The Study On The Neotectonic Deformation Of The Xianshuihe And East Kunlun Faults

The India–Asia collision leads to extrusion along the left lateral strike-slip faults that slice Tibet's east side and causes uplift of the Tibet Plateau.In this paper,we studied and compared the Xianshuihe fault zone in southeastern Tibet and the East Kunlun fault in northeastern Tibet.Based on the study,this paper discussed the new tectonic deformation and its dynamic mechanism in eastern Tibet.In the eastern Tibet Plateau,the Gongga complex,which is about 120 km in length and15-20 km in width,is developed along the Xianshuihe fault.Early studies suggested that the massif is a syn-kinematic granite with the fault.This paper divided the massif into 4 units: Pre-Cenozoic intrusions,mid-Jurassic metasediments,Oligo-Miocene migmatite zone,Intrusions less than 20 Ma ago.The Oligo-Miocene migmatite zone,about 120 km long and 1~3km wide,is developed along the Xianshuihe fault.There are two-staged anatexis and metamorphism of the migmatite zone.The first stage anatexis formed in 32~27Ma,which suggested that the Xianshuihe fault zone underwent intense thrust deformation during the Oligocene.The second migmatization occurred in25~20 Ma.Field investigation and microstructural studies showed a syn-kinematic anatexis of the second-staged migmatite,with intense developed horizontal lineations.This study confirmed that the initiation of slip along the Xianshuihe fault zone in 27~25 Ma.The analysis of tectono-thermal history shows that a 32-20 Ma rapid tectonic uplift stage in the Xianshuihe fault zone and its adjacent area.The uplift event also exists in the Longmenshan region,East Kunlun and the western Qinling area,reflecting a overall uplift of eastern Tibet plateau from Oligocene to early Miocene.This geological phenomenon doesn't support the extrusion model which shows gradually uplift and the orderly expansion of the Plateau.After the Oligo-early Miocene uplift and exhumation along the Xianshuihe fault zone,there occurred 20-12 Ma granite,which reflects partial melting or decompression melting under the fault zone.Since about ~13Ma,in middle Miocene,large-scale eastward extrusion caused strong sinistral strike slip along the Xianshuihe fault and the East Kunlun fault.The deformation formed a mylonite zone along the Xianshuihe fault zone.Then,the Xianshuihe fault connectivity in the Anninghe fault,the Zemuhe fault and the Xiaojiang fault,showing a clear kinematic trajectory.Since about ~13Ma,the east Kunlun fault connected to the Diebu-bailongjiang fault whichshowed left slip and thrust.However,the displacement and movement of the Diebu-Bailongjiang fault was blocked in the Motianling block.In the north margin of the Motianling block,therefore,formed the Hanan thrust fault.There are significantly different between the east Kunlun fault and Xianshuihe fault.Because the east Kunlun fault has not yet formed a connected and eastward extending fault zone.The Blocking resulted in slip rate decline and fuzzy structural feature of the east Kunlun fault...