The Study On The Drainage Evolution Of The Beida River In The Jiudong Basin And The Upward Growth Of The Jintanan Shan

Owing to the continuous convergence between India and Eurasia in the Cenozoic period,the Tibetan Plateau has been uplifted and expanded as the highest plateau in the world,making a great impact on the East Asian landscape and tectonics,and the global climate.Therefore,these issues on its growth history,extension process,driving mechanism,environmental effect,and so on,attracting wide attention from the international geoscientists,have been regarded as the current research projects in the geoscience field.A series of low mountains surrounding the margin of the Tibetan Plateau now have been uplifted as the youngest components of its extent.It is of great significance to explore their growth history for revealing the expansion process of this plateau.As a result,more attention from home and abroad has been paid to them,leading to one of the research focuses in geoscience field.Among these low mountains,the Jintanan Shan(in Chinese,Shan = mountain)occurred at the northeastern margin of the Tibetan Plateau extent,and constraining the Jiudong Basin,combined with the Qilian Shan,to the north and south,respectively.The geomorphology across there is thus characterized by the coupling between basin and mountain.In general,fluvial system can make a sensitive and positive response to surface uplift and climate change,and then register its series adjustments within geomorphic and sedimentary records.The Beida River drainage originating from the Qilian Shan flows northward through the Jiudong Basin and Jintanan Shan,and creates a series of wind gaps and fluvial terraces,thus providing good insight into the expansion process of the northeastern Tibetan Plateau.On the basis of these records,we have reconstructed the formation and evolution process of this drainage,and then made a discussion on the growth history of the Jintanan Shan in this thesis.Firstly,according to the detail investigation and comparison on the geomorphic and sedimentary records,these fluvial terraces formed by the Beida and Hongshuiba Rivers,which are two main tributaries of the Beida River drainage,have been identified in the seven wind gaps along the west-east Jintanan Shan.Secondly,the OSL dating method based on feldspar and quartz has been employed to establish the geochronological sequence of these terraces.Finally,based on above work,the formation and evolution process of the Beida River drainage was reconstructed,and then its driving factors were further analyzed to reveal the growth history of the Jintanan Shan.These systematic studies bring about preliminary advance in the following four aspects:(1)The Beida and Hongshuiba Rivers deriving from the northern Qilian Shan,defined as large rivers,cut northward through the Jiudong Basin and the Jintanan Shan,independently,resulting in terrace generation within the wind gaps of 6 to 4 and the wind gaps of 1 to 3,respectively.The Beida River drainage formed by the confluence of the two large rivers,however,created only one terrace along the Yuanyang Gorge.(2)Our dating results indicate that the gravel layer overlying the Jintanan Shan as the remain of alluvial plain was formed before 280 ka.Since then,the Beida and Hongshuiba Rivers have adjusted their channels through the Jintanan Shan several times until confluence,and consequently developing a series of wind gaps and terraces in the four periods of 220-150 ka,150-130 ka,130-100 ka,and <100 ka.(3)The formation and evolution process of the Beida River drainage has been preliminarily reconstructed by synthesizing above obtained results.The area between the Hei Shan and Heli Shan,characterized by flat topography and relative quiet tectonics,was thus occupied by a series of rivers and finally developed an alluvial plain by their sedimentary accumulation before 280 ka.After that,these rivers downcut into the original alluvial plain as the Jintanan Shan uplift,and creating seven river valleys.The most east and west valleys were identified to be formed by the two large rivers of the Hongshuiba River and the Beida River,while the remaining valleys may be attributed to the downcutting by local river deriving from the Jiudong Basin.In the following four periods of 220-150 ka,150-130 ka,130-100 ka,and < 100 ka,the two large rivers adjusted their channel through the growing Jintanan Shan via these valleys,resulting in an opposite migration.They finally merged to construct the Beida River drainage,which cut through the Jintanan Shan by the Yuanyang Gorge.Therefore,these valleys excavated into this mountain were defined here as wind gaps.(4)Further analysis for the driving factors reveals that the formation and evolution of the Beida drainage can be probably attributed to the coupling between tectonic uplift and cooling-drying climate change.Based on this,it is discussed that the landform of Jintanan Shan appeared in 280 ka,and its subsequent uplift process may be manifested as the process of east-west differential uplift.Based on this conclusion,we suggest in this thesis that the mountain topography of the Jintanan Shan may has be uplifted since 280 ka,and presenting a differential growth model in east-west direction.