The effects of quartz Dauphine twinning on strain localization in a mid-crustal shear zone

We use microstructural, electron backscatter diffraction (EBSD), and crystal vorticity axis (CVA) analyses to evaluate the effects of quartz Dauphine twinning on strain localization. We compare a granodiorite mylonite and a tonalite mylonite from the Grebe Shear Zone, a mid-crustal transpressional shear zone in Fiordland, New Zealand. EBSD analysis reveals identical quartz grain boundary migration dynamic recrystallization microstructures in both samples, with abundant Dauphine twinning only in the granodiorite mylonite. Bulk CVA patterns further distinguish the untwinned and Dauphine-twinned samples, indicating a transition from simple shear-to pure shear-dominated transpression, respectively. The quartz CVA pattern from the Dauphine-twinned sample indicates that twinned and untwinned grains record a transition from simple shear-to pure shear-dominated deformation, respectively. Pole figures of the Dauphine-twinned sample reveal high-temperature slip systems in the untwinned and Dauphine-twinned grains. We conclude that Dauphine twinning formed early in transpression, and rendered the twinned grains less deformable relative to the untwinned grains over time. The effectiveness of Dauphine twinning in localizing strain is therefore strongly influenced by its timing relative to the evolving shear zone kinematic deformation geometry.