The Research On The Lithospheric Conductivity Structure Of Eastern Qilian Mountain And Its Surrounding Regions

The Qinghai Tibet Plateau is the result of the collision between the India plate and the Eurasian plate since the Cenozoic,and the two plates are still colliding and squeezing with each other at a certain rate.Therefore,the Qinghai Tibet Plateau is also known as the change of the earth dynamics research and geological structure of natural laboratory,especially Qilian Mountains orogenic belt and neighboring blocks of the deep structure and physical state is one of the hot area of stuying the dynamics Qinghai Tibet Plateau.This paper is based on the project "Experimental Study of Standard Network of Continental Electromagnetic Parameters(SinoProbe-01)",and we try to study the deep electrical structure and material status of the crust and mantle by the array MT data collected in the eastern section of the Qilian Mountains and the surrounding area.The scope of the study area is:99.5°E-106.5°E,34.5°N-41.5°N,including West Qinling block,Qilian block,Alashan block and Ordos block.Based on the MT data processing and the two-dimensional inversion and threedimensional inversion method based on the NLCG algorithm,the electrical structure model of the crust and mantle in the eastern section of the Qilian Mountains and the surrounding area is obtained.The results show that the electrical structure of the Qilian block and the Alashan block and the West Qinling block is vertically stratified and laterally divided,and there is a large scale high-conductivity layer in the crust of each block.Among them,the high-conductivity layers of the Qilian block and the West Qinling block are mainly distributed in the middle and lower crust,and the high-conductivity layers of the Alashan block are mainly distributed in the upper and middle crust.The geoelectric model of the boundary faults in this study area,such as the north margin fault of the West Qinling,the Haiyuan fault,the Qilian piedmont thrust fault,the Longshoushan fault and the XiangshanTianjingshan fault,all are characterized for electrical gradient zones or low resistivity zone.Among them,the north margin fault of the West Qinling and the Haiyuan fault are super crust faults.Xiangshan-Tianjingshan fault and the Haiyuan fault are contacted in the deep crust,and they play an important role between the Qilian block and the Alashan block of the interaction in the regulating and absorption.The high-conductivity layers in the crust of the Qilian block tilt to the south,and they may be the electrical evidence of the southward subducting Alashan block and the northward obducting Qilian block,and the boundary between them is probably the Haiyuan fault(Haiyuan section).Combined with the heat flow and temperature in the middle and lower crust and other information,concluded that the large-scale high-conductivey layer in the middle and lower crust under the West Qinling block and Qilian block may be due to the existence of partial melts and saline fluids,and the high-conductivity layers have good circulation.According to the study of the relationship between the distribution of the high-conductivity layers and the distribution of the medium and strong earthquakes in the study area,it is found that most of the epicentral locations of the medium and strong earthquakes are located in the high and low resistivity interphase,and most of them are close to the high resistance region.This indicates that the low velocity and low density high-conductivity layers in the crust may have a material flow,and then induce an earthquake.