The microstructural and rheological evolution of shear zones
Nicholas J. Austin, 2011. "The microstructural and rheological evolution of shear zones", Deformation Mechanisms, Rheology and Tectonics: Microstructures, Mechanics and Anisotropy, David J. Prior, Ernest H. Rutter, Daniel J. Tatham
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Evidence of localized strain is ubiquitous in deformed lithospheric rocks. Recent advances in laboratory deformation techniques, including the use of torsion experiments, have enabled the coupling of microstructural and rheological evolution to be investigated in experiments run to strains approaching those reached in many natural shear zones. Further, the increased use of electron backscatter diffraction to quantify crystallographic preferred orientation (CPO) has significantly increased understanding of CPO formation and evolution. Combined, these laboratory and field observations support the assertion that a rock's microstructure is strongly linked to its rheology. However, complete quantification of the coupling between microstructure and rheology is complicated by the fact that rocks have inherently complex microstructures. This paper reviews recent work focused on quantifying the rates of microstructural evolution and the attainment of steady state for two key microstructural parameters: grain size and crystallographic preferred orientation. Theoretical considerations, laboratory measurements and field observations suggest that a full description of all relevant microstructural parameters, and the appropriate evolution equations for these parameters, may be needed to link microstructural and rheological evolution and therefore to quantify the bulk rheology of the lithosphere.
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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.