Comprehensive Study On Gravity-seismic Geological Profile And Teleseismic Tomography In The South China Sea

The tectonic position of the South China Sea is located at the junction of the Eurasian plate,the Australian plate and the Pacific plate,and its formation and evolution are affected by these three plates.The Cenozoic evolution model and dynamic mechanism of the South China Sea is one of the important issues in the study of tectonic evolution in the South China Sea.Exploring its evolution process has important scientific significance for the study of submarine spreading dynamics;Abundant oil and gas mineral resources have been formed during the expansion of the South China Sea basin.Studying the Cenozoic tectonic evolution of the South China Sea and analyzing its impact on oil and gas resources has important practical significance for the exploration of oil and gas mineral resources in the South China Sea.At present,researchers at home and abroad have not formed a unified understanding of the Cenozoic evolution model of the South China Sea.The complex tectonic evolution background of the South China Sea has bred a series of complex deep structures.The study of these deep structures can help us to explore the Cenozoic evolution process of the South China Sea basin.Gravity data is widely used in deep structure research because it contains structural information from the surface to the deep part of the earth.Artificial seismic data is widely used in structural research because of its high detection accuracy and high vertical resolution.However,the single use of gravity data to study deep structure has the disadvantage of poor vertical resolution,and the single use of seismic data has the problem of unclear deep reflection caused by insufficient energy.Using gravity data to constrain seismic data for studying deep crustal structure can take into account the advantages of the two methods and make up for each other’s defects.The Cenozoic evolution process of the South China Sea was influenced by the India Australia plate,the Eurasian plate and the Pacific plate.In order to explore the influence of the major plates around the South China Sea on its Cenozoic evolution,and to explore the dynamics and genetic mechanism of its opening,this process should be considered on a deeper and larger scale.It is far from enough to rely on the information provided by gravity data and artificial seismic data.From this point of view,the natural seismic travel time data include the influence of the earth’s deep structure on the seismic wave travel time,and the teleseismic travel time imaging method can meet the above research needs.Based on the above problems,this paper uses comprehensive geophysical data including gravity data,artificial seismic data and natural seismic travel time data to study the deep structure of the South China Sea and its surrounding areas,and then to explore the Cenozoic evolution process of the South China Sea.In this paper,the Cenozoic evolution model and its dynamic mechanism of the South China Sea and the paleo South China Sea are studied.The main research contents are as follows: the regional Moho depth is retrieved by using the latest high-precision satellite gravity anomaly data in the South China Sea and its surrounding areas;Using the forward fitting method of gravity anomaly constrained by seismic profile,the highprecision gravity anomaly data of the South China Sea basin are forward fitted,and the vertical crustal structure of the South China Sea basin is obtained;The high-precision natural seismic travel time data around the South China Sea are collected,filtered and preprocessed.The natural seismic travel time imaging inversion method is used to inverse the above travel time data,and the three-dimensional velocity structure of the upper mantle in the South China Sea and its surrounding areas is obtained;The highprecision three-dimensional velocity structure of the upper mantle in the subduction area of the eastern margin of the South China Sea was obtained by natural seismic tomography;Based on the above work,the Cenozoic evolution model and dynamic mechanism of the South China Sea are explored.The specific research work and achievements of this paper are as follows:(1)Study on the characteristics of Moho surface fluctuation in the South China Sea Based on satellite gravity data.Firstly,the latest high-precision free air satellite gravity anomaly data,topographic data and sediment thickness data in the South China Sea are collected,and the free air gravity anomaly is corrected by mesosphere correction and topographic correction;Using the gravity interface forward modeling method,the gravity responses of the sedimentary and seawater layers in the South China Sea basin are calculated,and the Bouguer gravity anomalies in the South China Sea and its surrounding areas are obtained by eliminating the free air gravity anomalies.The characteristics of Bouguer gravity anomaly show that