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Exploring deep Australia with the WOMBAT array

Nick Rawlinson, Brian Kennett and Hrvoje Tkalcic

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

 

Figure 1. Location of all short period passive seismic arrays deployed in southeast Australia over the last decade 

In the last decade, a dense rolling array of short period seismometers has been used to achieve a cumulative coverage of over 400 sites throughout southeast Australia (see Figure 1). This experiment, known as WOMBAT-SE, has recorded large volumes of passive seismic data for use in teleseismic tomography, ambient noise tomography, receiver function analysis and array studies of deep mantle and core structure. Station spacings vary between approximately 15-20 km in Tasmania, and 30-50 km on mainland Australia. Deployment times of early experiments in western Victoria and southeast South Australia are approximately 4 months, but since EVA (Figure 1), they have been extended to 8-12 months. The majority of instruments used have been ANU solid state short period recorders connected to vertical component L4Cs (natural frequency of 1Hz), but since SETA (Figure 1), three-component Lennartz LE-3Dlite sensors (also with a natural frequency of 1Hz) have been deployed.

To date, a variety of studies have been carried out with the recorded data, including teleseismic tomography, joint inversion of passive and active source data, ambient noise tomography, and analyses of exotic core phases. In this report, early results from a combined inversion of teleseismic arrival time residuals will be featured. Figure 2 shows a tomographic image at 150 km depth produced by the inversion of EVA, LF98, MB99, SEAL and SEAL2 datasets.

One of the clearest features in Figure 2 is the east to west change from slower to faster velocities from the Bendigo Zone to the Stawell Zone. It is tempting to interpret this as a change from Phanerozoic mantle lithosphere of oceanic origin to Proterozoic mantle lithosphere of continental origin. Expected changes in both composition and temperature between these two types of material make this a plausible argument. In addition, this approximate boundary has also been observed, albeit at lower resolution, using surface-wave tomography, which appears to clearly distinguish between cratonic western Australia and the younger orogens that characterise eastern Australia. However, it must remembered that recent overprinting effects, such as the hotspot-related Newer Volcanic Province in Victoria and magmatic processes related to the opening of the Bass and Otway basins and the Tasman Sea, may well have contributed significantly (via increased temperatures) to reduced wavespeeds observed in central and southern Victoria.

Figure 2: Preliminary image obtained from joint inversion of teleseismic arrival time residuals recorded at five arrays of the WOMBAT-SE project. PCF = Proterozoic Continental Fragment.