Inverted distribution of ductile deformation in the relatively “dry” middle crust across the Woodroffe Thrust, central Australia

Abstract

Thrust fault systems typically distribute shear strain preferentially into the hanging wall rather than the footwall. The Woodroffe Thrust in the Musgrave Block of central Australia is a regional-scale example that does not fit this model. It developed due to intracontinental shortening during the Petermann Orogeny (ca. 560–520 Ma) and is interpreted to be at least 600 km long in its E–W strike direction, with an approximate top-to-north minimum displacement of 60–100 km. The associated mylonite zone is most broadly developed in the footwall. The immediate hanging wall was only marginally involved in the mylonitization process, as can be demonstrated from the contrasting thorium signatures of mylonites derived from the upper amphibolite facies footwall and the granulite facies hanging wall protoliths. Thermal weakening cannot account for such an inverse deformation gradient, as syn-deformational P–T estimates for the Petermann Orogeny in the hanging wall and footwall from the same locality are very similar. The distribution of pseudotachylytes, which acted as preferred nucleation sites for shear deformation, also cannot provide an explanation, since these fault rocks are especially prevalent in the immediate hanging wall. The most likely reason for the inverted deformation gradient across the Woodroffe Thrust is water-assisted weakening due to the increased, but still limited, presence of aqueous fluids in the footwall. We also establish a qualitative increase in the abundance of fluids in the footwall along an approx. 60 km long section in the direction of thrusting, together with a slight decrease in the temperature of mylonitization (ca. 100 °C). These changes in ambient conditions are accompanied by a 6-fold decrease in thickness (from ca. 600 to 100 m) of the Woodroffe Thrust mylonitic zone.