The Influence of Lithospheric Thickness Variations Beneath Australia on Seismic Anisotropy and Mantle Flow

Abstract

Rapid plate motion, alongside pronounced variations in age and thickness of the Australian continental lithosphere, makes it an excellent location to assess the relationship between seismic anisotropy and lithosphere‐asthenosphere dynamics. In this study, SKS and PKS shear‐wave splitting is conducted for 176 stations covering the transition from the South Australian Craton to eastern Phanerozoic Australia. Comparisons are made with models of lithospheric thickness as well as numerical simulations of mantle flow. Splitting results show uniform ENE‐WSW aligned fast directions over the Gawler Craton and broader South Australian Craton, similar to the orientation of crustal structures generated during an episode of NW‐SE directed compression and volcanism ∼1.6 billion years ago. We propose that heat from volcanism weakened the lithosphere, aiding widespread lithospheric deformation, which has since been preserved in the form of frozen‐in anisotropy. Conversely, over eastern Phanerozoic Australia, fast directions show strong alignment with the NNE absolute plate motion. Overall, our results suggest that when the lithosphere is thin (<125 km), lithospheric contributions are minimal and contributions from asthenospheric anisotropy dominate, reflecting shear of the underlying mantle by Australia's rapid plate motion above. Further insights from geodynamical simulations of the regional mantle flow field, which incorporate Australian and adjacent upper mantle structure, predict that asthenospheric material would be drawn in from the south and east toward the fast‐moving continental keel. Such a mechanism, alongside interactions between the flow field and lithospheric structure, provides a plausible explanation for smaller‐scale anomalous splitting patterns beneath eastern Australia that do not align with plate motion.