Time-Lapse Record of an Earthquake in the Dry Felsic Lower Continental Crust preserved in a Pseudotachylyte-Bearing Fault

Abstract

The mechanisms of earthquake rupture in lower continental crust, below the usual frictional-viscous transition, remain uncertain. In addressing this problem, the study of pseudotachylyte (quenched frictional melt produced during seismic fault slip) and related structures from deeply exhumed rocks can provide direct observational constraints. A felsic granulite from the Musgrave Ranges (central Australia) exceptionally preserves pristine microstructures spatially related to a pseudotachylyte. This sample remained dry, without introduction of hydrous fluids, during pseudotachylyte development and subsequent exhumation. It was therefore unaffected by alteration and metamorphic re-equilibration. Fractures in the damage zone developed asymmetrically to either side of the pseudotachylyte and are marked by new, randomly oriented quartz, feldspar, and garnet grains. Pulverization of garnet occurred locally between intersecting fractures with powder injected into dilatant fractures in a quartz inclusion within the garnet host. Injection of pulverized material can explain the growth of new, compositionally different minerals (quartz, feldspar, kyanite, ilmenite, magnetite, and rutile) along dilatant fractures developed in a short-lived seismic event. The pseudotachylyte contains only clasts of quartz, suggesting an unusually high melting temperature. The sequentially developed microstructures provide a time-lapse record of thermomechanical processes during a single earthquake event, including initial rupture propagation with associated off-fault damage and local pulverization during very dynamic fluctuations in the local stress field; frictional heating and eventual melting during fault slip; flow and injection of melt; and rapid solidification (quenching) and crystallization of new minerals. This occurred under lower continental crustal conditions of ca. 650°C and 1.2 GPa about 550 Myrs ago.