Research On Satellite Gravity And Magnetic Data Processing Methods And Their Application In South China Tectonics

Satellite gravity and magnetic data have the advantages of being widely covered,rapidly updated,and not geographically limited.The data are widely used to discuss deep geological problems at global and regional scales,such as internal material distribution,structure,and tectonics.The South China Block(SCB)is located at the convergence of the three tectonic domains of the Paleo-Asian,Tethys,and Pacific.This area has experienced multi-stage tectonic events(e.g.,polymerization,fission,collision,orogeny,extension)that have led to a complex geological evolution process with characterized mineralization.Several milestones have been achieved in both the tectonics and mineralization of the SCB.However,it is limited by the ambiguous deep structure of the SCB,creating controversy about the coupling between the SCB and the eastern Tibetan Plateau(ETP),the Southwest part boundary of the Yangtze Block(YB),and the Cathaysia Block(CB),and the metallogenic background of different mineralization belts in the eastern South China Block(ESCB).The Moho surface,the Curie surface,the effective elastic thickness(EET),the lithospheric density structure,and the lithospheric thermal structure calculated from satellite gravity and magnetic data are important parameters reflecting the deep structure.However,the method of calculating the above parameters has many weaknesses when applied to large-scale regionals.For example,the Parker-Oldenburg density interface inversion method is affected by the large variation of lateral density difference and the difficulty of selecting a reference depth,leading to low resolution and inaccuracy of the inversion Moho surface.The inaccuracy of the inversion Moho surface affects the calculation of the EET and the lithospheric density structure.The method of constructing the lithosphere thermal structure is limited by the lack of measured geothermal heat flow data and the extremely heterogeneous spatial distribution,which causes a low level of reliability.In this paper,we obtained typical interfaces,3-D density and thermal structure to indicate the deep structure of the SCB by improving the processing method of satellite gravity and magnetic data.Then,we discuss the SCB issues in detail.Firstly,we quantified the accuracy and reliability of satellite gravity and magnetic data.Bouguer gravity anomaly(BGA)were derived using the earth gravity field model EIGEN-6C4 data and the gravity anomaly correction method in spherical coordinates.Then the reductionto-the pole(RTP)magnetic anomaly was obtained using the lithosphere magnetic field model EMAG2＿V3 data and the point-by-point with variable geomagnetic parameters method.Both data were quality-assayed.The quality assessment results indicate that the EIGEN-6C4 BGA and the EMAG2＿V3 RTP magnetic anomaly can be used for deep structure studies in the SCB.Secondly,we refinement and improvement of processing methods for satellite gravity and magnetic data.These methods are used to process satellite gravity and magnetic data in SCB tectonics.To improve Parker-Oldenburg density interface inversion method resolution and reliability,we propose the density interface inversion method with lateral variable-density and variable-reference-depth,which employs the above method to calculate the depth of the Moho surface in the SCB.Then,we apply the joint method of admittance and coherence to calculate the EET in the SCB based on the Moho surface depth,topography,and gravity anomaly data.Because the difference in crust-mantle material composition and temperature factors affect the velocity-density conversion relationship.We used the depth of the Moho surface,which is combined with seismic wave velocity and geothermal flow data to build a density reference model.Subsequently,the density reference model used the constraints gravity 3-D inversion method to get the reliable and accurate lithosphere density structure in the SCB.We use the power spectrum analysis of the EMAG2＿V3 RTP magnetic anomaly to estimate the depth of the Curie surface in the SCB.The functional relationship between the depth of the Curie surface and the terrestrial heat flow data is used to obtain integrated terrestrial heat flow data and improve the spatial resolution of terrestrial heat flow data in the SCB.Then,the SCB’s reliable lithospheric thermal structure by the 1-D thermal steady-state conduction equation,integrated geodetic heat flow data,and the relationship between seismic S wave velocity and temperature.Finally,we calculated the depth of the Moho surface,the depth of the Curie surface,the EET,and the density structure and thermal structure of the lithospheric by the processing of satellite gravity and magnetic data in the SCB.We combine regional geological data and previous research results to discuss the SCB’s three dispute issues separately.The specific understanding is as follows:(1)The coupling relationship between the SCB and the ETP: The SGB and the ETP show opposite movement,the Western Yangtze Block(WYB)crowding into the SGB in the deep.In the shallow part of the crust,rigid blocks(high-density)of the upper crust of the SGB collide with the YB to form the Longmenshan Fault Zone.In the deep part of the crust,the WYB is affected by thermal erosion(The SGB high-temperature and low-density indicated the partial melting fluids and the upwelling of deep thermal material),resulting in the progressive dissipation of low-temperature stable craton properties.(2)We speculate that the deep boundary of the Southwest section of the CB and the YB,along the Chenzhou-Linwu-Hezhou-Wuzhou-Beihai line.It lays the foundation for understanding the way and process of assembling the CB and the YB.(3)The characteristic metal metallogenic "source" of the major metallogenic belts in the ESCB has been defined from the perspective of mineral systems.The different metal mineralization corresponds to different metal "source".It indicates that the difference in metal "source" might be a significant factor in the spatially disparate distribution of mineralized metals.In summary,this paper investigates the serial processing method of satellite gravity and magnetic data,and proposes a density interface inversion method with lateral-variable-density and variable-reference-depth to improve inversion resolution and reliability.Then,we integrated seismic and geothermal data with satellite gravity and magnetic data to derive a series of important results about the SCB’s deep structure,and propose three new insights into its tectonics and mineralization.These research findings can be used to optimize the processing application level of satellite gravity and magnetic data in the geology field,and to better comprehend the SCB deep structure as well as provide important guidance for further exploration of strategic minerals.