Australia boasts arguably the richest Late Neogene to Quaternary faulting record in stable continental region(SCR) crust anywhere in the world. Variation in fault scarp length, vertical displacement, proximity to otherfaults and relationship to topography permits division of the continent according to fault character. Six on-shore “neotectonic domains” are recognised, with an additional offshore domain proposed by analogy withthe eastern United States. Each domain relates to a distinct underlying crustal type and architecture, broadlyconsidered to represent cratonic, non-cratonic and extended environments. In general, greater topographicexpression associated with faults occurring in extended crust relative to non-extended crust suggests ahigher rate of seismic activity in the former setting, consistent with observations worldwide. Using thesame reasoning, non-cratonic crust might be expected to have a higher rate of seismic activity than cratoniccrust. This distinction, together with the variation in fault character between domains, should be recognisedin attempts to identify analogous systems worldwide.A common characteristic of morphogenic earthquake occurrence in Australia appears to be temporal cluster-ing. Periods of earthquake activity comprising a finite number of large events are separated by much longerperiods of seismic quiescence, at the scale of a single fault and of proximal faults. In several instances there isevidence for deformation at scales of several hundred kilometres switching on and off over the last severalmillion years. What is not clear from the limited palaeoseismological data available is whether successive ac-tive periods are comparable in terms of slip, number of events, magnitude of events, etc. Regardless, thisapparent bimodal recurrence behaviour poses challenges for probabilistic seismic hazard assessment(PSHA). These rely on the simplifying assumption that large earthquake recurrence for long return periodsis not random (i.e. Poissonian). Presently, our ability to incorporate such time-dependent models is limitedby available data.
Australia is classified as a stable continental region (SCR) in termsof its plate tectonic setting and seismicity (Johnston et al., 1994).While such settings produce only approximately 0.2% of the world'sseismic moment release, large and potentially damaging earthquakesare not uncommon (e.g. Crone et al., 1997). In the last four decadesfive locations in Australia are documented as having experienced sur-face rupturing earthquakes (Fig. 1; Table 1).
130°E
130°E
120°E
120°E
110°E
110°E
10°S
20°S
30°S
40°S Precambrian Cratons
Phanerozoic Accretionary Terranes
Proterozoic Mobile Belts
Proterozoic-Paleozoic Fold Belts
Extended Continental Crust
SHmax directionEarthquake magnitude
4.0-4.99
5.0-5.99
> 6.0
Gawl
FlSeZo
S
Musgrave Mobile Belt
Albany-Fraser
4
Creek
Creek
Mobile Belt
Marryat
WA
km
North AustCraton
Pilbara Craton
Pilbara Craton
Yilgarn Craton
Pinjarra Orogen
Naturaliste Plateau
NW Shelf
Capricorn Orogen
CapricornOrogen
PatersonOrogen
King Leopold-Halls Creek
Orogen
Kimberley Craton
Tenna
NT
1 23
0 500 1,000 1,500250
Fig. 1. Simplified geological basement terranes of the Australian continent (modified from Shaw(M≥4) from 1841 to 2010 (http://www.ga.gov.au/earthquakes/searchQuake.do), major seismTable 1), and orientation of maximum horizontal stress (SHmax) (Hillis and Reynolds, 2000, 20
Low strain accumulation and release rates in SCRs relative to platemargin regions suggest that the recurrence of large earthquakes onindividual faults is typically thousands to hundreds of thousands ofyears or more (e.g. Adams et al., 1991; Clark and Leonard, 2003;Clark et al., 2008; Crone et al., 2003, 1997; Leonard and Clark,2011). A notable exception is the New Madrid Seismic Zone (NMSZ)in the central United States where the recurrence of M>7 eventsfor the last three seismic cycles, reportedly on one of several faultsresponsible for the 1811–1812 earthquakes (the Reelfoot Fault —
150°E
150°E
140°E
140°E
10°S
20°S
30°S
40°S
er Craton
AdelaideGeosyncline
ThomsonFold Belt
New EnglandFold Belt
Adelaide-KanmantooFold Belt
indersismine
SE SeismiZone
Lachlan Fold Belt
A
NSW
VIC
TAS
Mount IsaFold Belt QLD
Delamerian Fold Belt
ralian
5nt
NorthQueenslandFold Belts
et al., 1996 and Bain et al., 2004) showing locations andmagnitudes of historic seismicityic zones (Hillis et al., 2008; Leonard, 2008), historic surface ruptures (numbered 1–5— cf.03).
Table 1Details of documented surface rupturing earthquakes in Australia since 1960 (cf. Fig. 1).
Location Magnitude Year Reference(s)
1 Meckering, WA MS 6.8 1968 Gordon and Lewis (1980)2 Calingiri, WA ML 5.9 1970 Gordon and Lewis (1980)3 Cadoux, WA MW 6.1 1979 Lewis et al. (1981)4 Marryat Creek, SA MW 5.7 1986 McCue et al. (1987); Machette et al.
(1991, 1993); Crone et al. (1997)5 Tennant Creek, NTa MW 6.3–6.6 1988 Bowman et al. (1990); Crone et al.
(1992, 1997)
a Tennant Creek involved a series of three consecutive surface rupturing eventswithin one day.
3D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
Champion et al., 2001; Guccione et al., 1999; van Arsdale et al., 1998),has been five hundred years on average (e.g. Tuttle et al., 2002, 2005).While palaeo-liquefaction investigations of the Charleston seismic zonein South Carolina (USA) (Talwani and Schaeffer, 2001) and Charlevoixseismic zone in southeastern Quebec (Canada) (Tuttle and Atkinson,2010) also suggest a recurrence of large earthquakes on the order ofseveral hundred to several thousand years, although such studies arenot fault-specific (e.g. Obermeier et al., 1992). Similarly, frequent recur-rence of damaging ground shaking in the Kachchh rift region of north-west India has been attributed to earthquake activity on several faults(e.g. Bhatt et al., 2009), potentially strongly influenced by the~400 km distant Eurasian/Indian plate margin (Shulte and Mooney,2005; Stein et al., 2001). An associationwas proposed between extend-ed continental crust (failed rifts and passive margins) and the height-ened intra-plate earthquake activity that might give rise to thismoment release (Gangopadhyay and Talwani, 2003; Johnston et al.,1994), but analysis of an updated catalogue later showed this to lackstatistical significance, at least for the historic period (Shulte andMooney, 2005). In the case of Peninsular India, Vita-Finzi (2004) pro-poses that lithospheric-wavelength buckles relating to stresses arisingin the Himalayan Orogen influence the localisation of seismicity. Alink with seismicity has not been demonstrated in other continentswhere similar lithospheric buckling is apparent (e.g. Cloetingh et al.,2005; Nikishin et al., 1993; Quigley et al., 2010; Sandiford andQuigley, 2009).
In general, the intra-plate regions of the world, including Australia,are under-explored in terms of their neotectonic and palaeoseismic re-cords. The reasons for this are primarily a low perception of hazard anda relatively low rate of large, surface-rupturing (morphogenic) earth-quakes. Associated with the latter is the issue of preservation of faultscarps in the landscape. Obtaining evidence from a representative num-ber of seismic cycles for a given fault is often challenging and expensive;in erosional environments evidence for all but themost recent event(s)may have been removed, while in depositional environments shallowpalaeoseismological investigations may only be able to reach stratarelating to the most recent event(s). In addition, age constraint maybe difficult to obtain due to the limitations of conventional geochrono-logical techniques. For example, the range of 14C datingmight span onlyone, if any, of the last few large earthquake events. As a consequence,important conclusions regarding the potential for damaging ground-shaking in an area are often necessarily based upon models supportedby minimal data, or by analogy with deforming regions elsewhere(so-called analogue studies). Accordingly, an improved understandingof rupture behaviour in a tectonically-analogous setting permitsgreater confidence when making such extrapolations.
Australia boasts arguably the richest Late Neogene to Quaternaryfaulting record in all of the world's SCR crust (Clark et al., 2011a;Quigley et al., 2010). This record, spanning tens of thousands ofyears, occasionally affords the opportunity to examine the behaviourof SCR faults over multiple seismic cycles. In this paper we present areview of long-term SCR fault behaviour in Australia, and propose ageo-spatial “domains”model that might be used to capture variabilityin the characteristics of large earthquakes in different geological
settings. A brief overview of neotectonic deformation in SCR Australiais followed by an introduction to the neotectonic domains concept,and each domain is then defined in terms of its geological/tectonicsetting and fault characteristics. Analysis of the underpinningneotectonic data focusses on fault length and vertical displacementas key parameters, and long-term patterns are discussed in both thespatial and temporal context. Drawing upon this information, amodel of rupture–recurrence behaviour for SCR faults is proposedwhich may be testable in SCR areas worldwide. The complementarynature of the current study is identified in light of previous work(e.g. Johnston et al., 1994), and commentary on the implications foranalogue studies is presented.
2. The ‘neotectonic’ era in Australia
In this contribution two terms are used that require careful defini-tion. A ‘neotectonic fault’ is defined as a fault that has hosted displace-ment under conditions imposed by the current Australian crustalstress regime, and hence may move again in the future. Similarly,‘neotectonic displacement/deformation’ is defined as displacement/deformation under conditions imposed by the current crustal stressregime. These definitions recognise, after Machette (2000), thatassigning an ‘active/inactive’ label to a fault in a slowly deformingarea based upon the occurrence (or non-occurrence) of an event inthe last few thousands to tens of thousands of years is not a useful in-dicator of future seismic potential.
The most significant plate-wide neotectonic deformation consistentwith contemporary stress trends throughout the Australian plate isthought to have occurred during the Late Miocene (ca. 10–5 Ma), poten-tially at a lower bulk strain rate than present (Hillis et al., 2008; Sandifordet al., 2004). This LateMiocene re-organisation coincides temporally withsignificant changes at the Australian Plate margins (Dyksterhuis andMüller, 2008; Hillis et al., 2008), including the onset ofmountain buildingin the New Zealand Southern Alps (Sutherland, 1996;Walcott, 1998) andin Papua New Guinea (Hill and Hall, 2003; Hill and Raza, 1999; Packham,1996). Keep et al. (2002) recognise the same deformation event in theTimor Sea (Fig. 2), correlating it to the main compression phase inPapua New Guinea (Hill and Raza, 1999) and a proposed collision be-tween a micro-continental fragment and the Banda Arc (Richardson andBlundell, 1996). The authors also relate this collision to reactivation ofstructures in the Carnarvon Basin on the Northwest Shelf and theEromanga Basin in central eastern Australia (cf. Etheridge et al., 1991)(Fig. 2). A locally more intense event in the Timor Sea at ca. 3 Ma is pro-posed to reflect collision of the Australian and Eurasian plates (Keepand Haig, 2010; Keep et al., 2002; Packham, 1996). Deformation struc-tures relating to this Pliocene event havenot beendocumented elsewherewithin the Australian Plate.
Structural and sedimentary evidence from southeast Australiaprovides clear examples of the influence of plate reorganisation oncrustal stress and neotectonism (e.g. Dickinson et al., 2001, 2002;Hillis et al., 2008; Sandiford, 2003b; Sandiford et al., 2004). A majorunconformity found in all southeast Australian basins (e.g. Gippsland,Otway — Fig. 2) is related to substantial regional-scale tilting, uplift,folding and reverse faulting of Late Miocene and older strata(Dickinson et al., 2002). Pliocene and Quaternary strata overlyingthe unconformity contain neotectonic structures consistent with thecurrent in situ stress field, as determined from seismicity and down-hole stress measurement (e.g. Dyksterhuis and Müller, 2008; Hillisand Reynolds, 2000, 2003; Hillis et al., 2008).
Although local sources of stress cannot be precluded everywhere(e.g. Dentith and Featherstone, 2003), the neotectonic data presentedhere suggest that ongoing deformation within the Australian Plate isprimarily a response to distant plate boundary interactions (Hillis etal., 2008; Sandiford et al., 2004). Indeed, the current intra-plate stresscondition has been satisfactorily modelled in terms of plate boundaryforces in combination with mantle basal tractions (Burbidge, 2004;
Fig. 2. Offshore basins (solid grey lines) and selected (dominantly) onshore basins (dashed black lines) of the Australian continent (modified from Bain et al., 2004; Blake andKilgour, 2000). Note onshore terrestrial components suggesting uplift in the Otway, Gippsland and Southern Carnarvon basins.
4 D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
Coblentz et al., 1995, 1998; Dyksterhuis and Müller, 2008; Reynoldset al., 2003). A possible implication is that small changes in plate mar-gin interactions might have noticeable effects in terms of activitywithin the continent.
3. A long-term record of large (morphogenic) earthquakes inthe landscape
Erosion rates derived from cosmogenic 10Be abundances in geo-graphically disparate lithologies across Australia are ubiquitously low,but also non-zero. Low relief regions of the western two-thirds ofAustralia (Fig. 2) are characterised by erosion rates of 0.2–5 m/Ma(Belton et al., 2004; Bierman and Caffee, 2002; Chappell, 2006; Fujiokaet al., 2005; Quigley et al., 2010), higher relief areas of eastern Australiaby rates of up to 30–50 m/Ma (Heimsath et al., 2000, 2001; Tomkins etal., 2007; Weissel and Seidl, 1998; Wilkinson et al., 2005), and in theFlinders Ranges by rates locally up to 122 m/Ma, but averaging around40 m/Ma (Bierman and Caffee, 2002; Chappell, 2006; Quigley et al.,2007a,b). In general, erosion rates appear to correlate primarilywith re-gional relief, and at secondorder to local relief. In the case of the FlindersRanges, anomalously high bedrock erosion rates have been linked to re-lief and neotectonism, and correlate negatively with climate (Quigleyet al., 2007a; Quigley et al., 2007b). The first order topographic controlon erosion rates implies that fault scarps, the footprint of large surfacerupturing (morphogenic) seismic events, might be recognisable in thelandscape for hundreds of thousands of years or more in central andWestern Australia, but for only several tens of thousands of years to ahundred thousand years in the Flinders Ranges and eastern Australia.In regions with extremely low erosion rates, such as on the NullarborPlain (Fig. 2), it has been claimed that a seismic record spanning thelast 15 Ma has been preserved essentially intact (Hillis et al., 2008). In-direct evidence for tectonic uplift, such as enhanced rates of erosion(Quigley et al., 2007a), and deformed fossil planation surfaces andbedrock–sediment interfaces (Celerier et al., 2005; Quigley et al.,2007c, 2010) can also be significantly long-lived, sometimes survivingthe entire neotectonic era.
