Earth and Planetary Science Letters 231 (2005) 163–176 www.elsevier.com/locate/epsl

Contrasts in lithospheric structure within the Australian craton—insights from surface wave tomography

S. FishwickT, B.L.N. Kennett, A.M. Reading

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Received 14 September 2004; received in revised form 3 January 2005; accepted 7 January 2005

Editor: V. Courtillot

Abstract

Contrasts in the seismic structure of the lithosphere within and between elements of the Australian Craton are imaged using surface wave tomography. New data from the WACRATON and TIGGER experiments are integrated with re-processed data from previous temporary deployments of broad-band seismometers and permanent seismic stations. The much improved path coverage in critical regions allows an interpretation of structures in the west of Australia, and a detailed comparison between different cratonic regions. Improvements to the waveform inversion procedure and a new multi-scale tomographic method increase the reliability of the tomographic images. In the shallowest part of the model (75 km) a region of lowered velocity is imaged beneath central Australia, and confirmed by the delayed arrival times of body waves for short paths. Within the cratonic lithosphere there is clearly structure at scale lengths of a few hundred kilometres; resolution tests indicate that path coverage within the continent is sufficient to reveal features of this size in the upper part of our model. In Western Australia, differences are seen beneath and within the Archaean cratons: at depths greater than 150 km faster velocities are imaged beneath the Yilgarn Craton than beneath the Pilbara Craton. In the complex North Australian Craton a fast wavespeed anomaly continuing to at least 250 km is observed below parts of the craton, suggesting the possibility of Archaean lithosphere underlying areas of dominantly Proterozoic surface geology. D 2005 Elsevier B.V. All rights reserved.

Keywords: lithosphere; Archaean; Proterozoic; Australia; surface wave tomography

1. Introduction structure within a large cratonic region. Global studies [1,2] illustrate large-scale correlations between lithos- The Australian continent provides an excellent pheric properties and the age of the crust, but platform to investigate the variations in lithospheric acknowledge that the detailed picture of lithospheric structure is more complicated than any simple relationship [1]. Within the Australian continent the T Corresponding author. Tel.: +61 2 6125 4324. large differences in seismic structure beneath the older E-mail address: [email protected] (S. Fishwick). Precambrian shield regions compared to beneath the

0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.01.009 164 S. Fishwick et al. / Earth and Planetary Science Letters 231 (2005) 163–176 younger Phanerozoic regions have been seen in broad-band recording in Australia, the stable con- seismic data since the 1960s [3]. The craton consists tinental interior and scarcity of large urban areas make of an amalgamation of blocks of Archaean and much of the continent ideal for the deployment of Proterozoic age (see Section 1.1), and is bounded to temporary seismic recorders. This has been exploited the east by the Phanerozoic region of the continent. A to provide continent-wide recording of the adjacent more robust image of the lithospheric structure seismicity, and hence the potential for excellent beneath the craton will enable the formation and surface wave imaging of the region. evolution of the Australian continent to be discussed in greater detail. 1.1. Tectonic setting Surface wave tomography, exploiting regional earthquakes, is an ideal tool to develop a more The shield region of central and western Australia detailed picture of the seismic structure within the consists of three main cratonic blocks (the West, continent. A wide distribution of events and recorders North and South Australian Cratons) separated by is required to produce a reliable tomographic image. either orogenic belts, which may represent the accre- The active plate boundaries to the north and east of tionary margins of these terranes, or more recent Australia (through Indonesia, Papua New Guinea, Phanerozoic sedimentary cover [4] (Fig. 1). Solomon Islands, Vanuatu, Fiji, Tonga and New In the northern part of the West Australian Craton Zealand) give a wide distribution of frequent earth- is the Pilbara Craton; one of the oldest regions in quakes. A limited number of additional events to the Australia with tectonic evolution beginning around south of Australia, and events from the 908E ridge 3.5 Ga. The Yilgarn Craton covers the majority of the system in the Indian Ocean, allow us to record southern part of the West Australian Craton and is one earthquakes from most azimuthal directions. Although of the largest blocks of Archaean crust that survives there are only a few permanent seismic stations with today. Located between the Pilbara and Yilgarn

