The microstructural and rheological evolution of shear zones

Abstract

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.

Influence of shear heating on microstructurally defined plate boundary shear zones

“…A related concept is the "paleowattmeter" introduced by Austin and Evans (2007) and 159 Austin (2011). This is based on the partitioning of the rate of mechanical work between 160 shear heating and microstructural damage, governed by a parameter they call λ.…”

The influence of microstructure on deformation of olivine in the grain‐boundary sliding regime

“…Regardless of these subtleties, the simple system of equations presented above reasonably captures the transient behavior of olivine deformation at high strain and could be incorporated into larger‐scale models with relative ease. Additionally, these equations could be modified to include a temperature dependence for grain‐size evolution (for a review see Austin [2011]), modified to include a temperature dependence for strain rate [e.g., Hansen et al , 2011], and scaled to more Mg‐rich olivine [ Zhao et al , 2009]. …”