Two phases of pseudotachylytes in Ra Chu transect of South Tibetan Detachment System and its implications to fault activities

Tectonic pseudotachylytes preserve key information of fossil earthquakes. They provide valuable insights into the activity and tectonic evolution of ancient faults. The >2000 km long South Tibetan Detachment System is one of largest fault systems in the world, and the exposure of its seismogenic zone can help understand formation of earthquakes. Our structural analysis reveals two phases of pseudotachylytes under different deformation conditions in the leucogranite of Ra Chu transect. Field investigations identify two types of pseudotachylytes: (1) M-PT type pseudotachylyte associated with mylonitization, and C-PT type pseudotachylyte that formed during cataclasis. The former develops parallel to the pervasive foliation of the host rock, while the latter crosscuts the foliation at a high angle. Microstructural observation suggests that both types of pseudotachylytes are produced by frictional melting. Geochemical analysis has revealed selective melting of minerals and trace element migration during the formation of pseudotachylytes, necessitating the involvement of external fluids. These fluids play an important role in process of earthquake nucleation by reducing the strength of fault zones. Geochronological constraints correlate the formation of pseudotachylytes to regional tectonics: Firstly, M-PT formed within the brittle-ductile transition zone at ~17 Ma and underwent ductile deformation together with the host rock. As the fault zone exhumed, C-PT and cataclasis superimposed in the shallow brittle zone at ~14 Ma. The coexistence of these two types of pseudotachylytes records the rapid cooling and exhumation history of the South Tibetan Detachment System over several million years, indicating that seismic sliding occurred repeatedly in the same section of a long-lived fault zone.