KIMBA 97,98 Coral Sea

SKIPPY 93-96

QUOLL 99 WAcraton 00-01, 02-03 Tasman Sea

Archean basement Archean sediments Proterozoic sediments TIGGER 01-02 Proterozoic basement Paleozoic basement

Fig. 1. Overview of the geology of the Australian continent, highlighting the major Archaean and Proterozoic regions. The locations of the broad-band seismometers used in the temporary deployments (SKIPPY, KIMBA, QUOLL, WACRATON, TIGGER) are shown with pentagons and diamonds. S. Fishwick et al. / Earth and Planetary Science Letters 231 (2005) 163–176 165

Cratons, the Capricorn Orogen reflects the suturing of Bikini and Eniwetok nuclear explosions in the the two cratons during the Palaeoproterozoic [5]. Marshall Islands and the LONGSHOT explosion at The North Australian Craton comprises an amal- Amchitka Island in the Aleutian’s. Cleary [3] gamation of blocks of Precambrian age including the observed early, fast, travel time anomalies within Kimberley Craton in the west, Mt Isa Inlier towards shield areas, and late, or slow, anomalies in south- the east and Arunta Inlier in the south. The majority of eastern Australia and Tasmania. Surface wave studies the Kimberley Craton is overlain by ancient sediments by Goncz and Cleary [9] in the 1970s found large with little of the original basement exposed. The differences between the two regions, noticeably a low- Kimberley is thought to have amalgamated with the velocity zone at approximately 130 km deep in rest of the North Australian Craton during the Palae- eastern Australia, which is absent beneath the shield oproterozoic Halls Creek Orogeny. The Mount Isa area of central and western Australia. Small regions of block contains Late Archaean and Early Proterozoic the continent have been studied in some detail using crust, while the Arunta Inlier in the south of the craton arrays of short period recorders and body wave is dominantly Proterozoic in age, and contains tomography (see, e.g., [10]), however the comparison remnants of the accretionary margin that existed on of structure across the whole continent has been made the southern edge of the craton at this time [4]. The possible through surface wave inversion studies using tectonic evolution of the Arunta continues into the data from temporary broad-band deployments (e.g., Palaeozoic, through to the Alice Springs Orogeny SKIPPY, KIMBA) [11]. (400–300 Ma) which uplifted deeper crustal rocks to Zielhuis and van der Hilst [12], using the parti- the surface [6]. tioned waveform inversion (PWI) method of Nolet The South Australian Craton is dominated by the [13], first used the early SKIPPY deployments in the Gawler Craton in the central region, containing east of the continent, and were able to provide a clear Archaean to Mesoproterozoic basement. To the west image of the low-velocity zone in the east, and also a basement is covered with recent sediments on the pronounced increase in the thickness of the high- Nullarbor Plain. To the east is the Curnamona velocity lid under the Proterozoic continental regions. province, which, although mainly covered by sedi- Simons et al. [14] investigated a possible region- ments, is delineated by aeromagnetic data. Where alisation of the model, comparing areas based on exposures exist, the basement is of late Palaeoproter- geological province. At this stage data were limited in ozoic age. Western Australia, but within central Australia it The amalgamation of the Australian shield appeared that there was no obvious relationship occurred during the Proterozoic, and was dominated between seismic signature and lithospheric age. More by the long-lived accretionary margin on the southern recently, Simons and van der Hilst [15] again show edge of the North Australian Craton. To the east of that at scale lengths less than 1000 km the relationship the Precambrian cratons are a number of Phanerozoic between the age of the crust and the thickness of the units. The nature of the boundary between the ancient lithosphere is very complicated. shield regions and the relatively younger eastern An alternative method of surface wave tomography part of the continent, often referred to as the Tasman has also been applied to the Australasian region. Line, is presently under discussion, both in the Based on the waveform inversion technique of Cara crustal location and meaning [7], and the mantle and Leveque [16] and the continuous regionalisation extension compatible with the present seismological algorithm of Montagner [17], Debayle [18] created an models [8]. automated tomographic inversion scheme. The Cara and Leveque inversion scheme uses secondary 1.2. Previous studies observables derived from the waveform by cross- correlation, rather than directly inverting the wave- The contrast in seismic structure beneath the form itself. The latest work [19] using this inversion Phanerozoic eastern margins of Australia and the scheme has a dataset of over 2000 paths, and looks at Precambrian shield area was indicated in early work both azimuthal and polarization anisotropy, investi- on travel time anomalies within Australia from the gating the change from a complex pattern of 166 S. Fishwick et al. / Earth and Planetary Science Letters 231 (2005) 163–176 anisotropy in the uppermost layer to a smoother response to 62.5 s, while the STS-2 has the longest pattern below 150 km deep probably related to the flat response extending to approximately 125 s. large-scale plate motion. Data from the permanent seismic stations in the The various approaches to surface wave tomog- region, and the Southern Alps Passive Seismic raphy yield similar results on a large scale; however Experiment (SAPSE) in New Zealand [21], have also the small-scale features of the models are quite been used to supplement the path coverage obtained different. Choices made for regularisation and param- from the temporary deployments. The number of eterisation of the inversion procedure affect the paths into the permanent stations has been limited in models (see e.g., [20]), while at depth different an attempt to avoid potential bias in the tomographic treatments of the higher modes account for some of models produced by too large a concentration of the differences. Furthermore, lack of adequate path similar paths into a single station. Instead data from coverage in western Australia has made interpreta- permanent stations are used to improve the azimuthal tions of the intracratonic areas difficult. coverage throughout the region and provide additional New data from Western Australia improve the data through areas where path coverage is otherwise continental coverage significantly and now allows a limited. comparison of the lithospheric structures between the three main cratonic regions, and an investigation of 2.1. 1D path-specific models the relationship between mantle structure and surface geology. The initial stage of the inversion of the surface wave data is to extract a 1D model characterising a specific path from source to receiver. The procedure of 2. Data and methods calculating the 1D models is split into two distinct parts: firstly a waveform selection process, and The data used for the surface wave tomography secondly the waveform inversion. The waveform consist of recordings from broad-band seismometers selection process has the goal of submitting only throughout the Australian region (Fig. 1). Previous high-quality seismic waveforms to the subsequent studies (Section 1.2) used data from the SKIPPY, inversion. The seismogram must contain the appro- KIMBA and QUOLL experiments (undertaken by the priate portion of the data, the fundamental and higher Research School of Earth Sciences, Australian mode Rayleigh wave-train on the vertical component, National University) alongside data from permanent for an event at sufficient distance (N1000 km) from the seismic recording stations in the Australasian region. recorder to avoid the effects of complications from the The present study benefits greatly from the inclusion source region. The quality of the seismogram is of additional data from the most recent temporary assessed using estimates of the signal (As) to noise deployments of portable broad-band seismometers, (An) ratio at four frequency bands within the periods of WACRATON and TIGGER. interest (50 to 140 s). The waveform is used if the A variety of portable instruments have been signal-to-noise ratio As /AnN3 for at least two of the deployed during the experiments. A combination of frequency bands. Gu¨ralp CMG-3ESP, CMG-40T and Streckeisen STS-2 Having identified suitable seismic waveforms, the seismometers have been used. Reftek 72A-07 record- inversion process is now controlled by a modified ers were used in the older experiments, while a version of the automated procedure of Debayle [18]. combination of Refteks and Nanometrics Orion We attempt to find the 1D shear velocity model that recorders have been used since WACRATON. The gives the best fit between the secondary observables varying responses of these instruments must be for the real seismic waveform and the corresponding known as well as possible for successful application quantities for a synthetic seismogram calculated for of the waveform inversion procedure. The majority of the current 1D model. In order to create an efficient the seismometers have flat responses to ground inversion procedure, the secondary observables are velocity between 30 Hz and 0.033 Hz (30-s period), chosen to reduce the non-linearity of the Rayleigh although some of the CMG-3ESPs have a flat waveform data with respect to the model parameters, S. Fishwick et al. / Earth and Planetary