3.1. The neotectonic domains concept
Low landscapemodification rates acrossmuch of the Australian con-tinent provide an opportunity to assess long-term seismogenic fault be-haviour overmultiple seismic cycles. The Australian landscape reveals amarked disparity in the crustal response to imposed tectonic stresses.For example, neotectonic faults inWestern Australia are typically wide-ly spaced, are not associated with historic seismicity, and displace a lowundulating landscape by less than 10 m (e.g. Clark, 2010). In contrast,faults in theMt Lofty Ranges are closely spaced, are commonly associat-ed with historic seismicity, and host neotectonic displacements of up toa couple of hundred metres (e.g. Sandiford, 2003a, 2003b). A general-ised summary of the fault characteristics data from across Australia ispresented in Table 2. In an attempt to capture this variability, Clark(2006) proposed six preliminary seismicity source zones spanningcontinental Australia based on the landscape expression of approxi-mately 200 neotectonic features (mainly faults with surface expres-sion). Each earthquake source zone (domain) contained ‘neotectonicfaults’ (i.e. those which have hosted deformation under the currentAustralian crustal stress regime — Clark et al., 2011a; cf. Section 2)that it was contended shared common recurrence and behaviouralcharacteristics, in a similar way that earthquake source zones are de-fined using the historic record of seismicity (e.g. Gaull et al., 1990).
A review of existing published (and unpublished) data and informa-tion, alongwith recent advances in digital elevation data resolution andcoverage, has facilitated the compilation of an expanded inventory ofover 300 features that may relate to Neogene to Quaternary morpho-genic earthquakes in Australia. Fault characteristics, derived primarilyfrom assessment and measurement of the topographic expression ofneotectonic features in the landscape, provide an opportunity to reviseand update the domains of Clark (2006). In a similar fashion to Johnstonet al. (1994) continental-scale geologic and geophysical data sets havebeen used to provide guidance in terms of the gross division of crustalunits (e.g. Bain et al., 2004; Blake and Kilgour, 2000; FrOGTech, 2005;Murray, 1997; Palfreyman, 1984; Shaw et al., 1996; Wellman, 1976),while a range of larger-scale data sets has been used to refine domainboundaries between regions of apparently dissimilar long-term fault
Table 2Relative comparison of neotectonic fault characteristics across Australia, which serve as a basis for defining neotectonic domains.Modified after Clark et al. (2011a).
Neotectonic region Western/central Australiaa Nullarbor Flinders/Mt Lofty Ranges Eastern Australia Mesozoic extended crust
Completeness of faultdata
High in south-west, lowelsewhere
Moderate to high Moderate in Mt Lofty andnorthern Flinders Ranges,low in southern FlindersRanges
Moderate to high in Victoria,moderate to low elsewhere
Moderate to high in onshoresettings
Number of known/suspected faultsb
~80–100 40–50 40-50 (a dozen moreoccur on the Eyre andYorke peninsulas)
~40–50c 40–50
Relative density of faults Low Low to moderate Moderate to high Low to high (clustered) Moderate (high inGippsland/Otway)
Relationship of faults torelief building
No relation between faults andlarge-scale topography, exceptperhaps in the Carnarvon Basin
No relationbetween faults andsubduedlarge-scaletopography
Ranges bound by majorreverse faults
Typically no relationbetween faults andlarge-scale topography, butlocally fault-bound ranges
Typically no relationbetween faults andlarge-scale topography, butlocally fault-bound ranges
Relative historicseismicity rate
Moderate to high in SW, lowelsewhere
Very low High Moderate (low in the north) Moderate to high in onshoresettings
Relationship of historicseismicity to faults
Concentrated where there havebeen historic surface ruptures,rarely associated withpre-historic fault scarps
No association withpre-historic scarps
Well defined belt of highseismicity, locallyassociated withpre-historic fault scarps
No clear association withpre-historic fault scarps
Local clear association withpre-historic fault scarps
Post ca. 10 Madisplacement on faults
10 m or less Generally 10 m orless, rarely 30 m
Many with displacementsup to ~100–150 m
Many less than 10 m, someup to 100–200 m
Many less than 50 m, someup to 200 m
Number of post ca. 10 Maruptures on anindividual fault
Few (b5?) No data (likely tobe b5–10)
Many (several tens ormore)
Few to many Many (several tens or more)
Examples Hyden, Meckering, Lort River Roe Plain,Mundrabilla
Wilkatana, Milendella,Burra, Para
Khancoban, Cadell, LakeGeorge, Lake Edgar, D'AguilarRange
Rosedale Monocline,Fergusson Hill Monocline,Rough Range Fault
a Relationships based upon the better-studied southwest part of this region.b Most regions are under-explored, and the degree of investigation is not consistent within or between regions. For example, large areas of northwest Western Australia are
shown as having few neotectonic features, largely as a consequence of the region having been little studied.c Several features in Victoria are based upon underground mine records of faulted Late Tertiary (10–3 Ma) basalts (e.g. Canavan, 1988), where faulting has no surface expression.
Further records associate concentrations of historic seismicity with large fault scarps.
5D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
behaviour. A seventh, entirely offshore domain is proposed based uponanalogue studies with the central and eastern USA (Wheeler, 1995,1996, 2009; Wheeler and Frankel, 2000). The resulting neotectonic do-mains, which reflect the regional variation in the response of theAustralian continental crust to the imposed tectonic forces, arepresented in Fig. 3. The power of the domains approach lies in the abilityto extrapolate characteristics of neotectonic deformation from a fewwell characterised features to a larger number of features about whichlittle is known. For example, one of themost important physical charac-teristics of a fault rupture is its along-strike length, which can be relatedto the magnitude of the causative earthquake (e.g. Wells andCoppersmith, 1994). Estimates of themaximummagnitude earthquake(Mmax) that a domain is capable of producing can be determined fromfault length data (e.g. Biasi and Weldon, 2006; Stirling et al., 2002;Wheeler, 2009) after careful consideration of the possibility ofsegmented rupture. This provides the potential to significantly enhanceunderstanding of seismic hazard at time scales more representativethan the snapshot provided by the historic record of seismicity.
In the sections that follow, the domains are defined and describedin terms of their geological and neotectonic characteristics, with ex-amples from the best characterised parts of each domain. A compre-hensive record of the neotectonic data compilation and revision canbe found in Clark et al. (2011a), and a copy of the spatial data that un-derpins the Neotectonics Database is available in KMZ format on thepublisher's website or from the authors.
It is important to recognise that the neotectonic data possess anumber of inherent biases (cf. Table 2). These include (i) a significantsampling bias towards the southern part of the continent (Fig. 3)related to spatial data quality and proximity to major populationcentres (the ultimate goal of this work is assessing hazard to commu-nities), and (ii) a relative difference in scarp preservation potential
between northern and southern Australia as a result of climate andits influence on erosion/deposition and vegetation.
3.1.1. Domain 1 — Precambrian craton and non-reactivated Proterozoiccrust
A significant proportion of the basement of the western two-thirdsof the Australian continent is composed of Precambrian bedrockterranes which may be considered ‘cratonic’ in nature (cf. Shaw et al.,1996) (cf. Fig. 1). The topography is generally flat to undulating, anderosion rates are typically less than 5 m/Ma (e.g. Belton et al., 2004;Jakica et al., 2010; Quigley et al., 2010).
Domain 1 comprises highly structured Archean and non-reactivated Palaeoproterozoic crust that was largely cratonised priorto the Mesoproterozoic (e.g. Palfreyman, 1984; Shaw et al., 1996).The domain essentially encompasses the Yilgarn, Pilbara and Kimber-ley Cratons in Western Australia (WA), the Northern Australian Craton(predominantly across the Northern Territory — NT), and the GawlerCraton in South Australia (SA) (Fig. 1). Fault scarps in this domain aretypically spatially isolated, less than 40 km long, and with less than10 m of neotectonic displacement. However, exceptions exist in thenorthwestern Yilgarn Craton (WA), where scarps tend to be longerand closer together (e.g. Mt Narryer region — Williams, 1979)(Fig. 4a), and in the eastern Gawler Craton (SA), where closely-spacedscarps on the Eyre Peninsula are arranged in an en echelon pattern (cf.Crone et al., 2003; Dunham, 1992; Hutton et al., 1994; McCormack,2006; Miles, 1952; Robert, 2007; Weatherman, 2006 — Fig. 4b). Manyof these features may relate to stress concentration at the margins ofthe cratons.
The seismic character of Domain 1 is typified by neotectonic faultsobserved in the southwest of the Yilgarn Craton in southern WA(Clark, 2010) (Figs. 1 and 3). Faults developed in Archean crust with
Eastern Australian PhanerozoicAccretionary Terranes
Eastern Extended Continental Crust
Western Extended Continental Crust
4
5
6
7
neotectonic feature
DOMAIN
150°E140°E130°E120°E110°E
150°E140°E130°E120°E110°E
10°S
20°S
30°S
40°S
10°S
20°S
30°S
40°S
Fig. 3. Neotectonic domains map of the Australian continent (updated from Clark et al., 2011a). White lines represent suspected or known neotectonic features (primarily topo-graphic fault scarps) recorded in the Geoscience Australia Neotectonics Database (http://www.ga.gov.au/earthquakes/staticPageController.do?page=neotectonics).
6 D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
demonstrated Quaternary displacement, including the Meckering,Calingiri, Cadoux, Hyden, Dumbleyung (Fig. 4c) and Mt Narryer faults(Fig. 4a), are associated with discrete scarps up to 40 km long and lessthan 5 m high (Clark et al., 2008; Crone et al., 2003; Estrada, 2009;Gordon and Lewis, 1980; Lewis et al., 1981; Twidale and Bourne,2004; Williams, 1979). More than fifty additional features with topo-graphic characteristics similar to the known fault scarps have beenidentified in 10 m resolution digital elevation data in southwest WA(Clark, 2010). Twenty one of these have been the subject of groundreconnaissance and were found to exhibit characteristics consistentwith neotectonic displacement (e.g. drainage diversion, disruptionor impedance, disturbed Quaternary sedimentary deposits, enhancedlocal slope erosion, etc. — Clark, 2010). Surface deformation relatingto a small modern rupture has been documented in southwest WAusing interferometric synthetic aperture radar (Dawson et al., 2008).
In cases where early or pre-Neogene units have been exposed intrench excavations (e.g. Meckering, Hyden), the total verticalneotectonic displacement across these units appears to be significant-ly less than 10 m (e.g. Clark, 2010; Estrada, 2009; Gordon and Lewis,1980; Lewis et al., 1981). Excavations across the recently discoveredDumbleyung Fault (Clark, 2010) reveal evidence for at least three sur-face ruptures in the last 100 ka (Estrada, 2009), similar to that docu-mented on the Hyden Fault scarp (Clark et al., 2008; Crone et al.,2003). In apparent contrast, excavations of the 1968 MeckeringFault scarp identified evidence for only the 1968 event and no surfaceruptures during the preceding several hundred thousand years (Clarket al., 2011a). Inter-event slip rates can be as high as 300 m/Ma (Clarket al., 2011a), but slip rates averaged across Tertiary strata are ordersof magnitude lower, less than 10 m/Ma (Clark et al., 2008).
It should be noted that there is no evidence in the neotectonic re-cord (Clark, 2010) to suggest that the region of southwest WA knownas the ‘Southwest Seismic Zone’ (Doyle, 1971) (cf. Fig. 1), so-calledbecause of an anomalous concentration of earthquake epicentres inthe past 50 years, is associated with a high level of neotectonic activ-ity. Furthermore, models attempting to reproduce the pattern of
historic epicentres concentrated in proximity to the Australian conti-nental margin (e.g. Sandiford and Egholm, 2008) do not predict thepattern of neotectonic scarps.
3.1.2. Domain 2 —Sprigg OrogenFollowing the breakup of the supercontinent Rodinia in the Late
Neoproterozoic (Betts and Giles, 2006), and the formation of a passivemargin along the east coast of Proterozoic Australia, a series of five con-vergent margin orogenic belts formed (the Delamerian, Lachlan,Thomson, New England, and North Queensland Orogens) and accretedto the eastern seaboard from the Cambrian to the Jurassic (Direen andCrawford, 2003; Glen, 2005) (cf. Fig. 1). The Flinders and Mount LoftyRanges of South Australia (Fig. 5), which this domain is defined to ac-count for, overlie the westernmost, and oldest, of the accreted orogenicbelts— the Cambrian to Early Ordovician Delamerian Orogen. Themar-gins of this domain essentially mimic the extent of the sedimentaryrocks that were deposited in the Neoproterozoic Adelaide Rift Complex(Adelaide Geosyncline— cf. Fig. 1) during supercontinent breakup, andwhich were subsequently incorporated into the Delamerian Orogen(Drexel et al., 1993; Preiss, 1987).
