Rheological and microstructural evolution of quartz in the mid-crust

Abstract:

Deformation of the mid-crust is largely controlled by the rheology of quartz. While there is extensive experimental data on quartz rheology, feedbacks between grain-scale microstructure and rheology complicate the interpretation of naturally deformed samples. In this study we investigate the coupled processes of dynamic recrystallization and CPO evolution in a mid-crustal quartz mylonite from the Blue Ridge in Virginia. EBSD maps were constructed for 42 individual quartz microlithons, which represent domains that were originally monocrystalline. Within these microlithons the degree of dynamic recrystallization is highly variable, ranging from 10-100%. Shear strain within individual domains, inferred from empirical relationships between strain and recrystallization fraction, ranges from ~0.5-5.5. We document a systematic inverse relationship between shear strain and the strength of the neoblasts’ CPO. We also show that the neoblasts’ CPO is always inherited from the parent porphyroclast and is not reset by the macroscopic deformation kinematics. Recrystallized grains exhibit minimal internal distortion, indicating the dislocation density is low. On the basis of these observations, we conclude that deformation proceeded first by dislocation creep, and subsequently by another mechanism such as diffusion creep, which involved grain-boundary sliding. The large strains required to equilibrate the microstructure, and hence the rheology, suggest that natural crustal shear zones experience long and transient periods of strain weakening.