| The South China Sea is located in the Western Pacific Sea and Mediterranean-Himalaya tectonic junction, is the key region of the Tethys tectonic domain and the Pacific tectonic domain interaction. It is not only important for palte tectonics, but also for the correlation between adjacent Ryukyu arc trench system-Taiwan-Philippines and the East China Sea and South China Sea geological to Study on the cause of formation.The East Asia continental margin subduction zone is the interaction of the Eurasian plate and Philippines plate. The plate tectonic boundary from the south to the north respectively is the Manila subduction zone and the Ryukyu subduction zone. Plate subduction, collision and hyperplasia of lithospheric dynamics control the deformation evolution and tectonic subduction zone. The relationship between the western edge of Philippines tectonic plate and Eurasian plate is very complex. The northwest border is Philippines plate to the beneath the Eurasian plate subduction. The structure characteristics of the western edge of Philippines plate and its internal structural units, not only helps to reveal the effects of plate subduction of back arc regions, to analysis of marginal basins in the rifting and extension process and geodynamic mechanism, but also it is the important window of tectonic evolution background of the South China sea. The Philippines Sea and the South China Seaevolution pattern discussion may exist for the lithospheric magnetic anomaly pattern plate evolution patterns formed. The overall analysis of magnetic characteristics and different scale magnetic characreristics of lithosphere will provide some evidence for the study of plate tectonics and the evolution history.In this paper, the study object is the whole combination of the South China Sea and the Philippines Sea, and the magnetic anomaly model NGDC-720was used to analyze their lithosphere magnetic features. The study results could offer some of the evidence of the correlation between them and as well as their evolution process. It can provide some ideas and evidence to put them together as the study areato study the causes of abnormal magnetic from the larger scale using the magnetic anomaly characteristics, in a sense, it has certain practical value and theoretical significance of this study.The main work finished in this paper is showing as below:The geology and geophysics background of the study area were collected and integrated, and the heat flow data, seafloor crust ages and the earthquake sources locations data were processed, which supplied some analysis for the regional tectonic background.The three components of the magnetic anomaly characteristics were given with help of the NGDC-720magnetic model, introduce the calculation method of modulus and the modulus calculation method of vertical first derivative by the three component of magnetic field data, and discuss the characteristics of the modulus vertical first derivative from the three aspects of the relationship between modulus vertical first derivative and the direction of magnetization, the central character of derivative and the interference of multiple anomalies. Trough model experiments and theoretical calculations, it proved that the modulus vertical first-order derivative ratio modulus better relationship. And the magnetic modulus and the derivative of the modulus were calculated and then analyzed the results in multi-scale to discus the possible cause reasons of the magnetic anomalies in the study area.The total magnetic anomaly△T was obtained by calculation of the three components parameters of the magnetic anomaly and the distribution characteristics were then analyzed. The basic theory of the EMD method was introduced basically, which was then used to total magnetic anomaly in the study area, the magnetic anomaly and EMD decomposition of the shallow magnetic anomaly and deep source radial power magnetic anomalies spectrum and power spectrum are analyzed, then the field source depth estimation way was given.. Results of the mid or long wavelength and short wavelength magnetic anomalies which related to the lithosphere deep and shallow information were taken for the deep discussion, and some awareness were obtained.The magnetic susceptibility inversion theory was introduced in details, This paper adopts a magnetization similar to seismic tomography rate inversion method, it will become a grid unit size and location known underground media division, through the relationship equation established underground physical parameters and magnetic anomaly data, physical parameters using optimization inversion method of underground of each grid cell distribution, completed the forward model. At the same time as the magnetic susceptibility imaging division model unit number is far larger than the observed data points, and is therefore an underdetermined problem; according to the inversion evaluation theory, we construct the model constraint conditions, the change in the Cartesian coordinates along3axes