The Study On The Formation And Geomorphic Evolution Of The Yellow River Along The Lajia Gorge

The formation processes of large river and its drainage landform evolution have long been of great interest to researchers,attracting wide attention from geomorphological field.Over the past few years,the study on the river system evolution has been upgraded to focus upon the drainage development in the whole catchment scale,as the advance in dating and provenance tracing technology.Despite this progress,it still faces the challenges from single re-organization model and insufficient details.The key problem lies in lacking the relationship among these drainage patterns under different stages of geomorphic development.The Anyemaqen Shan(Shan=mountain in Chinese)and Xiqing Shan were deeply downcut by the Yellow River between Xinghai and Zoige basins,creating the Lajia Gorge.The Yellow River between the Xinghai and Zoige basins cuts through the physiographic transition between the gentle landform in the southern Tibetan Plateau and the high relief in the north.The landscape across these basins are characterized by an alternation between basin and mountain,thus providing excellent materials to reconstruct the drainage evolution progress from subsidence to excavation.In order to reveal the drainage evolution progress from basin subsidence to excavation along Lajia Gorge.Firstly,we collected the geomorphic information and sedimentary component,and then created ten geomorphic cross-sectional sequences.Secondly,the cosmogenic nuclide,OSL,and AMS-¹⁴C dating was employed to establish the geochronological sequence of the geomorphic and sedimentary records within its catchment.Based on above works,the evolution process among these drainage patterns under different stages of geomorphic development was re-constructed.Finally,we evaluated the pattern of the Yellow River traverse the Lajia Gorge by comparing the geomorphic evolution process with regional tectonic activities,climate records and subsidence of erosion basis.On the basis of above works,the following are our preliminary conclusions:(1)A planation surface overlay on the replat of Lajia Gorge,which may correspond to the planation surface(main surface)of the Tibetan Plateau;A set of fluvial gravel layer overlay on the valley created a erosional surface,the gravel layer connected with the basinfill top of Xinghai basin gently,indicating that a river has been injected into the basin before the gorge is traversed,but the river is distinguished from the Yellow River.Serious of typical Yellow River terraces are created under this surface.The oldest terrace represents the timing,in which Lajia Gorge traversed by Yellow River.(2)Combined with the previous works and our chronological results,the planation surface was formed at 8-3.6 Ma and the broad valley surface was crated at 200 ka ago,while the age of the highest terrace became younger along the Lajia gorge.While the age of the highest terrace in the Zoige basin is 2.5 ka,younger than the age of the basinfill top.(3)Based on the geomorphic characteristics,sediment component and chronology sequences,this paper reconstructed the formation and geomorphic evolution of the Yellow River along the Lajia gorge:on the early Miocene,Xinghai and Gonghe Basin became a basin complex,much of detrital materials from ranges were accumulated in basin complex;Later,a serious of widespread tectonic uplift initiated in the northeast margin of the Tibetan Plateau during 8-3.6 Ma,not only interrupted the planation,but also drove the Gonghenan Shan uplift and divided the basin complex;About 3.6 Ma ago,a large-scale river passed through Xinghai basin and then traverse Gonghenan Shan into Gonghe basin.Its source extended upwards to Zoige Basin,but did not connect the basin.On the contrary,long-term lateral erosion crated a erosional surface before 0.2 Ma,which overlay a set of fluvial sediments;Longyang Gorge was traversed at about 0.2 Ma,the Gonghe basin,which controls the base level of the upper reaches of the rivers,ended its subsidence.The rapidly declining erosion basis and the uplifted Anyemaqen Shan and Xiqin Shan drove the river gradually to extend upwards,finally connected with the Zoige Basin before 13 ka,whereafter the Yellow river were formed.