Science Letters 231 (2005) 163–176 167 and reduce the dependence on the starting model [16]. In previous work, the starting model for the inversion procedure has been a smoothed version of PREM (preliminary reference Earth model) [22], with a crustal average along the path calculated from 3SMAC [23]. Although the Cara and Leveque inversion scheme improves the domain of recovery to a larger perturbation from the starting model than PWI [24], we now introduce another step to improve our set of 1D models. Rather than calculate the 1D model from only one starting model, we perform inversions from seven different starting models. The new starting models are still based on PREM, but have perturbations of À4.5, À3, À1.5, +1.5, +3, +4.5% in the upper mantle between 50 and 200 km, before slowly converging back to PREM at 500 km depth. Using multiple starting models enlarges the domain where we can recover a 1D model. For example, a dominantly oceanic path can be very slow, and hence using starting models with negative perturbations from PREM gives an increased chance of recovering the 1D model. The 1D path-specific model that we use in the tomographic inversion is a weighted average of the group of similar final models that are accepted through the inversion scheme. The consistency of the group of models from the multiple inversions gives an additional means to assess the reliability of the wavespeed profile with depth, this information is used in weighting the information from each path within the tomographic inversion. For this work, we obtain 1D models for 2116 paths. 1400 Paths are from temporary stations deployed in the Research School of Earth Sciences experiments, with additional 716 paths to the perma- nent stations and SAPSE deployment. The present data limit the potential bias introduced by concen- trations of similar paths into a few permanent stations, and importantly give a more uniform path coverage Fig. 2. Maps illustrating data coverage. (a) The path density for the previous work of Debayle and Kennett [19]. (b) The path density for (Fig. 2) allowing comparisons of structure throughout the present study. The new data show improved coverage in both the Australian continent. Western and central Australia. (c) Each source–receiver path is displayed. 2.2. Tomographic inversion parameterisation scheme of Yoshizawa and Kennett The 1D models representing a path-average for [25] where a local basis function, in this case a each source–receiver pair are used within a tomo- spherical B-spline, is defined throughout the region graphic inversion to derive a model of the 3D velocity with knot points at uniform intervals. This approach structure for the Australasian region. We here use the provides a smooth representation of wavespeed 168 S. Fishwick et al. / Earth and Planetary Science Letters 231 (2005) 163–176 variations with scale lengths determined by the choice damping is applied we minimise the prediction error, of knot spacing. but no a priori information will be used to single out The 1D models can be regarded as an average under-determined parameters [27]. along the path. The reconstruction of the 3D model Originally the a priori reference model used in can be achieved by a linear inverse problem relating the damping was a global reference model, where the velocity perturbation at a particular point to the wavespeed only varies with depth, e.g., PREM or path information at the same depth. The tomographic AK135 [28]. However, previous tomographic inversion is solved using a damped least squares models for the Australian region have revealed inversion scheme [26]. The data are weighted to sub-continental velocities much faster than the control the relative contribution on each path based on reference model, while beneath the oceans the our estimate of the reliability of the 1D model. velocities are far slower than the global model. Tests Damping is used to control the trade off between on synthetic structures suggest that a least squares model variance and resolution (or prediction error and inversion with model-norm damping towards a solution length). If the damping is large we will homogeneous reference model, will struggle to minimise the under-determined part of the solution, recover small scale features within the contrasting but will not minimise the prediction error. If no regions. We therefore take an alternative approach to the inversion using a multi-scale approach (see e.g., [29]) to the tomography (Fig. 3). We first create a broad-scale model using knot points at 88 intervals, to obtain the large-scale structure in the region. The resulting model is then used as the reference model -10 for the final 28 inversion, and we now damp towards the large-scale features within the data, rather than towards a global reference model.

