Geometry of a large-scale, low-angle, midcrustal thrust (Woodroffe Thrust, central Australia)

摘要

The Musgrave Block in central Australia exposes numerous large-scale mylonitic shear zones developed during the intracontinental Petermann Orogeny around 560-520Ma. The most prominent structure is the crustal-scale, over 600km long, E-W trending Woodroffe Thrust, which is broadly undulate but generally dips shallowly to moderately to the south and shows an approximately top-to-north sense of movement. The estimated metamorphic conditions of mylonitization indicate a regional variation from predominantly midcrustal (circa 520-620 degrees C and 0.8-1.1GPa) to lower crustal (similar to 650 degrees C and 1.0-1.3GPa) levels in the direction of thrusting, which is also reflected in the distribution of preserved deformation microstructures. This variation in metamorphic conditions is consistent with a south dipping thrust plane but is only small, implying that a 60km long N-S segment of the Woodroffe Thrust was originally shallowly dipping at an average estimated angle of 6 degrees. The reconstructed geometry suggests that basement-cored, thick-skinned, midcrustal thrusts can be very shallowly dipping on a scale of many tens of kilometers in the direction of movement. Such a geometry would require the rocks along the thrust to be weak, but field observations (e.g., large volumes of syntectonic pseudotachylyte) argue for a strong behavior, at least transiently. Localization on a low-angle, near-planar structure that crosscuts lithological layers requires a weak precursor, such as a seismic rupture in the middle to lower crust. If this was a single event, the intracontinental earthquake must have been large, with the rupture extending laterally over hundreds of kilometers.