The Evolution Of Drainage Pattern And Its Relation To Tectonic Movement In The Upstream Yangtze Catchment

Landscape is the product of the internal tectonic activity and the surface external process.Tectonic geomorphology studies the interlinks among tectonic activity,climate change and surface processes.The fluvial process and evolution of river landscape are sensitive to tectonic activity and climate change.The rate of strike-slip faults and the related structural styles can lead to dislocation,capture,reorganization of drainage systems.The study area is located in the central part of the Qinghai-Tibet Plateau with dense rivers,including the Yellow River,Lancang River,the upstream of the Yangtze River-Tongtian River and Yalong River.Two groups of valleys are distinguished with NW-SE and SW-NE direction,respectively.Some drainage have developed an inverted drainage pattern with nearly right-angle intersection.This unique drainage pattern could be caused by tectonic activity.The main fault in the area is the northwestern part of the Ganzi-Yushu strike-slip fault with some secondary faults.The faults have a northeast-thrusting movement,along with mainly strike-slip motion.These faults have been active/reactive since Quaternary,leading to a series of destructive earthquakes,such as the Mw 7.1 Yushu earthquake in 2010.In this paper,we combined geomorphic indexes such as slope,topographic relief,steepness index,stream length-gradient index,and Hypsometric integral,with topographic analysis and field survey data(morphology and sediments)to investigate the response of the regional drainage system to tectonic activity.The understandings list below:1)Through the calculation and analysis of macro-geomorphic indexes such as slope and relief,combined with fault distribution and kinematics of strike-slip faults,we infer that there was a pull-apart basin controlled by the Yushu-Ganzi fault,Batang fault,and Xiaosumang-Xialaxiu fault.The basin was broken by the structural activity led by the later strike-slip faulting,such as normal faults,reverse faults,folds,graben,producing the interval distribution of second basins and ranges finally.2)The southwestern tributaries of the Tongtian River in the region(the Duoqun-Ningchaqu basin,the Dengailongqu basin,the Yequ river basin,and the Batang river basin)are mostly gentle broad valleys with small ks and SL,indicating the characteristics of old valley.In contrast,the SW-NE direction channels are mostly steep V-shaped valleys,with high values of ks and SL,which may be due to the uplift of the NE-trending structures and the subsequent increases of river erosion in the later period.The HI value also reflects the tributaries along Tongtian river are formed in the later period characterized by the young geomorphology,while the NW-SE rivers have developed for a long time with the mature geomorphic features.3)Combined the calculation of geomorphic indexes,topographic analysis and field survey data,it is inferred that rivers in the area were original NW-SE flowing parallel drainage system,and it is replaced by inverted trellis drainage pattern.The activity of the strike-slip faults led to some parts uplifted and some sinked,which deflected the rivers.The NE-trending thrusting of the fault enhanced the erosion of the SW-trending tributary of Tongtian River,and captured the ancient SE-trending rivers finally,forming recent inverted drainage pattern with nearly right-angle intersection.