Continental scale shear wave splitting analysis: Investigation of seismic anisotropy underneath the Australian continent

The structure of the upper mantle beneath the Australian continent is investigated using teleseismic shear wave splitting to extract seismic anisotropy. Measurements have been performed on data recorded at 190 sites with portable broadband seismic recorders, spanning almost the entire surface of the continent since 1992. The average time span of the various deployments, primarily designed for surface wave tomography, is 6 months, which is rather limited for shear wave splitting analysis. However, the data set provides a full continental scale survey using the reasonably favourable distribution of seismicity to Australia. Seismic anisotropy has the potential to provide insights into the lithospheric structure and the possible mechanical coupling between the crust and the upper mantle, but prior results for Australia have indicated relatively small levels of splitting and a complex pattern. These results are confirmed with our new and far more extensive measurements across the whole continent. The pattern of seismic anisotropy from shear wave splitting beneath Australia is rather complex and is not correlated with the almost north-south absolute plate motion (APM) from recent models. Deviation of the asthenospheric mantle flow around the lithospheric roots associated with the extensive Archaean and Proterozoic zones of central and western Australia could be occurring, and so mantle flow-related anisotropy cannot be completely ruled out. Despite the limited geological outcrop, especially in Phanerozoic eastern Australia, that is almost entirely covered by sedimentary basins, some relationships can be highlighted between the orientation of the polarization plane of the fast S-waves and structural trends along, for instance, the Halls Creek orogen bordering the eastern edge of the Kimberley basin or along the New England and Lachlan fold belts in the southeastern part of the continent. Such relationships might account for anisotropy frozen in the lithosphere during post-tectonic thermal relaxation. Along the poorly constrained and controversial Tasman Line (TL), the geological boundary between Precambrian and Phanerozoic Australia, directions of anisotropy measured at some stations located in the close vicinity of the 'line' appear to exhibit a curvilinear trend somewhat similar to that of the TL, suggesting that fossil deformation associated with the TL might be recorded in the pattern of seismic anisotropy.