Azimuthal Anisotropy of the Antarctic Upper Mantle

We examine upper mantle anisotropy across the Antarctic continent using 103 new shear wave splitting measurements obtained from teleseismic SKS, SKKS, and PKS phases combined with 56 previously published results. An eigenvalue technique is used to estimate the fast polarization direction and delay time for each phase arrival, and high-quality measurements are stacked to determine the best-fit splitting parameters at each seismic station. Delay times generally range from 0.3 - 1.0 seconds across Antarctica. Grid NE-SW fast polarization directions are found across much of Antarctica, and a clockwise rotation in fast polarization direction is broadly evident moving from west to east across the continent. The most notable departures from the Grid NE-SW trending fast polarization directions are observed in near-coastal regions, including the Amundsen Sea Embayment, Lützow-Holm Bay, and Dronning Maud Land. While fast polarization directions in the interior of East Antarctica are perhaps best attributed to relict deformational fabrics frozen into the lithosphere, upper mantle anisotropy can likely be attributed to a combination of both active asthenospheric flow and relict lithospheric fabrics throughout West Antarctica, the Transantarctic Mountains, and coastal East Antarctica. Upper mantle anisotropy may reflect numerous processes in Antarctica, including shear due to plate motion, large-scale geodynamic convection, relict deformational fabrics, and active tectonism. Aside from reflecting shear due to plate motion or mantle convection, anisotropy observed across much of West Antarctica can be attributed to Jurassic Cenozoic tectonism, including subduction along the paleo-Pacific margin of Gondwana, the breakup of Gondwana, the extension of the West Antarctic Rift System, and mantle plume impingement in Marie Byrd Land. Anisotropy in the Transantarctic Mountains is perhaps best associated with the late Cretaceous initiation of extension in the West Antarctic Rift System and localized Cenozoic extension in the Ross Sea. We attribute relatively uniform fast polarization directions in the interior of East Antarctica to Precambrian tectonism and variable shear wave splitting measurements in the near-coastal regions of East Antarctica to fabrics associated with the rifting of Gondwana.