3D S-wave velocity pattern in the upper mantle beneath the continent of Asia from Rayleigh wave data

Abstract

Group velocities of Rayleigh waves in the 10–150 s period range along about 1200 paths across Central Asia and Siberia are used to obtain group velocity maps of the region. To image the lateral variations of group velocities we used the tomography method developed earlier for spherical surface [PEPI 122 (2000) 19]. The method is supplemented by the opportunity to estimate the resolving power of the data. The locally averaged dispersion curves have been then inverted to vertical S-wave velocity profiles up to depth of 500 km at different sites in the region, which thus image a 3D velocity distribution. In order to reduce the number of unknown parameters we assumed the relationships between S-wave and P-wave velocities and densities to be those of the PREM model. The 3D S-wave velocity pattern is displayed in the form of maps at depths from 50 to 350 km with a step of 50 km, and of 2D vertical velocity sections along some profiles crossing different tectonic units. The most remarkable feature of the 3D velocity distribution is the presence of a high-velocity rise at the depths of 250–350 km beneath southern Siberia and western China, which borders upon the low-velocity asthenosphere along the longitude about 105°E. Collision of the high- and low-velocity substances results in pressing-out the viscous asthenosphere material in the weakened zone of the Baikal rift, where the lithosphere is thinned. The rise of the asthenosphere in this zone probably produces mantle upwelling, and leads to uplift of the Khangay Plateau.