Late Quaternary kinematic characteristic of the back range faults at the Middle Longmenshan fault zone

The Longmenshan fault zone is a huge multiple thrust fault at the middle on the eastern margin of the Qinghai-Tibet Plateau.It consists of a lot of thrust faults and nappes.It is the boundary between the very active Qinghai-Tibet block and inactive Sichuan block.The latest activity of the fault zone has been attracting much attention of many scientists at home and abroad.The Longmenshan fault zone consists of the back range,the central and the front range faults,which differ from each other in size and activity.Meanwhile,the activity of the whole fault zone is characterized by segmentation.In this study,we made a detailed field investigation on the back range faults at the middle segment of the fault and collected many samples from the strata covering the fault or from fault zone materials for TL and ESR dating.According to the observation of the back range faults at the middle Longmenshan fault zone and dating results,it can be concluded that the back range faults have the tendency to age from the Maowen fault to the Guasiling fault,that is from eastsouthern fault to westnorthern fault.It shows that the back range faults also have an advancing thrust characteristic during late Cenozoic.The main fault,that is the Maowen fault,has been active since late Pleistocene.However,there also is extensional deformation besides thrust deformatiom at the back range faults.This may provide important information for the study of the kinematics and geodynamics of the Qinghai Tibet Plateau.