Microstructural evolution and rheology of quartz in a mid-crustal shear zone

- Dynamically recrystallized quartz from mid-crustal mylonite is characterized.
- Crystallographic preferred orientation fabric intensity decreases with strain.
- Mid-crustal shear zones experience long, transient periods of strain weakening.

We present microstructural and crystallographic preferred orientation (CPO) data on quartz deformed in the middle crust to explore the interaction and feedback between dynamic recrystallization, deformation processes, and CPO evolution. The sample investigated here is a moderately deformed quartz-rich mylonite from the Blue Ridge in Virginia. We have created high-resolution crystallographic orientation maps using electron backscatter diffraction (EBSD) of 51 isolated quartz porphyroclasts with recrystallized grain fractions ranging from 10 to 100%. Recrystallized grains are internally undeformed and display crystallographic orientations dispersed around the orientation of the associated parent porphyroclast. We document a systematic decrease in fabric intensity with recrystallization, suggesting that progressive deformation of the recrystallized domains involves processes that can weaken a pre-existing CPO. Relationships between recrystallization fraction and shear strain suggest that complete microstructural re-equilibration requires strains in excess of γ = 5. Variation in the degree of recrystallization implies that strain was accumulated heterogeneously, and that a steady-state microstructure and rheology were not achieved.