HR: 08:15h
AN: T31D-02 INVITED [Abstracts]
TI: New Constraints on Mantle Flow in Subduction Systems
AU: * Fouch, M J
EM: fouch@asu.edu
AF: Arizona State Univ., Dept. of Geological Sciences, Tempe, AZ 85224
United States
AU: Lassak, T M
EM: teresa.lassak@asu.edu
AF: Arizona State Univ., Dept. of Geological Sciences, Tempe, AZ 85224
United States
AU: Roth, J B
EM: jeffrey.roth@asu.edu
AF: Arizona State Univ., Dept. of Geological Sciences, Tempe, AZ 85224
United States
AU: Smith, C M
EM: christine.smith@asu.edu
AF: Arizona State Univ., Dept. of Geological Sciences, Tempe, AZ 85224
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, Laboratoire de Dynamique des Systemes Geologiques, Paris, 75252
France
AU: Anglin, D K
EM: karen.anglin@asu.edu
AF: Arizona State Univ., Dept. of Geological Sciences, Tempe, AZ 85224
United States
AB:
From a compositional, dynamical, and structural standpoint, subduction zones are arguably Earth's most complex tectonic
setting. Significant uncertainty remains regarding the extent of coupling between lithospheric plates and surrounding
mantle, as well as the extent to which the mantle wedge is hydrated in subduction systems. A key observation used to address
these issues is measurements of seismic anisotropy, which while generally straightforward to observe via shear wave
splitting, are less straightforward to interpret particularly in regions of compositional and structural complexity. To this
end, we have surveyed several Pacific subduction zones to provide new constraints on subduction zone seismic anisotropy. We
have also performed numerical modeling to estimate the effects of hydration of the mantle wedge on observations of shear
wave splitting in subduction systems.
From an observational standpoint, we have obtained over 400 new shear wave splitting measurements from shear phases that
sample the Japan, Izu-Bonin, and Cascadia subduction systems. Across the Japan and Izu-Bonin regions, fast polarization
directions range from convergence-parallel to trench-parallel and suggest complex mantle deformation. We infer that the
total strength of anisotropy is 1-2% and is generated primarily from the mantle wedge and the subducting lithospheric slab
with a component of subslab anisotropy generated by diverted mantle flow beneath the Philippine Sea Plate. Conversely, fast
polarization directions for southern Cascadia exhibit less variation and are generally close to convergence-parallel across
the region. In this area, anisotropy appears to be stronger, on the order of 3-4%, and is located primarily in the mantle
wedge and overriding plate. The observed shear wave splitting variations, combined with relative delay time measurements
that will be presented separately in this session, are likely due a combination of mantle flow variations and the existence
of a serpentinized and/or hydrous olivine wedge.
From a modeling standpoint, we have examined the effects of high stress and hydration states to predict shear wave splitting
as a result of mantle silicate lattice-preferred orientation (LPO) development in mantle flow models 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 populations of seismic raypaths traversing the model within the
subduction zone. Results demonstrate that, using the combined observations of variations in fast polarization directions and
splitting times, it is possible to resolve a shift from anhydrous to hydrous mantle in subduction zone settings when sampling
of the mantle wedge is very good. The implications of these results suggest that new seismic experiments in subduction
systems are required to fully evaluate potential competing effects between changes in mantle flow and hydration state in
subduction zone mantle wedges.
DE: 3902 Creep and deformation
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005