The core of the domain comprises the Sprigg Orogen (Sandiford,2003b), which reactivates structures of the Delamerian Orogen, andcontinues to build relief today along the axes of the Flinders andMount Lofty Ranges. It is thus distinct from all other reactivatedProterozoic crust within the Australian continent. The seismic characterof this domain is defined by the faults detailed in Sandiford (2003b),Celerier et al. (2005) and Quigley et al. (2006), being closely-spaced,intimately connected to youthful topography (Fig. 5), and hosting upto two hundred metres of neotectonic displacement. A strong correla-tion exists between contemporary seismicity and broad neotectonicuplift in this domain, which may have built 50% of its modern reliefwithin the neotectonic era (Braun et al., 2009). Indeed, taking intoaccount the intermittent nature of faulting in Australia (e.g. Croneet al., 2003), Quigley et al. (2006) and Sandiford (2003b) suggests that30–50% of the present-day ~800 m elevation of the Flinders Ranges
Fig. 4. Digital elevation image (SRTM 90 m) showing examples of known fault scarps within Domain 1. (a) Mount Narryer on the northwest Yilgarn Craton (WA), (b) eastern mar-gin of the Eyre Peninsula (SA), and (c) southwest Yilgarn Craton (‘Southwest Seismic Zone’) (WA). White lines indicate suspected or known neotectonic features; red lines identifyDomain 1 features mentioned in the text.
7D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
Fig. 5. Digital elevation image (SRTM 90 m) of the Flinders/Mount Lofty Ranges. White lines indicate suspected or known neotectonic features; red lines identify Domain 2 featuresmentioned in the text. Eyre Peninsula fault scarps (Domain 1) are seen in the west of the image.
8 D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
relative to adjacent piedmont slopes has developed in the last 5 Ma.Cosmogenic 10Be concentrations in exposed bedrock surfaces and allu-vial sediments in the same area support the concept of ongoing reliefgeneration (Quigley et al., 2006, 2007b).
The ranges are bound on the east and west by reverse faults thatvary from a few tens to almost 100 km long and tend to be spaced sig-nificantly less than a fault length apart, often linking or arranged in anen echelon pattern (Fig. 5). Many of these faults thrust Proterozoic
and/or Cambrian basement rocks above Quaternary sediment(Celerier et al., 2005; Hillis et al., 2008; Quigley et al., 2006; Sandiford,2003b; Sprigg, 1946; Williams, 1973) (Fig. 6a–c). In the few instanceswhere the full thickness of overthrust colluvium can be estimated,total neotectonic throws are in the order of 100–200 m (Bourman andLindsay, 1989; Celerier et al., 2005; Preiss and Faulkner, 1984; Quigleyet al., 2006; Williams, 1973). Slip rates on individual faults have beenestimated to be in the order of 20–150 m/Ma (Sandiford, 2003b).
c
a
Mesozoic-Cainozoic
Neoproterozoic-Ordovician
Palaeo- to Meso-Proterozoic
FlindersRanges
GawlerCraton
Mount LoftyRanges
Pooraka Formation - 85 ± 11 ka
Pooraka Formation
W E
PoorakaFmtApilla
Tillite
Holocene-Recent
cmetres metres
Pooraka Fm talus breccia
b
EW
EmerooQuartzite
Pooraka Fm river gravels
Pooraka Fm river gravels
active scree slopes
Present stream bed
b
scree
scree
Willawortina Formation
Eyre Formation
Neoproterozoicquartzite
metresa
Fig. 6. Major geological provinces within the Flinders/Mount Lofty Ranges (dashed lines indicate structural form) and schematic cross-sections showing reverse displacement onthe (a) Paralana, (b) Wilkatana, and (c) Burra faults.Modified from Celerier et al. (2005) and Quigley et al. (2006).
9D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
Faults in this domain with documented Pliocene or youngerdisplacements include the Wilkatana, Burra, Milendella, Para, Paralana,Willunga, Morgan, Gawler/Concordia, Ochre Cove/Clarendon, Eden-Burnside, Bremer, Williamstown-Meadows and Ediacara (Alley andLindsay, 1995; Binks, 1972; Bourman and Lindsay, 1989; Clark et al.,2011a; Quigley et al., 2006, 2010; Sandiford, 2003b; Williams, 1973)(Fig. 5). Fault-propagation folding is well developed in Miocene strataproximal to the Willunga and Gawler faults (Lemon and McGowran,1989; Green, 2007).
Recurrence data exists for the Wilkatana Fault (Fig. 6b). This struc-ture is considered by Quigley et al. (2006) to have hosted a minimumof three surface rupturing earthquakes in the last 67 ka, involving atotal of approximately 15 m of slip. A single event slip estimate bythese authors in excess of 3.8 m implies earthquakes with magnitudesof greater than Mw 7.0. Single event displacements of 1.8 m have beenmeasured on the Alma and Williamstown-Meadows Faults, with thelatter being associated with a rupture ~25 km long (Clark et al.,2011a). It has been proposed that a detached Cambrian bedrock slab~7 m long overlying Pleistocene colluvium on the footwall of theMilendella Fault came to be in its current setting as the result of a singlelargemagnitude seismic event (Bourman and Lindsay, 1989; Clark et al.,2011a; Reid, 2007).
3.1.3. Domain 3 — reactivated Proterozoic crustDomain 3, together with Domain 1, encompasses the broader Prote-
rozoic basement of the central and western parts of the continent. It ischaracterised by numerous eroded orogenic belts that were active dur-ing the Proterozoic assembly of Australia. These orogenic belts, or mo-bile belts, encircle the Domain 1 cratonic nuclei (cf. Fig. 1) and have ahistory of multiple tectonic reactivations (e.g. Betts and Giles, 2006;Fitzsimons, 2003; Neumann and Fraser, 2007). While there is much di-versity in the timing and the pervasiveness of the last reactivation ofeachmobile belt (ranging fromMesoproterozoic to Devonian/Carbonif-erous— e.g. Haines et al., 2001; Betts and Giles, 2006), neotectonic fea-tures appear similar in character. Scarps tend to be longer and higherthan in Domain 1, and there is an indication that the spatial arrange-ment of scarps perhaps reflects the strong linear structural grain pres-ent in the underlying mobile belts (e.g. Betts and Giles, 2006;Fitzsimons, 2003). An excellent long-term record of deformation ofthis type of crust is evident on the Nullarbor Plain (Hillis et al., 2008)(cf. Figs. 1 and 2), which is floored by the Proterozoic Albany–Fraser/Wilkes Orogen (Fitzsimons, 2003). The Nullarbor Plain preserves an ex-traordinary neotectonic record that paradoxically reflects the long-termtectonic stability of the underlying Proterozoic basement terranes. Nu-merous linear fault scarps that cut the plain are visible in 90 m SRTM
(a)
(a) (c)
Fig. 7. Digital elevation image (SRTM 90 m) of (a) Nullarbor Plain. (b) Roe Plain Scarp where it crosses both the Nullarbor and Roe Plains. (c) Topographic profiles across the RoePlain Scarp. Red lines indicate suspected or known neotectonic features.
10 D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
11D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
digital elevation data (Clark et al., 2011a; Sandiford and Quigley, 2009;Sandiford et al., 2009) (Fig. 7). Assuming approximately uniform sur-face lowering of the limestone plain of ~5 m/Ma (Stone et al., 1994),displacements in the last 15 Ma have been in the order of a few tensof metres at most, and more commonly less than 10 m. Slip rates aver-aged over the neotectonic era are, therefore, in the range of 1–3 m/Maor less. This conclusion is supported by the observation that severalscarps do not continue from the Nullarbor Plain onto the ca. 2 Ma RoePlain surface (James et al., 2006) (Fig. 7b–c). Consistent with observa-tions in other parts ofWestern Australia, these scarps are not associatedwith the building of regional relief (i.e. mountain ranges).
While little neotectonic data exists for the Capricorn, Paterson,King Leopold and Halls Creek Orogens (Fig. 1) they might be expectedto behave in a similar fashion by virtue of their structural and tectonicsimilarity to other mobile belts within the domain (cf. Betts and Giles,2006; Betts et al., 2002; Fitzsimons, 2003; Karlstrom et al., 2001;Shaw et al., 1996; Wade et al., 2006) and the demonstrated suscepti-bility of this crustal type to reactivation throughout geologic time(e.g. Betts and Giles, 2006; Neumann and Fraser, 2007; Occhipintiand Reddy, 2009). Again, orientation of these structures with respectto the stress field is a major consideration in determining the poten-tial for any neotectonic reactivation.
Western Tasmania has been tentatively placed in Domain 3 (cf. Fig. 3)based upon thewidespread exposure of Neoproterozoic rocks, metamor-phism and deformation fabrics (e.g. Berry et al., 2008), and its pre-Delamerian Orogeny correlation with crust west of the Tasman Line(Direen andCrawford, 2003; Glen, 2005). Despite a long Palaeozoic defor-mation history, and the proposal that analogous Proterozoic basementfloors the Lachlan Fold Belt to the north in Victoria (Cayley et al., 2002,2011) (cf. Fig. 1), we maintain that the character of the scarps is moreakin to those of Domain 3 than those in Domain 4 (Clark et al., 2011b;cf. Section 3.1.4). Furthermore, there is no record of the early extensionalhistory that might align it with Domain 2, nor the strong organisation ofscarps.
3.1.4. Domain 4 — eastern Australian Phanerozoic accretionary terranesThis domain includes the five Phanerozoic Tasmanide orogens
(Glen, 2005) (Fig. 1), and excludes western Tasmania (part of Domain3 — cf. Section 3.1.3) and the Sprigg Orogen (Domain 2). Theneotectonic record from this domain is restricted almost entirely tothe Lachlan and Delamerian Orogens. Given significant historic seis-micity in the high-relief and relatively high-erosion New Englandand North Queensland Orogens (cf. Fig. 1) it seems likely that furtherfeatures remain to be discovered in these regions. However, largechanges in crustal stress orientation and possibly magnitude (Hillisand Reynolds, 2003) centred on the New England Orogen may actto suppress neotectonic activity. It is possible that the surface expres-sion of earthquakes that nucleate in Thomson Orogen crust issubdued or suppressed by the cover rocks of the Eromanga Basin(cf. Figs. 1 and 2). Alternatively, the low seismic activity ratesrecorded in historic times within the Thomson Orogen may be atrue indication of long-term low levels of crustal deformation.
3.1.4.1. Eastern Highlands. The Eastern Highlands are characterised bysome of the greatest relief (cf. Fig. 2), and some of the higher bedrockerosion rates on the continent. Values for the Eastern Highlands fallpredominantly in the range of 20–50 m/Ma (e.g. Bishop andGoldrick, 2000; Bishop et al., 1985; Heimsath et al., 2000, 2001;Tomkins et al., 2007; Weissel and Seidl, 1998; Wilkinson et al.,2005). With few exceptions, comparable bedrock erosion rates andtectonic relief production rates might be expected to result in poorfault scarp preservation. This general expectation is borne out by ex-amples such as the Khancoban–Yellow Bog Fault, which superposesPalaeozoic granite over some 500 m of Cenozoic fluvial gravels(Moye et al., 1963; Sharp, 2004), yet has little topographic expression.The Tawonga (Beavis, 1960; Beavis and Beavis, 1976; Hills, 1975;
Sharp, 2004) and Kiewa faults (Beavis and Beavis, 1976) also fallinto this category, with hundreds of metres of Cenozoic displacementlikely, although what proportion of this can be attributed to theneotectonic era is unclear. Further to the southwest, in the goldfields ofthe central Victorian highlands, neotectonic faulting is often known onlyfrom displacement of the base of Pliocene and younger basalt flows thatcap palaeochannel fill sediments (i.e. deep leads) (e.g. Canavan, 1988;Kotsonis and Joyce, 2003a,b).
Holdgate et al. (2008) present evidence from the southern EasternHighlands that resurrected the idea of a punctuated post-EoceneKosciuszko Uplift (cf. Andrews, 1910; Browne, 1967; Sprigg, 1946)that continued into the Late Pliocene and potentially into the Pleistocene(cf. Norvick, 2011; Sharp, 2004). It is possible that this period of uplift isresponsible for adding several hundredmetres of local relief to the high-lands, and may relate to the pulse of deformation seen in south-eastAustralian offshore basins in the interval 10–5 Ma, associated with thereorganisation of the crustal stress field into its present configuration(Dickinson et al., 2001, 2002; Hillis et al., 2008; Sandiford et al., 2004).The Lake George Fault east of Canberra, which impounds up to 200 mof Late Miocene and younger sediment (Abell, 1985; Coventry, 1976;Singh et al., 1981), and faults of the Lapstone Structural Complex (LSC)west of Sydney (Branagan and Pedram, 1990; Pickett and Bishop,1992) (Fig. 8),may also have accumulatedmuch of their several hundredmetres of neotectonic displacement during this time (Clark, 2009; Clarket al., 2009). Palaeomagnetic data suggest that folding and uplift relatingto the LSC had largely ceased by the Late Pliocene (Bishop et al., 1982with age recalculated by Pillans, 2003). However, the preliminary find-ings from a small ephemeral lake impounded by the Kurrajong Fault inthe northern LSC (Fig. 8) indicate that neotectonic displacement maybe limited to less than 20 m of the greater than 100 m of throw on thisstructure (Clark et al., 2009).
