HR: 09:30h
AN: T31A-06    [PDF]
TI: Geodynamic and Seismic Characterization of Mantle Flow in the Izu-Bonin Subduction System
AU: * Lassak, T M
EM: Teresa.Lassak@asu.edu
AF: Department of Geological Sciences, Arizona State University, Box 871404, Tempe, AZ 85287-1404 United States
AU: Fouch, M J
EM: fouch@asu.edu
AF: Department of Geological Sciences, Arizona State University, Box 871404, Tempe, AZ 85287-1404 United States
AU: Hall, C E
EM: chall@gps.caltech.edu
AF: California Institute of Technology, Seismological Laboratory, MS 252-21, Pasadena, CA 91125 United States
AB: The goal of this study is to provide new constraints on mantle flow in subduction zones as inferred by patterns of seismic anisotropy. In particular, we are implementing improved methods of interpreting seismic anisotropy measurements in terms of lattice-preferred orientation (LPO) induced by mantle flow. In this study, we focus on the dynamics of the Izu-Bonin (IB) subduction system, which contains a plate geometry that can be approximated to first order by 2D mantle flow models. This simple configuration, combined with recent new measurements of shear wave splitting in the region [Anglin and Fouch, this session], provides an important opportunity to evaluate the role of water on flow and strain development in a hydrated mantle wedge. Current and previous observations of seismic anisotropy in the IB mantle wedge, suggest convergence-parallel fast polarization directions, consistent with models of deformation in a "dry" mantle system. However, the IB mantle wedge is most likely hydrated, and some models of deformation in a hydrated mantle predict convergence-orthogonal fast polarization directions. This dichotomy thus provides an ideal setting in which to test hypotheses regarding the effect of water on mantle flow, strain, and seismic anisotropy. To this end, we use a finite element approach to calculate kinematic models of flow in the mantle wedge. We use these flow models to calculate LPO in the mantle wedge following the method developed by Kaminski and Ribe [EPSL, 2001] that incorporates the combined effects of intracrystalline slip and dynamic recrystallization on textural development. We utilize the resulting textures to predict shear wave splitting for a range of seismic raypaths by solving the Christoffel equation for each increment of a raypath and integrating the resulting predicted shear wave splitting experienced by each ray. A significant advantage of this approach is the ability to provide more appropriate representation of a range of mantle mineralogies, resulting LPO development, and predicted shear wave splitting for regions within the mantle flow model. While our current modeling efforts utilize an isoviscous mantle rheology, we are developing more appropriate models with temperature-dependent rheologies and hydrated mantle conditions. In addition, we will examine the role of water on LPO development and predicted shear wave splitting. Finally, we will compare the range of results of predicted shear wave splitting with shear wave splitting measurements currently in progress.
DE: 7218 Lithosphere and upper mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8162 Rheology--mantle
SC: Tectonophysics [T]
MN: 2003 Fall Meeting