HR: 17:15h
AN: T24B-06    [Abstracts]
TI: Sensitivity of Lower Mantle Seismic Anisotropy Beneath Subduction Zones to Mantle Viscosity Structure and Mineral Inherited Deformation
AU: * Nippress, S
EM: nippress@liverpool.ac.uk
AF: University of Liverpool, Department of Earth and Ocean Sciences, University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP United Kingdom
AU: Kusznir, N
EM: sr11@liverpool.ac.uk
AF: University of Liverpool, Department of Earth and Ocean Sciences, University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP United Kingdom
AU: Kendall, M
EM: kendall@earth.leeds.ac.uk
AF: University of Leeds, School of Earth Sciences, University of Leeds, Leeds, LS2 9JT United Kingdom
AB: Until recently, the mantle between 410km and the top of the D'' was thought to isotropic. Wookey et al (2002) report shear-wave splitting observations generally between 3 and 6secs from deep focus events in the Tonga-Kermadec subduction zone, recorded in Australia and show that the observations are generated by near-source anisotropy located in the top-most lower mantle. We investigate, using geodynamic modelling and seismic ray tracing, the effect mantle viscosity structure and mineral inherited deformations have on the predicted magnitude of shear-wave splitting in the top-most lower mantle. We use finite element (FE) modelling to predict subduction zone stresses arising from incompressible fluid flow driven by the body forces arising from the negative buoyancy of the subducting slab. We model the flow and stress field at the point in time when the subducted slab reaches the 660km phase transition, and not the development of subduction. The FE model is driven dynamically by the excess density (50kg/m$^{3}$) of the subducting slab within the upper mantle, rather than using plate-like velocity boundary conditions. Large deviatoric stresses (maximum values 40 MPa) are generated in a broad region (lateral wavelength 800km) in the topmost lower mantle. These stresses could induce mineral alignment in a broad region (lateral wavelength 800km) in the topmost lower mantle below the slab. We model finite strain accumulated by a mantle parcel as it propagates through the FE fluid flow models. Strain fields are mapped into seismic anisotropy and we then ray trace from the base of the subducting slab to predict shear-wave splitting along ray paths in the top-most lower mantle to teleseismic distances of $30\deg$-$60\deg$. For a viscosity model proposed by Steinberger (2000), which has viscosity increases at 410km, 660km and in the lower mantle, 5-10secs of shear-wave splitting is predicted compared to only 0-5secs for a uniform mantle viscosity. For the Steinberger viscosity model, we predict that shear-wave splitting reduces from 7-10secs to 4-6secs when deformation above 410km is ignored, and this further reduces to 2-3secs when all deformation accumulated above 660km is zeroed. Similar patterns are seen when other viscosity profiles are used. The dependence on the magnitude of the predicted shear-wave splitting on the mapping of finite strain to anisotropy has been explored, and compared to LPO calculations in the upper 410km of our subduction models. Predicted shear-wave splitting magnitudes using the Steinberger viscosity structure with the effect of mineral inherited deformation included are comparable to those observed from the Tonga-Kermadec subduction zone.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8166 Stresses--deep-seated
DE: 7207 Core and mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting