Geomorphological Evolution And Valley Development Of The Lower Jinsha River During Late Cenozoic

The Jinsha River is the upper strethes of the Yangtze River which is the largest river in China.It flows firstly from north to south through the Longitudinal Range-Gorge Region of the Hengduan Mountain,parallelling to the gorges of the upper Mekong River and upper Salveen River.However,after approaching Shigu in Yunnan Province,it suddenly turned to the northeast,seperated with the Mekong and Salveen River,forming the famous“First Bend”of Yangtze River.Although there are numerous researches on the formation and evolution of the Jinsha River since the Early 20^th Centuty,but so far,the development pattern and the formation time of the Jinsha River are still a controversial issue.In addition to the complexity of this problem,the different methods and evidencs adopted by different scholars are also an important factor.In the light of the planation surface being the main paradigm of geomorphology,and an important geomorphologic marker for the regional long-term landscape evolution,providing constrainsts for the starting points of modern valley development.At the same time,the modern geological histories of a river were mainly survived in the landforms and the related sedimentary of its valleys,such as fluvial terraces,alluvial fans,and dammed lake sediments,they are all the products of valley development,not only faithfully records the development history of the valley,it also contains rich information about the regional tectonics and climate change.In this paper,based on studies of the planation surfaces,fluvial terraces,and dammed lake sediments,we discussed the processes of geomorphologic evolution and vally development of the lower Jinsha River.The main conclusion are as follows:?1?In order to identify the planation surfaces,a combined method of DEM-based fuzzy logic and topographic and river profiles analysis was employed in this study.Based on a comprehensive analysis of four morphometric parameters:slope;curvature;terrain ruggedness index?TRI?;and relative height?RH?,we established the relevant fuzzy membership functions.For slopes,between0oand 10o,the value of the membership degree?MD?was set to 100%;between 10oand 30othis was changed linearly from 100%to 0%;?30oslopes were set to 0%.For the curvature,?1 m^-1 or?-1 m^-1 values were set to 0%;and-1–-0.2 m^-1 and 0.2–1.0 m^-1 values were changed linearly from100%to 0%,respectively.For the TRI,values between 0 m and 80 m were set to 100%;values between 120 m and 300 m were changed linearly from 100%to 0%;and values?300 m were set to 0%.For the RH,?60 m values were set to 0%;?250 m values were set to 100%;and?60 m and?250 m values were changed linearly from 0%to 100%.?2?Based on these fuzzy membership functions,we calculated the membership degree?MD?of the study area,which represent the likelihood of a region belonging to planation surface.Afer comparing with the planation surfaces identified by interpretation of Google Earth imagines and field investigation of some representive areas,results show that patches with a MD?80%and an area?0.4 km² represent the PS remnants in the lower Jinsha River.They consist of 1764 patches with an altitude,area,mean slope,and relief of mostly 2000–2500 m above sea level?asl?,0–10km²,4–9o,0–500 m,respectively,covering 9.2%of the study area's landscape,dipping to southeast,decreasing progressively from northwest to southeast in altitude,and no clear relation between each patch's altitude and slope,or relief.All these results indicate that they are remnants of once regionally continuous PSs which were deformed by both the lower crust flow and the faults in upper crust,and dissected by the network of Jinsha River.So,the valleys of Jinsha River were incised slowly by the river itself during the past prolonged geological time.?3?Topographic and river profiles analysis,and hypometic analysis,combined with field investigation show that at least three PSs?PS1–PS3?well developed along the main valleys of the lower Jinsha River,these three PSs especially survived well in the Yongren-Huili region,indicating several phases of uplift then planation during the Cenozoic era.Based on the incision amount deduced from projection of relict river profile on PSs in the Yongren—Huili region,together with erosion rates,breakup times of the PS1,PS2,and PS3 were estimated to be 3.47 Ma,2.19 Ma,and1.45 Ma,respectively,indicating appearance of modern Lower Jinsha River started between Pliocene to early Pleistocene.?4?In the lower reaches of the Jinsha River,fluvial terraces are not well developed and poorly preserved,but the lacustrine sediments are widely distributed along the valleys,with different characteristics and altitudes in each reach.According to the cosmogenic nuclide,ESR,and OSL dating technologies,the ages of the lacustrine sediments and fluvial terraces are all younger than1.89 Ma,indicating that the modern valleys of the Jinsha River have appeared since that time.The age of the lacustrine sediments represented by the Xigeda Group gradually became younger from high to low,indicating they are all the dammed lake sediments deposited on the hillslopes following the river incision.?5?The formation and development of the river valley record detailly the response process of the river system to regional tectonics and climate change.For example,the tectonic movement controls the amplitude and rate of river incision by changing the landscape relief and slope.The rarely,sporadically distributed narrow fluvial terraces and the widely distribued dammed lake sediments indicate strong tectonic activities in this region.In the context of strong tectonic activies,the valleys of the lower Jinsha River were formed not only following a pattern of incision,and then grade,and incision again,but also largely affected by disturbance of the catastrophic random events.Frequent climate change during Quaternay period can control the incision and accumulation process by changing the hydrological conditions such as river runoff and sediment.Most fluvial terraces are formed during the transition period from cold to warm.However,the formation of the dammed lake terrace and the cut-in-fill terrace is rather complex,which may be mainly controlled by the channel-hillslope coupling mechanism and the complex response of the valley itself.