Crust And Upper Mantle Structure And Deformation Of Northern Vietnam And The Northern South China Sea

The South China Sea(SCS),located at the convergence of the Tethys-Himalayan tectonic domain and the western Pacific tectonic domain,has a complicated tectonic background and strong magmatic activity.Therefore,the formation mechanism and evolution history of the South China Sea have always been the frontiers of geoscience research.Previous studies confirm that the formation of the South China Sea cannot be explained by a single dynamic mechanism.The collision extrusion model,the back-arc spreading model,the paleo-South China Sea subduction model,and the mantle plume model all have some unexplainable problems.In addition,there are disputes over the India-Eurasian collision suture,the initial activity time and distribution of the Hainan mantle plume,and how the crustal weakening material beneath the southeastern Tibet Plateau extends further to the southeast.However,most studies focus on the South China Sea Basin,northern Vietnam and South China while lack of understanding of the tectonic connection between South China and Vietnam.Moreover,the large-scale regional models do not have enough resolution for northern Vietnam and the northern South China Sea,making it difficult to explain the fine structure of this region.As the intersection of two global tectonic systems,the fine 3D structure of the crust and upper mantle in northern Vietnam and the northern South China Sea is essential for understanding the magmatic activity and tectonic evolution in the region.In this study,we used the 2010 continuous waveform of 65 stations in northern Vietnam and southern South China to obtained the 3D S-wave velocity structure of the crust and uppermost mantle through ambient noise data analysis and direct surface wave tomography method.The checkerboard test results show that the lateral resolution of the model is about 100-150 km.Our model shows the velocity in the shallow crust presents obvious lateral inhomogeneity.For example,the Beibu Gulf Basin and the Yinggehai Basin show obvious low-velocity characteristics,while northern Vietnam,Qinzhou and Hainan Island show obvious high-velocity characteristics.And in the middle crust and lower crust,the two flanks of the Red River Fault have different velocity structure,the southern part shows a significant low-velocity anomaly,while the northern part shows a high-velocity anomaly.Therefore,we regard the Red River Fault zone is the collision suture between the Indochina block and the South China block.In addition,the velocity difference between the Yangtze block and Cathaysia block also provides a reference of the amalgamation suture.We suggest that the low velocity of the lower crust and uppermost mantle beneath the Yinggehai Basin might related to the upwelling of hot mantle material formed by the eastward subduction and retreat of the Indian plate.And the observed high-velocity upheaval feature beneath the Leiqiong area and Beibu Gulf Basin may represent the ancient magmatic underplating,which has a relation to the Hainan plume.We combined the ambient noise and earthquake surface wave dispersion data in 2010 to further obtain the 3D S-wave velocity structure and azimuthal anisotropy structure of the crust and upper mantle beneath northern Vietnam and the northern South China Sea through direct surface wave phase velocity dispersion travelltime inversion.The results show that the crustal velocity model is basically consistent with the tomography from ambient noise data.In the shallow crust,the anisotropy in northern Vietnam is stronger than that of southern South China,and the direction of fast waves is mainly parallel to the fault zone,and the anisotropy is mainly parallel to the strike of the fault zone,indicating that the anisotropy in the shallow depth is mainly formed by the joint plane or structure parallel to the fault strike and controlled by the movement of the fault zone.Deeper results show a large number of low-velocity anomalies in the upper mantle of northern Vietnam and South China continents,and the low-velocity anomalies in northern Vietnam extends across the Red River Fault to the southern South China block,which be related to the upwelling of mantle material caused by the eastward subduction and retreat of the India plate or the delamination of lithosphere beneath the Indochina block.The upwelling mantle material migrates laterally at the bottom of the lithosphere,thus forming the E-W anisotropy.And the eastward migrating material is blocked by the high-velocity anomalies and turned to form divergent anisotropy.For the high velocity anomalies in the upper mantle with strong azimuthal anisotropy,we suggest it may be the preserved fossil anisotropy.We indicate the high velocity anomalies of the crust and lithosphere in the Leiqiong area with weak azimuthal anisotropy conform a plume-strengthened lithosphere relate to the Hainan plume,and a paleo-mantle plume head might beneath the Leiqiong area.Over all,the structure and deformation of the shallow part of northern Vietnam and the northern South China Sea are mainly controlled by the strike-slip movement of the fault zone,and the deformation of the upper mantle is the result of slab subduction,lithospheric delamination and mantle plume activities.For the formation mechanism of the South China Sea,our results show the magma activities of the Hainan plume,the extrusion of the Indochina block and the subduction and retreat of the Paleo-Pacific plate all provide important driving forces for the breakup of the South China continental margin and the formation of the South China Sea.We obtained the S-wave velocity models and density models of northern Vietnam and the northern South China Sea using the joint inversion method of gravity data and surface wave data,and compared them with the inversion results from separate data.The joint inversion velocity model is basically consistent with the velocity model obtained by the single surface wave inversion.The joint inversion model mainly adjusts the velocity in the area with relatively poor path coverage,and the velocity perturbation is within 3%.The density model of the joint inversion has similar feature with single surface data inversion both of surface wave data and gravity data,but the absolute value is different.The joint inversion model adjusts the local extrema of the single data inversion model,and the results are relatively stable and reliable.In addition,the gravity anomalies obtained by the forward inversion of the joint inversion velocity model are basically the same as the original gravity data,the difference of the two data is within 10mGal.Therefore,the comparison results of different models show that the velocitydensity models we obtained are self-consistent,and can well constrain the variations of the velocity and density of the crust and upper mantle.Our researches use different data to study the velocity structure of the crust and upper mantle in northern Vietnam and the northern South China Sea.The models well reveal the magmatic activity and tectonic deformation in this region,and provide a referred model for the study of studying the tectonic evolution history of the second perimeter of the South China Sea.