DYNAMIC RECRYSTALLIZATION AND QUARTZ CPO EVOLUTION IN A MID-CRUSTAL MYLONITE FROM THE VIRGINIA BLUE RIDGE

The rheology of quartz exerts an important control on the evolution of the continental crust. At conditions typical of the middle to lower crust, quartz deformation generally proceeds by dislocation creep processes. As dislocations accumulate in a crystal, the associated strain energy promotes dynamic recrystallization, in which the migration of grain boundaries and/or the migration of dislocations to sub-grain boundaries generates crystals with a lower free dislocation density. This process reduces grain-size and may induce strain-weakening by triggering a switch to grain-size sensitive deformation mechanisms, with profound implications for crustal rheology. However, data relating dynamic recrystallization, strain, and CPO evolution from natural samples are limited. Accordingly, we have investigated the processes of dynamic recrystallization, grain-size reduction, and the evolution of CPO in a quartz-rich mid-crustal mylonite. The sample targeted in this study is from the Rockfish Valley Shear Zone in the Blue Ridge province of Virginia, a 1-3 km thick zone of mylonitic deformation representative of mid-crustal shear zones. Crystallographic analyses from quartz porphyroclasts document an asymmetric type-I cross-girdle pattern typical of non-coaxial plane-strain deformation. The isolated quartz porphyroclasts exhibit variable degrees of recrystallization, from less than 20% to fully recrystallized. We have created high-resolution maps from over 30 individual porphyroclasts; these results demonstrate an inverse correlation between the degree of recrystallization and CPO fabric strength for individual porphyroclasts. Moreover, the recrystallized grains display orientations randomly dispersed about their parent grain. This suggests an important role for grain-size sensitive deformation mechanisms like diffusion creep and grain boundary sliding that are capable of weakening pre-existing fabrics. Variation in the degree of recrystallization also implies that strain accumulated heterogeneously, and that a steady-state rheology was not achieved. These observations indicate that natural crustal shear zones experience long and transient periods of strain weakening.