Paleoseismic slip records and uplift of the Longmen Shan, eastern Tibetan Plateau

Fault rocks and their structures are the keys to understanding faulting mechanisms and earthquake generation processes. Histories of fault zone evolution and their seismic activities are also recorded as overprinted structures in fault rocks. Recognition of ancient seismicity is aided by multiple lines of evidence preserved in fault rocks.

The Yingxiu-Beichuan fault (YBF), in the Longmen Shan, eastern Tibetan Plateau, was repeatedly reactivated during its long-term activities and bears widespread brittle fault rocks, such as fault gouge, fault breccia, cataclastic rocks and pseudotachylyte. As a reliable tracer of paleoearthquakes, pseudotachylytes are recognized in the Pengguan Complex both from the Wenchuan earthquake Fault Scientific Drilling (WFSD) cores and at surface outcrop. Microstrctural evidence of embayed and rounded fragments, flow structure, vesicles and microlites of various morphologies, indicate that the pseudotachylytes were generated as results of frictional melting during earthquakes. While cataclasite and fault gouge with fragments of angular shapes and varied sizes, which are also considered as evidence of seismic slip, are observed within the fault zones. These results show that the main fault zones in the WFSD drilling cores record ancient seismic slip events.

The YBF consists of 180-280 m-thick fault rocks, and dips to 73-63° above 700 m-depth, while varies to 40-30° below 700 m-depth revealed by the investigations of the WFSD cores and the surface outcrop. This demonstrates that the YBF is a typical listric thrust fault.

Based on the lithological studies and U-Pb dating, three faulted sets of alternating Neoproterozoic Pengguan Complex (granite, volcanics) overthrusting upon the late Triassic Xujiahe Formation were recognized in the WFSD-2 drilling cores. This implies that the Longmen Shan fault belt consist of a series imbricate thrust sheets, which has strong crust shortening and uplifting effect. Besides, all the fault rocks in the thrust faults record seismic slip, indicating that the accumulation effect of long-term seismic activities give rises to the rapid uplift of the Longmen Shan, among which the YBF plays an important role.