Radial seismic anisotropy as a constraint for upper mantle rheology

Seismic shear waves that are polarized horizontally (SH) generally travel faster in the upper mantle than those that are polarized vertically (SV), and deformation of rocks under dislocation creep has been invoked to explain such radial anisotropy. Convective flow of the upper mantle may thus be constrained by modeling the textures that progressively form by lattice-preferred orientation (LPO) of intrinsically anisotropic grains. While azimuthal anisotropy has been studied in detail, the radial kind has previously only been considered in semi-quantitative models. Here, we show that radial anisotropy averages as well as radial and azimuthal anomaly-pattems can be explained to a large extent by mantle flow, if lateral viscosity variations are taken into account. We construct a geodynamic reference model which includes LPO formation based on mineral physics and flow computed using laboratory-derived olivine rheology. Previously identified anomalous v(SV) regions beneath the East Pacific Rise and relatively fast V-SH regions within the Pacific basin at similar to 150 km depth can be linked to mantle upwellings and shearing in the asthenosphere, respectively. Continental anisotropy at shallow (similar to 50 km) depth is under-predicted, and these deviations are in quantitative agreement with the expected signature of frozen-in, stochastically-oriented anisotropy from past tectonic episodes. We also consider two end-member models of LPO formation for wet and dry conditions for the asthenosphere (similar to 150 km). Allowing for lateral variations in volatile content, the residual signal can be much reduced, and the inferred volatile patterns underneath the Pacific appear related to plume activity. In deeper layers (similar to 250 km), anisotropy indicates that small-scale convection disrupts plate-scale shear underneath old oceanic lithosphere. We suggest that studying deviations from comprehensive geodynamic reference models, or residual anisotropy, can provide new insights into the nature and dynamics of the asthenosphere. (C) 2007 Elsevier B.V. All rights reserved.