| The Jiangnan orogenic belt,Located between the Yangtze and Cathaysia Block,is a key window to reveal the evolution and dynamics of the South China.In recent years,a great deal of deep exploration work has been carried out on scientific issues such as intracontinental mineralization,large igneous provinces and important tectonic units in South China.In 2019,magnetotelluric has completed the 0.5°×0.5° array network of East South China and the key long profile observation of the eastern and central Jiangnan orogenic belt.However,the magnetotelluric profiling study of the key structures in the western section of the Jiangnan orogenic belt has not yet begun,and its boundary and southwestern extension are still controversial.This paper presents an inversion study of two geomagnetic profiles in the 107°-113°E and 22°-30°N regions,with the aim of improving the reliability of the two-dimensional(2D)inversion electrical structure and providing a more accurate deep structural interpretation of the western section of the Jiangnan orogenic belt.In this paper,a new 2D magnetotelluric inversion scheme is proposed:(1)According to the existing geological and geophysical knowledge of the western section of the Jiangnan orogenic belt,a macroscopic prior electrical model is constructed.(2)The theoretical response of the prior model is calculated through 2D and three-dimensional(3D)forward modeling.(3)Comparison of the induction vectors of the theoretical response and the measured data to determine the best qualitative mode of interpretation of the transverse boundary.Extraction of electrical principal axis angles with phase tensor information from theoretical 3D response.Test and determine the tensor impedance rotation that best matches the theoretical 2D response.(4)Through the 2D inversion of the rotated three-dimensional response with different regularization operators,regularization factors and data combinations,the optimal 2D inversion parameters of the prior model are obtained.Through this scheme,the best 2D inversion strategy of measured data is obtained and applied to the inversion of measured data.Finally,reliable electrical structure is obtained.The electrical structure shows that the Yangtze block and Cathaysian block as a whole are relatively low to medium resistance in the study area,while the Jiangnan orogenic belt is a high resistance block.There are large-scale low-resistivity anomalies below Moho surface in Yangtze block.Based on the comprehensive geological data and other geophysical research results,this paper obtains the following understanding:(1)The low resistivity anomaly in the deep part of the Yangtze block is the trace of the southeastward subduction of the lower crust of the Yangtze,and its formation is spatially and temporally related to the northwest-trending thrust nappe of the Jiangnan orogenic belt and the east-west extrusion of materials from the eastern margin of the Qinghai-Tibet Plateau.(2)The Precambrian basement in the southwest extension direction of the western section of the Jiangnan orogenic belt gradually changes from deep to shallow,which is presumably caused by the difference of collision and compression strength between the central and western sections of the Jiangnan orogenic belt. |