3.1.4.2. Murray Basin. Significantly lower relief and the widespreadpreservation of retrogressive Pliocene marine strandline deposits inthe Murray Basin to the west of the Eastern Highlands (Fig. 9) suggestsignificantly lower erosion rates. Consequently, fault scarps might beexpected to be better preserved within the basin relative to the high-lands. However, thematerials in which fault scarps are developedwith-in the basin are typically partially indurated Cenozoic sediments ratherthan bedrock, and so are highly susceptible to degradation resultingfrom fluvial, lacustrine and even aeolian processes. Examples ofknown Neotectonic features include the Danyo, Iona and NeckarbooRidges in the central Murray Basin (Sandiford, 2003b), and the CadellTilt Block in the eastern Murray Basin (Bowler, 1978; Bowler andHarford, 1966; Harris, 1938; Rutherfurd and Kenyon, 2005), which isdefined along its eastern margin by faults at Cadell and Echuca South(Fig. 9). All of the above features are the broad scale-expression withinCenozoic basin fill sediments of reactivated Lachlan Fold Belt basementfaults — the “resurgent tectonics” of Hills (1961).
The Cadell Fault provides the only palaeoseismic data for Domain4 (Clark et al., 2007; McPherson et al., 2012) (Fig. 9). During theNeotectonic era (i.e. the last 5–10 Ma), two periods of displacement,each involving ~15–20 m of uplift, have occurred on the CadellFault. The earlier active period is postulated, on the basis of evidencefrom seismic reflection data, to have occurred during the depositionof the Late Miocene to Pliocene Calivil Formation (cf. Brown andStephenson, 1991). This timing for reactivation of the fault is consis-tent with evidence for a widespread pulse of activity on Australianfaults that coincides with the establishment of the current stress re-gime (Dickinson et al., 2002; Hillis et al., 2008; Sandiford et al.,2004). Following a prolonged period of relative tectonic quiescencespanning almost the entire Quaternary and involving 5 m or less ofuplift, the Cadell Fault entered a new phase of activity. At approxi-mately 70 ka the Murray River system started aggrading in responseto renewed uplift across the Cadell Fault (Stone, 2006). This renewedperiod of activity, which lasted until 20 ka, involved perhaps six or
Fig. 8. Digital elevation image (SRTM 90 m) of the southeast Eastern Highlands. White lines indicate suspected or known neotectonic features; red lines identify Domain 4 featuresmentioned in the text.
12 D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
more morphogenic earthquakes. The slip rate on the fault, averagedover as many as five complete seismic cycles between ca. 70 and20 ka is ~500 m/Ma (0.5 mm/a). Assuming full length rupture, theaverage recurrence for Mw 7.3–7.5 events is 8.3 ka. More than two av-erage seismic cycles have lapsed since the last seismic event on thefault. Accordingly, the fault is inferred to have expended the store ofelastic strain in the surrounding crust, and relapsed into a quiescentperiod.
Slip rates and total neotectonic displacement values are pres-ently unknown for the remaining features in Domain 4. Accord-ingly it is not yet possible to assign a seismic character todomain-wide faulting with any confidence. However, it is not im-plausible that faults within this domain behave similarly to theCadell Fault, at least in terms of pronounced temporal clusteringof surface rupture that might cumulatively amount to tens to acouple of hundred metres of displacement over the neotectonicera. The previous example of the Kurrajong Fault within theLapstone Structural Complex suggests caution in applying blan-ket rules within the domain.
3.1.5. Domain 5 — eastern extended continental crustAustralia's passive margins are fringed by extended continental crust
resulting from the Late Mesozoic break-up of the super-continentGondwana (Veevers, 2000). In most places this crust is submerged.
However,Mesozoic extensional basins (or parts thereof),which preserverich neotectonic records are exposed onshore in the southeast(Gippsland and Otway Basins) and along the western margin (Perthand Carnarvon Basins) of the continent (Fig. 2).
The main regions of neotectonic activity in Domain 5 centre on anaulocogen (a failed intra-cratonic rift — Burke, 1977) that developedbetween Tasmania and the mainland during the Mesozoic separationof Australia and Antarctica, and which reactivated during the openingof the Tasman Sea (Veevers, 2000). This aulocogen is overlain by theGippsland, Bass and eastern Otway Basins, and extends along thewestern margin of Tasmania through the Sorell Basin (Fig. 2). Seismicdata show Neogene reverse reactivation of Mesozoic normal faultsthroughout these basins (Blevin et al., 2005; Cummings et al., 2004;Dickinson et al., 2002; Hill et al., 1994; Holdgate et al., 2003). Al-though palaeoseismic data is lacking, slip rate data, typically averagedover the last hundred thousand to a few million years, is available(e.g. Gardner et al., 2009; Sandiford, 2003a). The Sorell Basin is in-cluded in the domain as it is generally considered to be continuouswith the western Otway Basin (Blevin and Cathro, 2008; Borehamet al., 2002; Krassay et al., 2004). However, the prominent inversionstructures that are characteristic of the Gippsland and Otway Basins,and to a lesser degree the Bass Basin (e.g. Blevin et al., 2005), appearto be muchmore subtle to absent in the Sorell Basin, suggesting loweractivity (Boreham et al., 2002). This is perhaps analogous to the
Fig. 9. Digital elevation image (SRTM 90 m) of the Murray Basin. White lines indicate suspected or known neotectonic features; red lines identify Domain 4 features mentioned intext. Dashed blue line defines approximate margin of the Murray Basin.
13D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
relationship between the active Carnarvon Basin and comparativelyless active Perth Basin (cf. Section 3.1.6).
3.1.5.1. Otway Basin. Neogene inversion of Mesozoic to Tertiary basinsin southeastern Australia has resulted in the formation of upland sys-tems, such as the Otway Ranges, which are defined at the surface bybroad Pliocene to Quaternary anticlines, monoclines, synclines andrare reverse faults (Hill et al., 1994). On the northern flanks of theOtway Range in southern Victoria, the remnants of a Pliocenestrandplain rise ~120 m over a series of ENE-trending faults andmonoclines to elevations of ~250 m (Sandiford, 2003a, 2003b;Sandiford et al., 2004; Wallace et al., 2005) (Fig. 10). Along with itscorrelatives in the Murray and Gippsland Basins (Holdgate et al.,2003; Wallace et al., 2005), this strand plain developed during fallingsea levels following a 6 Ma high stand at ~65 m above present daysea level (Brown and Stephenson, 1991), implying almost 200 m offault-related tectonic uplift. The parallelism and conformable natureof most Pliocene strandlines across Victoria has been taken as evi-dence for minimal tectonism in the interval 6–3 Ma (Wallace et al.,2005).
Significant deformation of Neogene strata has occurred within theOtway Basin subsequent to the deposition of regionally extensivestrandlines at ca. 5–6 Ma (Sandiford, 2003a; Sandiford et al., 2004). To-pographic profiles along strandlines that drape faults on the western
margin of the Otway Ranges (e.g. Fergusson Hill and Simpson anti-clines) suggest time-averaged uplift rates of 40–50 m/Ma (Perincekand Cockshell, 1995; Quigley et al., 2010; Sandiford, 2003a; Stirling etal., 2011). Sandiford (2003a) presents topographic and geologicevidence for amajor pulse of deformation in theOtway Ranges betweenca. 2 and 1 Ma. This implies that the long-termuplift rates above are notrepresentative of uplift rates over the last million years. Nopalaeoseismic data (as distinct from neotectonic data) exist forindividual earthquakes due largely to a predominance of folding, as op-posed to discrete surface faulting (e.g. Edwards et al., 1996), perhapscombined with relatively little deformation in the last million years(cf. Sandiford, 2003a).”
3.1.5.2. Gippsland Basin. Neotectonism in the Gippsland Basin has a sim-ilar expression to the Otway Basin, with folding being more commonthan faulting at the surface (Barton, 1981; Beavis, 1975; Dickinson etal., 2002; Gloe, 1960; Sandiford, 2003b). In the onshore basin, coal de-position ceased as a result of Late Miocene compression (Holdgate etal., 2007). On structural highs, the uppermost coal seam (the YallournSeam) is overlain by the same Late Pliocene unconformity that denotesthe onset of the neotectonic era offshore (cf. Sandiford et al., 2004). Theeroded subcrop surface of the coal seamswas buried less than 5 Ma agoby a series of 10–40 m thick outwash fan deposits known as theHaunted Hill Formation (HHF) (Bolger, 1991; Holdgate et al., 2008).
Fig. 10. Digital elevation image (SRTM 90 m) of the onshore Otway Basin. The Otway Ranges are composed of Cretaceous sediments uplifted during inversion of Otway Basin ex-tensional structures, and represent the major topographic high. Note Pliocene strandlines which have been uplifted on the western side of the ranges, and which provide a datumfor slip-rate estimation. The Miocene-Recent Newer Volcanics provide a similar datum to the north of the ranges. White lines indicate suspected or known neotectonic features; redlines identify features mentioned in text.
14 D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
Slip rate estimates slightly in excess of those of Otway Basin faults(~65 m/Ma) have been obtained from the preliminary results of cosmo-genic radionuclide (CRN) burial and exposure dating of Haunted HillFormation sediments folded over the intra-basin Snake Ridge andMorwell Monoclines (Clark et al., 2011a) (Fig. 11). The slip rate on thebasin-bounding Yallourn Fault, also based upon CRN dating of upliftedHHF gravels, is greater than 63 m/Ma.
Long-term slip rate estimates for the Gippsland Basin are subjectto the same uncertainty as those in the Otway Basin, stemmingfrom apparent episodic pulses of regional deformation. In the easternonshore Gippsland Basin an angular unconformity between theJemmy's Point formation (Pliocene) and the overlying Haunted HillFormation (Late Pliocene to Quaternary) implies significant post-3 Ma deformation (Holdgate et al., 2003). Seismic reflection evidencecombined with palynological age constraint suggests that a major ep-isode of deformation involving folding ceased at 1.0 Ma in theoffshore Gippsland Basin while continuing onshore until approxi-mately 0.2–0.25 Ma (Holdgate et al., 2003). Based upon this chronol-ogy, the same authors estimate an uplift rate of 50–80 m/Ma for theRosedale Fault (Fig. 11) over the interval 1.5–0.25 Ma. Assuming con-stant slip from 1.5 Ma to present, the rate becomes 88 m/Ma, compa-rable to that estimated for the Yallourn Fault. Gardner et al. (2009)obtain slip rates on the Waratah Fault (Fig. 11) of 10–40 m/Ma fordisplacements across both 125 ka and Pliocene marine terraces. Incontrast to the aforementioned faults, the Gardner et al. (2009) re-sults are consistent with periodic rupture behaviour.
3.1.6. Domain 6 — western extended continental crustThis domain comprises extended Precambrian and Palaeozoic
crust flooring five marginal and offshore basins (Perth, Carnarvon,offshore Canning, Browse, and Bonaparte; Fig. 2) and their Mesozoicand Cenozoic (±Late Palaeozoic) sedimentary fill rocks. Similar toDomain 5, most neotectonic activity focusses on the basins that pri-marily relate to the Mesozoic fragmentation of Gondwana (Longleyet al., 2002; Veevers, 1972, 2000), particularly those on the NorthwestShelf (Fig. 1). However, ‘subtle’ Neogene reactivation of some basinsegments formed predominantly as the result of Palaeozoic rifting(e.g. Fitzroy Trough of the onshore Canning Basin, Petrel Trough ofthe Bonaparte Basin, Southern Carnarvon Basin, Northern PerthBasin; Fig. 2) has been noted (Baillie and Jacobson, 1995; Craig etal., 1984; Denman and van de Graaff, 1977; Keep et al., 2007). Thesouthern Perth Basin, which developed primarily in the Mesozoic, isalso less active than basins further north, and may be considered tobe similar to the Sorrell Basin (cf. Domain 5 — Section 3.1.5). Withthe exception of the Fitzroy Trough, the onshore Canning Basin is ex-cluded from this domain (Figs. 2 and 3), as it is considered that thePrecambrian basement, rather than the basin-fill sediment, dictatesthe seismic character. This assertion is supported by the occurrenceof earthquake hypocentres (e.g. Denham et al., 1979; Everinghamand Smith, 1979) at depths equalling or exceeding basin sedimentthickness (e.g. Jonasson, 2001; Yeates et al., 1984). Analysis of dredgesamples from the Naturaliste Plateau suggests basement characteris-tics compatible with classification in Domain 3 (Halpin et al., 2008).
Fig. 11. Digital elevation image (SRTM 90 m) of the onshore Gippsland Basin. The Strzelecki Ranges are composed of Cretaceous sediments uplifted during inversion of the GippslandBasin, and represent the major topographic highs. Note location of open cut coal mines where inverting structures have brought coal deposits to within 50 m of the surface. Whitelines indicate suspected or known neotectonic features; red lines identify Domain 5 features mentioned in text. Thick dashed grey lines define approximate basin margins.
15D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
However, the degree of extension of the underlying Proterozoicbasement (Borissova, 2002) suggests that its allocation to Domain 6is appropriate. Parts of the Browse/Bonaparte Basins may requireclassification into a new non-intra-plate domain, given the affinityof deformation patterns found there with those to the north associat-ed with the Timor collision (Keep et al., 2007).
While no palaeoseismological information exists for single earthquakeevents, the neotectonic character of this domain is very similar to theGippsland and Otway Basins (cf. Sections 3.1.5.1 and 3.1.5.2), with largeanticlines overlying inverting normal faults, as exemplified by the CapeRange and Rough Range Anticlines (Malcolm et al., 1991). In the case ofthe Fitzroy Trough, seismic reflection imaging shows that the majornorthern bounding fault has been reactivated to an extent during theNeogene (Keep et al., 2007), and is also associatedwith a high level of his-toric seismicity (Fig. 1). This subsidiary style of deformation may domi-nate in the Palaeozoic parts of the domain (e.g. the Perth Basin andsouthern Carnarvon Basin proximal to the shield).
