| The southern margin of the Ordos Basin which connected to the Qinling orogenic belt is a well-known transition zone between an active zone and a stable block.It has a unique structural location,strong structural deformation and later transformation,and has always been a hot spot for academic research.As the Ordos Basin is an important multi-energy base in my country,knowing the structural evolution and transformation of its southern margin and the basin-mountain relationship between the Qinling orogenic belt and the Ordos Basin can provide strong support for the exploration of various minerals in this area.Magnetotelluric method has been proven to be one of the effectively passive sources geophysical methods to detect the crustal structure,and does not cause damage to the ground.It is one of the friendly geophysical exploration methods for the southern edge of the Ordos Basin,where is full of cities.Therefore,we used the magnetotelluric data collected at 33°?35°N,108°?110°E in the "Central Orogenic Belt Lithospheric Electrical Structure Research Project" and the "Deep Exploration Technology and Experimental Research Special"(Sino Probe).Through the analysis of the data,it is found that the shallow part of the study area is dominated by two-dimensional east-west structure,and the deep part has obvious three-dimensionality.We established 2 profiles along 108°E and 109.5°E for two-dimensional inversion,and obtain the electrical model.It shows that the northern Weihe Basin fault and the northern Qinling Mountains fault are characterized by electrical gradients,and segments the region into three structural units.Furthermore,we used 69 measuring point data in the area for 3D inversion,and obtain a reliable 3D electrical model.The model shows that the entire crust in the study area is distributed in east-west strips with high and low resistance,and its boundary is consistent with the location of the deep fault.The southern front of the Ordos block is dominated by high resistance characteristics,and its middle-lower crust has obvious low resistance.The surface layer of the Weihe Basin is shown as low-resistance body,and the middle-lower crust of the southwest Weihe Basin also appears a low-resistance anomaly to the upper mantle.The internal electrical structure of the Qinling orogenic belt is quite different,where the north is characterized by huge thickness and high resistance,while low resistance anomalies appear in the lower crust of the south.Combined with other geophysical exploration results and geochemical results,it is shown that the low resistance of the middle-lower crust in the southern Ordos block may be a manifestation of the coexistence of saline fluids and local melting,and its origin may be related to the north-eastward extrusion of the Qinghai-Tibet Plateau;the low-resistance at the surface of Weihe Basin may be a manifestation of Quaternary sediments,which may confirm its long-term geological history of deposition,and the deep low-resistance layer may be a manifestation of the upwelling of mantle material;the huge thickness resistance of the North Qinling is likely to be the stable crystalline basement and the low-resistance layer of the lower crust of the South Qinling may be geological fluids with a certain scale,which is discussed that the lithology and structure distribution in the Qinling orogenic belt may be uneven due to long-term geological movements. |
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