HR: 0800h
AN: T21B-0533    [Abstracts]
TI: Finite Element Models of Viscous Flow in the Mantle Wedge Above a Subducting Slab for Different Relative Subducting and Overriding Plate Motions
AU: * Sherrington, H F
EM: hkfs@u.washington.edu
AF: University of Washington, 310 Condon Hall Box 351310, Seattle, WA 98195 United States
AU: Willett, S
EM: swillett@u.washington.edu
AF: University of Washington, 310 Condon Hall Box 351310, Seattle, WA 98195 United States
AB: A series of models of viscous flow in the mantle wedge above a subducting slab are generated with a two-dimensional finite element code used to solve for time-dependent, viscous deformation of the mantle in response to relative motion of the slab and the overriding plate. Resultant velocity fields are used to compute finite strain for mantle particles traversing the wedge; these patterns of finite strain may serve as a proxy for mantle fabrics formed through lattice preferred orientation (LPO) of olivine crystals. Boundary conditions applied to the wedge model to represent relative motions of the subducting and overriding plates have a strong influence on the appearance of mantle wedge flow. In particular, distinctive flow patterns result from an overriding plate velocity that is either faster or slower than the subducting plate velocity. The resulting geometry of mantle flow is further modified in a three-plate scenario, where two overriding plates move at different velocities along the top of the mantle wedge. Flow fields generated from these boundary conditions are computed for both simple, isoviscous and isothermal cases as well as more complex rheologies such as a power-law relationship between stress and strain rate with temperature-dependent viscosity. For the case of isoviscous and isothermal flow fields, migration of the overriding plate away from the trench results in mantle material being drawn from deeper depths than the case of a stationary overriding plate. In addition, preliminary results indicate that temperature-dependent viscosity may result in even steeper particle paths from deeper in the mantle into the wedge corner. This distinction may have important implications for the thermal structure of the mantle wedge. Finite strain is computed for particles traversing each of these flow fields, yielding a predicted olivine LPO for each case. Important differences in these LPO fields imply that seismic waves traversing the mantle wedge above a subducting slab may yield waveforms with a distinctive signature reflecting the specific geometry and characteristics of the mantle flow field through its particular pattern of olivine LPO. Combining seismic anisotropy measurements with flow modeling may thus provide important constraints for flow in the mantle wedge above a subducting slab.
DE: 8100 TECTONOPHYSICS
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting