Microstructures, Mechanisms and Rheology
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.