Azimuthal Anisotropy From Multimode Waveform Modeling Reveals Layering Within the Antarctica Craton

The isotropic structure of the crust and upper mantle under Antarctica has been constrained by many studies. However, the depth dependence of seismic anisotropy, a powerful tool to characterize deformation and flow, is still poorly known. Here, we modeled three-dimensional (3-D) variations in azimuthal anisotropy under Antarctica using a multi-mode Rayleigh waveform fitting technique. We first searched the model space with a reversible-jump Markov Chain Monte Carlo approach to find path-averaged vertically polarized shear wave velocity profiles that fit fundamental and higher mode Rayleigh waveforms. We then inverted them to obtain a 3-D velocity and azimuthal anisotropy model across the region down to 600 km depth. Our results reveal that the east-west dichotomy found in other studies is not only characterized by different wave velocities but also by different anisotropy directions, likely reflecting the different deformation histories of the two blocks. Azimuthal anisotropy was found to be present in the top 300 km only and peaks at 100 - 200 km depth under the East Antarctica craton. Additionally, depth changes in fast direction were observed within the craton between 75 km and 150 km depth, suggesting layering is present. We speculate this layering relates to the formation history of the craton.