In parts of the Carnarvon and northern Perth Basins adjacent to thecratons, where structures relating to Palaeozoic extension dominateover those related toMesozoic extension, and evidence for Neogene ex-tension is sparse, neotectonic features appear to be intermediate incharacter between those associated with basins (this domain) and cra-tons (cf. Domain 1 — Section 3.1.1). Clusters of scarps, coinciding withmapped basement structure, occur in the Middalya/Kennedy Range/Wandagee area east of Cape Cuvier, and the Toolonga area east ofShark Bay (Fig. 12a). The Toolonga group of scarps include features
greater than 60 km in length and with up to 30 m of vertical displace-ment. Regressive marine strandlines of unknown age have beenuplifted across these features, with the total uplift across the faultgroup being in the order of 90 m (Clark et al., 2011a). Assuming thatthe ridges are early Pliocene in age, similar to those in the MurrayBasin in eastern Australia (cf. Section 3.1.4.2.; Fig. 9),modest surface up-lift rates are obtained. The onshore Canning Basin (including the FitzroyTrough), and the Bonaparte Basin, might also be placed in this interme-diate category, although only one possible neotectonic feature is cur-rently known from this region.
3.1.6.1. Carnarvon Basin. In the Carnarvon Basin a series of asymmetricanticlines has developed as fault propagation folds above blindwrench and oblique reverse faults (Hillis et al., 2008; Hocking,1988; McWhae et al., 1956). The trend of the anticlines and their un-derlying faults changes from northwest in the southern basin throughnortherly in the central basin to northeast in the northern basin, inpart mimicking the cratonic margin and the trends of underlyingMesozoic wrench structures (Keep et al., 2002; Malcolm et al.,1991) (Fig. 12a). The growth of fault propagation anticlines is gener-ally dated as Miocene and younger (e.g. Barber, 1988; Crostella andIasky, 1997; Hearty et al., 2002; Keep et al., 2000, 2002). Thesestructures are typically not related to the major basin-boundingfaults, as demonstrated by the Darling Fault in the southern PerthBasin (cf. Jakica et al., 2010). Seismic reflection imaging suggeststhat the loci of reactivation are dominantly steep, flower-type
(b)(a)
(c)
Fig. 12. (a) Digital elevation image (SRTM 90 m) of the southern Northwest Shelf region showing major topographic uplift relating to neotectonic inversion of Carnarvon Basin exten-sional structures. Lines indicate suspected or knownneotectonic features; red lines identify Domain 6 featuresmentioned in the text. Dashed grey lines define approximate basinmargins.(b) Digital elevation image (10 m) of the Cape Range Anticline and adjacent structures, with location of elevation traverses indicated. Note that anticline crest elevation decreases to thesouth. Inset location shown on subpanel a. (c) Surveyed elevation traverses showing relative position of wave-cut marine terraces (Tantabiddi, Jurabi, Milyering, Muiron) on the westernflank of the anticline. Note apparent increase in elevation of correlative terrace surfaces to the south, consistent with anticline growth in this direction.
16 D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
structures, and that significant inversion and the formation oftranspressional anticlines are restricted to one or two major faultswithin the flower (Keep et al., 2000, 2002; Malcolm et al., 1991).Emerged Pleistocene marine terraces on the Cape Range Anticline(Fig. 12b–c), overlying the Learmonth Fault (Crostella and Iasky,1997), and anticlinal folds in offshore Plio-Quaternary sea floor sedi-ments (van de Graaff et al., 1976; Wyrwoll et al., 1993), indicate thatNeogene deformation has continued until at least the last interglacial(~120 ka). Faults in the entirely offshore Browse Basin appear to besimilar in character (Keep et al., 2002; Symonds et al., 1994). Evi-dence interpreted to suggest that the last interglacial marine terracehas been tectonically uplifted to form the Cape Cuvier Anticline(Denman and van de Graaff, 1977; Veeh et al., 1979) has recentlybeen challenged (O'Leary et al., 2008). Preliminary 36Cl cosmogenicradionuclide dates from a marine terrace 10–15 m above presentsea level on the Cape Range (the Jurabi Terrace of van de Graaff etal., 1976) are consistent with a Marine Isotope Stage 7 (ca. 200 ka)age for formation of the surface (Clark et al., 2011a). In contrast tothe last interglacial (MIS Stage 5), where sea level may have reachedup to 3–4 m higher than present (e.g. Zhu et al., 1993), sea level is notthought to have exceeded present levels during MIS Stage 7 (Siddallet al., 2003). This implies uplift rates on the underlying LearmonthFault of a few tens of metres per million years, similar to faults inthe eastern Australian inverting basins (cf. Section 3.1.5).
continental crust from the edge of the continental shelf (or adjacentneotectonic domain) out to the Continental/Oceanic Crust Boundary(Blake and Kilgour, 2000; FrOGTech, 2005). This “passive margin” do-main formed as the result of the Mesozoic break-up of the supercon-tinent Gondwana (Veevers, 2000). While no neotectonic data hasbeen compiled to characterise this domain, an appreciable numberof Australian historic earthquakes of magnitude 5 and above are lo-cated either within this domain, or near the boundary of this domainand adjacent domains. This is particularly apparent along the westernmargin of the continent (Fig. 13). Similar crust on the eastern sea-board of the United States hosted the M7+ Charleston earthquakesequence (Johnston, 1996; Talwani and Schaeffer, 2001), flaggingthis domain type as a potential source of damaging ground-shaking(e.g. Wheeler, 1995, 1996, 2009; Wheeler and Frankel, 2000).
3.2. Comparative analysis of neotectonic domains data
The previous sections document regional variation in thelong-term response of the Australian continental crust to the imposedtectonic forces, and in the character of the neotectonic features thataccommodate that response. For example, compare the isolated,low-displacement faults of the southwest of Western Australia
Fig. 13. Distribution of historic earthquakes (M≥4; 1841–2010) relative to neotectonic domain boundaries.
17D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
(Clark, 2010), the en echelon network of high-density, relativelyhigh-displacement faults in the Flinders/Mount Lofty Ranges(Sandiford, 2003b), the anastomosing network of highly interlinked,relatively high-displacement faults in the Carnarvon, Otway andGippsland Basins (Holdgate et al., 2003; Sandiford, 2003a), and therelatively isolated, moderate- to high-displacement features arrangedin belt-like patterns in the Murray Basin and Eastern Highlands(Beavis, 1960; Sandiford, 2003b; Sharp, 2004). Objective derivationof domains by statistically analysing the neotectonic data to test forgeographic variance of characteristics (i.e. cluster analysis) is notpossible owing to large uncertainties and variability in the availabledata. However, it is reasonable to assume that variation arises as aresult of the interaction between the crustal stress field (e.g. Hillisand Reynolds, 2003) and continental crust of varying character(e.g. Johnston et al., 1994). Long-term fault behaviour might there-fore be extrapolated from regions rich in data to those poor in databy mapping regions of similar crustal properties, with the caveatthat changing stress conditions (magnitude and orientation) maymodify the activity level.
Data from individual faults can be used to establish patterns (in spaceand time) of rupture, and provide a basis for assessing the legitimacy ofextrapolating neotectonic fault data from a given region to faultselsewhere. The neotectonic data compiled herein contains two semi-quantitative variables useful for characterising fault behaviour — faultlength and vertical displacement. As discussed below, these variables are
subject to a level of uncertainty in their derivation. They also possess acommon trait of non-normal statistical distribution. Attempts to normal-ise the data were unsuccessful and therefore valid multivariate statisticalanalysis of the data was not possible. Nonetheless, these two key vari-ables can be analysed as ordinal level data and applied in the estimationof seismicity parameters such as maximum magnitude earthquake(Mmax).
The data set used in this section was accessed from the GeoscienceAustralia Neotectonics Database in September 2011 (see supplementaryKMZ file), and so updates that used by Clark et al. (2011a). Subject to anongoing quality control process, all features within the database areassigned a level of confidence (A–D) denoting the likelihood of a correctidentification of the feature as being neotectonic; with A representing adefinite neotectonic feature and D a deformation structure that showsno evidence for neotectonic activity. Future work will almost certainlydemonstrate that some features have been misidentified and are notneotectonic. However, for the purpose of this analysis all structures inthe database, with the exception of those demonstrated not to relate toneotectonic faults (i.e. category D), are given equal weighting. For sim-plicity, domains are henceforth referred to in an abbreviated form, withD1, D2, etc. representing Domain 1, Domain 2, etc.
3.2.1. Fault lengthOne of the most important physical characteristics of a fault rup-
ture is its along-strike length, which can be related to the magnitude
Fig. 14. Cumulative frequency histogram of Australian SCR fault length data showing a positively skewed distribution. Inset table shows summary statistics.
18 D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
of the causative earthquake (e.g. Leonard, 2010; Wells andCoppersmith, 1994). Estimates of the maximum magnitude earth-quake (Mmax) that a structure is capable of producing can be deter-mined from fault length data (e.g. Biasi and Weldon, 2006; Stirlinget al., 2002;Wheeler, 2009) after careful consideration of the possibil-ity of segmented rupture. In general, the fault lengths reported heremight be expected to be underestimates as vertical displacement ta-pers towards the tails of ruptures, resulting in lower discoverabilityin digital elevation data (the primary datasets in which Australiansurface-rupturing faults have been identified). Fault length, or moreaccurately surface rupture length, are defined herein as the along-strike distance from tail to tail of discrete geomorphic features con-sidered to represent one or more surface-rupturing earthquakeevents. As most scarp features are not related to mapped faults, andunderlying geological structure is incompletely known, no assump-tions are made with regards to fault segmentation.
0
20
40
60
80
100
120
140D1 D2 D3
Neotect
Fau
lt L
eng
th (
km)
8 4 4
57 172 155
25 38.5 53.5
min
max
med
64 40 50n
Fig. 15. Box and whisker plot of fault length data for each neotectonic domain. Boxes denot90th and 10th percentiles. Data table shows minimum, maximum, median and number of d(also Fig. 13).
The population distribution for fault length data is presented inFig. 14. It shows a strong positive skew with 90% of faults shorterthan 100 km and 75% less than 60 km in length. Median fault lengthis 35 km, with a modal value of 19 km. Fault length data for each de-fined domain are presented in Fig. 15. Domains D1, D2 and D5 exhibitshort fault lengths, all with a relatively limited range of variability. D2has a maximum fault length more than twice as long as that in D5.Fault lengths in the remainder of the domains are more variable,but are generally longer.
3.2.2. Vertical DisplacementVertical displacement is ameasure of the vertical separation between
two sides of a fault as a result of movement on the structure. In conjunc-tion with temporal information on fault movement (if known), verticaldisplacement can be used to estimate slip rate (e.g. Murata et al., 2001;Philip et al., 2001). Estimates of neotectonic displacement also provide
D4 D5 D6onic Domain
10 3 6
190 80 124
52.5 30 59
38 21 17
e 75th and 25th percentiles, central point indicates median value, and whiskers defineata points (n) for each domain. Colours correspond to accompanying domains in Fig. 3
0
20
40
60
80
100
120
0 20 40 60 80 100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
500
520
540
560
580
600
Vertical Displacement (m)
Fre
qu
ency
0%
20%
40%
60%
80%
100%
FrequencyCumulative %
n = 230 Vertical Displacement (m)
Median 15
Mode 30
Minimum 1
Maximum 600
Fig. 16. Cumulative frequency histogram of Australian SCR vertical displacement data showing a positively skewed distribution. Inset table shows summary statistics.
19D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
afirst order indication of the number of large earthquakes that a fault hasgenerated in the current stress regime, given certain assumptions re-garding single event slip and segmentation (Clark, 2010; Leonard,2010; Leonard and Clark, 2006, 2011). It is important to reiterate thatthe total geomorphic displacement across a fault reported here cannotalways be attributed to the neotectonic era or to neotectonic displace-ment (cf. Section 3.1.4.1). Differential erosion across a fault, or inheri-tance of relief from pre-neotectonic times, cannot be readilydiscriminated in some regions, particularly in eastern Australia.
Here we adopt vertical displacement in lieu of fault slip due to apaucity of fault dip data and a data set dominated by features definedusing digital elevation data. Uncertainty in the identification andmeasurement of scarp heights using elevation data might be expectedto vary according to the acquisition system and resolution of thesource data (e.g. 1 m LiDaR versus 90 m SRTM). Spatial resolution
0
20
40
60
80
100
120
140
160
180D1 D2 D3
Neotecto
Ver
tica
l Dis
pla
cem
ent
(m)
1.3 1 1.5
50 250 75
5 35 10
min
max
med
64 40 50n
Fig. 17. Box and whisker plot of vertical displacement data for each neotectonic domain. Boxdefine 90th and 10th percentiles. Data table shows minimum, maximum, median and numbFig. 3 (also Fig. 13).
issues can bias the displacement values, either through non-detection of smaller, more subdued surface displacements in thelandscape, or by over- or under-estimation of observed displace-ments. It is also important to note that for most faults exhibiting geo-morphic expression in the landscape vertical displacement values aretypically minima, as erosion will act to reduce relief across a scarpwith time. The timing and number of movements associated with agiven displacement is usually unknown, but the latter can be estimat-ed using standard relations (e.g. Wells and Coppersmith, 1994).
