Research On The Deep Electrical Structure Of The Middle Section Of Jiangnan Orogen-Xuefeng Mountain Structural Belt And Its Adjacent Area

The Jiangnan orogen is formed of the Yangtze block and the Cathaysia block in the South China continent during the collision of the Mesoproterozoic and Neoproterozoic.It has experienced the fragmentation of paleo-microplates,intracontinental collision orogens,and the influence of tectonic movements under the limits of the peripheral plates.The structural deformations in the orogen are complex and diverse,with highly developed faults,folds,and unconformities,forming structural features different from the Yangtze and the Cathaysia blocks.This paper mainly uses the magnetotelluric sounding(MT)data collected in the Xuefeng Mountain structural belt and its neighboring areas to carry out two-dimensional inversion of the long profile and three-dimensional inversion of the whole area,so as to establish the electrical structure model of the study area.Through the analysis of the deep electrical structure and characteristics of the area,the deep structure of the Xuefengshan structural belt in the middle of the orogen,the main faults,the contact relationship of the Yangtze and the Cathaysia blocks and the dynamic mechanism of the area are studied and discussed from the perspective of electrical properties.The analysis of dimensional by MT data shows that the shallow part of the Xuefengshan structural belt has two-dimensional attributes,its deep structural features are complex,and there are three-dimensional effects.The overall main axis direction is about NE40° through the G-B decomposition of the impedance tensor,which is in line with the surface geological structure in the study area.The electrical structure model shows that massive high-resistance is widely distributed in the study area,and the high-resistance extension is relatively large,which is cut by the major fractures in the area,showing obvious characteristics of "lateral block".The main high conductivity below the Enshi fault on the west side of Xuefeng Mountain belongs to the lithospheric scale and has a tendency to extend upwards,which may represent upwelling mantle material.The overall high resistance of the orogen reflects the stable Precambrian tectonic background of the Yangtze Paleoblock in South China.The Jiangshao fault is the eastern boundary of the Jiangnan orogen.The edge of the low-resistance is distributed along the fault,and the deep part migrates to NW,indicating that the Cathaysia block has subducted under the Yangtze block.From the two-dimensional model,this subduction has reached the west side of the basement uplift zone of Xuefeng Mountain.The overall electrical characteristics show that the Xuefengshan structural belt is distributed in a NEE-direction arc with its basement uplift belt as the center,and the Xuefengshan uplift is characterized by thick-skin thrusting nappe involved in the detachment of the basement.The degree of deformation of the Xuefeng Mountain's surface from west to east has become more complex,with the characteristics of stratigraphic unconformity as a typical example,showing a trend of gradual change.The “staggered” distribution of the high and low resistance bodies on the east side of Xuefeng's electrical model reflects that the shallow faults in the area thrust in the SE direction on the west side of Chengbu and Xinhua,and thrust toward the NW direction on the east side,forming a fault hedge structure.The Jishou and Kaili faults in the middle section of the orogen may have tended to SE earlier,and the nearly vertical tectonic morphology in the current crust may be caused by the development of fault strike-slip and ductile shear due to the change of the principal stress direction of the subduction of the paleo-Pacific plate.In addition,the low resistance near the western boundary of the Jiangnan orogen reflected in the electrical model may be the basement detachment surface corresponding to the depth of Xuefeng Mountain.The dynamic mechanism of the Xuefengshan tectonic belt is the stress transfer caused by collisions between continental blocks and intracontinental tectonic movements in different periods,and it is controlled by the long-range effects of mantle material upwelling,magmatism,and movement between peripheral plates.The early power may be derived from the secondary mantle movement induced by the secondary or tertiary global dynamic system.The tectonic movement since the Phanerozoic is mainly due to the unbalanced state and difference of the deep material between different blocks under the tectonic limits of the peripheral plate.