the amplitude of Bouguer gravity anomaly in the South China Sea is large in the basin area and small in the land area around the basin;There is a Bouguer gravity anomaly gradient zone from Yinggehai basin to Xisha Trough basin in the northern South China Sea.Using the Parker Oldenburg gravity interface inversion method and the Bouguer gravity anomaly data of the South China Sea,the Moho depth characteristics of the South China Sea and its surrounding areas are obtained.The depth fluctuation characteristics of Moho Surface reveal that the shape of Moho surface in the South China Sea basin is similar to the seabed topography,and its depth fluctuation shows the characteristics of "shallow in the middle,deep around;shallow in the ocean,deep in the continent".The gradient zone found in Bouguer gravity anomaly shows in Moho depth that there is an obvious Moho uplift zone in Yinggehai Basin and Xisha Trough basin.This paper infers that the Moho uplift zone is caused by mantle upwelling,which is closely related to the Cenozoic evolution of the South China Sea.(2)Study on crustal structure of the South China Sea Basin Based on highprecision gravity seismic section data.Gravity anomaly data and seismic profiles are from CNOOC.They entrusted the Institute of Oceanography,Chinese Academy of Sciences to collect four high-precision gravity seismic geoscience sections in the basin area;The density data mainly come from the previous research data,including the rock density in the land area around the South China Sea,the density converted from the formation velocity in the basin area,and the rock density obtained from our field survey in the south of the South China Sea.The gravity anomaly forward fitting method is used to study the crustal structure of the South China Sea basin.The specific process is as follows: first,the Moho obtained from seismic profile and regional gravity anomaly inversion is used to rigidly constrain the shallow stratum and Moho surface,and the density of each stratum is adjusted with reference to the collected density parameters.When the fitting curve is consistent with the measured gravity anomaly curve,the crustal structure corresponding to each geological section is obtained.The results of geoscience transect study show that the South China Sea basin is composed of seawater layer,Cenozoic sedimentary layer(post-spreading sediment,spreading sediment and pre-spreading sediment),crust(upper crust and lower crust)and mantle from top to bottom;The variation law of density value of each layer is: from shallow to deep,gradually increasing.It is worth noting that there are a large number of abnormally high-density bodies above the Moho surface in the South China Sea basin,which may be formed by the melting and reconsolidation of the crust by the upper mantle during the Cenozoic expansion of the South China Sea;The thickness of the sedimentary layer in the spreading period of the South China Sea gradually thinned from northeast to southwest,and its density also showed that the Northeast was greater than the southwest.The above phenomena reveal that the Cenozoic seafloor spreading of the South China Sea was carried out from northeast to southwest,resulting in earlier sedimentation,greater sediment thickness and greater sediment density in the northeast of the South China Sea basin.(3)Study on the three-dimensional velocity structure of the upper mantle in the South China Sea and its surrounding areas based on high-precision natural seismic travel time data.Firstly,the principle and data processing flow of fast marching teleseismic tomography are studied;Then the high-quality natural earthquake travel time data of the South China Sea and its surrounding areas from 2006 to 2020 are collected;After data screening,first break picking and crustal correction,the data are converted to a format that meets the inversion requirements;The checkerboard test method is used to verify the resolution and stability of the observation system composed of source and station;Finally,the three-dimensional velocity structure of the upper mantle in the South China Sea and its surrounding areas is obtained by threedimensional velocity inversion.The inversion results show that there is a good coupling between the deep velocity structure and the surface tectonic units.The high-speed anomaly is mainly distributed in the island arc and land area,while the low-speed anomaly is mainly distributed in the marine area.The important discovery is that there is a NW trending low-speed anomaly from the Qinghai Tibet Plateau to the north of the South China Sea.The depth of the low-speed anomaly is about 200-500 km,which may represent the lateral mantle flow caused by the India Eurasia plate collision.We infer that the mantle flow is the main driving force of the Cenozoic seafloor spreading in the South China Sea.The mantle flow flows along the Red River fault and is divided into two branches before entering the South China Sea.One