The population distribution for vertical displacement data ispresented in Fig. 16. It shows a very strong positive skew with 90% offeatures having displacements less than 100 m, and 75% with lessthan 40 m. Median displacement is 15 m, with a modal value of 30 m.Fig. 17 presents vertical displacement data for each neotectonic domain.These data indicate that domains D1, D3 and D6 are characterised by
D4 D5 D6nic Domain
2.5 10 1
600 250 300
42 45 15
38 21 17
es denote 75th and 25th percentiles, central point indicates median value, and whiskerser of data points (n) for each domain. Colours correspond to accompanying domains in
20 D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
relatively small median vertical displacements, with D1having the low-est median value of 5 m. Both D1 and D3 exhibit quite low variability.Domains D2, D4 and D5 similarly have comparable median values, allof which exceed 30 m, and show much greater variability.
4. Discussion
4.1. Discrimination of domains
The domains are not statistically distinct with respect to the keyneotectonic fault variables (length and displacement) (Figs. 14–17), lim-iting the confidence with which conclusions may be drawn from thedata. However, the measures of central tendency (in this case best de-scribed by the median) and the variability surrounding them areinstructive.
The shortestmedian fault lengths occur in D1 andD5 (Fig. 15). In thecase of D1, this is likely to be a fundamental property of the highly struc-tured cratonic crust which is being deformed (cf. Dentith andFeatherstone, 2003; Dentith et al., 2009; Everingham and Gregson,1996; Hills, 1961; Pandey et al., 2008). For example, Dentith et al.(2009) show the 1968 Meckering surface rupture to be strongly con-trolled by complex bedrock architecture, with individual rupture seg-ments initiating and terminating at intersections between lithologicunits and regional dyke and fault sets. Unpublished aeromagnetic dataover the Hyden Scarp, also in southwestWestern Australia, show a sim-ilar pattern (Geoscience Australia, unpublished geophysical data). Inthe case of D5, the result is counter-intuitive as several systems oflengthy depocentre-bounding faults are evident (e.g. Nelson andHillis, 2005; Williamson et al., 1991). This may be a key observation,as intra-basin faults comprise the bulk of records for this domain, andare length-limited by the numerically sub-ordinate basin-boundingfaults. Furthermore, the larger basin-bounding fault systems are typical-lymapped as segmented structures (VandenBerg, 1997), and have beenincorporated as such in the database. For example, in the GippslandBasin the Rosedale and Budgeree faults are co-linear and bound thenorthern margin of the Balook Block. Similarly, the Bass-Almurta andYarragon fault segments bound the northern margin of the NarracanBlock (Fig. 11). In the absence of palaeoseismological data, any com-ment on how segmentation affects rupture length is purely speculative.
The median scarp length of 38.5 km in D2 is nearly 10 km longerthan for D1 and D5. The interquartile range of 21–57 km is consid-ered to be a reasonable reflection of the seismic potential of D2given the existing palaeoseismic data (cf. Section 3.1.2). However, apossible single event displacement of ~7 m on the Milendella Fault(Clark et al., 2011a; Reid, 2007), while likely to far exceed the averagesingle event displacement, would potentially require a rupture lengthsignificantly in excess of the 54 km measured along strike of theMilendella Fault.
Median fault lengths from D3, D4 and D6 are broadly similar (all~55±5 km; Fig. 15), and are approximately 20 km longer than inD1, D2 and D5. The 75th percentile length value for D4 is 73 km,which is consistent with the ~80 km single event rupture length pos-tulated for the Cadell Fault (Clark et al., 2007; McPherson et al., 2012).The similarity in length potentially reflects the structural characteris-tics of the Proterozoic to Phanerozoic fold belts and mobile beltswhich constitute the basement of the domains (the Perth Basin andsouthern Carnarvon Basin are floored by the Proterozoic PinjarraOrogen (Fitzsimons, 2003). Palaeoseismological data for domain D4is restricted to that from the Cadell Fault, and indicate the possibilitythat long scarps might relate to commensurately long (and large) sin-gle event ruptures (cf. segmentation). The larger range and generallyhigher values for domain D6 potentially relate to extensive structuraloverprinting during the reactivation of the mobile belt at the time ofGondwana break-up.
Vertical displacement data distinguish the central and westernparts of the continent (D1, D3 and D6) from the eastern parts (D2,
D4 and D5) (Fig. 17; cf. Fig. 3). The central and western domains aretypified by smaller displacements with less variability, while the east-ern domains show larger median displacements with much highervariability. This characterises the relative accommodation of strainbetween cratonic and non-cratonic parts of the continent, and isreflected in the long-term slip rates measured on the neotectonicfaults, and in the relief of the landscape. The proportion of older,colder and thicker crust in the ancient cratons and mobile belts thatdominate the central and western regions (e.g. Collins et al., 2003)may well contribute to the smaller displacements. The smaller dis-placements may also reflect the relative age of the structures andtheir consequent degree of preservation in the landscape. In theFlinders/Mount Lofty Ranges, enhanced crustal heat flow has beenimplicated in mechanically weakening the crust (Celerier et al.,2005), which potentially contributes to the large displacements andhigh fault density recorded in D2. This mechanism on its own is intu-itively difficult to reconcile with the observation that the hypocentresof instrumentally recorded earthquakes tend to be deeper in D2 thananywhere else in Australia (e.g. Leonard, 2008) as a relatively highheat flow environment might be expected to result in thinning ofthe brittle seismogenic crust and relatively shallower hypocentres.Alternatively, neotectonic strain localisation within D2 might betterrelate to the inherent susceptibility of the listric extensional architec-ture that underlies the domain (Flöttmann and Cockshell, 1996) to re-verse reactivation and inversion (e.g. Johnston et al., 1994).
4.2. Patterns in long-term fault behaviour
Both spatial and temporal patterns in large earthquake occurrenceare suggested by the neotectonic data presented herein, and have inpart guided the discrimination of domains. In most cases interpreta-tion of patterning is speculative, as data is sparse.
4.2.1. Spatial patternsEach neotectonic domain is characterised by a unique spatial ar-
rangement of features (primarily faults and folds), reflecting the vary-ing response of the crust to the imposed stresses. Structures in D1 aretypified by those in the Yilgarn Craton of southwestWA. Thesemanifestas surface-expressed fault scarpswhich trend approximately N–S; closeto perpendicular to the prevailing maximum compressive stress fieldtrend (SHmax) (Clark, 2010) and parallel to the broad structural trendsof the Precambrian terranes that comprise the craton (cf. Wilde et al.,1996). Near to the boundaries of D1, linear and belt-like arrangementof scarps is apparent. This trend is also noted on the northwest and east-ern boundaries of theGawler Craton section of D1 (Fig. 3), andmight re-flect stress concentration associated with crustal rheology changesbetween cratons and marginal mobile belts.
Linear spatial arrangements are more common in D3; perhapsreflecting the strong structural grain common to orogenic and mobilebelts (Figs. 3 and 7). Similar to D1, scarps in D3 are again generallyoriented perpendicular to the local SHmax (cf. Hillis and Reynolds,2003). Scarp length in D4 is similar to D3 (Fig. 15), and linear spatialarrangements also dominate D4. Data from the Cadell Fault(McPherson et al., 2012) demonstrate that the longer rupture lengthsin the eastern non-cratonic domains can potentially relate to largerearthquakes. In D4 scarps are commonly arranged as linear fault com-plexes, which are offset from nearby complexes in an en echelon fash-ion (cf. Caskey and Wesnousky, 1997). A mix of trends is apparent,from perpendicular to SHmax, to those with a more N–S orientation,the latter relating in part to the dominant structural grains in the dif-ferent orogens that constitute D4 (cf. Fig. 1).
The western part of D4, within the Delamerian Orogen componentof the Tasmanides (Figs. 1 and 3), comprises basement structures thatare similar in character and arrangement to the eastern part of D2.However, as the axis of the Flinders/Mount Lofty Ranges isapproached within D2, the character changes dramatically to much
21D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
shorter, more closely spaced, and higher displacement faults arrangedin linear and en echelon patterns with respect to one another. It islikely that many of these faults link, or at least interact strongly inthe subsurface (e.g. Paul et al., 1999; Preiss and Faulkner, 1984;Somerville et al., 2008). Along the axis of the ranges the predominantscarp trend is perpendicular to SHmax.
DomainD5 and the northern part (Northwest Shelf region) of D6 arecharacterised bywell-linked anastomosing networks of faults and folds.While the fault properties are quite different between these domains,they share a partially inverted basin crustal architecture. In the PerthBasin and Southern Carnarvon Basin (D6) the arrangement of faults issimilar to the Sorell Basin (D5), with onlyminormodification of the pri-mary extensional architecture apparent. The geometry of, and relation-ships between, neotectonic faults in the inverted basins around themargins of Australia is largely inherited from the original extensionalbasin architecture (e.g. Hocking, 1988; Keep et al., 2000, 2002;Williamson et al., 1991). Faults bound topography (e.g. Otway,Strzelecki and Cape Ranges, and the Flinders/Mount Lofty Ranges) andform interlinked anastomosing networks that extend, in some cases,for hundreds of kilometres. There is an indication that the level ofneotectonic activity in a given inverting system bears a relationship tohow long prior to compression the last major basin-structuring exten-sional event occurred. For example, there is a progression of increasingneotectonic activity from the parts of the Carnarvon Basin formedmainly in the Palaeozoic, to those where extension continued into theMesozoic, and perhaps early Cenozoic. Similarly, the Fitzroy Trough de-veloped themajority of its structural architecture in the Palaeozoic, wasrelatively little effected by Mesozoic Gondwana break-up, and is todayless neotectonically active than other parts of the Canning Basin thatwere involved inMesozoic break-up (e.g. Keep et al., 2007). Similar con-clusions may be drawn from extended margins around Australia(Blevin and Cathro, 2008).
4.2.2. Temporal patternsTemporal patterns in large SCR earthquake occurrence may be in-
ferred at the scale of a single fault (Clark et al., 2008; Crone et al.,1997), of clusters of faults (Leonard and Clark, 2006; Leonard andClark, 2011), and at the domain scale (Braun et al., 2009; Clarket al., 2011a; Holdgate et al., 2003; Paine et al., 2004; Sandiford,2003a). While the suite of neotectonic fault behaviours may varyacross Australia, as implied by the neotectonic domains model, oneindividual fault characteristic appears to be common to mostAustralian intra-plate faults studied: active periods comprising afinite number of events are separated by typically much longer pe-riods of quiescence (Crone and Machette, 1997; Crone et al., 2003;e.g. Clark and Van Dissen, 2006; Clark et al., 2007, 2008, 2011b;Estrada, 2009) (Fig. 18). Data and modelling from elsewhere in theworld identify similar episodic behaviour on faults with low sliprates (e.g. Friedrich et al., 2003; Marco et al., 1996; Ritz et al., 1995;Wallace, 1987) and suggest that the time between successive clustersof events (deformation phases) is highly variable but significantlylonger than the inter-event times between successive earthquakeswithin an active phase (e.g. Chéry and Vernant, 2006; Chéry et al.,2001; Li et al., 2009; Marco et al., 1996; Stein et al., 1997). This char-acteristic has been referred to as Wallace-type behaviour (cf. Wallace,1987), and may be conceptualised using a model similar to that pro-posed by Friedrich et al. (2003) (Fig. 19).
The sparse data available suggests that an active period (e.g. t1,t2, t3 — Fig. 19a) might constitute half a dozen events (e.g. CadellFault) or less, and perhaps as few as two or three in WesternAustralia (e.g. Hyden — Clark et al., 2008; Crone et al., 2003). It alsosuggests that the slip rate in an active period might be several ordersof magnitude greater than that over the long-term (e.g. Clark et al.,2007, 2008). Published data on faults in the west of Australia (D1)suggest that the interseismic intervals between large events in an ac-tive period might be in the order of 20–40 ka (Clark et al., 2008;
Crone et al., 1997, 2003; Estrada, 2009). Recently acquired data onthe Cadell Fault (Clark et al., 2007; McPherson et al., 2012) indicatemore frequent rupture in D4; three times in the interval ca. 70–45 ka, and 3 times in the interval ca. 45–25 ka. It is inferred thatthree uplift events of similar magnitude (Clark et al., 2007;McPherson et al., 2012) had occurred on the Cadell Fault prior tothe diversion of the Murray and Goulburn Rivers (Bowler, 1978;Bowler and Harford, 1966), suggesting that these events are likelyto have been spaced thousands of years apart, similar to the Meersand Cheraw faults in the western central United States (Crone andLuza, 1990; Crone and Machette, 1995). This contrasts with theNew Madrid Seismic Zone in the intra-plate central United States,where sequences of large earthquakes have occurred on averageevery ~500 years for at least the last two seismic cycles in the cur-rent active period (Tuttle et al., 2002). Quiescent intervals can besufficiently prolonged, in the western and central parts of Australiain particular, that most or all relief relating to an active periodmight be removed by erosion prior to the next active period (Clarket al., 2007, 2008; Crone et al., 2003).
