RHEOLOGICAL CONSTRAINTS ON THE 96-MILE SHEAR ZONE, GRAND CANYON USA

We present new rheological constraints on the 96-mile shear zone in Grand Canyon National Park. The 96-mile shear zone is a major crustal-scale structure near the boundary of the Yavapai and Mojave terranes. The kinematics and strain rate on this major fault can provide significant constraints on orogenesis during ~1.4 Ga. The shear zone is made primarily of highly deformed Vishnu schist with 2-10 mm lenses of pure quartz that display textbook evidence for subgrain rotation dynamic recrystallization. The quartz CPO opening angle thermometer indicates a deformation temperature of 450°C, consistent with the microstructure. Grain size paleo piezometery using EBSD-derived grain maps indicate a stress of 122.9 MPa. Strain rate was modeled using several Quartz flow laws with a range of reasonable pressures. These model results show strain rates spread across several orders of magnitude when error propagation is included. The model results center at a strain rate 10^-11 s^-1. We calculated offset across the shear zone using the relationship between strain rate and shear strain rate, ε = γ/√3. Our results suggest that the offset was on the order of 25 km/m.y. If the shear zone was 30 m wide. This result is very likely an overestimate due to strain localization within quartz bands, but nevertheless they suggest that the 96-mile shear zone was a high-stress, high strain rate structure capable of offsetting a significant portion of the crustal column in a short time frame.