-30 3. Surface wave tomographic images

The tomographic images illustrated in Fig. 4 are from the final models produced from the multi-scale a 120 150 scheme, with knot points at 28 intervals. The models are plotted as perturbations from the 1D global reference model AK135 [28]. At all depths the same colour scale is used, with saturation at perturbations F -10 greater than 7.5%. Models are shown at 50-km depth slices from 100 to 300 km. By 100 km depth, the fast wavespeeds character- istic of an ancient shield region are seen throughout the cratonic area. Slightly fast velocities are also seen

-30 in parts of eastern Australia, with much slower velocities on the very eastern margin of the continent. The largest variations in shear wavespeed are seen at a depth of about 150 km, with variations of slightly b 120 150 more than 15% between the fastest and slowest velocities. The potential causes of the velocity Fig. 3. Multi-scale tomography. (a) The image of the tomographic model calculated with 88 knot points, damped to a global reference variations are discussed in more detail in Section model. (b) The final image, with 28 knot points, using the model 4.1. Below 200-km depth the fast velocities become plotted in panel (a) as a reference model. more fragmented and the magnitude of the wavespeed S. Fishwick et al. / Earth and Planetary Science Letters 231 (2005) 163–176 169

Fig. 4. Images of the tomographic S-wave velocity models at 100-, 150-, 200-, 250- and 300-km depth. A map of the main geological units is included for reference. The models are plotted as perturbations of shear wavespeed from the global reference model AK135. The colour scale is the same for all five tomographic images. anomalies starts to decrease. It is apparent that some 3.1. Resolution of the fast wavespeed anomalies at these depths are not associated with the continuation of the lithosphere As an aid to assessing the reliability of the and have their origin in the sub-lithospheric mantle tomographic models, we illustrate results from a features. selection of synthetic tests. From the previous studies 170 S. Fishwick et al. / Earth and Planetary Science Letters 231 (2005) 163–176 of the Australian region it is clear that we expect to simulated through added noise relative to the weight- see features of different scale lengths, e.g., contrasting ing for the path in the actual inversion at this depth; velocities of craton against ocean compared to intra- paths with good higher mode information will there- cratonic variations. As simple checkerboard tests only fore have less noise. The 38 input structure is not indicate resolution for a particular scale of anomaly recovered particularly well at 250-km depth (Fig. 5b); [30] we create a multi-scale input structure (Fig. 5a), although we see some recovery of the general pattern for the tomographic inversion step, as an approxima- the recovered amplitudes are lower and there is tion for the type of structure we hope to resolve. A set significant smearing. Using larger size anomalies of 1D models are created from this multi-scale input (Fig. 5c) we see a better recovery of the amplitudes by averaging along the actual paths and these are then of the imposed anomalies, although there is still some employed to attempt to reconstruct the imposed smearing. The recovery of regions with neutral wavespeed structure. A component of noise has been velocity is poorer, this is partly due to the implicit added relative to the weighting for the path used in the smoothing of the spline parameterisation used to actual inversion as a means of simulating the represent the output model, which is different to that limitations of the waveform inversion step. As in the used to create the input. These resolution tests actual inversions the path-specific models for the illustrate that at greater depths in our model the synthetic input are weighted by an estimate of the amplitudes of the anomalies are likely to be more reliability of the path information. reliable for larger features. The results from this synthetic test at 100 km are Our results indicate the recovery of synthetic illustrated in Fig. 5. Comparison of the output model structures for particular depth slices of the tomo- with the input model highlights some important of the graphic inversion. While we see reasonable recovery tomographic results. The sharp contrast at 1488 east is of the structures within the continent for this test, we recovered well, this is a similar structure to the have to recognise the assumptions used within the contrast associated with the Tasman line in the real tomographic inversion. In this study we assume great data. However, to the west of Australia the same circle propagation and only consider the sensitivity to contrast is not recovered nearly as well, due to the structure on the ray path. In reality the seismic waves relatively poor azimuthal variation in path coverage in are influenced by a zone around the path. Neglecting this dominantly oceanic region (Fig. 2). Within the this influence zone means that the realistic resolution Australian continent the pattern of high and lower is slightly less than the idealized resolution seen from wavespeed anomalies is recovered, although there is the synthetic tests [25]. some smearing of the velocities. The small-scale The tests we have performed provide a useful features are not that well resolved to the south and assessment of potential horizontal resolution. How- west of the continent, again this is not surprising given ever it is more difficult to track resolution in depth the more limited path coverage in these regions. We through the two-stage surface wave inversion. In the limit our main discussion to features within the second tomographic step each depth slice of our continent, where resolution at this depth is sufficient model is calculated separately, and therefore any to resolve structures with horizontal scale lengths of a vertical smearing arises from the construction of the few hundred kilometres. original 1D path-specific models. It is therefore The multi-scale models are appropriate for the difficult to provide a simple resolution test because shallower mantle, but by 250-km depth we are below the result will be heavily dependent on the specific the lithosphere for most of the continent and expect to form of any anomalies. see a simpler pattern of anomalies. At such deeper The reliability of the deeper part of 1D models is levels standard checkerboard tests should provide markedly improved when higher mode information is useful information on the potential resolution. We available [31], and so such paths are given higher illustrate the results of two different resolution tests weighting. The distribution of events in depth to the with different size anomalies (3 and 58) separated by north and east of Australia is favourable for such regions of neutral velocity (Fig. 5b,c). Once again, the higher mode contributions, but the shallow ridge influence of error from the waveform inversion is events to the south are dominated by fundamental .Fswc ta./ErhadPaeaySineLtes21(05 163–176 (2005) 231 Letters Science Planetary and Earth / al. et Fishwick S.