The earthquake recurrence behaviour model presented in Fig. 19can also be used to express the relative probability for the occurrenceof a future event on an SCR active fault (Fig. 19b), underpinned bythree assumptions (after McCalpin, 2009): (1) morphogenic earth-quakes occur due to progressive strain build-up, and so are apredictable periodic process; (2) the periodicity of a characteristicearthquake can be described as a function of its recurrence intervals;and (3) as more time elapses since the last characteristic earthquake,the probability of the next characteristic earthquake increases. If aneotectonic fault does not experience a surface-rupturing earthquakefor a very long time, say more than 100 ka (or at least two mean seis-mic cycle intervals for an active period), our expectation of an eventin the near future will be at some very low or “background” level. Itis likely that most if not all of the relief relating to previous ruptureshas been removed or masked by erosional or depositional (e.g. aeolian)processes, for the majority of such faults (e.g. Meckering, TennantCreek, Marryat Creek). These faults might not be recognised as activeprior to the first event in a new active period. Subsequent to the firstevent in a new active period our expectation of a future rupture, saywithin the following several tens of thousands of years, is greatly elevat-ed (Fig. 19b). This expectation will decay with successive ruptures untileventually reaching the “background” level again after the occurrenceof only a finite number of events. The points critical to understandingthe hazard posed by such SCR faults, and modelling this hazard proba-bilistically, become: 1) is the fault in question about to enter an activeperiod, in the midst of an active period, or in a quiescent period?; 2)how many large events might constitute an active period, and howmany ruptures has the fault generated so far in its current active period(should it be in one)?; and 3) what is the “mean” recurrence interval inan active period, and what is the variability around this mean? This“mean” might be incorporated statistically into probabilistic seismichazard assessments (e.g. Stirling et al., 2011). Additional points worthyof consideration include: (4) what is the inter-seismic repose time be-tween active periods; (5) are the inter-seismic periods Poissonian in na-ture while the seismic cycles are slip predictable?
When considering the data presented in Figs. 14–17, and theresulting fault behaviour model depicted in Fig. 19, it is importantto be cognisant of the sampling bias inherent in the dataset. The ma-jority of features in the neotectonic database are known due to theirassociation with significant and/or distinct topographic relief. In es-sence, they are the active SCR faults that are most discoverable, andbecause of this, they are the ones that have been the subject ofstudy. The bias in the dataset of active SCR faults is, therefore, domi-nated by the faults that have most recently been the most active. Ac-tive SCR faults that are well into a quiescent phase may be nearly“invisible” in the landscape, and hence are relatively unstudied andtherefore under-represented in any mapping (Fig. 3) and subsequent
?
?
?
?
>50 ka
>50 ka
>80 ka
?
?
?
?
?
Meckering,Domain 1
Marryat Ck,Domain 3
Tennant Ck,Domain 1
Roopena,Domain 2
Milendella,Domain 2
Wilkatana,Domain 2
Burra,Domain 2 Mundi Mundi,Domain 2
Cadell/Sth EchucaDomain 4
Lake EdgarDomain 3
Hyden,Domain 1
Lort River,Domain 1
Dumbleyung,Domain 1
~25 ka~17 ka ~50 ka
~70 ka~25 ka
~2000 ka
>100 ka ?
>250 ka ?
Additional eventsTimes unknown
Additional eventsTimes unknown
Additional eventsTimes unknown
Possibly 1-2 eventsTimes unknown
Additional eventsTimes unknown
Additional eventsTimes unknown
~30 ka
~30 ka
~100 ka
~30 ka ~50 ka
~7 ka
~7 ka ~22 ka
~9 ka ~16 ka
~24 ka
~63 ka
~15 ka
1968
1986
1988
~35 ka
~90 ka >250 ka ?
Time unknownOld event
Ungava,Canada
Cheraw,Colorado
Meers,Oklahoma
NMSZ,Mississippi
Bhuj,India
1030-102(historical)
Est imated t ime of earthquakes in years(logari thmic scale)
North America
Rift-related
Australia
1.2 ka 2.9 ka >~120 ka
~8 ka ~12 ka ~25 ka
>>1000 ka ?
>>1000 ka ?
Additional eventsTimes unknown
A.D. 1450A.D. 1812
A.D. 1819
A.D. 900
A.D. 893
104 105 106
Fig. 18. Compilation of surface-breaking earthquake recurrence data for SCR settings (expanded after Crone et al., 2003). Lort River and Dumbleyung data from Estrada (2009),Wilkatana, Burra and Mundi Mundi data from Quigley et al. (2006), Lake Edgar data from Clark et al. (2011b) and Cadell data from Clark et al. (2007). Australian examples arecited with their relevant neotectonic domain (cf. Fig. 3).
22 D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
data collection (Figs. 14–17). Nonetheless, focussed investigation onsuch SCR faults could yield data pertinent to testing the fault behav-iour model depicted in Fig. 19. Another limitation of the SCR faultdataset is that there are few active faults worldwide where fault dis-placement behaviour over several active and quiescent phases hasbeen determined.
There are a number of possible drivers of episodic fault behaviour. Ifmore than one active fault exists in a region, and the fault density is suf-ficiently high (e.g. a fault spacing that is one rupture length or less),then episodic behaviour might be the result of stress interactions be-tween faults. It is plausible that such behaviour might occur in domainsD2, D4, D5, D6 and perhaps D3. For example, the surface expression ofthe Cadell Fault links in with the Mount William Fault system to thesouth (Harris, 1938). However, the much lower neotectonic fault
density in domains D1 and D3 suggests an alternative mechanism.Chéry and Vernant (2006) were able to produce episodic slip behaviourin a single isolated fault embedded in an elastic lithosphere, in whichfault stress fluctuations may occur, loaded by plate motion. In thiscase, deep post-seismic viscous strain can be the source of crustal tran-sient strain that in turn allows earthquake clustering (Kenner andSegall, 1999; Meade and Hager, 2004). Possible causes of fault stressfluctuations include climatic change (e.g. pore pressure variation,erosion-depositional processes — Hertzel and Hampel, 2005), orperhaps more importantly, self-induced fault weakening/healing(Ben-Zion et al., 1999). Chéry and Vernant (2006), assuming thatweak-ening or strengthening processes are gradual and due to repeatedearthquakes (Ben-Zion et al., 1999; Sleep and Blanpied, 1992), foundthat if the fault weakening time is short enough (less than several
1 2 3 4 n
Rupture
“elevated”
“background”
Pro
babi
lity
of fu
ture
rup
ture
rela
tive
to “
back
grou
nd”
Time
Dis
plac
emen
t~x
t2
t1
t3>10t>10t<2x
presentPliocene?
?
Most recent active period
Penultimate active period
Second penultimateactive period
Long term slip rate
(b)
a
b
quiescent period
active period
Fig. 19. (a) Generalised fault slip diagram for Australian SCR faults based upon data from the Cadell Fault (Clark et al., 2007; McPherson et al., 2012). Three active periods of faultgrowth (earthquake occurrence) are denoted by t1, t2, and t3. These active periods are relatively short lived, and are composed of only a few ruptures each (ca. b6 events per activeperiod). Inter-event times between successive ruptures within an active period may range up to several thousand to several tens of thousands of years. We have adopted a char-acteristic earthquake rupture model based upon palaeoseismic data from the Cadell (Clark et al., 2007; McPherson et al., 2012) and Lake Edgar (Clark et al., 2011b) faults. Longquiescent periods separate the active periods, and the length of the quiescent periods can range from many tens of thousands of years to greater than a million years; (b) detailof (a) expressed in terms of the probability of a future event.
23D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
tens of thousands of years), fast slip rate excursions occur, as well as acomplete lack of activity.
Braun et al. (2009) suggest a model for the southwest of WesternAustralia whereby uniform contraction in the ductile lower crust andupper mantle is accommodated by essentially random brittle fracturein the upper crust. This model is supported by the generally non-clustered distribution of palaeoscarps within D1 (Clark, 2010). How-ever, the remarkable sequence of three surface breaking earthquakeseast of Perth in 1968, 1970 and 1979 (Meckering, Calingiri andCadoux — Gordon and Lewis, 1980; Lewis et al., 1981) raises ques-tions of how sections of upper crust “unload” in response to this uni-form contractional strain, and at what scale. The Meckering, Calingiriand Cadoux scarps (Fig. 4) are 70–100 km apart; too distant for staticstress changes to have promoted rupture (cf. Caskey and Wesnousky,1997). Furthermore, the ruptures were sufficiently temporally sepa-rated that dynamic stress changes are unlikely to have promoted rup-ture. The observations are consistent with the postulate that blocks ofupper crust on the scale of ~104 square kilometres can unload in thespace of a decade. In the case of the southwest of Western Australia,this process may be facilitated by a mid- to upper-crustal architecturecharacterised by fundamental sub-horizontal structural discontinuities(most notably at ~10 km and ~25 km depth) (Dentith et al., 2000;
Drummond and Mohamed, 1986; Everingham, 1965; Goleby et al.,1993) that are compartmentalised by major moderately-dipping faultsystems, forming the “superterranes” of Wilde et al. (1996). With onlytheMeckering, Calingiri and Cadoux ruptures as examples, it is not possi-ble to draw conclusions with any certainty. However, the potential is thatwithinD1peculiarities of the geology and crustal structuremight result inthe emergence of spatial and temporal patterns in fault rupture thatmight not be expected from the apparently random distribution of spa-tially isolated scarps at the domain-scale.
Leonard and Clark (2011) used the scaling relations of Leonard(2010) and reasonable estimates of landscape modification rates inthe southwest of Western Australia to generate a catalogue of causa-tive palaeo-earthquakes from the neotectonic scarp inventory in thatregion (cf. Clark, 2010). The rate of morphogenic earthquakes wascompared with the rates estimated from the historic record of seis-micity for the same region (the South West Seismic Zone — Fig. 1),and it was clear that the last 40 years have been anomalously highin terms of seismic moment release (Leonard and Clark, 2006,2011). This result is consistent with the low relief landscape in thesouthwest of Western Australia, and implies that seismicity is migra-tory, probably on the timescale of a few hundred years, and at a spa-tial scale of greater than several fault lengths (i.e. tens of kilometres).
24 D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
However, conclusions relying upon the historic catalogue of seismic-ity that extend significantly beyond the short window of observationmust be treated with caution as it has been proposed that many of theearthquakes comprising the short historic record of plate interiors areaftershocks of large earthquakes that occurred hundreds of years pre-viously (Stein and Liu, 2009).
There is a body of evidence emerging that the temporal clusteringbehaviour observed from individual fault studies in Australia may besymptomatic of a larger picture of the more or less continuous tecton-ic activity from Late Miocene to Recent being punctuated by “pulses”of activity in specific deforming regions (Quigley et al., 2010).Holdgate et al. (2008) present evidence from the southeast EasternHighlands (Figs. 1 and 2) that resurrected the idea of a punctuatedpost-Eocene Kosciuszko Uplift (Browne, 1967) that continued intothe Late Pliocene and possibly the Pleistocene. It is possible that theinitiation of this uplift might relate to the pulse of deformation seenin SE Australian offshore basins in the interval 10–5 Ma associatedwith the reorganization of the crustal stress field to its present config-uration (Dickinson et al., 2001, 2002; Hillis et al., 2008; Sandiford etal., 2004). Further deformation episodes are constrained to theinterval 6–4 Ma in southwest Victoria (Paine et al., 2004). In contrast,the parallelism and conformable nature of most Pliocene strandlines(e.g. Figs. 9 and 10) across Victoria has been taken as evidence forvery little tectonism in the interval 6–3 Ma (Wallace et al., 2005). Aperiod of uplift is constrained to have occurred between 2–1 Ma inthe Otway Ranges (Sandiford, 2003a). In the eastern onshoreGippslandBasin an angular unconformity between Jemmy's Point formation(Mid-Pliocene) and the overlying Haunted Hill Formation (Late Plioceneto Quaternary), suggests significant post 3 Ma deformation (Wallace etal., 2005), whichwas considered to have ceased at 1.0 Ma in the offshoreGippsland Basin while continuing in the eastern onshore basin until ap-proximately 250 ka (Holdgate et al., 2003). Mild deformation appears tohave continued in the offshore basin until the Mid-Pleistocene (ca.470 ka) (Mitchell et al., 2007), and to within the last ca. 125 ka in thesouthern onshore basin (Gardner et al., 2009).
Given the high sensitivity of the stress state in the Australian con-tinent to distant boundary forcings (Coblentz et al., 1995, 1998; Hilliset al., 2008; Sandiford et al., 2004), it is possible that the pulses relateto small changes at the plate margins, such as subduction of a seamount, changes in the nature of convergence on Australia's EastTimor margin (e.g. Keep et al., 2002; Keep and Haig, 2010), or pro-cesses such as the onset of extension episodes in the Taranaki Basinin New Zealand (e.g. King and Thrasher, 1992). If proven, this wouldhave profound consequences for how large earthquake occurrenceshould be assessed in Australia, as the fundamental assumption ofmorphogenic earthquakes occurring due to progressive strain build-up, and so being in some predictable way periodic, is not satisfied(cf. McCalpin, 2009; Ward and Goes, 1993). An alternative hypothesisis that the boundary conditions have remained essentially static andthe locus of deformation has migrated across the Australian continentwith time as stress changes in one piece of unloaded crust destabilisean adjacent piece of crust. There also remains the possibility thatcombinations of these processes operate.
4.3. Implications for stable continental region (SCR) analogue studies
A fundamental implication of the neotectonic domains modelpresented here is that intra-plate fault characteristics are not univer-sal in their applicability in analogue studies. Careful choice of subjectfaults or structures within analogous crust of similar stress fieldcharacter is required to extrapolate meaningfully to imperfectlycharacterised areas. Johnston et al. (1994) identified continentalcrust worldwide that might be considered SCR (for analogue pur-poses), and proposed a fundamental distinction in the seismicity ofextended and non-extended parts of this crust. According to their def-inition, non-extended crust includes continental shields, platforms
and Palaeozoic and Mesozoic fold belts, whereas extended crust in-cludes intra-continental rifts of all ages and passive margins no youn-ger than ~25 Ma. In terms of the Johnston et al. (1994) schema,Domains 1, 3 and 4 from the present study represent non-extendedcrust, while Domains 5, 6 and 7 represent extended crust. Domain 2is difficult to categorise as it is an actively uplifting compressionalorogen (Quigley et al., 2007b) which may owe much of its structural(and seismic) character to an inherited extensional rift architecture.
