Role of Phyllosilicates on Rheological Weakening of Detachment Shear Zones (Raft River Mountains, Utah)

Abstract

Phyllosilicates, such as muscovite, are mechanically weak mineral phases due to their ability to shear easily along their basal plane. During deformation, phyllosilicates precipitate and recrystallize, and tend to form interconnected networks that may localize and partition strain. To investigate the role of phyllosilicates on strain localization we quantify phyllosilicate distribution, spatial arrangement, interconnectivity, and volume in quartzite mylonites from the Miocene detachment shear zone associated with the Raft River metamorphic core complex (NW Utah). The detachment shear zone is localized in the Proterozoic Elba quartzite, which unconformably overlies an Archean basement complex and consists of an alternating sequence of white quartzite (~90% quartz, ~10% muscovite) and muscovite-quartzite schist. We use image analysis of petrographic thin sections and X-ray computed tomography to quantify muscovite content, grain shape, spatial distribution, and interconnectivity in samples with varying amount of muscovite collected across the detachment shear zone. Our preliminary results suggest that muscovite grains vary in both shape (rods vs. wafers) and distribution (interconnected vs. isolated) across the detachment shear zone. These changes seem to correlate with different quartz microstructures. The dominant quartz recrystallization mechanism is subgrain rotation, and in the samples with interconnected muscovite grains, the amount of quartz that is recrystallized is less than in the samples with isolated muscovite grains. Quartz relict grains have a higher aspect ratio in the samples with isolated grains than in the sample with interconnected muscovite grains. Our preliminary results reveal a relationship between quartz deformation mechanisms and phyllosilicate content/distribution in the Raft River detachment quartzite mylonites.