HR: 0800h
AN: S11B-0552    [Abstracts]
TI: Radial seismic anisotropy as a constraint for upper mantle rheology
AU: Becker, T W
EM: twb@usc.edu
AF: Department of Earth Sciences, University of Southern California, MC0740 3651 Trousdale Pkwy, Los Angeles, CA 90089-0740, United States
AU: Kustowski, B
EM: kustowsk@seismology.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St, Cambridge, MA 02138, United States
AU: * Ekstrom, G
EM: ekstrom@ldeo.columbia.edu
AF: Department of Earth and Environmental Sciences, Lamont-Doherty Earth Observatory, P.O. Box 1000, Palisades, NY 10964, United States
AB: 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 and anomalies from improved seismological maps can be explained to a large extent by novel mantle-flow computations. Our models incorporate LPO formation and a laboratory-derived olivine creep law and were recently shown to produce LPO heterogeneity as observed in mantle xenoliths. In accordance with seismology, we predict the strongest an\-iso\-tropy under oceanic plates. Continental anisotropy at shallow depths is under-predicted, which we interpret as the signature of past tectonic episodes. Previously identified anomalous v_SV regions beneath the East Pacific Rise and relatively fast v_SH regions within the Pacific basin at ~150~km depth can be linked to mantle upwellings and shearing in the asthenosphere, respectively. Radial anisotropy amplitudes potentially provide a strong constraint for mantle rheology and lateral viscosity variations, which are difficult to infer with other methods.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7270 Tomography (6982, 8180)
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8162 Rheology: mantle (8033)
SC: Seismology [S]
MN: 2007 Fall Meeting