Anisotropy in the Indian Ocean upper mantle from Rayleigh- and Love-waveform inversion

Abstract

A tomographic method is applied to Love- and Rayleigh-wave seismograms in order to address the problem of upper-mantle elastic anisotropy in the Indian Ocean. The first step in our approach is a waveform inversion: for each path of the study, a 1-D depth-dependent model compatible with the waveforms of the fundamental mode and several higher modes of Love and Rayleigh waves is obtained. Then, the models related to the different paths are inverted in order to retrieve 3-D velocity heterogeneities and anisotropy. In this paper, both the radial anisotropy and the azimuthal anisotropy of S waves are investigated in detail. We find a significant radial anisotropy in the uppermost 300 km of the mantle with an overall amplitude smaller than what is found in global studies. Azimuthal S-wave anisotropy is also present. The directions of fast S-velocities show rather simple patterns from 100 to 300 km depth, and strong correlations with the direction of absolute plate motion (APM) are found. In the uppermost 100 km, the pattern of fast-velocity directions is complex and does not correlate simply with plate motions. From 100 to 200 km depth, a correlation with APM is found in most oceanic regions. The main lack of correlation is located in the vicinity of La Reunion and Mauritius islands. In this region, the anisotropy exhibits a complex pattern between 100 and 150 km, possibly due to a disturbing effect of the hotspot. At 200 km depth and deeper, azimuthal anisotropy vanishes progressively beneath most of the oceanic regions. No significant azimuthal anisotropy is found beyond 300 km depth. The depth extent of both azimuthal and radial anisotropy, the amplitude of radial anisotropy and the pattern of azimuthal anisotropy support the idea of a preferential orientation of olivine crystals in a low-viscosity zone beneath the lithosphere. This region would be located between 100 and 200 km in oceanic areas, and possibly deeper, between 200 and 300 km, in continental areas, not well resolved in this study.