is small, the model is smooth, which constructs the inversion objective function. In the study area, two typical profiles (N20°E110°-N20°E110°, N15°E120°-N15°E135°) were selected and the magnetic susceptibility inversion was taken for them. The inversion results were then introduced and analyzed, and the initial geological reasons were given.Based on the regional magnetic anomalies distribution information and the anomalies information from the two profiles and combining the other material, the author tries to discuss the correlation between the lithosphere of the Philippine Sea and the South China Sea.Based on these work, some initial conclusions are drawn.According to the three components of the NGDC720magnetic model, the magnetic anomaly belts features of the Philippine Sea basin are with regular distribution, which shows a NW and NNW direction trends and with large amplitudes. But for the South China Sea, the regularities is not so obviously, which shows a rough distribution and with NW, NE or EW trends. And it is also with heavily faults and with smaller magnetic anomaly amplitudes than that of the Philippine Sea. From the vertical component of the magnetic anomaly, the Philippines marine magnetic anomalies of vertical component of overall appears strip, strip to roughly the same, the direction is NWW; vertical component anomaly is from-550to350nT, and magnetic anomaly in the West Philippines basin to nine-Palau ridge of positive and negative abnormal strip phase distribution is very clear, this provides some evidence for the Philippines sea back arc spreading evolution. And between the Kyushu Palau ridge and Maliyana ridge and south of this Parisi Vera basin in negative anomaly, intermediate with a north-south direction is abnormal bands, corresponding to the the rift valley basin. And in the South China Sea, the vertical component of magnetic anomalies are also present certain band characteristics, but with Philippines sea slightly messy, strip to Philippines sea, it runs roughly northeast. Vertical component anomaly is from-550to350nT, from land to sea, from the sea to the basin edge gradually increasing trend, especially in the basin central abnormal value maximum, and the character variation, the distribution is more complex, which has a close relationship with the different structural units. In addition, there are the South North negative associated anomalies. The overall characteristics of magnetic anomaly reflects the tectonic and magmaticactivities in the South China Sea more violent and irregularity. From the distribution of the overall form of abnormality mainly divided into two directions, namely NE and NW two directions, and the fault strike and the South China Sea is very consistent, the NE and NW.According to the total magnetic anomaly and the extracted multi-scales information, the deep and shallow lithosphere magnetic distribution trends of the Philippine Sea and the South China Sea area are not same. In the area of the South China Sea, there are stronger NE and EW direction magnetic anomaly belts than that in the total magnetic anomaly results, which reflects that there are upper crust formation in NE and EW direction after the seafloor spreading in the later period of the South China Sea formation. However, the deep magnetic anomaly distribution trends are mainly in NW or NNW directions and the faults breaking is not apparent. In the area of the Philippine Sea, there is similar situation, except for the agreements of the shallow magnetic anomaly belts distribution trends of the Philippine Sea basin, there are obvious difference between the lithosphere deep and shallow magnetic structures in the Parece Vela basin and the Shikoku basin, which are with apparent SN trends in shallow and NW or NNW trends similar with the Philippine Sea basin in deep. All the results show that the South China Sea, the Parece Vela basin and the Shikoku basin have the similar evolution process with that of the Philippine Sea basin. The magnetic susceptibility inversion results of the two profiles reveal the effecting relation between the Manila trench and the Philippine Sea trench. We think that after the collision of the Philippine Sea with the Eurasia continent, the Philippine Sea unwrap in the north part of the Manila trench near the Luzon troughs and is with heavily deformation, then it forms overlapped tectonic structure. In the most west part, the north side of the Manila trench is formed, which obstruct and slow down the west direction effects of the Philippine Sea plate. And in the other hand, the South China Sea here subducts downward along with the Manila trench with steep dip angles. In the Philippine Sea trench, the Philippine Sea Plate subducts downward and forms the subduction belts of the southwest Philippine Sea plate. For the South China Sea, because of the shielding effect from the Philippine islands, the influence to the South China Sea is much more decreased here. |
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