Paleoseismic Slip Records and Uplift of the Longmen Shan, Eastern Tibetan Plateau

We document the geological fault zone geometry and internal architecture of the Yingxiu-Beichuan fault (YBF) in the Longmen Shan, eastern Tibetan Plateau, which hosted the catastrophic 2008 Wenchuan earthquake. The YBF has been repeatedly reactivated during its long-term activity and contains widespread brittle fault rocks. Pseudotachylytes are recognized in the Pengguan Complex both from the Wenchuan earthquake Fault Scientific Drilling (WFSD) cores and at a surface outcrop. Microstructural evidence of embayed and rounded fragments, flow structures, vesicles, and microlites of various morphologies indicate that the pseudotachylytes were generated by frictional melting during earthquakes. Cataclasite and fault gouge with fragments of angular shapes and varied sizes observed within the fault zones are also considered as evidence of paleoearthquakes. These imply that the main fault zones in the WFSD cores record ancient seismic slip events. Geological field mapping at the surface and in WFSD cores shows that the YBF is a listric reverse fault, with a 155-290-m thick fault zone that dips 73 degrees-68 degrees at depths <700m and 30 degrees at depths >700m. Three faulted sets of alternating Neoproterozoic Pengguan Complex overthrusting upon the Late Triassic Xujiahe Formation in the WFSD-2 core imply that the main tectonic framework of Longmen Shan consists of a series of imbricated thrust sheets that have accommodated strong crustal shortening. These suggest that the accumulated crustal shortening caused by imbricated thrusting and long-term seismic activity along the YBF and deep concealed faults is an important process responsible for the rapid uplift of the Longmen Shan.