Research On Tectonic Geomorphology In Active Extensional Region ——A Case Study In The Zhongba Rift,Southern Tibet

The southern Tibet rift system is a series of?NS trending rifts,which is one of the most significant tectonic features within the Tibet Plateau.The Zhongba rift zone is located in the western part of the southern Tibet rift system,which has well-developed active faults and clear faulted landforms.However,the study on the activity of the fault is still lacking,which hinders our understanding of the deformation mechanism of the entire southern Tibet rift system.Based on the main research methods of tectonic geomorphology,this thesis carries out detailed study of the Zhongba rift zone.The characteristics of main geomorphic factors and basin erosion rate in the rift zone are obtained,and the relationship between regional geomorphic development characteristics and regional tectonic activities is discussed accordingly.The main achieves are as follows:(1)The Zhongba rift is located in the northern part of Zhongba County and the Yarlung Zangbo suture belt with a total length of approximatlely 150 km.It belongs to the middle segment of the Zhongba-Dawatso rift belt,wihch is composed of active normal fault systems on both sides of the rift belt along the NW-NNW direction from the south of Taruocuo to the north of the Gangdise Mountains.Field investigation shows that the Zhongba fault has the characteristics of normal component,such as obvious geometric shape,clear faulted traces,and the phenomenon of vertical displacements of alluvial fans and terraces.(2)Based on GIS spatial analysis technology,we use the Digital Elevation Model(DEM)data to extract geomorphic parameters in this area,including topographic slope,topographic relief,channel steepness index,and knickpoints.Combined with the geometric characteristics of the Zhongba fault and topographic and geomorphological characteristics in the rift,we have analyzed tectonic activity of the faults on both sides in the Zhongba rift.It indicates that the tectonic activity of the western side tends to weaken as the segment approaches the Gangdise Mountains in the south.The other side decreases from the high activity in the central region to the two ends.Then,we conclude that the tectonic activity of the western side may be stronger than that of the eastern side.(3)Using the method of cosmogenic nuclide ¹⁰Be,we have studied erosion rates of 7 basins from modern river sands on both sides in the Zhongba rift zone.The millennium-scale erosion rates in this area are obtained.The erosion rates on the western side of the rift from south to north are(7.86±1.25)×10^-2 mm/yr,(1.92±0.22)×10^-2 mm/yr,and(2.34±0.31)×10^-2 mm/yr.The other side are(8.06±1.47)×10^-2 mm/yr,(3.78±0.46)×10^-2 mm/yr,(1.73±0.20)×10^-2 mm/yr,and(3.88±0.44)×10^-2 mm/yr.The results show that the higher the concentration of ¹⁰Be in the basin,the lower the erosion rate.Comparing the correlation between the river geomorphological parameters and the average erosion rate of the basin in the Zhongba rift zone,it is found that the erosion rate decrease with the increase of the average slope and steepness of the basin.The spatial difference between the erosion rate and topographic factors is a mismatch with previous research.In this regard,it is necessary to further consider other factors,such as landslides.In addition,glacier cover may overestimate the rate of ¹⁰Be generation during the calculation,which affects the calculation results.(4)Based on the study of the tectonic and geomorphic characteristics of Zhongba rift zone,we find that the research method of tectonic geomorphology can reflect the information on tectonic activity and geomorphic evolution.The factors controlling the river geomorphic parameters,erosion rate,and activity in the Zhongba rift zone are discussed.We believe that climate change and regional lithology in the study area have little influence on the change of geomorphic parameters,and the role of glacier erosion on geomorphic evolution needs to be further discussed in detail.Tectonic activities may play a key role in maintaining the topographic and geomorphic evolution of the Zhongba rift zone.