Seismic Slip in the Lower Crust Inferred from Granulite-related Pseudotachylyte in the Woodroffe Thrust, Central Australia

Abstract

Pseudotachylytes generated in granulite facies rocks are found in the Woodroffe thrust, central Australia. Petrologic evidence and structural and textural features show that these pseudotachylytes contain typical melting-origin features including injection occurrence, rounded and embayed clasts, and microlites within the fine-grained matrix, which formed contemporaneously with the wall granulite facies rocks under conditions of ~8 kbar and ~650–700°C. These granulite-related pseudotachylyte (G-Pt) veins are overprinted in subsequent deformation stages by three other types of pseudotachylytes including mylonite-related (M-Pt) and ultramylonite-related (Um-Pt) pseudotachylytes that are then transcut by cataclasite-related pseudotachylytes (C-Pt). The overprinting occurred following uplift of the lower crust where the G-Pt veins formed through the brittle-plastic transition zone where the M-Pt and Um-Pt veins occurred within the brittle-dominated regime of the C-Pt generation zone in the upper crust. The coexistence of multiple generations of large voluminous C-Pt, M-Pt, Um-Pt, and G-Pt veins indicates that numerous large earthquakes accompanying distinct seismic slip which produced the pseudotachylytes occurred over an extended period of seismicity and various depths of the crust within the Woodroffe thrust zone. The spatial distribution and structural features of the multiple-stage pseudotachylytes suggest that coseismic slipping caused by large earthquakes which nucleated in the brittle-dominated seismogenic zone propagated downward through the brittle-plastic transition zone into the plastic-dominated granulite facies depth from the upper to lower crust.