Long magnetotelluric sounding profile for the study of crust and upper mantle beneath chinese continent

As a sub-project of SinoProbe, a 2000-km-long MT profile across Tibetan plateau, Sichuan basin and crystalline rock area in south China will be deployed from the northeast margin of Tibetan plateau to the southeast coast of China. By now, the deployment across west Qinling orogen and Qilian block has been finished. The primary result shows that the electrical structure of this region is featured by blocks in horizontal direction and layers in vertical direction with a widely spread layer of high conductivity in the mid-lower curst. Regional fault zones distributed in this area all present as electrical gradient zones or low resistivity zones in the electrical model. The border of different electrical blocks shows a high coherency with the division of geological blocks. It is concluded from the primary electrical model that the north margin of west Qinling fault zone is a large-scale plate boundary featured by left-lateral slip. The upper and middle crust of mid-Qilian block consists of a large amount of fractures, which might be formed by the crushing process in the north-south direction. The south-dipping conductive layer in the transition zone between north Qilian and Hexi corridor might be the electrical trial of Qilian block’s northward obduction and Alxa block’s southward subduction. But the boundary between them might not be the north margin of north Qilian fault zone but the south margin of Longshoushan fault zone. The conductive layer in the crust of west Qinling orogen is inferred to be caused by both partial melting and salt aqueous fluid, while the conductive layer in the mid-Qilian block might be just caused by salt aqueous fluid. There are two explanations for the formation of conductive layer in the transition zone between north Qilian and Hexi corridor. From the tectonic aspect, it might be formed by detachment process. But considering the relatively low conductivity and limited scale of this layer, it also might be caused by salt aqueous fluid upwelling from both north and south margin of north Qilian fault zone. Judging from the distribution of conductive layers in the crust of north Qilian block, large scale of partial melting does not exist in this region. A significant electrical interface exists within the depth range of 50-60 km, which is also the depth of moho determined by seismic observation. The depth of electric moho in the northern part of north Qilian fault zone is shallower than the depth determined by seismic observation.