A Study On The Slip Rates Of Longpan-Qiaohou Fault Zone In Late Quaternary

The Longpan-Qiaohou fault zone, the western boundary of the Northwest Yunnan region, is located at the western edge of Sichuan-Yunnan rhombic block. The slip rates of Longpan-Qiaohou fault zone in Quaternary are important to understanding the tectonic evolution of Northwest Yunnan region and even Sichuan-Yunnan rhombic block, as well as the earthquake diseater mitigation efforts in Yunnan Province.Based on analysis, collection and summary of previous achiecements, This thesis aims to analysis geometric characteristics of the fault through interpreration of satellite images and fieldwork investigations, to obtain displacement of geomorphic surfaces by geomorphology interprets and DGPS measurements and calculate the slip rates combined with dating techniques such as TL and14C dating, to get its paleoearthquake sequence through trench works. The major researth results of this thesis are presented below.(1)Geometric characteristics of the Longpan-Qiaohou fault zoneWe generate3D visualization of satellite remote sensing images by using the ALOS remote sensing images and1/50000DEM, and interpret the tectonic geomorphology of this fault. After fieldwork investigations, we clearly understand the geometric distribution characteristics.This fault, which has a length of120km and a15°-20°trend, starts from the southeast Yanfengchang, extends by north of the Shangjing village, Shangji village and Duanjia village, continues along the Shaxi basin, Jianchuan basin, Jiuhe basin and reachs Longpan along Jinsha River valley.According to the geometric distribution characteristics, the Longpan-Qiaohou fault zone can be divided into5segment, Dongfule segment, Jianchuan segment, Jiuhe segment, Hongwen segment and Longpan segment. Dongfule segment and Jianchuan segment are in left-step echelon, with Shaxi basin lain between as a pull-apart basin. Jianchuan segment and Jiuhe segment are parallel, which means that development of Jianchuan basin is not only associated with the Longpan-Qiaohou fault, may be affected by the Lijiang-Xiaojinhe fault. Jiuhe segment and Hongwen segment are in Left-step echelon, with Jiuhe basin lain between as a pull-apart basin.(2)The slip rates of Jianchuan southwest segment in Longpan-Qiaohou fault zoneThe activity patterns of Longpan-Qiaohou fault zone is mainly strike-slip at mountainous section and dip-slip at basin section since late Quaternary. The rivers across the fault in Jianchuan southwest segment can be divided into four grades. The first level river is tens of kilometers long and its sinistral displacement is about1000m. The second level river is over ten kilometers long and its sinistral displacement is about500m. The third level river is several kilometers long and its sinistral displacement is about145m. The fourth level river is a few hundred meters long and its sinistral displacement is about38m. The displacements of rivers can be divided into several grades, which means the Longpan-Qiaohou fault is active during the Quaternary.There are6terraces in Jianchuan basin. T1terrace is8±5m above river and its age is4～7Ka B.P. T2terrace is20±10m above the river and its age is33～38Ka B.P. T3terrace is40±10m above the river and its age is33-61Ka B.P. T4terrace is60±10m above the river and its age is61～113Ka B.P. T5terrace is80±20m above the river and its age is82～153Ka B.P. T6terrace is110±20m above the river and its age is150～350Ka B.P.The horizontal slip rate of Jianchuan southwest segment in Longpan-Qiaohou fault zone is3.10～6.45mm/a. The vertical slip rate of this segment is changed with time. Its vertical slip rate is0.05～0.13mm/a since middle Middle Pleistocene,0.13～0.24mm/a since late Middle Pleistocene, and0.20～0.45mm/a since Late Pleistocene.(3) preliminary study on paleo-earthquake of Jianchuan southwest section in Longpan-Qiaohou Fault zoneTwo M>6.5paleo-seismic events were exposed by the trench in the north of Houjing village, one of which happened before10010±40a B.P. and the other one happened between6130±30a B.P. and6130±30a B.P. The vertical displacement of the first event is about0.72m and the second event is about lm. Combined with previous studies, We conclude that three M>6.5earthquakes have happened in Jianchuan basin since Holocene, which are happened at1751,6230±130a B.P. and10737±468a B.P. The recurrence interval is about5300a.