Research On ²⁶Al/¹⁰Be Burial Dating And Landform Development In The Litang River Terrace

Since the Cenozoic era,the continuous collision of the Eurasian plate has led to large-scale deformation and continuous uplift of the Tibet plateau and its surrounding areas,forming a low-undulation landfall zone in the inland of the plateau and in the southeast.At present,there are still different views on the genesis of the landform on the southeastern margin of the Tibet plateau,mainly including the continuous deformation model of lower crustal pipeline flow and the block lateral extrusion model.Litang river is located in the southeastern edge of the Tibet plateau and is located in a terrain slope of 4000-3000 m.The upper reaches of the river flow through the broad flat plateau,the river valley is wide and slow;The downstream flows through the Heng Duan mountain area,the river valley is narrow and deep.There are Litang fault and Xiaojinhe-Lijiang fault around Litang river,and the tectonic activity is strong in the region.In recent years,digital topographic and geomorphic factor analysis,especially the longitudinal section of bedrock channel,has been increasingly applied in the study of geomorphic evolution and tectonic deformation in active tectonic areas.The development process of river landform can be studied by reconstruction of the longitudinal section of ancient river.According to the longitudinal section of modern river,the knickpoints of the river can be found,and the longitudinal section of ancient river can be reconstructed by projection,so as to obtain the lower cutting amount of ancient river and the multiple uplifts of the knickpoint at different stages.Therefore,this paper chooses Litang river basin as the research area,focuses on the chronological study of the river terrace in Nazidina section of Litang river,and studies the longitudinal profile of Litang river by combining with the analysis of digital topography and geomorphic factors.Through field investigation,it was found that three levels of river terraces(T1,T2 and T3 from the new to the old)were reserved in the section of Litang river in Nazidian,and the tug-of-war heights were 78 m,390 m and 490 m,respectively.The sediments in the above terraces were dated by Isochronous burial dating,and the results were 0.19±0.01 Ma,0.24±0.04 Ma and 0.59±0.05 Ma,respectively.Based on 90 m resolution DEM data,the concave degree and steepness index of Litang river basin were calculated by using the river erosion model in Topo Toolbox,and the knickpoints of the main stream were found by using the knickpointfinderfunction.The results show that there are two knickpoints in Litang river main stream.The knickpoint 1 is located at the reservoir near the village of Kajiwa,with an elevation of 2700 m.Knickpoint 2 is located near Queruduo village,with an elevation of 3200 m.The steepness coefficient of the main stream and the tributary below the knickpoint is obviously less than that above the knickpoint.After ruling out the influence of local structure,lithology and base level decline,this paper considers that the two main stream knickpoint are caused by traceable erosion caused by the increase of the downstream Xiaojinhe-Lijiang fault activity rate.In order to obtain the cutting rate of the river near the fault,this paper used Topographic Analysis Kit(TAK)software to reconstruct the corresponding paleochannel longitudinal profile by combining the elevation and concave degree of the residual channel above the knickpoint.According to the reconstructed elevation of the ancient river,the cutting amount of the ancient river in the corresponding period of T1 and T3 at the position of the Xiaojinhe-Lijiang fault was 200 m and 680 m,respectively.In addition,the reconstruction results of the longitudinal section have a good correspondence with the elevation of the terrace of Nazidian section,so the age of the terrace sediment can be used to represent the development period of the ancient river channel.Results,combined with the corresponding assumptions to knickpoint 1 and knickpoint 2 to Lijiang-Xiaojinhe fault of minimum horizontal migration rate is respectively 360km/Ma and 220 km/Ma,knickpoint 1 and 2 in the Lijiang-Xiaojinhe fault in the beginning of the incised age were 0.36 Ma and 0.9 Ma,Litang fault location of the Lijiang river in a small river knickpoint 1 and knickpoint 1 paleochannel profile of incised rate is 0.5 mm /a and 0.75 mm/a.In summary,the Xiaojinhe-Lijiang fault is at 0.36 Ma and 0.9 Ma,and there may be two tectonic events.The Xiaojinhe-Lijiang fault began to move,which led to the adjustment of Litang river longitudinal profile,and the knickpoint was formed near the fault and gradually traced to the source of erosion.The rate of fault activity then increases further,and new knickpoint form and travel upstream in the same way.To0.59 Ma and 0.2 Ma,the two knickpoint passed around the Nazidian section respectively and formed T3 and T1 terraces.Subsequently,the channel continues to be traceable to the source in the form of knickpoint,reaching the current position and presenting an unbalanced longitudinal profile of the channel.