Fig. 5. Synthetic tests of the horizontal resolution for the tomographic inversion. Top—input structures (a) 100-km depth: contains multi-scale anomalies, maximum perturbations are F8%; (b) 250-km depth: simple checkerboard pattern with 38 anomalies, the same size as the smaller scale features used at 100 km, maximum perturbations are now F5%; (c) 250- km depth: larger checkerboard anomalies, 58, maximum perturbations are F5%. Bottom—the recovered structures for each tomographic inversion. The same colour scale is used for all images. 171 172 S. Fishwick et al. / Earth and Planetary Science Letters 231 (2005) 163–176 modes. The half width of the resolution kernels for a recently been applied to the Australian continent particular 1D model will be in the order of 30 km [34]. between 75 and 200 km depth. This means that the Although temperature is likely to be the strongest largest wavespeed anomalies have the possibility of a control on the seismic velocity, other variables such as small level of leakage between the sections at 50-km water and partial melt [35] and grain size [36] will depth intervals as displayed in Fig. 4. The quite have an effect on the seismic velocities observed distinct geographic patterns in the different images within the mantle. Due to the complicated interactions suggest that such leakage is not that significant for of these variables it is beyond the scope of this paper regions with good data coverage. In the shallowest to perform quantitative calculations to calculate part of our models there may be some influence from possible mantle temperatures or to estimate chemical crustal structures. However, the shortest period used variability. However, Faul and Jackson [37] have in this study is 50 s and at this period the sensitivity developed a new formulation for the combined kernels are not strongly sensitive to structure in the influence of temperature and grain size on shear crust (as illustrated in [19], Fig. 2). wavespeed based on recent experimental work at seismic frequencies; their modelling based on the shear velocity distribution in our tomographic images 4. Discussion and conclusions indicates variations in temperature of approximately 600 8C between the oceans and the oldest parts of the 4.1. Velocity variations Australian craton at a depth of 150 km. We concentrate here on the location of anomalies, The tomographic images illustrate large velocity and the potential relationship with surface geology. variations within the Australian region, with varia- tions of about 15% at 150-km depth. Below we briefly 4.2. Intracratonic structure discuss the potential causes of the velocity variations within the upper mantle. At a depth of 100 km fast velocities, about +4% Goes et al. [32] combine results from many mineral relative to AK135, are seen throughout much of the physics studies to consider the effect of temperature continent. The fastest velocities are seen in western and composition on seismic velocities, and then Australia beneath the Pilbara Craton and northern estimate temperatures in the mantle from P and S parts of the Yilgarn Craton. Previous work has tended tomographic results within Europe. Considering only to concentrate anomalies in western Australia towards the anharmonic temperature effect they calculate the southern part of the Yilgarn Craton, probably due changes in shear wave velocity of less than 1% per to the limited path coverage being dominated by paths 100 8C. For the range of shear wavespeeds that we to the permanent station, Narrogin (NWAO), in the have imaged (Fig. 4) a purely anharmonic interpreta- southwest corner of the continent. tion would lead to unreasonably large temperature To the east of the main cratonic region we also see variations in the upper mantle. However, including the fast wavespeeds that are related to the depth extent of effects of anelasticity increases the influence of Phanerozoic lithosphere. On the eastern margins of temperature on shear wavespeed to between 1 and the continent, as seen in the early work by Zielhuis 4% per 100 8C [32], thus reducing the temperature and van der Hilst [12], the very low velocities near the variations that would be required to explain the northeast coast of Australia and around the Bass Strait imaged wavespeed variations. correspond spatially with areas that have undergone The effects of composition are calculated to be Neogene volcanism. much smaller than the effect of temperature [32], and The fast velocities in western Australia continue to shear wavespeed variations alone are unlikely to be greater depth. Beneath the Yilgarn Craton a large fast able to resolve chemical variations within the mantle. wavespeed anomaly is clearly seen to at least 250-km Deschamps et al. [33] propose a method using depth. This feature does not appear to be continuous tomographic models and gravity data to infer from east to west across the craton, but has slower chemical variations within the mantle, which has velocities in