CENOZOIC FAULT ACTIVITY OF THE SOUTHERN SEGMENT OF THE LONGMENSHAN THRUST BELT:EVIDENCE FROM LOW-TEMPERATURE THERMOCHRONOLOGY DATA

The Longmen Shan,located at the eastern margin of the Tibetan plateau,is a steep and high exhumation area.In recent years,the 2008 Wenchuan M W7.9 earthquake and the 2013 Lushan MS7.0earthquake occurred,and researchers presented a lot of low-temperature thermochronology data of the Longmen Shan and adjacent area.In this paper,we provide 4 ZFT ages and 4 AFT ages for the southern segment of the Longmenshan Thrust Belt(LTB),where the low-temperature thermochronology data are still few.Combining with previous researches,we get the Cenozoic exhumation history of the Baoxing Massif,located at the southern segment of the LTB,and new knowledge about the Cenozoic activity of the southern segment of the LTB.The Baoxing Massif began quickly cooling in the early Cenozoic,with the cooling range exceeding 225℃,while the cooling range of the Pengguan Massif in the central segment of the Longmen Shan is between 185~225℃.The four AFT ages in the Baoxing Massif are between 2.7~5.0Ma,which are younger than that in the Pengguan Massif,and it indicates that the late-Cenozoic cooling rate of the Baoxing Massif is bigger than that of the Pengguan Massif.Under this assumption that the surface temperature is 15℃ and the paleo-geothermal gradient is 30℃/km,the average exhumation rate from 3~5Ma to present is about 0.63~1.17mm/yr.The low-temperature thermochronology data indicate that the differential exhumation is concentrated in the Beichuan-Yingxiu Fault and the Jiangyou-Guanxian Fault in the central segment of the LTB,while it is dispersed in a wider region along the two branches of the Shuangshi-Dachuan Fault and the faults and folds to the east,in the southern segment.