The Geomorphic Characteristics Of Xiying River In The Northeastern Margin Of Qilian Mountains And Its Geological Significance

The Qilian Mountains,as the northern boundary of the Tibetan Plateau,is one of the key areas to study the uplift and expansion process of the Plateau.The terraces of Xiying River in the eastern part of Qilian Mountains are well developed and well preserved.The uplift process of the Qilian Mountains may be recorded by the development and evolution of the terraces.The evolution process of the terraces may provide an important basis for the study of the uplift and expansion of the Tibetan Plateau.Through detailed field investigation and chronological study of river terraces in Xiying River,the following understandings have been obtained:(1)Through detailed field investigation and measurement,7 terraces of the Xiying River Basin were determined,and the chronological framework of the river terraces was established by using the methods of Cosmogenic Nuclide Burial Dating and Optically Stimulated Luminescence Dating.Terraces T7-T1 were Separately formed at 1.54 Ma B.P.,1.12 Ma B.P.,0.77 Ma B.P.,0.40 Ma B.P.,75.01 ka B.P.,49.21 ka B.P.and 4.72 ka B.P..(2)According to the characteristics of terrace sediments and the composition of the section,it is considered that almost all the 7 terraces are pedestal terraces.The gravel in the gravel bed are sub-roundness,with good roundness and general sorting.The height above the river of the terraces are generally more than 20 m.According to the comparison of the age of the terraces,marine oxygen-isotope values and grain-size variations,it is believed that the main controlling factor for the formation of Xiying River terraces is not climate change but tectonic movement.The age of formation of river terraces is corresponding to several times of uplift of the Tibetan Plateau,and there are also clear evidences from other adjacent rivers.(3)The downcutting rate of Xiying River of each stage reflects the process of tectonic uplift in the northern margin of Qilian Mountains since middle Pleistocene.The history of downcutting shows that the average uplift rate of the northern margin of Qilian Mountains since 1540 Ka is 0.14 mm /a,and shows a gradually enhanced trend overall: 1700-780 Ka,the uplift rate is about 0.10 mm /a;780-49.21 ka,the uplift rate accelerated to 0.15 mm /a;since 49.21 ka,the uplift rate has accelerated to0.65 mm / a.The multi-stage uplift of terraces is caused by that the Indian Ocean plate is constantly subducting into the Eurasian plate.This is also the fundamental cause for the formation of 7 terraces,and the trend is accelerating.(4)Based on the average rate since 1540 ka,the northeastern margin of the Qilian Mountains has been uplifted by at least 360 m since the Quaternary,which indicates that the northeastern margin of the Qilian Mountains is still in a rapid stage uplifting state during the Quaternary.By comparing with the Quaternary uplift of the main body of the Tibetan Plateau,Gonghe-Guide area and Ordos Plateau,it is shown that from the main body to the northeast margin of the Tibetan Plateau,there is an decreasing trend in the rate of uplift amplitude since the Quaternary.