Effect of temperature on the B- to C-type olivine fabric transition and implication for flow pattern in subduction zones

摘要

The effect of temperature on the B- to C-type fabric transition in olivine was investigated by large-strain shear deformation experiments. A new series of deformation experiments was conducted at 1273-1373 K, 2.0 GPa and strain-rates ranging from 3.8 x 10(-4) to 3.2 x 10(-5) s(-1) under water-saturated conditions, and combined with previous results at 1473 K, the influence of temperature on deformation fabrics was investigated. Two types of olivine fabrics were identified, one in which the olivine [001] axis is subparallel to the shear direction and the (010) plane is parallel to the shear plane (B-type), and another one in which the [001] axis is parallel to the shear direction and the (100) plane is parallel to the shear plane (C-type). The B-type olivine fabric occurs at higher stress than the C-type, but the stress magnitude at which the B- to C-type fabric transition occurs decreases significantly with decreasing temperature. The temperature dependence of the transition stress is parameterized using a range of exponential flow law that is constrained by the experimental study. When the experimental results are extrapolated to lower temperatures, the fabric transition between B- and C-type is predicted to occur at similar to 1 MPa and similar to 1000 K. This prediction is consistent with the olivine fabrics in naturally deformed peridotites found in plate-convergent regions. The present study indicates that when some amounts of water (> 200 ppm H/Si) are present, the B-type fabric will dominate in low temperature regions of the upper mantle whereas the C-type (or E-type) fabric will dominate at high temperatures. The complex seismic anisotropy in subduction zones can be explained by changes in the dominant type of olivine fabrics caused by spatial variations in physical and chemical variables, such as temperature and water content rather than by complexities in mantle wedge flow pattern. (c) 2006 Elsevier B.V. All rights reserved.