ANTARCTICA-II - UPPER-MANTLE STRUCTURE FROM VELOCITIES AND ANISOTROPY

To improve the lateral resolution of three-dimensional seismic wave ve locity models in Antarctica and the surrounding oceans, we have analys ed direct earthquake-to-station Rayleigh-wave data observed on the ver tical high-gain long-period and the very long period components of sev en GEOSCOPE stations located in the southern hemisphere and three othe r stations at equatorial latitudes. The phase velocities of Rayleigh w aves along 400 well-distributed paths are obtained in the period range 60-300 s, by fitting the data with synthetic seismograms computed wit h known source parameters in a reference earth model represented by th e Preliminary Reference Earth Model (PREM). Corrections for shallow la yers have been carefully applied to the observed phase velocities. The geographical distributions of phase velocities and azimuthal anisotro py are then computed with the tomographic method without any a priori regionalization developed by Montagner (Ann. Geophys., 4(B3): 283-294, 1986). The results show some new and important features of Antarctica and the southern hemisphere. The locations of velocity anomalies are well resolved. The eastern part of Antarctica corresponds to a craton- like structure down to depths of about 250 km, and the highest velocit ies are observed in Enderby Land, where some of the oldest rocks in th e world have been sampled. The low velocities are located along the ri dges encircling the Antarctic continent. The lowest velocities appear in some areas corresponding to hotspots (Crozet, Kerguelen, Macquarie and Balleny Islands). Also, an elongated low velocity is found on the western flank of the Transantarctic Mountains, which might be related to the existence of a rift zone similar to the African rift. The Austr alia-Antarctica Discordance (AAD) presents slow velocities near the su rface but fast velocities below the lithosphere. These main features a re discussed in the framework of the Gondwana hypothesis and the earli er supercontinent. The first azimuthal anisotropy results are also dis cussed. Anisotropy values are smaller within the Antarctic continent t han in the surrounding oceans. They are also small in the AAD but part icularly large in the areas around it, suggesting an active tectonic p rocess characterized by a downward flow at depth, a good candidate for a cold spot or a new subduction zone.