Constraining the Relative Chronology of Repeated Large Earthquakes in the Longmen Shan Thrust Belt, Eastern Tibetan Plateau

Abstract The Yingxiu–Beichuan Fault Zone (YBFZ), located in the Longmen Shan Thrust Belt (LSTB) on the eastern Tibetan Plateau, is highly seismically active. However, direct evidence of large earthquakes at depth is scarce. Here, we present rock magnetic, microstructural, and geochemical analyses of four fault zones from the Wenchuan Earthquake Fault Scientific Drilling borehole 2. Results show that fault gouges have high magnetic susceptibility, contain neoformed magnetite and monoclinic pyrrhotite, providing direct evidence for repeated large earthquakes with frictional heating temperatures of ∼500–900°C at depth. These earthquakes occurred in a reducing, sulfide‐bearing fluid environment throughout the seismic cycle. The increasing abundance of neoformed ferromagnetic minerals with depth could be related to more recent large earthquakes propagating at greater depths, supporting a structural model of vertical stacking and eastward propagation of the LSTB. This study provides key constraints on the deep seismic history and tectonic evolution of the LSTB.