The Electrical Resistivity Structure Of The Wuyi Orogenic Belt In South China And Its Tectonic Implications

This thesis mainly used four magnetotelluric profiles' data to do deep electrical structure research and conducted a detailed study of the Wuyi orogenic belt and its adjacent areas.And the electrical structure provides some new comprehension of the crust-mantle structures and geodynamic processes of this region.The impedance tensor data of all measuring points are obtained by fine pretreatment.Consequently,we have got 163 high-quality broadband MT and 15 long-period MT sounding data.Concerning MT data processing.Meanwhile,we have analyzed the dimensionality of the survey area through Swift decomposition,Bahr decomposition,and impedance phase tensor decomposition.We believe that the investigation area has an overall three-dimensionality.In addition,we have used the GB decomposition method to decompose and analyze the data of all magnetotelluric measurement points with multiple frequencies and multiple measurements.We found that the concentration of the electric spindle of each survey line is not homogenous,and it is difficult to find a unified electrical spindle angle.The geological structure in South China is complicated and diverse.Because of the complexity of the geological structure in the research area,usual one-dimensional and two-dimensional inversion methods cannot meet the needs of complex geological structure interpretation.To obtain more reliable geoelectrical distribution models,we also deploy the Niblett-Bostick method to calculate the penetration depth of each frequency data at each MT sounding data.Finally,we chose the inversion depth as 100 km.Since the length of the survey line in the study area exceeds 500 km and the scale of the inversion profile is relatively massive,it is difficult to divide the boundary of the anomalous resistivity.In this paper,we uses the total horizontal directional derivative method to define the boundary of the anomaly and establishes a model for forward and inversion research.Additionally,we have analyzed the reliability of the total horizontal directional derivative method,verifying it with measured data exerting drilling constraints.It proved the effectiveness of the total horizontal derivative in identifying the anomalous boundary of the electrical profile.It is also beneficial for the judgement in the anormality of acquired data and the selection of the inversion model.After preprocessing and analyzing the magnetotelluric data,this paper implemented the three-dimensional inversion of the measured data.The 3D inversion results,compared with the previous research data,have a robust 3D resistivity structure model.Based on these electrical structure models and the existing geological and geophysical data in the survey area,we have the following results of the electrical structure and dynamic process in this area:(1)The underground electrical structure of the Wuyi orogenic belt can be roughly divided into two electrical layers from shallow to deep: the first layer is a high-resistance layer with a resistivity ranging from several hundred to several thousand ohm meters;the second layer is a relatively low-resistance layer,whose resistivity ranges from ten to hundreds of ohm meters.There are apparent differences in the electrical structure between the Wuyi block and its east and west adjacent areas.The lithosphere in the research area is not a simple "thick in the west and thin in the east" layered structure.This study also found that the shallow high-resistance layer has an uneven distribution of high resistance electrical conductors.(2)The four magnetotelluric profiles across the Wuyi orogenic belt.The inversion results show that the geoelectric structure under the Wuyi orogenic belt and its neighboring areas is not only stratified from up to down,but also varies significantly from east to west.Among them,there is a low-resistance anomaly along the Jiangshao fault penetrating the crust into the deep mantle at the western boundary of the Wuyi block.The section from west to east called the Ganjiang fault zone,the electrical structure difference on both sides is very obvious.The overall fault presents a high dip angle,and the angle becomes larger when the deep part extends to the low-resistance layer.The electrical structure of the eastern part of the profile shows that the lithosphere on both sides of the Zhenghe-Dapu fault has apparent differences.The resistivity of the Wuyi block in the study area is lower than the southeast coastal block.(3)Discontinuous low-resistance-low-velocity layers were found within 10-30 km of the crust in the Wuyi orogenic belt and adjacent areas in the study area.The low-resistance anomaly may be related to the fluid generated by the metamorphism of the lower crustal rocks.After the fluids in the study process rise to the middle crust,they are enriched under the Cathaysia crystalline base.(4)The research results show that the Bouguer gravity anomaly in the Wuyi tectonic corridor and adjacent areas is low,and the terrain is higher in elevation.Combining the characteristics of the low resistance of the mantle of the Wuyi block and the distribution of high-conductivity layers in the crust,it is inferred that the formation and evolution of the Wuyi block may have experienced the delamination of the lithosphere,and the asthenosphere upwelled deep in the study area.The existing geological structure of the Wuyi block is not the original structure left by the Early Paleozoic orogenic movement,but is the result of later transformation.,The high-altitude topography of the study area is related to the upwelling of the asthenosphere.(5)The paleo-Pacific subduction in the Mesozoic Era caused a large amount of water to enter the South China block.The addition of a large amount of water to the bottom of the lithosphere caused the mantle convection to accelerate,and at the same time,reduced the viscosity of the lithosphere,resulting in a decrease in the stability of the bottom of the lithosphere.The base of the lithosphere became unstable under the action of gravity,and it disintegrated and settled.The sinking of the upper mantle also created space for the upwelling of the asthenosphere.The uplift of the Wuyi Mountain surface in the Mesozoic Era may be related to the continuous heating of the lithosphere by the upwelling of the asthenosphere.