The Deep Roots of Geology: Tectonic History of Australia as expressed by Mantle Anisotropy

Australia is an old stable continent with a rich geological history. Limitations in sub-surface seismic imaging below the Moho, however, mean that is unclear to what extent, and to what depth, this rich geological history is expressed in the mantle. Studies of seismic anisotropy, which reflect past/present mantle deformation, can offer potential insights. One commonly employed technique is shear wave splitting, in which the wave polarisation is measured. New such results from seismic arrays deployed across central Australia, reveal a pattern of anisotropy that is consistent with past deformation of the Australian lithosphere that has been preserved for over 300 million years. Another informative technique is to use scattered surface waves, called Quasi-Love waves, that can detect lateral gradients in seismic anisotropy. The first such study for the region finds that scatterers are preferentially located near (1) the passive continental margins, and (2) the boundaries of major geological provinces within Australia. Such lateral anisotropic gradients within the continental interior imply pervasive fossilized lithospheric anisotropy, on a scale that mirrors the crustal geology at the surface. Beneath the continental margins, lateral anisotropic gradients may indicate small-scale dynamic processes in the asthenosphere, such as edge-drive convection, that are tied to the margins.