Late Quaternary Structural Deformation Characteristics Of The Northern Margin Fault Of Toleshan In The Middle And Western Qilianshan

The Qilianshan（means Mountains）,as the front of the northward intense compression uplift and tectonic deformation of the Qinghai Tibet block,provide an important reference for us to understand the process and mechanism of tectonic deformation within the continent,especially the latest neotectonics activity and deformation characteristics.Previous research has mainly focused on the active fault zones and their structural deformation characteristics in the northern margin of the Qilian Mountains and the Hexi Corridor Basin,as well as the southern margin and the edge of the Qaidam Basin.However,there is insufficient understanding of the geometric images,structural transformations,deformation styles,and gradual structural relationships with active structures on both sides of the Qilian Mountains.This paper conducts a quantitative study on the late Quaternary structural deformation characteristics of the Tole Mountain northern margin fault in the central western section of the Qilian Mountains.On the basis of indoor data collation and satellite image interpretation,through field investigation,we selected typical fault landforms for UAV aerial survey,data processing and dislocation value measurement,and conducted a chronological study on the corresponding geomorphic surface.According to the sedimentary characteristics of different landforms such as alluvial fan and river terrace,we used such technical methods as cosmogenic nuclide dating,photoluminescence dating and 14C dating,Quantitatively obtain the structural deformation characteristics of different segments of the northern margin fault of the Tole Mountains,and then explore the structural styles and deformation modes of active faults in the central and western segments of the Qilian Mountains.The main insights gained are as follows:The northern edge of the Tole Mountain fault is a reverse fault-fold zone developed in the central western section of the Qilian Mountains,with a total length of about 280kilometers and an overall strike of 315°.It can be roughly divided into three sections,namely the Babaohe section,Qilian section,and Jingtie Mountain section.Fault activity disrupts multi-level river terraces,alluvial and proluvial platforms,forming rich fault scarps,terraces,or platform arched deformation and other fault landforms.Among them,the Babao River section is distributed along the southern edge of the Babao River in the upper reaches of the Heihe River,with a total length of about 70km and a strike of 280°.It is mainly characterized by the steep slopes of the alluvial and proluvial terraces of the tributaries on the southern side of the Babao River and the deformation of the high platform reverse fault fold;The Qilian section has an intermittent length of about 85km and a strike of 315°,with the most typical staggered landforms in the Yeniugou area.It develops along the southern edge of Ganzhou in the upper reaches of the Heihe River,mainly manifested as terrace or platform compression thrust fold deformation and multi-level fault scarps;The Jingtie Mountain section is approximately 125km long and intermittently distributed,with a strike of 280-325°from Baishagou to Gahai,and a strike of approximately 310°from Gahai to Zhulongguan in Qiqing Township.The fault landforms are mainly characterized by steep fault ridges.The Optically stimulated luminescence,cosmogenic nuclide exposure age and ¹⁴C dating methods are used to define the ages of different terraces or platforms that were dislocated along the Tole Mountain fault.At the same time,drone aerial DEM image data was used to obtain terrain profiles of different levels of river terraces or alluvial platforms with fault or fold deformation,and the vertical sliding rate and horizontal shortening rate of different fault segments were obtained.Among them,in the Babao River section,the vertical velocity of the T2-T3 river terraces with fault faulting has been 0.59±0.07 to 0.66±0.01 mm/a since the Yaodonggou and Hongnicao reached 8.04ka,It is an active fault in Holocene;Qilian section:Since the arrival of 75.72 ka in Quancigou,Hongtu Gou,and Tiemule Gou,the vertical velocity of the T2-T4 level river terrace or platform has been 0.31±0.01 to 0.55±0.08 mm/a,and its horizontal shortening rate is 0.69±0.09 to 0.74±0.01 mm/a,Mainly active faults in Holocene.The vertical velocity of the alluvial-proluvial bodies in the nine Qingyang areas of the Jingtieshan section has been 0.33±0.07 mm/a since 15.95 ka.Overall,the vertical velocity along the fault is relatively stable,concentrated between 0.5-0.6 mm/a,Mainly active faults in Late Pleistocene.The above research results are roughly equivalent to the deformation rates of the reverse fault-fold zone on the northern edge of the Qilian Mountains and the active fault zone on the northern edge of the Qaidam Basin,reflecting the distributed structural deformation characteristics of the Qilian Mountains during the Late Quaternary.By using the theory of fault related fold deformation,the deformation characteristics of platform folds were analyzed.It was found that the northern edge of the Tole Mountain fault has a relatively steep front wing,a nearly planar fold top,and a shallowly dipping rear wing,which is consistent with the deformation characteristics of terrace folds in fault bending folds.it is believed that there is a shovel shaped fault bending fold deformation caused by the migration of knee fold zones in the northern edge of the Tole Mountain fault.Based on the results of artificial seismic exploration and magnetotelluric exploration conducted by predecessors in the research area,combined with the surface structural deformation characteristics obtained in this study,comprehensive analysis suggests that the central western section of the Qilian Mountains extends westward from the Lenglongling Fault and can be divided into three late Quaternary active fault zones:the Toleshan Fault,the northern edge of the Toleshan Fault,and the Sunan Qilian Fault.The deep and shallow structural relationships and deformation characteristics between the above faults are mainly characterized by a strike slip high angle compression thrust fault combination in the Tuoleshan Babaohe area,while the Qilian Yeniugou area shows a compression extension and folding deformation style of the reverse fault towards the interior of the basin.