branch of the mantle flow enters the Yinggehai Basin and Xisha Trough along the Red River fault and extends to the northwest sub basin of the South China Sea.The branch of the mantle flow causes the Moho uplift of the Xisha Trough and the expansion of the northwest sub basin;The other branch of mantle flow flows eastward,which is blocked by the subduction of the ancient Pacific plate and then flows southward.Together with the previous branch of mantle flow,it controls the expansion of the eastern and southwestern sub basins of the South China Sea.A retained high-speed anomaly body was found in the mantle transition zone below Borneo and the Philippines.Combined with the exposed ophiolite and ophiolite belt,it is inferred that the high-speed anomaly body is a relic plate of the ancient South China Sea.The ancient South China Sea subducted and disappeared under Borneo,and the closure direction is from West to East.A high-speed anomaly body has also been found in Indochina Peninsula,which is inferred to be the remains of paleo Tethys.Based on the above results,this paper retraces the suture of the ancient South China Sea and the ancient Tethys.(4)Study on deep structure of subduction zone in the eastern margin of the South China Sea Based on high-precision natural seismic travel time data.Firstly,the natural seismic travel time data from 1960 to 2020 in the subduction area of the eastern margin of the South China Sea were collected,and the original data were screened,the first break was picked up and the crust was corrected.Then,the high-precision threedimensional velocity structure of the upper mantle in this area was retrieved by teleseismic travel time tomography.The inversion results show that the South China Sea plate subducts eastward to the lower part of the Philippine island arc,and the angle of the subducted plate gradually decreases from north to south,revealing that the plate tearing occurred during the subduction of the South China Sea plate eastward,and the tearing position corresponds to the latest expansion ridge of the South China Sea basin;The subduction depth of the South China Sea plate also shows the characteristics of from north to south from deep to shallow,indicating that the expansion and subduction of the South China Sea plate are earlier in the north than in the south.The above findings reveal that the eastward expansion of the South China Sea plate is blocked by the Philippine Sea plate.(5)Study on Cenozoic evolution model of the South China Sea Based on comprehensive geophysical data.By comprehensively analyzing the results of the Moho inversion interpretation in the South China Sea,the geological geophysical comprehensive interpretation results of the geoscience section,the three-dimensional velocity structure of the upper mantle in the South China Sea and its surrounding areas,and the high-precision three-dimensional velocity structure of the upper mantle in the subduction area on the eastern margin of the South China Sea,the Cenozoic evolution model of the South China Sea basin is explored,and the relevant geological understanding is put forward:(1)The lateral mantle flow caused by the India-Eurasian plate collision is the main driving force for the Cenozoic seafloor expansion of the South China Sea basin;(2)Before the lateral mantle flow enters the South China Sea,it is divided into two branches: eastward and southeastward.The convergence of the two branches of mantle flow in the South China Sea affects the formation of the South China Sea.The transition of the spreading ridge during the evolution of the South China Sea basin may be caused by the convergence of the branches of mantle flow;(3)The ancient South China Sea closed in a scissor like manner from west to East,subducted and disappeared under Borneo,and its suture line extended eastward from the southern tip of Indochina Peninsula through Borneo and Palawan Island to the Philippine island arc.The drag to the southeast during the subduction and extinction of the ancient South China Sea may be the reason for the clockwise change of the expansion ridge of the Eastern Sub basin of the South China Sea;(4)The subduction and disappearance of the ancient South China Sea and the southeast extrusion of Indo China Peninsula caused by the collision between the Indian plate and Eurasian plate may be the main reason for the inconsistent direction of the expansion ridges of the eastern and southwestern sub basins of the South China Sea.The distribution of oil and gas resources in the South China Sea is affected by the Cenozoic tectonic evolution.The distribution of oil and gas resources generally shows the characteristics of "outside oil and inside gas,north oil and South gas".The oil fields are mainly distributed in the continental margin area with deep Moho surface,and the heat flow value is relatively low.The gas fields are mainly distributed in the deep-water area with shallow Moho surface,and the heat flow value is high.