Cenozoic Uplift Of The Central Yunnan Fragment Southwestern China,Revealed By Apatite(U–Th)/He Dating

The central Yunnan fragment which located on the southeastern margin of the Tibet Plateau is one of the regions with the strongest crustal deformation the Tibet Plateau.The eastern boundary of the central Yunnan fragment is the Xiaojiang fault;the southern part is connected with the Indosinian block with the Honghe fault as the boundary;the western part is bounded by the Jinsha River fault;and the northern part to the Lijiang-Xiaojinhe fault is the structural boundary.In this study,we used low-temperature thermochronology to determine the time of eastern central Yunnan fragment uplift.Prior assessments of central Yunnan fragment formation were not very consistent,with estimates ranging from about early pleistocene to the late Eocene.So it is still controversial and other means are needed to obtain comprehensive estimates.Previous studies have been conducted in the western and central parts of the central Yunnan fragment.However,there are not good estimates for the eastern part.Our sampling located in the eastern part of the central Yunnan fragment.There have been no prior reports on low-temperature thermal chronological research in this area.To better understand the history of denudation in the region,we used low-temperature thermochronology to study samples collected along vertical profiles.Sample data from the Jiaozi Mountain area in the eastern part of the central Yunnan fragment allowed us to plot an elevation-age map.As age increased,elevation also increased in a positive linear relationship.The cooling rate from 37.9 Ma to 36.5 Ma was much higher than 36.5 Ma-25.7Ma,implying that intensive uplift occurred prior to 36.5 Ma.The sampling locations were very close to Xiaojiang fault,which may have affected tectonic activity in the region.Xiaojiang fault is the southern part of the Xianshuiriver-Xiaojiang fault belt that is made up of several faults: The Xianshuiriver-Xiaojiang fault zone sequence is north to south.Xianshuiriver faults was initiated at 9 Ma,which implied that Xiaojiang fault activity should have occurred after 9 Ma.The apatite(U-Th)/he ages from our study area were older than 23 Ma,on the other hand.Thus,in the eastern part of the central Yunnan fragment,tectonic activity did not appear to account for the uplift prior to the Neogene.Based on the age of uplift on the eastern central Yunnan fragment,together with previous low-temperature thermochronology research on the western and eastern central Yunnan fragment,we concluded that the central Yunnan fragment uplifted prior to 36.5 Ma,in a west to east sequence.The uplift caused a change in palaeo-geographical terrain,which may have altered the ancient river systems of the southeast Tibetan Plateau.we can better understand the characteristics of tectonic activities in the region.