The relationship between the internal structure of the Wenchuan earthquake fault zone and the uplift of the Longmenshan

The 2008 Wenchuan earthquake ( M W 7. 9)occurred at the eastern margin of the Tibetan Plateau,producing 270km and 80km-long co-seismic surface ruptures with different kinematics features,along the Yingxiu-Beichuan and Guanxian-Anxian faults, respectively. Fault rocks are the products of fault activities. The characteristics of fault rocks provide information on fault activities and their evolution history. Here,we focus on the outcrop of the Yingxiu-Beichuan fault in Bajiaomiao village,Hongkou town and results from WFSD-1drill cores. Based on field survey,X-raydiffraction analysis,microstructure and drill cores analysis,a ～240m-wide fault zonewas confirmed as the Yingxiu-Beichuan fault zone from the outcrop,corresponding to the～105m-wide fault zone at depth, as determined from the WFSD-1 drill cores,which presents the characteristics of a multiple cores model. Fault rocks,including fault breccia,fault gouge and cataclasite were identified in both the outcrop and drill cores,while pseudotachylyte was only visible at the outcrop. The location of the Principal Slip Zone of the Wenchuan earthquake,which does not entirely follow the ancient fault zone, obliquely crossing the Yingxiu-Beichuan fault zone,might indicate that the Wenchuan earthquake fault and Yingxiu-Beichuan faultdo not belong to the same fault system. Based on fault rocks studies,there are four kinds of fault mechanisms along the Yingxiu-Beichuan fault zone, such as frictional melting, thermal pressurization, mechanical lubrication and elasto-hydrodynamic lubrication. Thermochronometry research indicates that the Yingxiu-Beichuan fault activity has controlled the rapid uplift of the Longmenshan since it formed at 15～10Ma ago. The fact that the internal structure shows five different fault rock units in the fault zone may be related to the different episodes of the Longmenshan uplift.