Global upper mantle tomography of seismic velocities and anisotropies

Abstract

A data set of 2600 paths for Rayleigh waves and 2170 paths for Love waves enabled us to retrieve three-dimensional distributions of different seismic parameters. Shallow layer corrections have been carefully performed on phase velocity data before regionalization and inversion at depth. The different seismic parameters include the five parameters of a radially anisotropic medium and the eight azimuthal anisotropic parameters as defined by Montagner and Nataf. It is found that the lateral heterogeneities of velocities and anisotropies in the upper mantle are dominated down to 250-30 km by plate tectonics with slow velocities below ridges, high velocities below continents and a velocity increasing with the age of the seafloor. Anisotropy is present in this whole depth range and the directions of maximum velocities are in good agreement with absolute plate velocities. Below 300 km, there is a sharp decreasing of the amplitude of lateral heterogeneities of seismic velocities and anisotropies. Below 450 km, lateral heterogeneities display a degree 2 and to a less extent a degree 6 pattem. Therefore, between 250 km and 450 km, there is a transition region where vertical circulation of matter is possible as shown by subducted slabs and "plumes" of slow velocities but which probably separates two types of convection. The first one is closely related to plate tectonics and to the distribution of continents. The second one dominates below 450 km and is characterized by two downgoing and two upgoing flOWS.

New evidence for dislocation creep from 3-D geodynamic modeling of the Pacific upper mantle structure

“…Other observational constraints on mantle viscosity such as long-wavelength geoid anomalies [7] suffer similar limitations. Only observations of seismic anisotropy in the upper mantle [8,9], if caused by lattice preferred orientation (LPO), suggest the upper mantle being dominated by dislocation creep [1]. Maybe mantle melts and shape preferred orientation of mantle minerals may also induce anisotropy [1], although these mechanisms are not very likely to cause the globally observed anisotropy.…”

The state of the upper mantle beneath southern Africa

“…The first seismic tomography models which were derived from body waves and long-period surface wave data supported the tectosphere model with high-velocity continental roots extending to over 400 km depth (e.g. Woodhouse and Dziewonski, 1984;Montagner and Tantimoto, 1991). However, more recent work which includes both long and short period surface wave data (e.g.…”

Global azimuthal seismic anisotropy and the unique plate-motion deformation of Australia

“…Differences in the thickness of the high-velocity lid underlying continents as imaged by seismic tomography, have fuelled a long debate on the origin of the 'roots' of continents [1][2][3][4][5] . Some of these differences may be reconciled by observations of radial anisotropy between 250 and 300 km depth, with horizontally polarized shear waves travelling faster than vertically polarized ones 2 .…”