Pseudotachylytes in the Longmen Shan fault zone and fault weakening mechanisms

Fault generated pseudotachylytes are considered as fossil earthquakes, preserving important information about faulting mechanisms and earthquake generation processes, which are of great significance to understanding fault behavior and seismicity. Integrating the surface outcrops and study of the Wenchuan earthquake Fault Scientific Drilling (WFSD-1 and WFSD-2) cores, this paper presents detailed analyses on the structures and mineral composition of the pseudotachylytes in the Longmen Shan fault zone. These pseudotachylytes and related cataclasitic rocks are well preserved in the Neoproterozoic Pengguan Complex along the southern Yingxiu-Beichuan fault. They are black, brown to gray in appearance, with thicknesses ranging from several millimeters to tens of centimeters, appearing as fault veins and injection veins. Multiple lines of microstructural evidence, such as embayed fragments, honeycomb-like vesicles, viscous flow structures and microlites or aggregates with various morphologies of acicular, lath-like, plume-like, fibrous and granulous, indicate that the pseudotachylytes were formed as results of frictional melting during the seismic slip. This implies that melt lubrication played a most important role in the fault weakening process. Thermal pressurization and mechanical lubrication could also occur at different slip stages. Besides, different colored pseudotachylyte veins, observed at both macroscopic and microscopic scales, may indicate different alteration or devitrification degrees, and their overprinting relations suggest they were multiply generated. This means that the pseudotachylyte-generated earthquake events had happened repeatedly in the Yingxiu-Beichuan fault zone. Recent research shows that the pseudotachylytes were produced at similar to 10-15 km-depth in the Late Triassic. Combined with fault gouge and fault breccia in the Xujiahe Formation, the similar to 180-280 m-wide Yingxiu-Beichuan fault rocks zone represents the products of long-term fault activities. Known as the steepest topographic boundary in the Longmen Shan area, the crust shortening caused by the long-term seismic activity of the Yingxiu-Beichuan fault may be responsible for the rapid uplift of the Longmen Shan.