Interplay between Seismic Fracturing and Aseismic Creep in the Woodroffe Thrust, Central Australia - Inferences for the Rheology of Relatively "Dry" Middle Continental Crust

The over 600 km long mid-crustal Woodroffe Thrust developed during the intraplate Petermann Orogeny around 550 Ma. Ductile shearing was accommodated along a shallowly south-dipping plane with a top-to-north shear sense. Due to late-stage open folding of the thrust plane, a nearly continuous N-S profile of 60 km length in the direction of thrusting is now exposed. Metamorphic conditions of 500 °C - 600 °C at roughly 0.8 GPa and the rare availability of aqueous fluids are well constrained along the entire transect, providing an ideal framework for studying variation in microstructure and texture in the dominantly "dry" felsic middle continental crust. During the ductile thrusting quartz dynamically recrystallized by subgrain rotation and grain boundary migration, whereas plagioclase and K-feldspar underwent fracturing and neocrystallization. Quartz crystallographic preferred orientation (CPO) shows progressive variation from dominant prism to mixed rhomb/basal slip, while plagioclase and K-feldspar CPO is random. Flow stresses, estimated from quartz grain size piezometry, decrease towards higher metamorphic grade and deeper structural levels, yielding a strain rate that is up to two orders of magnitude faster than the geological average of 10-13 - 10-15 s-1. On this basis, a qualitative rheological model is developed explaining the observed cyclic interplay between ductile shearing (mylonitization) and brittle fracturing (pseudotachylyte formation) in the relatively "dry" middle continental crust, where the syn-deformational influx of water has not been pervasive.