HR: 15:10h
AN: T23D-06    [Abstracts]
TI: Seismic Characterization of Mantle Flow in Subduction Systems: Can We Resolve a Hydrated Mantle Wedge?
AU: * Lassak, T M
EM: Teresa.Lassak@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287-1404 United States
AU: Fouch, M J
EM: fouch@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287-1404 United States
AU: Hall, C E
EM: chall@gps.caltech.edu
AF: California Institute of Technology, Seismological Laboratory, Pasadena, CA 91125 United States
AU: Kaminski, E
EM: kaminski@ipgp.jussieu.fr
AF: IPG Paris and Universite Paris 7 Denis Diderot, Laboratoire de Dynamique des Systemes Geologiques 4 place Jussieu, Paris, 75252 France
AB: The goal of this study is to provide new constraints on the resolvability of mantle flow in subduction zone settings as inferred by observations of seismic anisotropy. We are motivated by the multitude of seismic anisotropy observations in subduction systems that exhibit a broad range of shear wave splitting parameters. While fast polarization directions have typically been interpreted as a proxy for flow or maximum finite extension, experimental studies suggest that olivine slip systems change under hydrous conditions. The simple assumed relationship where fast directions are interpreted as parallel to mantle flow in anhydrous regions is not appropriate if the relative importance of slip systems varies, as would be expected in the mantle wedge of a subduction zone. For instance, fast polarization directions have been interpreted as orthogonal to mantle flow in some hydrated mantle regions, but the conditions that could produce seismic observations of flow-orthogonal fast directions due to hydrated mantle sources have not yet been examined. To this end, we predict shear wave splitting as a result of mantle silicate lattice-preferred orientation (LPO) development during mantle flow. We evaluate two-dimensional (2D) mantle flow models for LPO development using a theory 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 population of seismic raypaths traversing the model by solving the Christoffel equation for each increment of a raypath and integrating the resulting predicted shear wave splitting recorded by each ray. For a model where LPO evolved to steady state under dry simple shear conditions, complete realignment of LPO after a transition to hydrated olivine rheology requires an additional $\sim$300% strain. However, we observe resolvable significant changes in fast directions ($>$10$\deg$) after only $\sim$125% strain. Using these results as a guide for regions of subduction systems that likely contain a transition from anhydrous to hydrated mantle, we expect that only very limited regions of the mantle may develop coherent LPO that reflects the hydrated system. We can test the hypothesis that trench-parallel fast directions indicate wet LPO textures by determining how far from the trench wet deformation conditions must hold, and whether this is consistent with estimates of where water release from the slab occurs. We are currently applying this analysis to a range of 2D subduction zone flow models to directly evaluate these effects and determine under what scenarios hydrated mantle may be imaged by shear wave splitting observations.
DE: 8199 General or miscellaneous
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 7218 Lithosphere and upper mantle
DE: 3210 Modeling
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting