S M C O U E E NR, A S T E P H E N S O D
N ., D E N H A I \ 4
given by Richardsonet al. (1979)show remarkably
good agreementbetweenpredictedand observeddirections of axesof maximum compressionover the Australian continent(lig. 10)if not alwayseverywhereelse.
Their model E1 (fig. 10a)drivesthe platessolelyby a
ridge-push force and, in the region in question, the
horizontal deviatoric stressesare compressive.Similar
stressstates are given by their models 827 and E3t
(fig. 10b) in which the plates are driven by different
combinationsof ridge-pushforces,forcesat subduction
and continental convergencemargins, and viscous
drag forceson the baseof the plate.
>--
The Richardson el ø/. models produce deviatoric
stresses
that are of the order of 10 MPa or lessbut this
is a consequence
of the adoption of nominal valuesfor
the various forces. Boundary stressesare typically
setat about 10 MPa whereasDavies(1978)and Hanks
(1977)arguethat thesestresses
arethe orderofhundreds
of MPa. Theselatter estimatesare of a magnitudewhere
they may support or annul the stressfields generated
by the local tectonic structure and generateseismicity
that may not otherwiseoccur. One of the important
aspectsof thesemodelsis that they canreproduceregional variations in the orientation of the stressfield that
are of comparablemagnitudeto those observed.
a)
b)
Figure 10
(a) Principal horizontal stress in the lithosphere far model El (ridge
push only) of Richardson et al. (1979). Principal stress axes v)ithout
arrows iÌenote compressían. (b) Principal horizontal tleyiatoric stress
in the lithosphere for model E3l (ridge push, continental convergence,
slab pull anil yiscous drag on the base of the lithosphere) of Richardson
et al. (1979).Principal stressaxes without arraws denote compression.
Axes with arrows denole tension.
is not a singleentity but that the Australasianpart may
consist of severalplates whose boundariesare def,ined
by three broad zones of seismicity seen within the
continent; Fitch et al. (1973)argued that the lithosphere acts as an efficient stressguide such that the
seismicity within the continent reflects the response
of the continent to variable forces acting on the plate
boundaries.
Estimatesof the stressesgeneratedat plate margins are
little more than guessesand stressesassociatedwith
density and structural anomalieswithin the plate interior are likely to be at leastasimportant. The separation
of the local and regional stressf,reldsremains problematical as the above-discussed
examplesshow. In the
east and west of Australia this regional field appears
to be predominantly east-west but it is more nearly
north-south in south and central Australia. This regional or global stressfield must thereforeexhibit some
azimuthal variation. Also, the superpositioningof a
regional stresscould be at variance with the previous
argument, that the Australian continent is relatively
free from a contemporaryforce f,reldunlessthesestress
levelsare below the brittle strengthof the upper mantle.
Nevertheless,it is interesting that some of the models
The Indian-Australian plate experienceshigher levels
of seismicitythan do other intraplate regionssuchasthe
Antarctic and American plates. From an analysis of
plate motions, Minster and Jordan (1978)deduceda
compressional
deformationof about 1 cm yr- 1 occurring in a northwest-southeastdirection across the
Ninety East Ridge. Weisselet al. (1980)also showed
from the deformation of recent sediments.that the
plate has been undergoing deformation since the late
Miocene,possiblyin responseto the changingboundary
conditions along the northern margin of the plate.
They suggestthat an increasein resistiveforces along
the Himalayanboundary could resultin the transmission of horizontal compressioninto the plate interior
and that this could qualitativelyexplainthe east-west
and northwest-southeastcompressivedirections in
westernAustralia.
We havenot examinedherethe geologicalevidencefor
the past stressstate of the Australian continent but
there appears to be very considerableevidence that
suggeststhat compressionwas more the rule than the
exceptionsince at least Palaeozoictime (e.g.Powell
et al., 1984; Burek et al., 1978),both before and after
the Mesozoicbreak up of Gondwanaland.If correct,
the ridge-pushforcesare unlikely to play a major role
in determining the overall stressfield.
One platetectonicsforcenot considered
in the Richardson et ø1. model is the stress associatedwith the
change in curvature of the Earth's surface as the
continents change latitude (Turcotte and Oxburgh,
1973).As a plate movesfrom high to low latitudesthe
radius of curvature decreases
and the associatednorthsouth stresses
in the upper half of the plate should be
tensional near its centre and possibly compressional
near its edge.Magrritudesof thesestresses
have been
estimated to approach several kbars (Turcotte and
Oxburgh,i973). Dooley (1983)suggested
that the Aus-
738
S T A T EO F S T R E S S
W I T H I NT H E A U S T R A L I A N
CONTINENT
tralian continent as part of Gondwanaland,may have
spentmuch of its time moving from low to high latitudesand that this may explain the geologicalevidence
in latePalaeozoicand
for the predominantcompression
Mesozoic time. In more recent time the continent has
been moving equatorward,ye| compressionremains
the dominant stress.The associatedstressespostulated
by Turcotte and Oxburgh are basedon elastic shell
theory. This is inappropriate for processesoperating
and the actualstressstate
on thesevery long timescales
is likely to be much less than the above estimates.
