Vectorial tomography—II. Application to the Indian Ocean

Abstract

‘Vectorial tomography’ which enables lateral heterogeneities and anisotropy to be retrieved simultaneously is applied to a dataset of dispersion velocities of Rayleigh and Love waves in the Indian Ocean. Crustal corrections are applied to take account of lateral variations in shallow parameters. It is shown that the dataset is explained by fewer parameters when azimuthal anisotropy is introduced than when only lateral variations of dispersion velocity are considered. Through the hypothesis of orthotropy (existence of a local symmetry axis), we display three‐dimensional models of S‐wave velocity and anisotropy characterized by its amplitude and two angles. The regions of slow velocities are generally associated with low anisotropy and to a vertical symmetry axis suggesting ascending flow. On both sides of these regions, the symmetry axis is generally horizontal or sub‐horizontal. Shields display high velocities at least down to a depth of 200 km. Moreover, Australia is characterized by high anisotropy down to the same depth. Horizontal directions of anisotropy can change within a depth range of 50km, suggesting a complex convection pattern. At a depth of 300km, the correlation with surface tectonics is very tenuous and directions of anisotropy seem to diverge from (or converge towards) a slow velocity anomaly located in the Central Indian Basin. Copyright © 1988, Wiley Blackwell. All rights reserved