New geochronology constraints on timing and depth of the ancient earthquakes along the Longmen Shan fault belt, eastern Tibet

Abstract

Pseudotachylyte is an ideal target to directly date ancient earthquake associated with regional faulting. Here we perform step-heating 40Ar/39Ar, clay mineral K-Ar, and zircon fission track (ZFT) analyses on the pseudotachylyte samples collected from the Yingxiu-Beichuan co-seismic rupture of the Longmen Shan fault belt (LSFB), to provide time constraints for the tectonic evolution of the LSFB during the Indosinian orogeny. 40Ar/39Ar results from the matrix show that the frictional melting occurred 226-235 Ma ago. Combined with mylonite dating of the host rock, the age of the ancient earthquakes is constrained at 231-238 Ma, with a formation depth of 10-14 km. As a response to the earthquakes, a series of soft-sediment deformation structures are widely preserved in the Middle and Late Triassic strata along the Yingxiu-Beichuan fault (YBF), indicating that the LSFB was seismically active since the late Anisian and persisted episodically until the end of the Indosinian orogeny. Both clay mineral K-Ar and ZFT analyses record a younger deformation or alteration with an age of ca. 195 Ma, corresponding to a post-collisional orogeny. These new data represents the first direct evidence of the regional thrusting (YBF) in the central LSFB during the Indosinian orogeny, concurrent with the initial ductile deformation of the western boundary fault. Tectonic inheritance then strongly influenced the evolution of the LSFB as most of the Mesozoic faults are reactivated by major Tertiary tectonic deformations.