Late Quaternary Tectonic Activity And Deformation Model In The Northern Margin Of Qaidam Basin

The uplift of the Tibetan Plateau caused by the collision between the Eurasian plate and the Indian plate in the early Cenozoic is one of the major geological events in the history of the earth.With the continuous collision,the uplift and expansion of the plateau have created the present tectonic framework of the whole Eurasian continent.The Qilian Shan,sandwiched between the Alxa block and the Qaidam block,is the front of the expansion of the Tibetan Plateau and the latest component of the plateau.There are a large number of late Quaternary active faults and folds in the Qilian Shan area,with strong tectonic activity and complex deformation patterns.It is an ideal place for studying the expansion and deformation of the plateau.Previous studies on the active tectonics of the Qilian Shan were mainly focused on the northern margin of the Qilian Shan and the Hexi Corridor,but there is a lack of in-depth research on the equally important southern edge of the Qilian Shan,that is,the northern margin of the Qaidam Basin.Therefore,in order to have a deeper understanding of the tectonic deformation of the Qilian Shan and expansion of the plateau,it is necessary to conduct in-depth research on the active tectonics in the northern margin of Qaidam Basin.In this study,we select the basin-mountain boundary faults and piedmont folds in the northern margin of the Qaidam Basin as the research objects,focusing on the Dachaidan fault and southern Zongwulong Shan fault,the first row of folds in the piedmont-Shidiquan anticline and Delingha anticline.By studying the geometric and kinematic characteristics of active faults and folds,the active tectonic geometric image of the northern margin of Qaidam Basin has been constructed and perfected.Based on this work,the deformation mode of the northern margin of the Qaidam Basin is discussed.The main results of the thesis are included as follows:(1)The southern Zongwulong Shan fault is a boundary reverse fault between Qilian Shan and Qaidam Basin,with a total length of 95 km and an overall strike in the EW direction.The results show that the age of the geomorphic surface is 43 ka,20ka and 11 ka,which can be well compared with that of the northeastern margin,and its formation is mainly controlled by climate factors.The southern Zongwulong Shan fault is a Holocene active reverse fault,the vertical slip rate of the fault is 0.41 ± 0.05 mm/a and the horizontal shortening rate is 0.47 ? 0.80 mm/a since late Pleistocene.(2)The Dachaidan fault is located at the southern margin of Qaidam Shan in the middle section of the northern margin of Qaidam,forming the basin-mountain boundary between Qaidam Shan and Qaidam Basin.According to its geometry and activity,the Dachaidan fault is divided into three segments.The eastern and Western segments are thrust faults,and the middle segment is mainly dominated by dextral strike slip and thrust.The dextral slip rate of the middle segment of the Dachaidan fault is 2.04 ± 0.33mm/a since 6.5ka.The vertical slip rate is 0.18 ± 0.02mm/a since 114 ka,0.41 ± 0.06mm/a since 14 ka,and 0.33 ± 0.08mm/a since 6.5ka,the overall vertical slip rate is between 0.17?0.41mm/a(3)The Shidiquan anticline is located in a small zonal intermountain basin bounded by Zongwulong Shan in the north and Hongshan in the south.The anticline is asymmetric fold with steep south limb and gentle north limb,and is controlled by a blind reverse fault with dipping northward.The age of the Fan3 alluvial fan,which constitutes the main body of the anticline,is 158.32±15.54 ka through cosmogenic nuclide dating.Based on the comparative analysis of regional tectonic activities,we believe that the formation of Shidiquan anticline responded to the Gonghe movement of about 150 ka B.P.in the northeastern margin of Tibetan Plateau,and is the reaction of Gonghe movement in the northern margin of Qaidam Basin.The uplift rate of Shidiquan anticline is 0.06 ± 0.01mm/a and the shortening rate is 0.05 ± 0.01 mm/a.(4)The Delingha anticline is located in front of the Zongwulong Shan in the northeastern margin of Qaidam Basin,forming the first row of fold in the Piedmont.The anticline is 25 km long and 6km wide,trending NWW.It is an asymmetric fold with a short and steep north limb and a long and gentle south limb.According to the deformation and age of the geomorphic surface in the western section of the anticline,the shortening rate of the anticline since 142 ka is 0.22±0.03mm/a,and the uplift rate is 0.51±0.06mm/a.There are two wind gaps and water gaps in the eastern segment of Delingha anticline,geomorphic evidence shows that the anticline propagate eastward.In the eastern section,six geomorphic surfaces from Q1 to Q6 are divided.According to the cosmogenic nuclides exposure age of the geomorphic surface and the distance of different geomorphic surfaces along the anticline ridge,the eastward propagation rate of Delingha anticline is obtained as 17mm/a.Assuming that the anticline propagate in one direction,the anticline initiated at 1.47 Ma according to its length.Affected by the eastward propagation of the anticline,the drainage system across the anticline is continuously abandoned and deflected laterally,forming the current distribution pattern of wind gap and water gap.(5)The sum of the shortening rates of southern Zongwulong Shan fault and Delingha anticline yields a total shortening rate of 0.69?1.02 mm/a,which represents the total shortening rate of the basin-mountain boundary fault in the northern margin of the Qaidam Basin and the first row of fold in the Zongwulong Shan piedmont.GPS data reveal that the total crustal shortening rate across the Qilian Shan is 5-7mm /a.Therefore,the crustal shortening of basin-mountain boundary fault and the first row of fold on the southern edge of of Zongwulong Shan accounts for about 10 ? 20% of the total crustal shortening of the whole Qilian Shan.(6)The formation of ?150 ka B.P.geomorphic surface in Shidiquan anticline and Delingha anticline in the northern margin of Qaidam Basin was in response to the Gonghe Movement.The Gonghe movement is widespread in the northern margin of Qaidam Basin,which is mainly reflected in the formation of the latest fold,the abandonment of tectonic terraces and geomorphic surfaces,and the accelerated deformation of pre-existing folds.(7)The Elashan fault,the Riyueshan fault,the right-lateral strike-slip section of the Dachaidan fault,and the right-lateral strike-slip fault at the northern margin of Zongwulong Shan constitute the right-lateral strike-slip fault system in the Qilian Shan area.The right-lateral strike slip segment of Dachaidan fault,especially the small-scale right-lateral strike slip fault in the northern margin of Zongwulong Shan fault,is smaller than that of Elashan and Riyueshan faults,and may be in the early stage of right-lateral strike slip deformation.The formation of the right-lateral strike-slip section of the Dachaidan fault may be the result of the differential thrust of the eastern and western piedmont.The right lateral strike slip fault system in Qilian Shan mainly regulates the differential movement of different blocks.