Cenozoic Stratigraphic Structure And Crustal Deformation Of The Southern South China Sea

The South China Sea(SCS)is located at the junction of the Eurasian,Pacific and India-Australia plates.It is a unique marginal sea on the continental margin of the East Asian,where plate movements were active.Since the late Mesozoic,the SCS has experienced tectonic processes such as continental lithosphere extension,continental crust breaking up and seafloor spreading,and the southern SCS collided with Borneo in the Cenozoic.The complexity of geological processes makes the southern SCS became a key region to reveal the complex rifting-drift processes of the conjugate margins of the SCS.This article has interpreted a newly acquired multi-channel seismic profile with a total length of over 700km across the southern SCS.Six Cenozoic unconformities and four sedimentary sequences were identified according to the seismic reflection features of the sedimentary strata and the contact relationship between the strata.Meanwhile,the ages and corresponding tectonic events of the unconformities were determined based on drilling and previous research in this region,and the stratigraphic framework was constructed.Based on seismic profile interpretation,the structural domains of the SCS were divided according to the differences of tectonic units,crustal structures and the features of stratigraphic and faults.Based on the shallow constraints provided by the seismic profile,the density structure of the deep crust of the DZ02 profile is inverted by gravity data.Based on the calculation of normal fault offsets,the buried depth of sedimentary basement in the Nansha Block section along the profile and the depth data of the Moho,the extension factors of the upper and whole crust were calculated.Then,the extensional features and thinning mechanism of crust in the southern SCS were analyzed.This study is helpful to boost the understanding of the extension-breaking up processes and dynamic mechanism of the marginal sea crust and provide a possible basic theory and understanding for the exploration of oil and gas resources in the SCS.Through the research of this article,the following points are obtained.(1)Six significant unconformities were identified in the Cenozoic stratigraphic system of the southern continental margin of the SCS,namely Tg,T70,T60,T40,T20 and T10.These unconformities represent the important tectonic events that change the sedimentary environment in the processes of crustal extension in the SCS.Tg and T40 are two distinctive unconformities that are widely distributed throughout the South China Sea,and they represent the beginning of continental marginal extension and the end of seafloor spreading in the SCS,respectively.According to the identified unconformities and combined with the reflection characteristics of the sedimentary layer and the evolution stages of the SCS,the Cenozoic sediments on the southern continental margin of the SCS were divided into four stratigraphic units,namely,the syn-rift unit(Tg-T70),the syn-drift unit 1(T70-T60),the syn-drift unit 2(T60-T40)and the post-rift unit(T40-seabed).(2)Rely on the feature of the tectonic units,strata structures and faults pattern,the southern SCS is divided into five structural domains from northwest to southeast,namely,the Northern Margin Extension Domain,the Southwest Subbasin(SWSB)Oceanic Domain,the Hyper-stretched Domain,the Forebulge Domain,and Nansha Trough.Based on the deformation features of each structural domain,combined with the geological and deep data around the southern SCS and the crustal extension features of the Nansha Block,the structural response of regional tectonic events in the evolution processes of each structural domain is clarified.(3)Gravity data was used to perform a joint inversion of the crustal density structure along the DZ02 profile.The results show that the density of crust gradually decreases from land to sea,the density of the upper and lower crust is more uniform,and the density of the middle crust varies laterally.The major detachment faults did not penetrate the lower crust,but disappeared between the middle and lower crust,indicating that the lower crust is a ductile detachment layer.(4)Based on the faults identified in the seismic profile,the crustal extension factors of the Nansha Block were calculated by summating the normal fault offsets.The results show that the Nansha Block mainly experienced two extension episodes.The first extension episode was concentrated in the syn-rift period during the Palaeocene-Early Oligocene(Tg-T70),accounting for about 65%of the total extension lengths.The second extension episode was appeared in the syn-drift period during the Early Oligocene-Middle Miocene(T70-T40),that is,during the seafloor spreading of the SCS,and the extension accounted for about 27%of the total crustal extension lengths.(5)We compared the extension factors of the upper crust and the whole crust,and found that there was a difference between the extension factors of the upper crust and whole crust,which was the result of the depth-dependent extension of crust.We proposed a crustal extension model combining the distal depth-dependent extension and the distal uniform extension in the southern SCS.The extent of the depth-dependent extension zone may depend on the magnitude of the lateral gradient force and the degree of weakness of the ductile crust.(6)Based on the comparison between the DZ02 seismic profile and the others seismic profiles on its east side that across the Nansha Block,and it is found that the extent of the crustal extension of the southern continental margin is gradually increasing from the east to the west.It is related to the later formation of the spreading centre and longer period extension on the west side of the southern continental margin.Also,since the syn-rifting period,the extent of crustal extension of the west side of the Nansha Block has always been greater than that of the east side.We suggest that this is the result of the lack of rigid blocks and the weaker crust in the west side.