Upper mantle seismic anisotropy of South Victoria Land and the Ross Sea coast, Antarctica from SKS and SKKS splitting analysis

We determine shear wave splitting parameters of teleseismic SKS and SKKS phases recorded at 43 broad-band seismometers deployed in South Victoria Land as part of the Transantarctic Mountain Seismic Experiment (TAMSEIS) from 2000 to 2003. We use an eigenvalue technique to linearize the rotated and shifted shear wave particle motions and determine the best splitting parameters. The data show a fairly consistent fast direction of azimuthal anisotropy oriented approximately N60°E with splitting times of about 1 s. Based on a previous study of the azimuthal variations of Rayleigh wave phase velocities that show a similar fast direction, we suggest that the anisotropy is localized in the uppermost mantle, with a best estimate of 3 per cent anisotropy in a layer of about 150 km thickness. We suggest that the observed anisotropy near the Ross Sea coast, a region underlain by thin lithosphere, results either from upper mantle flow related to Cenozoic Ross Sea extension or to edge-driven convection associated with a sharp change in lithospheric thickness between East and West Antarctica. Both hypotheses are consistent with the more E-W fast axis orientation for stations on Ross Island and along the coast, subparallel to the extension direction and the lithospheric boundary. Anisotropy in East Antarctica, which is underlain by cold thick continental lithosphere, must be localized within the lithospheric upper mantle and reflect a relict tectonic fabric from past deformation events. Fast axes for the most remote stations in the Vostok Highlands are rotated by 20° and are parallel to splitting measurements at South Pole. These observations seem to delineate a distinct domain of lithospheric fabric, which may represent the extension of the Darling Mobile Belt or Pinjarra Orogen into the interior of East Antarctica.