The present study provides an analysis complementary to that ofJohnston et al. (1994). Whereas the extended versus non-extendeddichotomy is tested by Johnston et al. (1994) using the Global SCRcatalogue of seismicity, here we test against the record of morpho-genic earthquakes in the Australian landscape. The use of apalaeoearthquake catalogue has the advantage of enabling assess-ment of faulting characteristics at time-scales commensurate withthe recurrence time of large, morphogenic earthquakes in the SCRsetting.
The major tenet of the Johnston et al. (1994) model is that extend-ed crust is more seismically active than non-extended crust. Notwith-standing the biases inherent in the neotectonics data (cf. Section 3.1),the distribution and characteristics of faults currently in the Australianneotectonic record broadly support this assertion (compare D2, D5and D6 with D1, D3 and D4) (cf. Shulte and Mooney, 2005).
The neotectonic data also permit further sub-division of the ex-tended and non-extended crustal classes proposed by Johnston etal. (1994). In extended crust domains (e.g. D5, D6), the age ofmajor rifting appears to be important in terms of the record ofneotectonic activity (i.e. fault rupture length and amount of reliefproduction), and thus has implications for estimation of Mmax andmorphogenic earthquake recurrence rate. Palaeozoic intra-cratonicrifts and passive margin components preserve less evidence forlarge earthquakes than those rifted in the Mesozoic. This trend isoften mimicked by contemporary seismicity, for example, compareactivity in the Palaeozoic Fitzroy Trough with that in the MesozoicGippsland Basin aulocogens, or the Palaeozoic southern versusMesozoic northern Carnarvon passive margin basins (cf. Figs. 1 and2). The distinction becomes even clearer when one considers mostPrecambrian rifts. There is no evidence for neotectonic reactivationof the Mesoproterozoic rift in which the McArthur Basin (NT – D1)sits, nor extensional structures related to the NeoproterozoicAmadeus Basin (NT – D3) (Fig. 2). However, in the latter case,there is evidence consistent with neotectonic reactivation of com-pressional structures formed at the northern and southern marginsof the basin in the Petermann Ranges and Alice Springs orogenies(cf. Korsch and Lindsay, 1989).
Domain 2 is anomalous within the framework presented above. Thedomain formed as an intra-cratonic rift that developed into a marginalbasin (similar to the Otway Basin), during the Neoproterozoic toCambrian breakup of the supercontinent Rodinia (Drexel et al., 1993;Glen, 2005; Preiss, 1987). Interpretation of seismic imaging suggeststhat the fundamental structural architecture of the domain developedat this time; Cambrian platform and trough sequences overlying Prote-rozoic (Adelaidean) basement were normally displaced across majorsoutheast-dipping normal faults that sole into a basal reflector at12–13 km depth (Flöttmann and Cockshell, 1996). Reverse displace-ment during subsequent contractional orogenic events (i.e. theDelamerian, Alice Springs and Sprigg orogenies) accrued on a spatiallyrestricted subset of these faults corresponding to the topographic axesof Kangaroo Island, and the Flinders/Mount Lofty Ranges (Fig. 5). Bal-anced cross-sections derived from geological mapping suggest that acontractional architecture, developed during the Delamerian Orogeny,predominates at the surface, at least along the main topographic axes(Jenkins and Sandiford, 1992; Paul et al., 1999). Smaller-scale, or lessprogressed, analogues of the Flinders/Mount Lofty Ranges can perhapsbe seen in the Otway Ranges (Otway Basin— Fig. 10) and the Narracanand Balook Blocks (Gippsland Basin — Fig. 11). The fundamental
25D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
differences that result in D2 accommodating up to one third of the seis-mic strain across the entire Australian continent (e.g. Braun et al., 2009)are the orientation of the Adelaidean palaeo-rift almost perpendicularto the contemporary SHmax (Fig. 1), and perhaps enhanced heat flowresulting in a thermally and mechanically weakened crust (Celerier etal., 2005; Neumann et al., 2000). In these respects D2 does not fit neatlyinto a simple extended versus non-extended framework, and thereforemay be unusual amongst the world's SCR crust.
As we have shown in our analysis of the neotectonic data (Figs. 15and 17), the non-extended shield, platform and fold belt division ofJohnston et al. (1994) can be readily subdivided into domains D1,D3, and D4. In particular, reactivated Proterozoic fold belts (D3) andPalaeozoic fold belts (D4) are characterised by neotectonic structuresthat are longer and have larger displacement than those in D1 craton-ic (shield) crust. Displacement data also suggest that structures inPalaeozoic fold belts may have accumulated more strain in theneotectonic era than those in Proterozoic fold belts, at least in termsof the implied amount of slip that has accrued on the faults. This as-sertion could be tested by summing the moment represented byeach fault scarp in each domain and dividing by the area of the do-main. However, it should be reiterated that the amount of displace-ment on any given fault that can be apportioned to the neotectonicera is often unknown.
Analogues between the Australian domains and SCR crust else-where in the world (cf. Johnston et al., 1994) are readily apparent.For example, poly-phase deformation of a compressional nature is acommon feature in the post-rift evolution of many passive marginsand rifts (D5/D6 analogues) (Balasubrahmanyan, 2006). Archean cra-tonic nuclei fringed by Palaeoproterozoic mobile belts (D1 analogues)comprise a large portion of the geology of Peninsula India (Hoffman,1989) and North America (Kroner and Cordani, 2003). Meso- andNeoproterozoic mobile belts involved in the accretion of thesupercontinent Rodinia (D3 analogues) are found worldwide(e.g. Cawood and Buchan, 2007; Collins and Pisarevsky, 2005),as are Phanerozoic accretionary terranes associated with theamalgamation of the supercontinent Gondwana (D4 analogues)(Hoffman, 1989). Below, we briefly discuss some analogouscrust from the North American SCR as a detailed example of theutility of the domains approach.
The Archean and Palaeoproterozoic core of the North Americancontinent (including the Superior, Wyoming, Hearne, Rae, Nain andSlave Provinces and interstitial Proterozoic orogenic belts) (Davisand Bump, 2009) can be considered analogous in terms of crustalproperties to D1. This core is fringed on the southern and easternsides by reactivated Proterozoic mobile belts and orogenic crust anal-ogous to D3 (e.g. the Yavapai-Mazatzal Province and Grenville Belt)(Wheeler and Frankel, 2000). Further to the southeast Palaeozoicfold belts similar to those in D4 extend to coastal plain east of the Ap-palachian Orogenic Belt (van Arsdale et al., 1998). Mesozoicaulacogens extending through the Palaeozoic and into the Precambri-an shield (e.g. the Reelfoot Rift, the Southern Oklahoma aulacogen,the Ottawa Rift, the Saguenay Graben) can be considered similar toD5. The main orientation of the aulacogen with respect to the prevail-ing stress field might be used to further refine the expectation of anal-ogous crustal response. For instance, the small angle between themajor structures of the Reelfoot Rift and the SHmax is similar to the re-lationship expressed in the Gippsland Basin. The Reelfoot Fault (vanArsdale et al., 1998) and faults such as the Morwell Fault in theGippsland Basin are well suited for analogue studies.
The Mesozoic rifted passive margin of the eastern United States,containing the Charleston source zone (Clark et al., 2011a), mightbe considered analogous with parts of D6 (and perhaps the SorellBasin in D5), or possibly D7. However, in contrast to our observationsof such settings, Wheeler (1995, 1996) notes that Palaeozoic(Iapetan) passive margin crust to the west of and beneath Appala-chian orogenic crust in the northeastern USA appears to be more
active than Mesozoic rifted margin to the east, at least in terms ofthe historic seismic record. In this respect, the Appalachian orogenicbelt and underlying crust is similar to the Flinders/Mt Lofty Orogenicbelt (Sprigg Orogen) of D2.
5. Conclusions
Australia boasts arguably the richest Late Neogene to Quaternaryfaulting record in all of the world's SCR crust (e.g. Quigley et al.,2010). This record, spanning hundreds of thousands of years, affordsthe opportunity to examine the behaviour of SCR faults over several,and in some cases, many, seismic cycles. Based upon differences incharacter of the seismogenic faults across the continent, and guidedby variations in the geologic and geophysical characteristics of thecrust, we propose (after Clark, 2006) six revised onshore neotectonicdomains (Fig. 3). A seventh offshore domain is defined by analogywith the eastern United States. A simple statistical analysis of twosemi-quantitative parameters of the neotectonic fault dataset (lengthand displacement) reveals that, while the faults within each domainare not statistically distinguishable with respect to individual charac-ters, combinations of the central tendencies of the data are sufficient-ly distinct to justify the domains model. For example, faults occurringwithin Proterozoic mobile belts (D3) or Palaeozoic mobile belts (D4,D6) tend to be longer than faults elsewhere, with the implicationthat their values for Mmax would be higher. Faults in the cratonicparts of central and Western Australia (D1, D3) tend to have accumu-lated less neotectonic slip than those in non-cratonic easternAustralia (D2, D4, D5). Furthermore, faults in crust that has beenrifted (D2, D5, D6) appear to be more closely spaced than those innon-extended crust. These characteristics relate to the fundamentalproperties of the crust that is being deformed, which might be seenas a consequence of deformation occurring on pre-existing faults inthe intra-plate setting (cf. Sandiford and Quigley, 2009; Sandiford etal., 2004). The recognition that variation exists in neotectonic faultbehaviours according to crustal type allows for more confident as-sessment of global analogues for comparison.
Variations in spatial and temporal patterns in large earthquake oc-currence are inferred between domains. Scarps in D1 appear to be rel-atively short, randomly arranged, and spaced such that directinteractions between faults are unlikely (cf. Figs. 3 and 4). However,the three historic surface ruptures at Meckering, Calingiri and Cadouxsuggest that apparently random surface rupture occurs in response tounloading of sections of the upper crust at scales in the order of 104
square kilometres. Strain concentration at some margins of D1 cra-tons is evident in a more belt-like arrangement of scarps. Within Pro-terozoic and Palaeozoic mobile belt domains (D3, D4 and to someextent D6) linear arrangement of scarps is apparent, with the poten-tial for direct interaction. Extended domains (D2, D5 and to some ex-tent D6) are characterised by en echelon and anastomosing networksof faults where interaction between ruptures is highly likely.Bi-modal rupture behaviour has been noted in all domains wheredetailed palaeoseismic data is available (D1, D2, D4, D5). Variationbetween domains can be seen in the number of ruptures and the in-terval between ruptures in an active period, and in the inter-activeperiod intervals (Figs. 18 and 19). Active periods might constitutehalf a dozen events or less in non-cratonic SCR (e.g. D2, D4, D5, D6),and perhaps as few as two or three in cratonic SCR (e.g. D1, D3).Interseismic intervals between morphogenic earthquakes in an activeperiod might be in the order of a few thousand years to 20 ka innon-cratonic SCR (with the potential for recurrence as little as 100 sof years in extended domains — e.g. D4, D5, D6) and 20–40 ka in cra-tonic SCR. The duration of inter-active periods is poorly defined, butmay last for hundreds of thousands to millions of years. This behav-iour in individual faults seems to be mimicked at a larger scale inthat punctuated episodes of deformation have been noted withindeforming regions (Braun et al., 2009; Clark et al., 2011a; Holdgate
26 D. Clark et al. / Tectonophysics 566–567 (2012) 1–30
et al., 2003; Paine et al., 2004; Quigley et al., 2010; Sandiford, 2003a).This perhaps relates to subtle changes in stress forcing at the platemargins or migration of the locus of deformation with time.
Despite the difficulties presented by non-Poissonian rupturebehaviour and the potential that strain accumulation is not progressive,data regarding neotectonic faults in the SCR context have the potentialto impact probabilistic seismic hazard assessments (e.g. Somerville etal., 2008; Stirling et al., 2011). Knowledge of the distribution of struc-tures or settings that have responded in a particular way to crustalstress in the neotectonic era, as characterised by their fault lengthand displacement characteristics, influences the assignment ofMmax to earthquake source zones. The large earthquakes at long re-turn periods, which the neotectonic features primarily represent,are significant in terms of seismic hazard to critical infrastructuresuch as large dams, power-generation and emergency servicesfacilities.
The neotectonic domain (Section 3.1) and temporal clusteringmodels (Section 4.2.2) attempt to account for the spatial andtemporal variation observed in the occurrence and magnitude ofmorphogenic earthquakes within Australian SCR crust. Within thisframework the points critical to understanding the hazard posed bySCR faults, and modelling this hazard probabilistically, can besummarised as a series of questions: 1) is the fault in questionabout to enter an active period, in the midst of an active period, orin a quiescent period?; 2) how many large events might constitutean active period, and how many ruptures has the fault generated sofar in its current active period (should it be in one)?; and 3) what isthe “mean” recurrence interval in an active period, and what is thevariability around this mean? Additional points worthy of consider-ation include: (4) what is the inter-seismic repose time between ac-tive periods; (5) are the inter-seismic periods Poissonian in naturewhile the seismic cycles are slip predictable?
Acknowledgements
This paper is published with the permission of the Chief ExecutiveOfficer of Geoscience Australia. The authors gratefully acknowledgereviews on a draft of the manuscript by Dr. David Burbidge and Dr.Lisa Hall. Feedback from Dr. Rich Briggs at the USGS and an anony-mous reviewer greatly improved the manuscript.
Appendix A. Supplementary data
Supplementary data associated with this article can be found inthe online version, at http://dx.doi.org/10.1016/j.tecto.2012.07.004.These data include Google map of the most important areas describedin this article.
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