HR: 11:50h
AN: T12B-07    [Abstracts]
TI: Finite Strain in the Forearc Mantle: Testing the B-type Fabric Anisotropy Hypothesis
AU: * Kneller, E A
EM: ekneller@umich.edu
AF: University of Michigan, Department of Geological Sciences, 425 E. University Avenue, Ann Arbor, Mi 48109 United States
AU: van Keken, P
EM: keken@umich.edu
AF: University of Michigan, Department of Geological Sciences, 425 E. University Avenue, Ann Arbor, Mi 48109 United States
AU: Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Yale University, Department of Geology and Geophysics, 210 Whitney Avenue, New Haven, CT 06520 United States
AU: Park, J
EM: jeffrey.park@yale.edu
AF: Yale University, Department of Geology and Geophysics, 210 Whitney Avenue, New Haven, CT 06520 United States
AB: Seismic observations from many subduction zones show that the seismically fast direction is perpendicular to the direction of convergence. This is opposite of what is expected from models that assume flow is parallel to plate motion and the seismically fast axis of olivine [100] aligns sub-parallel to the shear direction (A-type fabric). Recent deformation experiments on olivine aggregates show that under low-temperature and high-stress conditions, the fast axis of olivine aligns sub-perpendicular to the shear direction (B-type fabric)(Jung and Karato, 2001; Katayama et al., 2004). B-type fabric has potential to explain convergence-perpendicular anisotropy in subduction zones with flow parallel to plate motion. Kneller et al. (2005) used combined data from deformation experiments on olivine aggregates and dynamical models of subduction zones to predict the distribution of B-type fabric in the mantle wedge. This study predicted that the forearc mantle has suitable thermal and stress conditions for B-type fabric and a rapid transition toward the backarc to conditions more suitable for other olivine fabrics. A vertical projection of the volcanic arc into the mantle wedge is predicted to mark the fabric transition between B-type and A-, E-, or C-type fabrics depending on water content. An important aspect not thoroughly investigated by our previous research is finite strain accumulation across the predicted fabric transition. In this study we present finite strain calculation for non-Newtonian subduction zone models with composite water-dependent rheology. This composite rheology includes experimentally based Peierls, dislocation, and diffusion creep. We predict greater than 100 % strain accumulation across 75 km for material traveling into the forearc mantle. This strain accumulation may be sufficient to produce a well developed B-type fabric. Furthermore, material enters the forearc mantle from a low-strain-rate thermal boundary layer at the base of the overriding plate and may have experienced little previous fabric development due to the dominance of diffusion creep. This would facilitate the development of B-type fabric by not requiring the destruction of a pre-existing fabric.
DE: 5120 Plasticity, diffusion, and creep
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8033 Rheology: mantle (8162)
DE: 8104 Continental margins: convergent
DE: 8162 Rheology: mantle (8033)
SC: Tectonophysics [T]
MN: Fall Meeting 2005