The 3-D Deep Electrical Structure And Crustal Deformation In The Eastern Qilian Mountains And Its Adjacent Areas

Since 55 Ma begin,the strong uplift of the Qinghai-Tibet Plateau has been caused by the collision and convergence of India and Eurasian plate,and the influence range of the convergence is beyond the Qinghai-Tibet Plateau,The eastern part of the Qilian orogenic belt,as a local plateau in the northeastern margin of the Qinghai Tibet Plateau,is located in the convergence area between the northward extension deformation of the northeastern margin of the Qinghai Tibet Plateau and the Eurasian continent.It is also the most concentrated area of three kinds of tectonic deformation movements in the northeastern margin of the Qinghai Tibet Plateau,which are intense lateral escape,strong north-south crustal shortening and rapid vertical uplift Qilian Mountain has become an important area for studying uplift and extension of the Qinghai Tibet Plateau due to its location and structural transformation.In this paper,the eastern part of Qilian Mountains and its adjacent areas are selected as the research area.The target area includes the hidden faults under the hinterland of Tengger Desert.The magnetotelluric survey work along long sections from the south of West Qinling Mountains to the north of Alxa block is completed.Based on the data measured from three sections,the refined data processing and two-dimensional and three-dimensional inversion calculation are carried out to obtain the two-dimensional and three-dimensional deep electrical structure in this area,Based on the results of the present three-dimensional crustal movement in the northeastern margin of the Qinghai Tibet Plateau,the characteristics of crustal deformation in the eastern part of Qilian Mountains,discusses the earthquake structure and earthquake environment of multiple earthquakes in the area,Combined with the acquired chronological framework,deformation model and evolution process of Cenozoic structural deformation in the middle,West and east parts of the Qilian Mountains,We analyze the contact relationship between the eastern part of the Qilian Mountains and the blocks on the north and south sides,and the mechanism of the uplift and northward expansion of the Qinghai Tibet Plateau,and discusse the influence scope of the northeastward expansion of the Qinghai Tibet Plateau,the location and change of the front of the plateau form and other scientific issues.Get the following knowledge:(1)Refined processing and inversion calculation of magnetotelluric data: in this paper,two long sections of high quality magnetotelluric data of hundreds of Hz to tens of thousands of seconds are obtained.The parameters of apparent resistivity,impedance phase,two-dimensional deviation angle,principal axis electrical trend angle,magnetic tilt and so on are obtained by using phase tensor decomposition technology and magnetic tilt graph technology,and qualitative analysis is carried out for them;multi variable two-dimensional inversion calculation is carried out by using NLCG method,and multi parameters are carried out by using modem software multiple initial modelsand three-dimensional inversion of belt topography.According to the qualitative results of early apparent resistivity and phase data analysis,as well as the regional geological structure,the most reasonable two-dimensional and three-dimensional electrical structure model is selected by comparing the results of two-dimensional and three-dimensional inversion.(2)The 3-D Deep Electrical Structure and Crustal Deformation in the Eastern Qilian Mountains and Its Adjacent Areas:The distribution characteristics of the electrical structure of the crust and upper mantle in the eastsection of Qilian Mountains and its adjacent areas vary greatly in the transverse direction along the two MT profiles,while the electrical structure of the same block is highly similar.The places where the electrical structure changes most correspond to the main active fault zones(such as WQLF,QHF,NQLF and LSSF).Qilian-West haiyuan fault is the largest and most important main boundary fault in the study area.The northern part of the fault is a large-scale and complete south deep North shallow "snuff bottle" or "Rugby" high resistivity structure,presumably a Gulang nappe.The upper crust of the Longxi block and the South Qilian block in the southern part of the fault is a high resistivity structure,and the middle and lower crust is characterized by low resistivity Mainly.The fault in the northern margin of West Qinling Mountain and Lajishan fault are relatively strong electrical boundary zones,which are large-scale fault zones in this area.The Shuangqiaogou-Huangyangchuan fault in LJS-N section is the associated fault of Qilian xihaiyuan fault in the upper crust,There is an obvious electrical boundary zone in the south of Minqin,which is supposed to be the Hongyashan-Sidaoshan fault concealed under the Tengger Desert.The fault may be the boundary between the Qinghai Tibet Plateau and the Alxa block together with the northern edge fault of Longshou mountain.It shows that the edge of Qinghai Tibet Plateau has crossed the Hexi corridor to the southern edge of Alashan block.(3)The seismic structures of 3 moderate-strong earthquakes in the Eastern Qilian Mountains and Its Adjacent Areas: The compressional collision environment between the Qinghai Tibet Plateau block and the Alxa block near the eastern part of the Qilian orogenic belt and the integral movement of the Gulang nappe may be the macro dynamic background of multiple medium and strong earthquakes in the area.During the northeastward pushing process of the block in the Qinghai Tibet Plateau,the Gulang nappe moved northward as a whole.In the middle and lower crust detachment zone,the 1927 M8.0 Gulang earthquake occurred first,followed by the 1954 M7.0 Minqin earthquake in the North front end,the 2016 Menyuan M6.4 earthquake in the South back end.(4)Deep electrical structure and crustal deformation of Qilian-West haiyuan fault zone and its two sides: West Qinling,South Qilian block and Longxi basin show a high and resistivity boby in the south of Qilian xihaiyuan fault.The low resistivity layer in the middle and lower crust has the characteristics of wave undulation in the occurrence depth,showing the bending trend of pushed deformation,which proves that The uplifting trend is mainly realized in the form of shortening the earth's crust.The Gulang nappe area to the north of Qilian-West haiyuan fault shows a complete high resistance structure,which is not easy to deform.It forms a Piedmont basin with a gentle slope on the landform,indicating that the electrical structure of different blocks plays an important role in controlling the three-dimensional crustal deformation and geomorphic formation in this area.(5)The Enlightenment of the Northeast extension of the Qinghai Tibet Plateau:The eastern section of the Qilian Mountain structural belt is mainly composed of Hongyashan-Sidaoshan fault,the northern margin fault of Qilian Mountain,Huangcheng-Shuangta fault,Qilian-West haiyuan fault and other faults,forming a "flower" structure that extends from the south to the north,showing obvious characteristics Horizontal northward extension and vertical extrusion features.The extension of the northeastern margin of the Qinghai Tibet Plateau is dominated by the whole crustal shortening and strike slip shearing of several high angle thrust nappes and strike slip faults in the shallow part of the northeastern margin of the Qinghai Tibet Plateau,and the deformation mode of low angle subduction of the Alxablock in the deep part,which is the NE extension mode of the Qinghai Tibet Plateau.