Antarctica II: Upper-mantle structure from velocities and anisotropy

Abstract

Abstract To improve the lateral resolution of three-dimensional seismic wave velocity models in Antarctica and the surrounding oceans, we have analysed direct earthquake-to-station Rayleigh-wave data observed on the vertical high-gain long-period and the very long period components of seven GEOSCOPE stations located in the southern hemisphere and three other stations at equatorial latitudes. The phase velocities of Rayleigh waves along 400 well-distributed paths are obtained in the period range 60–300 s, by fitting the data with synthetic seismograms computed with known source parameters in a reference earth model represented by the Preliminary Reference Earth Model (PREM). Corrections for shallow layers have been carefully applied to the observed phase velocities. The geographical distributions of phase velocities and azimuthal anisotropy 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 velocities are observed in Enderby Land, where some of the oldest rocks in the world have been sampled. The low velocities are located along the ridges 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 Australia-Antarctica Discordance (AAD) presents slow velocities near the surface but fast velocities below the lithosphere. These main features are discussed in the framework of the Gondwana hypothesis and the earlier supercontinent. The first azimuthal anisotropy results are also discussed. Anisotropy values are smaller within the Antarctic continent than in the surrounding oceans. They are also small in the AAD but particularly large in the areas around it, suggesting an active tectonic process characterized by a downward flow at depth, a good candidate for a cold spot or a new subduction zone.