New Activity Characteristics And Tectonic Significance Of The Western Margin Of Ganjia Basin Faults

The northeastern margin of the Tibetan is an important part of the uplift and tectonic deformation of the Tibetan Plateau.It is also the frontal area for the northward extrusion and expansion of the Tibetan Plateau block.It has developed a series of late quaternary strongly active fault zones and multiple cenozoic rifted basins.The Ganjia Basin is located at the junction of Xiahe County,Gannan Tibetan Autonomous Prefecture,Gansu Province and Tongren City,Qinghai Province.It is located in the northeastern part of the Tibetan Plateau and at the end of the fault zone of the northern margin of the Western Qinling fault.There are three north-south trending faults(east branch,middle branch,and west branch)developed in the west margin of the Ganjia Basin,which appear as three arc-shaped geomorphic stepped zone by higher in the west and lower in the east.Based on interpretation of high-resolution satellite remote sensing images,geological and geomorphological surveys,UAV aerial surveys,profile cleaning and ¹⁴C dating,evidences of new activities in late Quaternary were found along the west margin faults of the Ganjia basin.At the same time,combined with the investigation and analysis of the fault zone in and around the Ganga Basin,the tectonic evolution process of the Ganjia Basin and its relationship with the fault zone in the northern margin of the Western Qinling Mountains were studied,which provided a scientific basis for studying the structural transformation model of the western end of the fault zone in the northern margin of the Western Qinling Mountains.On the other hand,this work is very important for the scientific evaluation of the earthquake risk in this area.The results can provide a scientific basis for the earthquake disaster relief and planning in this area,so as to improve the ability of earthquake disaster prevention and reduce the loss of property from the people.Through researches and analysis,the paper has obtained the following main conclusions:(1)There are three north-south trending faults(east branch,middle branch,and west branch)developed in the west margin of the Ganjia Basin,which appear as three arc-shaped geomorphic stepped zone by higher in the west and lower in the east.Based on interpretation of high-resolution satellite remote sensing images,geological and geomorphological surveys,UAV aerial surveys,profile cleaning and 14C dating,evidences of new activities in late Quaternary were found along the west margin faults of the Ganjia basin.The geomorphic appearance of new activities of the faults shows as fault scarps with different fault levels,faulted terraces and alluvial platforms,as well as right-lateral movement of gullies and terraces,etc.The fault profile and slickenside revealed that the faults have dextral thrust properties and the Holocene strata on the faults has been dislocated.(2)According to the UAV measurement and dating data,the average horizontal right-lateral slip rate of west branch of the west margin faults of the Ganjia basin since the mid Holocene is 0.89 mm/a,while the average vertical slip rate is 0.30 mm/a.(3)The Ganga Basin mainly develops the Western Margin Faults of the Ganjia Basin(east branch,middle branch,and west branch),Baishiya Fault,Bajiaochengbei Fault,Nimalonggou North Margin Fault and Qielonggou Fault.The new activity of the NS-trending fault zone in the western margin of the plus basin is mainly affected by the dextral compressional movement of the NNW-trending uplift of the Zhamarigen Mountain.In the tectonic response at the western end,the Baishiya fault,the Bajiaochengbei fault,the northern Nimalonggou fault and the Qielonggou fault jointly form the transformation model of the horsetail-shaped fault at the western end of the northern margin fault zone of the Western Qinling Mountains.According to the evolution of the regional stress field and the development and deformation of the fault,the western end of the Western Qinling fault has roughly gone through three stages:the early compressional thrust stage,the middle forward compressional rear extensional stage,and the late end strike-slip extensional stage.(4)The sedimentary sequence of the Ganga Basin is basically continuous.It is speculated that it is a pre-existing intermountain basin in the western section of the West Qinling.Since the late Cenozoic,the tectonic deformation has been affected by the tectonic deformation at the end of the fault at the northern margin of the western Qinling from the northwest to the west and the Zhamarigen Mountain on the west side.The evolution process of the Ganga Basin can be divided into three stages:the early Cenozoic intermountain depositional basin stage;the late Cenozoic-early Quaternary unilateral fault depression stage and the late Quaternary normal fault migration structural deformation stage.(5)The deformation of the western end of the fault at the northern margin of the Western Qinling fault is a fault bundle that spreads out to the west,all of which are characterized by left-lateral normal faults.The new fault activities are from south to north,which from the Baishiya fault to the Qielonggou fault.The trend of migration and transformation shows that the fault in the northern margin of the Western Qinling will not end in the Ganga Basin.It will transform to the southwest and continue to expand.