SEISMIC AZIMUTHAL ANISOTROPY BENEATH THE AUSTRALIAN CONTINENT REVEALED BY SHEAR WAVE SPLITTING ANALYSES

Seismic azimuthal anisotropy beneath Australia is quantified using splitting of the teleseismic SKS, SKKS, and PKS ("XKS") phases to investigate asthenospheric flow and lithospheric deformation beneath one of the oldest continents on the Earth. A total of 511 pairs of acceptable XKS splitting parameters were obtained at 116 seismic stations. Unlike other stable continental areas in Africa, East Asia, and North America, where spatially slow-varying splitting parameters dominate, the fast orientations and splitting times observed in Australia show a complex pattern, with a slightly smaller than the normal average splitting time of 0.85 ± 0.33 s. On the North Australian Craton, the fast orientations are mostly N-S, which is parallel to the absolute plate motion (APM) direction of the Australian continent. Those observed in the South Australian Craton are mostly NE-SW and E-W, which are perpendicular to the maximum lithosphere horizontal collision direction. In east Australia, some of the NE-SW fast orientation can be attributed to the APM induced seismic anisotropy, and a few fast orientations mostly align with the strike of the orogenic belts. The observed spatial variations of the seismic azimuthal anisotropy, when combined with results from depth estimate using the spatial coherency of the splitting parameters and results from seismic tomography studies, suggest that the azimuthal anisotropy in Australia can mostly be attributed to simple shear in the rheologically transition layer between the lithosphere and asthenosphere, with local modulations of the mantle flow system by undulations of the lithosphere-asthenosphere boundary, as well as a spatially variable degree of contribution from lithospheric fabrics.