Differential Late-Cenozoic vertical motions of the Beichuan-Yingxiu Fault and the Jiangyou-Guanxian Fault in the central Longmenshan range and their tectonic implications

After the 2008 Wenchuan earthquake,several groups did some researches on theCenozoic exhumation history of the Longmenshan range.However,to the east of the Pengguan massif,the thermochronology data are still scarce and the exhumation history is unknown,as the previous researches mainly focus on the Pengguan massif.The research on the exhumation history of the rocks to the east of the Pengguan massif can not only help to understand the Cenozoic faulting histories of the faults at the Longmenshan Thrust Belt,but also make significant constraints on the uplift mechanism of the eastern margin of the Tibetan plateau.We add some fission track samples along the Beichuan-Yingxiu fault and the JiangyouGuanxian fault,on the basis of previous studies.The samples are dated at the Fission Track Lab of the National Chung Cheng University,with the external detector method.We get 6Zircon Fission Track(ZFT)data and 6Apatite Fission Track(AFT)data.At the hanging wall of the Jiangyou-Guanxian fault,the AFT data show significant difference divided by the Xiaoyudong fault.To the south of the Xiaoyudong fault,the AFT age is 39 Ma,while the ages are between 6—8 Ma to the north.The research reveals that the vertical activities of the Beichuan-Yingxiu fault and the Jiangyou-Guanxian fault since 8 Ma are much different separated by the Xiaoyudong fault:north to the Xiaoyudong fault,the average vertical slip rate along the Beichuan-Yingxiu fault and the Jiangyou-Guanxian fault is about 0.1mm·a-1 and about 0.55mm·a-1,respectively;south to the Xiaoyudong fault,the average vertical slip rate along the Beichuan-Yingxiu fault and Jiangyou-Guanxian fault is about 0.55mm·a-1 and about 0.1mm·a-1,respectively.The lateCenozoic vertical slip rate of the faults revealed by the thermochronology data is coincided with the co-seismic vertical displacement of the Wenchuan earthquake.The horizontal shortening across the Jiangyou-Guanxian fault(north to the Xiaoyudong fault)reaches 8~12km since8 Ma,which indicates that the crustal shortening performing as the thrust movement along the main faults is the one of the main causes for the uplift and exhumation of the Longmenshan range.Our results don′t support the lower crust thickening model for the uplift of the Longmenshan range.