Characteristics And Genesis Of Deep Crustal Structure In The Southern South China Sea

The South China Sea which is located at the junction of the Eurasian plate,the Pacific plate and the Australian plate is the largest and most complex marginal sea basin in the Western Pacific Ocean.The formation and evolution mechanism of the South China Sea has been concerned by geologists.Different models have been proposed for the dynamic mechanism of the South China Sea.The research area is located in the southern part of the South China Sea between the South China Sea Basin,Borneo and Palawan,and featured a thinned continental crustal structure.The latest oceanic drilling results show that there is no mantle exhumation during the rapid lithospheric extension,defining an endmember of passive continental margin between volcanic and non-volcanic types.The existence of high-velocity layers in the lower crust has been confirmed in the northern South China Sea.However,the existence of high-velocity layers in the southern South China Sea is controversial.The volcanic activities are mainly concentrated after the cessation of the spreading of the South China Sea.What's more,there is a sudden change in crustal thickness at the fault.The relationship between deep structure and shallow structure in the South China Sea Basin is also unclear.In order to solve the problems,the joint inversion of gravity and seismic profile and simulation of two-dimensional lithosphere are carried to analyze the characteristics of the crustal and lithospheric structures,especially the genesis of the lower crustal high-speed layer,the thermal structure,velocity and density structure.Based on typical seismic profiles,the stratigraphic sequence,fault activity and igneous rock age will be combined with the numerical simulation results to discuss origin of deep structure in the south of the South China Sea.1.The crustal structure model of the south of South China Sea is established by joint inversion of gravity and seismic profiles.The anomalous reflection characteristics of Moho are identified in the seismic profiles.The work carried out by constraining high resolution seismic data and free-air gravity data in the study area.The depth of Moho gradually shallows from the southern continental margin to the central basin.The depth of Moho in the basin changes greatly with a range from 10 km to 24 km.The depth of Moho in the west is shallower than the east,which may be related to the closure from east to west of the proto-South China Sea and the opening of the South China Sea from the east to west.In the west of Nanweixi Basin and Beikang Basin,Nansha Trough Basin and the West and south of Liyue Basin,the high-velocity layers are identified in the lower crust with a thickness of 0.5-8 km.2.The simulation of lithospheric structure reveals the deep structural characteristics of the southern South China Sea.Based on the data of topography,bouguer gravity anomaly,heat flow in the research area and the petrological parameters,a comprehensive two-dimensional geophysical-petrological lithospheric structure model is established.The depth of the lithosphere-asthenosphere boundary(LAB)in the southwestern sub-basin is nearly constant in 72 km(1330?).The LAB is about 75 km in Nanweixi Basin,and deepening to about 80 km in the southwestern part of Liyue Basin.This result is in good agreement with the analysis results of Rayleigh wave.Compared with the surrounding areas,the Nansha block is a low temperature block,and the Moho surface temperature is equal to or shallower than the CPD temperature.The abrupt change of P-wave velocity exists on the LAB,with the increase of 0.1-0.15km/s.The S-wave low-velocity zone exists in the 70-150 km range of Nansha block,and the columnar channel is not recognized.Therefore,the Hainan mantle plume is not supported in the South China Sea.3.The high-velocity layers can be divided into two types according to their genesis.The extensional factors in Beikang Basin,Nanweixi Basin and Liyue Basin are about 1.5-4.It is not enough to provide paths for sea water to enter the mantle and react with peridotite to produce serpentinization.The volcanic age is after the Mid-Miocene,so the high-velocity layers here are considered as the Cenozoic underplating magma,mainly after the expansion of the South China Sea.In Nansha Trough Basin,the maximum factor of crustal extension can reach 11.2.The serpentine is formed by the reaction of peridotite and sea water which enters the mantle through the fractures.The highest temperature at the high-speed layer is 420 C,which has the temperature condition for preserving serpentine.After the eruption of the volcano in the Mid-Miocene,the high-velocity layer here is a mixture of serpentinized peridotite and late magma after the cessation of South China Sea.4.The relationship between deep structure and shallow tectonics in the South China Sea and the genesis are discussed.The lithospheric thickness in the southern part of the South China Sea is gradually increasing from the oceanic basin to the continental margin.The upwelling of asthenosphere promotes the expansion of the South China Sea,accelerating the subduction and extinction of the Proto-South China Sea and the expansion of the South China Sea.Because of the upwelling of asthenosphere,the mantle undergoes decompression and melting to form MORB basalt and oceanic crust.At 23 Ma,ridge transitions have resulted in the formation of magma residues between old and new spreading ridges and the storage of partially melting magma.After the cessation of the South China Sea,the remnant magma erupted through the fractures to form volcanoes.The remnant magma at Moho formed the high-velocity layer of lower crust.At present,Nansha block is a low-temperature block,which cooled rapidly in the late expansion of the South China Sea.During the thermal subsidence stage,the crustal lithosphere contracted and Moho surface decreased to reach the equilibrium condition,and produced the small faults in the continental and ocean transition.