The Activity Of The South Segment Of Xiaojiang Fault In Late Quaternary

The southeast margin of the Tibetan Plateau(SETP)is the area where the most complex active tectonics lies and most frequent powerful earthquake erupts.Also,it becomes an increasingly heated discussion for researchers.At this point,Xianshuihe-Xiaojiang fault(XXF)is one of the significant boundary faults in SETP.Located in the southern tip of the XXF,which means it is also located in the intersection area of XXF and XJF,Xiaojiang fault(XJF)is the key to studying the mechanism of the kinematics and dynamics in SETP?There still exists some problems regarding XXF that few researches are carried out on the south segment of Xiaojiang fault(SXJF)while much importance is attached to the north and middle part of the XJF.Thus,the paper is intended to do some detailed research on geological and geomorphological circumstances,following with paleoearthquake and slip-rate works to explore the activity of SXJF in late Quaternary.Here are our conclusions :(1)The SXJF passes through Baiyun?Luoshuidong?Fangmaping?Longtan?Shanhua,also crossing Red River valley from the west slope of Niuguntang.It's about 70 km long.The overall strike is about N20°E,with a east arc in the middle.The fault is clear in linear on satellite image,which also appears to be a continuous and single feature.(2)It's rich in the tectonic landforms in the SXJF.There are many tectonic landforms,for example surface rupture,fault trough,scarp,dislocated gullies and hill-ridges,brow ridge surface,sag pond and so on.It demonstrates the strong activity of the SXJF,with multiple recurrences in situ.(3)We find many paleoearthquake events in SXJF.The Luoshuidong section has the best result.It defines three events,it's 5627±21 a BP to 12491±60 a BP,3908±38 a BP to 5627±21 a BP,495±20 a BP to 3908±38 a BP from old to new.Taking into consideration of the result in Fangmaping section,we can define the latest event is between 495±20 a BP and 574±36 a BP.Searching for the historical seismic data,we preliminarily consider the event occured on 11/30,1606,with epicenter located between Fangmaping and Shenxiandong reservoir,about M7.23-7.65.(4)We measure the slip rate in the Longtan section,and get the sinistral slip rate 7.02±0.20mm/a in late Quaternary.It is similar with the middle and north part of the XJF,suggesting there aren't significantly reduce in slip rate by the blocking of NW-strike fault belt.So,its activity may be underestimate of the SXJF.It may play a important role in adjusting the clockwise rotation as a single fault.We find it's a active fault in late Quaternary of the SXJF.The slip rate is 7.02±0.20mm/a,similar with some GPS results where the activity can continue southward,even cross the Red River fault.So,we carefully survey the satellite images south of the Shanhua and find some fault evidence.We may get the conclusion that the SXJF doesn't end at Shanhua,but stretching south,even crossing the Red River fault.The SXJF and its southward part may be the eastern boundary of regional clockwise rotation,not the Dien Bien Phu fault.