the central Yilgarn and faster zones at the S. Fishwick et al. / Earth and Planetary Science Letters 231 (2005) 163–176 173 western and eastern edges. In contrast, below 150 km, Craton make a simple relationship with the surface the large fast velocity perturbations are not seen geology unclear. In contrast, tomographic results from beneath the Pilbara Craton. The synthetic tests (Fig. P- and S-delay times in South Africa [40] indicate a 5b) suggest that at 250-km depth due to the small size simple relationship where the highest velocities are of the Pilbara Craton it would be difficult to resolve a confined to the region beneath the Archaean Kaapvaal fast velocity anomaly. We therefore must be cautious Craton, with lower velocities in the Proterozoic with any interpretation in this region, however the mobile belt to the south. Images from the Fennoscan- rapid change from fast to neutral velocities at dian region of the Baltic Shield are more complicated, shallower depths of between 150 and 200 km suggests body wave tomography [41] indicates no obvious that the Pilbara may not have as deep a lithospheric feature related to the Archaean–Proterozoic suture, keel as the Yilgarn Craton. but recent surface wave tomography [42] illustrates a In northern Australia, the Kimberley Block is different character of structure beneath the Archaean underlain by fast velocities to a depth of around 250 province in comparison to the Proterozoic, although km, and the region of faster velocities extends further faster wavespeeds are not observed at all depths south than is apparent from the surface geology. This within the Archaean region. Work in the Canadian deep anomaly may be related to older Archaean shield [43] also indicates no obvious correlation lithosphere, as isotopic data from diamondiferous between the depth of the lithosphere and the age of kimberlites suggest that Archaean lithosphere once the overlying geological units. existed beneath the Kimberley craton [38]. Recent Some variations in lithospheric structure are likely work [39] indicates a possible connection between the to be a result of the tectonic history of the continent edge of this anomaly and the occurrence of diamond since formation. Assuming the mantle root beneath deposits in the region. At 200 km a distinct break in the Precambrian shield is related to the formation of the fast anomalies is seen below the Canning Basin, lithosphere in the Precambrian, then it is also highly which separates the Kimberley and Pilbara Cratons, likely that this mantle has been affected by the and also to the east of the Kimberley Craton. The tectonic processes that occurred since that formation. other dominant region of fast velocities is a predom- For example, if a geological reconstruction suggested inantly north–south belt that runs from the edge of the multiple episodes of subduction, it is unreasonable to McArthur Basin, to the west of Mt Isa, and into the think that the lithospheric mantle would be unaffected eastern edge of the Arunta. This anomaly continues to by this series of events, and this may explain some of at least 250 km, and is very similar in magnitude to the complexity to the lithospheric structure that is that seen in the Yilgarn, suggesting the possibility of imaged in the present tomographic models. an Archaean lithospheric root in this region. Within the South Australian Craton there appear to 4.3. Shallow structure be variations in the extent of the lithosphere. Under- lying the Curnamona block fast velocities are seen to The image of our shallowest depth slice, at 75 km around 150 km. Below this depth the transition to (Fig. 6a), gives an intriguing picture of the uppermost positive anomalies moves to the west. Beneath the mantle structure underneath the Australian Craton. Gawler Craton fast velocities are seen to 250 km, The West Australian Craton is quite well defined by however this is a region where path coverage (Fig. 2)is the high seismic velocities. The highest velocities are limited and we see smearing in our resolution tests (Fig. seen beneath parts of the Pilbara, Capricorn Orogen 5). We are therefore less confident in the interpretation and the northern Yilgarn. Within the North Australian for this area. A current deployment of portable broad- Craton we generally see slightly fast velocities, band seismometers should provide more data for this particularly in the northern parts, the Kimberley, Pine area, and reduce the uncertainties in future work. Creek Inlier, McArthur Basin and northern Mt Isa The structure of the lithospheric mantle beneath Block. In contrast, in the southern parts of this craton, Australia is quite complicated, the location of the and within much of central Australia we see lowered fastest wavespeeds changes with depth, and uncer- velocities that are not expected within a cratonic tainties in the original age of the North Australian region. 174 S. Fishwick et al. / Earth and Planetary Science Letters 231 (2005) 163–176

