Obliteration of olivine crystallographic preferred orientation patterns in subduction-related antigorite-bearing mantle peridotite: an example from the Higashi–Akaishi body, SW Japan
S. R. Wallis, H. Kobayashi, A. Nishii, T. Mizukami, Y. Seto, 2011. "Obliteration of olivine crystallographic preferred orientation patterns in subduction-related antigorite-bearing mantle peridotite: an example from the Higashi–Akaishi body, SW Japan", Deformation Mechanisms, Rheology and Tectonics: Microstructures, Mechanics and Anisotropy, David J. Prior, Ernest H. Rutter, Daniel J. Tatham
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Large parts of the mantle wedge near subduction boundaries are likely to be hydrated and contain antigorite. This mineral is acoustically highly anisotropic and potentially has a strong influence on seismic properties of the wedge. The Higashi–Akaishi body of SW Japan is an exhumed sliver of partially serpentinized forearc mantle, ideal for studying the effects of antigorite on the development of tectonic fabrics in the mantle. Samples with less than 1% antigorite show strong B-type olivine crystallographic preferred orientation (CPO) patterns. In contrast, samples with >10% antigorite deformed during the same tectonic event show much weaker olivine CPO patterns lacking the flow-normal a-axis concentration. These microstructural data suggest that the development of antigorite during deformation weakens olivine CPO due to phase boundary slip and associated rigid-body rotation of olivine grains. Antigorite and similar sheet silicates are likely to be present to some extent in the mantle wedge of all convergent margins. Our results suggest that even if this amount is only a few percent, strong olivine CPO is unlikely to develop and any pre-existing CPO is likely to be destroyed. Under these conditions, olivine CPO is unlikely to contribute significantly to seismic anisotropy in the mantle wedge.
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This collection of papers presents recent advances in the study of deformation mechanisms and rheology and their applications to tectonics. Many of the contributions exploit new petrofabric techniques, particularly electron backscatter diffraction, to help understand evolution of rock microstructure and mechanical properties. Papers in the first section (lattice preferred orientations and anisotropy) show a growing emphasis on the determination of elastic properties from petrofabrics, from which acoustic properties can be computed for comparison with in-situ seismic measurements. Such research will underpin geodynamic interpretation of large-scale active tectonics. Contributions in the second section (microstructures, mechanisms and rheology) study the relations between microstructural evolution during deformation and mechanical properties. Many of the papers explore how different mechanisms compete and interact to control the evolution of grain size and petrofabrics. Contributors make use of combinations of laboratory experiments, field studies and computational methods, and several relate microstructural and mechanical evolution to large-scale tectonic processes observed in nature.