In addition to the stressfield associatedwith plate
tectonics there is the possibility that the regional
stressis in part an inherent property of the continent's
mechanicaland thermal properties.This is suggested
by the observationthat the compressionaxesare nearly
have
orthogonalto the coastline. Similar observations
beenmade elsewhere;for example,along the Atlantic
coast of the United States(Zoback and Zoback, 1980)
and along the south and south-easternshore of China
(Ding Yuanzhangper comm.,1983).The mechanismis
not the lateral structural contrasts betweencontinent
and ocean as discussedby Bott and Dean (1972),
nor the flexure resulting from sedimentloading (Walcotl. 1972;Turcotte et al., 1977).Rather,a more appropriate mechanismmay be the one proposedby Fleitout
and Froidevaux(1982)in which cold lower lithospheric
material, in this case the subcontinentallithosphere,
inducesa strongregional compressivestressthat is sufficient to support near-surfaceloads. Fleitout and Froidevaux cite the geophysicalevidencefor such a cold
lower lithosphereunder the EuropeanAlps in support
of their model. Similar evidencehas not been found
under the easternAustralian highlands,but neither has
the limited geophysicaldata been examinedwith such
a model in mind for this or any other provinceof the
Australiancontinent.
CONCLUSIONS
The seismig and < in situ > stressmeasurements,
as
emphasizedby most previous writers on the subject,
provide overwhelmingevidencefor the predominantly
compressivestate of the Australian continent,and that
this stressis being releasedby brittle failure only in the
upper and middle regionsof the crust.Geologicalevidence suggeststhat this compressivestate may have
existedthrough much of geologicaltime since at least
the early Palaeozoic.
The seismicityof Australia occursin threemain zones.
primarily with the
In easternAustralia it is associated
most deformed parts of the PalaeozoicFold Belt.
Seismicity occurs down to depths of about 20 km
and no eventshave been located with certainty in the
lower crust below the Moho at a depth of about 4050 km (Finlayson et aI., 1979).The South Australian
seismicityis restricted mainly to the late Proterozoic
Adelaide Geosynclineand most of the eventsoccur in
the upper crust within the thick sedimentarysequences
folded into the syncline.The reasonsfor the seismicity
in the Simpson Desert remain obscure, particularly
becausethe solid geology is obscuredby the Quater739
nary cover which blankets the area. The seismicity of
westernAustralia is diffuseand is well documentedonly
for the southwesternarea. Here, the activity is located
in the ArchaeanYilgarn Block and there is no obvious
correlationwith any major tectonicfeatures.
Much of the Australianseismicityappeaßto delineate
broad zones of past weakness,such as the Adelaide
Geosynclineor the Lachlan Fold Belt. That seismicity
still occurs in these areas may reflect the relaxation
of stress-differences
associatedwith thesepast tectonic
upheavals.As relaxation occurs at depth and the effective elastic thickness of the lithosphereis reduced,
deviatoric stressesin the upper crust actually increase
with time. During this relaxation process,however,
the lithospherecools,viscouscreeprates decrease,and
the layer increasesin strength.It remains uncertain
which of the two competing effectsdominates; the
migration of deviatoric stressinto the upper crust or the
increasein strength of the lower crust with time. The
Australian data does suggestthat the stress-differences
may ultimately exceedthe strengthof the crust and that
failure takes place long after the original teetonic process occurred. Superimposedupon this relaxation
model will be changesin the ambientstressstateinduced
by changingsurfaceloads suchas erosionand sedimentatior¡ or by changes in the plate tectonics regime
such as the changing boundary conditions along the
plate margin to the north of Australia. Relativelysmall
changesin the overall stresspattern could be enough
to trigger seismicity in regions where the relaxation
processhas resulted in stresslevels that are near the
critical failure limit.
It is curiousthat wherethe non-hydrostaticstresses
are
greatest- as deducedfrom the gravity field - levels
of seismicity are exceedinglylow. This indicates that
featuressuch as the basin and arch of central Australia
or the Perth Basin and Darling Fault of westernAustralia representstates of mechanical equilibrium between buoyancy,load, elastic and viscoelasticforces
on the one hand and horizontal compressiveforces
on the other hand and that theselast-mentionedforces
must be of the order of 100-200MPa. In the model of
the Perth Basin and the Darling Fault the resultant
of the local and regional stressfieldsleadsto maximum
compressionin the Yilgarn Block, east of the fault
and this is wheresomeof the major seismicityin western
Australiaoccurs.
The erosionof the easternAustralianhighlandsleads
to regionalisostaticrebound and the associatedstress
field can be evaluated quantitatively. The results are
tension near the surface of the central highlands and
compressionnear the surfaceon the flanks. At depth
this pattern is reversedwith compressionbelow the
highlandsand tension below the flanks. Instead the
observedstressstateis one of compressionthroughout
the upper crust. Again a regional compressivestress
must be invoked.
The natureof the regionalcompressionremainsobscure.
The observationsand modelssuggestthat the azimuth
of this compressiveforce is variable and that this force
may be nearly orthogonal to the coastline. Arguments
can be marshalled for and against a plate tectonics
K U R TL A M B E C KH, W S M ' O U E E N R
, A . S T E P H E N S OD
N, DENHAM
observations.
Theseassertions,
like many othersin this
paper,remainto be tested.
cause,where the compression is a consequenceof forces
acting on the plate margins and on the base of the
lithosphere. Another possibility is that this regional
field is a consequence of thermal and mechanical
properties of old continents along the lines proposed
by Fleitout and Froidevaux (1982), particularly if the
compressive force has operated through much of geological time as is suggested by numerous geological
A,cknowledgements
David Denham publisheswith the permissionof the
Director, Bureau of Mineral Resources,
Geology and
Geophysics.
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