The Formation Of Long Yang Gorge And The Evolution Of The Upper Yellow River System In Qinghai

The development of large rivers and the evolution of basin geomorphology have always been one of the hot topics in geomorphology and paleoenvironmental research.The Yellow River originates in the hinterland of the Qinghai-Tibet Plateau,and its upper course passes through the Zoige Basin in the eastern part of the plateau,then enters the northeastern part of the Qinghai-Tibet Plateau to the north,and its current course connects several mountain basins such as Xinghai Basin,Republican Basin,Gui De Basin,Jianzha Basin and Xunhua Basin,then enters the Linxia Basin and Lanzhou Basin in Gansu,and then enters the middle reaches of the Ordos Basin in Ningxia,Inner Mongolia,Shaanxi,Shanxi and other provinces to the north.Finally,it crosses Sanmen Gorge into the downstream river of Henan-Shandong and other provinces into the Bohai.With the continuous improvement of dating and source tracing technology,academic research on the Yellow River has expanded from the formation and development of the river to discussing the evolution of the hydrological pattern of the basin,and controversies continue.The reason for the controversy is that there are two weak links in the research process: firstly,the details of the evolutionary process are not sufficiently studied,and secondly,there is a single understanding of the regrouping pattern of the watershed system.The core issue is to clarify the interrelationship between the evolutionary process of landforms and the changes in the Yellow River river system.The upper reaches of the Yellow River are the key area for the study.Due to the influence of the uplift of the Tibetan Plateau and global climate change,the geomorphic pattern of the northeastern part of the Tibetan Plateau has undergone significant changes since the Late Cenozoic,which has an important influence on the formation of the Yellow River and the development the of the watershed system.At the same time,the continuous downcutting and erosion of the Yellow River through the northeast margin of the Tibetan Plateau is not only a response to climate change and tectonic activity but also has greatly shaped the formation and evolution of regional landforms.Therefore,the study of the deep-cut history of the upper Yellow River is an effective way to understand the evolutionary pattern of this large river and to understand the geomorphological evolution of the basin on the northeastern margin of the Tibetan Plateau.The study of how the river gorges between the upper basins was formed and the limitation of the age of incision is the basic work to explore the response of rivers to tectonic activity and climate change.At present,there are still controversies among previous authors about the evolution process of the river system and the age and way of river incision in the upper river section,and more research is needed.Based on this,in order to reveal the evolutionary transformation of the Yellow River system pattern in the upstream section from Republican to Gui De,this paper selects Longyang Gorge,the link between the Republican and Gui De basins in the upper reaches of the Yellow River,as the object of study,and complements and redefines the causes of the formation of Longyang Gorge and the time of its passage.Through detailed field investigations and stratigraphic observations,ESR and OSL dating analysis of the highest river terrace in the Longyang Gorge section,as well as source area tracing analysis of the granitic gravels alluvial to the terrace,the penetration date of Longyang Gorge is defined,and the mode and driving mechanism of the cut-through of Longyang Gorge are explored in the context of tectonic,climatic and erosional datum changes.On the basis of this,the evolutionary model of the watershed system in the upper reaches of the Yellow River is established by combing the previous research results with field investigations in the upper reaches of the Yellow River.Based on the work done above,the paper mainly gained the following insights:(1)The ESR dating results of the fine sand from the highest terrace sediments of the Yellow River Laxiwa Hydroelectric Station are approximately 138±13 ka and 151±21 ka,and OSL test age results were 153.88±10.41 ka and 157.00±10.36 ka,which indicates that the passage of this section of the river and the beginning of the formation of the Longyang Gorge is about 150,000 years old.(2)The U-Pb zircon dating and trace element experimental test analyses of alluvial granitic gravels in the highest terrace of the Laxiwa hydroelectric section of the Yellow River show that the U-Pb zircon dating results of the granitic bodies in the area crossed by Longyang Gorge and the granitic gravel samples in the overlying alluvial deposit of the terrace are similar,with a crystallization age of roughly 240.4 ± 1.5 Ma,and the enrichment and The granitic gravels deposited in the Laxiwa reach river terrane are granitic material carried by the river through the bedrock area during the formation of Longyang Gorge.It indicates that the bedrock was cut through and piled up on the highest terrace during the river passage,which indicates that strong tectonic activity occurred during the formation of Longyang Gorge,therefore,this paper suggests that the passage of Longyang Gorge is closely related to the Gonghe movement.(3)Analysis of the regional tectonic background and geomorphic evolution shows that the uplift and geomorphic changes on the northeastern margin of the Qinghai-Tibet Plateau have contributed to the important evolution of the water system pattern in the upper reaches of the Yellow River since the Pliocene.Based on this,the development and formation of the upper reaches of the Yellow River are integrated,and a model of geomorphic formation and evolution of the Yellow River is established under the mechanism of continuous retrogressive erosion and tectonic activity response.Until 1.8 Ma-1.7 Ma,the modern Yellow River appeared in Lanzhou-Linxia and began to cut into the Qinghai-Tibet Plateau;after about 1.2Ma,the Yellow River penetrated the Xunhua Basin and the Guide Basin due to Kunhuang movement and headward erosion.At that time,Qinghai Lake was still part of the outflow system and was regarded as the upstream of the river;at 0.15 Ma,the Gonghe movement occurred,and the Yellow River cut through Longyang Gorge,and the preliminary pattern of the upstream water system of the Yellow River was established.