Beneath Bass Strait: Linking Tasmania and Mainland Australia using Ambient Seismic Noise

Abstract

One of the most hotly debated topics in Australian geology pertains to the tectonic relationship between Tasmania and mainland Australia. Among the main difficulties in reconciling mainland Australian and Tasmanian tectonics is the lack of Precambrian exposure in the Lachlan Orogen (Victoria), which contrast with the West Tasmania Terrane that exhibits numerous outcrops of Proterozoic rocks, apparently excluding any tectonic affinity between them. Furthermore, the West Tasmania Terrane differs significantly from the East Tasmania Terrane in that the latter does not contain any evidence of Precambrian rocks and no evidence of a Proterozoic continental basement has been reported, either in outcrop nor inferred from geophysical surveys. Furthermore, the presence of Bass Strait and the Mesozoic and Cainozoic sedimentary and volcanic sequences that mask the older terranes, makes the link between Tasmania and southeast mainland Australia even harder to decipher. This has significantly impeded the ability of conventional surface mapping to unravel the tectonic history of this area, which remains one of the great challenges of Australian Earth sciences. The focus of this study is ambient seismic noise data from 24 broadband stations, which span northern Tasmania, several islands in Bass Strait and southern Victoria, thus allowing a dense coverage of surface wave paths that can be exploited to image the 3-D structure of the crust joining Tasmania and Victoria in high detail. To produce the highest quality Greenâ??s functions, careful processing of the data has been performed, after which Group and Phase velocity dispersion measurements have been carried out using a frequency-time analysis method on the symmetric component (average of the casual and acasual signal) of the empirical Green functions. The next step is to invert the dispersion curves for maps of Group and Phase velocity, which is expected to shed new light on the structure beneath Bass Strait.