75

-10 -10

-30 -30

a b 150 120 120 150

relative to ak135 -8 -6 -4 -2 0 2 4 6 8 -18 s 18 s

Fig. 6. (a) The image of the tomographic S-wave velocity model at 75 km depth. (b) The variations in mantle S-wave arrival times for paths of less than 208. Negative arrival times, indicating fast velocities, are plotted in shades of blue, positive arrivals (slow) are plotted in red.

The cause of these lowered velocities is difficult Western Australia, provides improved images of 3D to explain. At these shallowest depths the tomo- shear wavespeed and thus a comparison of structure graphic models might be affected by smearing of within the Australian Craton. The additional data from crustal structure. However the region of low velocity stations in Western Australia not only improve does not relate to any a priori information in definition of the Archaean cratons but also markedly 3SMAC, and if the low velocities were simply increase path coverage in central Australia compared related to deeper crust we would expect to see the with previous studies. At shallow depths, the mantle strongest feature in the northern Mt Isa region, and beneath the West Australian Craton has fast shear thus better correlation with work [45,46] on patterns wavespeeds. In contrast, below central Australia we of crustal thickness. see a zone of lower seismic velocity in both body The unusual slow velocities are confirmed by wave and surface wave results, which is unusual for a body wave data. Fig. 6b illustrates the pattern of cratonic region. arrival times of S waves from less than 208 plotted At depths greater than 100 km, we see fast along the propagation path [44]. In the central region velocities throughout the Australian Craton. There slow velocities are seen along paths from earth- are structures on scale lengths of a few hundred quakes near Tennant Creek to stations to the south kilometres within the cratonic lithosphere. Fast veloc- and southwest. Despite the slow propagation speeds ities extend deepest beneath the Yilgarn Craton. the attenuation along these paths is low and high Beneath parts of the North Australian Craton, includ- frequencies are retained. The body wave paths turn ing the Kimberley, fast velocities are also seen to well below the crust and the concordance with the depths of at around 250 km. If we assume a relation- surface wave results provides strong support for the ship between the age and depth extent of the litho- reduced wavespeeds to be in the uppermost mantle. sphere, this would suggest that a large part of the North Future work will aim to give better constraints on Australian Craton, to the west of the Mt Isa Block is the magnitude and location of this anomaly and underlain by Archaean lithosphere. possible trade offs between crustal and mantle The pronounced high wavespeed features below velocities. the Yilgarn Craton in Western Australia continue to about 250 km and then link to a zone of slightly fast 4.4. Conclusions wavespeeds, but with an east–west rather than north– south orientation (Fig. 4—300 km). It is therefore The inclusion of data from new temporary deploy- rather difficult to determine a particular depth to ments of broad-band seismometers, particularly in which the lithospheric keel continues. S. Fishwick et al. / Earth and Planetary Science Letters 231 (2005) 163–176 175

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