Low Temperature Thermochronology Constraint On Differential Exhumation History Of Both Sides Of Lancangjiang Fault At Downstream Mekong River

The uplift and evolution of the Tibetan Plateau is one of the most important geological events in the Cenozoic.The Chinese Mainland is sandwiched between the Indian plate,the Philippine Sea plate and the Pacific plate.Since the India Eurasia plate collision,the Tibet Plateau has been uplifting,which has contributed to the current tectonic and geomorphic characteristics of the plateau and its surrounding areas.The southeastern margin of the Tibet Plateau is located in one of the frontiers of the lateral extension of the plateau.The regional dynamic boundary is highly complex,and the tectonic activity is strong.Thanks to the gently descending terrain on the southeast edge,the Southeast Asian monsoon can enter the interior of the plateau,creating conditions for the development of Lancang(Mekong)River,Yarlung Zangbo River,Nujiang River,Jinsha River and other river systems.In the region,the Lancang River,Nujiang River and Jinsha River are nearly parallel and parallel to each other.The main rivers cut down deeply and narrowly,turn sharply,three rivers flow together,deep valleys alternate with high-altitude flat surfaces and other world-famous steep river erosion landforms.Therefore,the southeast margin of the Qinghai Tibet Plateau is an ideal place to study the interaction of tectonic process,geomorphic evolution and climate change.The formation of orogenic belt or plateau landform is the result of the joint action of lithospheric tectonic movement,atmosphere,and surface processes.The three processes of structure,climate and surface erosion constitute a dynamic and coupled interaction system.As the main agent of surface erosion,the formation and development of rivers have an important impact on geomorphic evolution.In addition,rivers are very sensitive to the changes of tectonics,climate and other factors,so the evolution of regional water system can usually reflect the characteristics of landform and recent tectonic changes.How to distinguish and evaluate the relative importance of single factor is a controversial and challenging scientific issue.According to previous studies,large-scale rapid undercutting occurred in the valleys of Lancang River section around 14-17 Ma,which led to the emergence of Lancang River.At the same time,they use numerical simulation experiments to find that the era of Lancang River undercutting is consistent with the era of Asian summer monsoon enhancement and propose that climate change is an important reason for the formation of Lancang River.However,this fast cut-off time has not been widely confirmed in the region.At the same time,Lancang River fault with roughly the same trend is developed along the Lancang River.However,sufficient attention has not been paid to the Lancang River fault in the research.All samples are collected from the Lancang River and the west side of the Lancang River fault,and there is no relevant data to support the east side.Whether the exhumation history on both sides of Lancang River fault is consistent is still a problem worth studying.In this paper,the Lincang area on the southeast edge of the Tibet Plateau is selected as the study area.Low temperature thermochronology samples are collected in this area.Using apatite and zircon(U-Th)/He dating technology,combined with onedimensional qtqt simulation and three-dimensional pecube simulation program,the differential denudation history on both sides of the Lancang River fault zone in the region is tentatively explored,and the geometric shape of the Lancang River fault in the region is roughly simulated,The denudation history of the hanging wall of the fault and its possible mechanism are discussed.The preliminary conclusions are as follows:(1)There are significant differences in apatite(U-Th)/He ages on both sides of Lancang River fault zone in Lincang area,southeast margin of Tibet Plateau.There is differential denudation on both walls of the fault,which has experienced different denudation history,and this differential denudation is controlled by the activity of the fault zone.We use pecube 3D simulation to roughly constrain the fault geometry in the region.(2)The hanging wall of Lancang River fault zone in Lincang area mainly experienced three rapid cooling events,which occurred in Late Cretaceous Paleocene,late Eocene early Oligocene and late Oligocene early Miocene respectively.We believe that the three events were mainly controlled by the oblique subduction of the Indian plate and the regional fault activity.