Depth of Ancient Seismicity Along the Woodroffe Thrust (Central Australia): Constraints From Pseudotachylytes in Peraluminous Gneisses

The Woodroffe Thrust (WT) is a regional-scale mylonitic shear zone that developed during the Petermann Orogeny (630–520 Ma) in lower to mid-crustal rocks of the Musgrave Ranges, central Australia. In the upper part the WT hosts the largest volume worldwide of tectonic pseudotachylytes (coseismic quenched frictional melts). The pseudotachylytes were only marginally reworked along the mylonitic belt marking the WT, which mainly derived from the footwall amphibolite-facies rocks. Mid-crustal conditions of deformation along a shallowly dipping (<6°) WT were previously inferred from estimates of the P-T conditions of mylonitization along a regional transect in the N-S direction of thrusting. However, the pressure estimates were subject to large uncertainties. To better constrain the ambient conditions of the ancient seismic faulting along the WT, we investigate pseudotachylytes within peraluminous gneisses, a rock type more sensitive to P-T changes in the range of interest. Microstructural analysis allows the sequence of minerals (corundum, sillimanite, cordierite, andalusite, kyanite and garnet) developed during melt quenching and subsequent solid-state growth to be established. Critical observations are the growth of andalusite during pseudotachylyte cooling, constraining faulting at <0.45 GPa, and of kyanite during the immediately following ductile reactivation of pseudotachylytes. Seismic faulting is inferred to have occurred at ambient conditions of ∼0.4 GPa and 450°C, that is at much shallower conditions than previously assumed. These new P-T estimates imply an inclination of 20–25° of the WT, if the main stage of seismic faulting and mylonitization along the WT were coeval.