Crustal structure in eastern Australia from ambient noise tomography of WOMBAT transportable array data

Posted by pkm at Feb 12, 2015 11:10 AM |
N. Rawlinson (1), S. Pilia (2), Y. Yang (3), P. Arroucau (4) & M. Salmon (5) - (1) School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, Scotland; (2) The Petroleum Institute, Petroleum Geoscience Department, Abu Dhabi, UAE; (3) Department of Earth and Planetary Sciences, Macquarie University, North Ryde 2109, Australia; (4) Centro de Geofisica, Faculdade de Ciências da Universidade de Lisboa. Lisbon, Portugal; (5) Research School of Earth Sciences, The Australian National University, Canberra ACT 2601

The WOMBAT transportable seismic array of eastern Australia is the largest passive seismic experiment of its type in the southern hemisphere, with over 700 stations deployed during the course of 17 separate array movements. Station spacings average 50 km on the mainland and 15 km in Tasmania, with the recent deployment of a broadband array in southern Victoria and northern Tasmania providing connectivity across Bass Strait. Although a variety of imaging techniques have been applied to data from WOMBAT, the focus of this presentation will be on results from ambient noise tomography.

One of the key debates in Australian geology is centred on the evolution of the proto-Pacific margin of east Gondwana in the late Proterozoic and Palaeozoic, which was responsible for a protracted period of accretionary growth outboard of the Archean and Proterozoic core of central and western Australia. However, direct access to the Palaeozoic rocks which underpin the so-called Tasman Fold Belt System (TFBS) is limited due to vast Mesozoic-Cainozoic sedimentary and volcanic cover sequences. The evidence obtained so far points to a complex period of crustal growth, which may have included entrainment of one or more exotic continental blocks, an amalgamation of multiple linear orogen-parallel accretionary events, the formation of a large orocline and several arc complexes.

With a relatively high density of stations covering over half of the TFBS, WOMBAT is well placed to unravel the complexities of Palaeozoic crustal accretion using ambient noise tomography. Long term cross-correlations of background noise (dominantly produced from diffuse oceanic sources) between station pairs yield estimates of interstation surface wave Green's functions which can be exploited for imaging purposes. We use a two-stage Bayesian transdimensional tomography scheme to convert Rayleigh wave dispersion data into 3-D crustal shear wave velocity; this approach does not rely on local linearisation to produce a solution, and permits the parameterization to adapt to the spatially variable information content of the data. The final result is a remarkably detailed snapshot of the crust, which at shallow depths clearly resolves sedimentary basins (e.g. Gippsland, Bass, Otway basins) as low velocity anomalies and crystalline basement associated with the Southern Highlands and Gawler Craton as high velocity anomalies. In the lower crust, a high velocity anomaly which extends southwards from Victoria into western Tasmania is interpreted as an exotic continental fragment that resisted subduction and resulted in the formation of a large orocline via differential roll-back. Evidence for this orocline is clearly observed via the eastward wrapping of the higher velocity Stawell Zone around the lower velocity Hay-Booligal Zone. Variations in the fast direction of azimuthal anisotropy, obtained from the same dataset, also strongly support the presence of a large orocline.

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