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Contrasts in Lithospheric Structure Within the Australian Craton—Insights from Surface Wave Tomography

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Contrasts in Lithospheric Structure Within the Australian Craton—Insights from Surface Wave Tomography 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.
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