Tectonic Evolution Of The South China Sea And A Numerical Modeling

The margin sea, located at the transition zone between continent and oceanbasin, is a key area for studying geodynamics and evolution of continental margin, butalso an accumulation area of hydrocarbon and gas hydrate. The South China Sea isthe largest margin sea of the West Pacific, and surrounded by three plates of Eurasian,Pacific and Indo-Australia. Under the interaction of three plates, the geologicalprocesses of SCS are very complicated and have so far failed to established auniversally applicable model of tectonic evolution and thus become urgent issue ofthe SCS research to be resolved.In this paper, based on the typical seismic profiles and gravity and magnetic data,the distributing feature and combination relation of main fractures in SCS andadjacent region were studied. The main fractures were classified according the scaleand epoch. Based on the combination relation of main fractures in SCS, four periodscan be divided: rift â… period, rift â… period, seafloor spreading period and post ofseafloor spreading period. Further, regional stress fields were described in each periodrespectively. Results show that, tectonic evolution of SCS in Cenozoic is under thecontrol of the Pacific tectonic domain and the tethyan tectonic domain, and fourdifferent evolution stages can be divided.Considering the existing evolution models in SCS, different numerical modelswere established for analyzing the most appropriate model for the open of SCS. Theresults of plan-view models show that because of the obstruction from subduction ofPacific plate, the mantle flow generated by the collision of India-Eurasia movedtowards south and southeast, resulting in NW-SE trending extension in the SCSregions. However, associated rapid sinistral movement along the Ailao shan-RedRiver shear zone and the East Vietnam Boundary Fault, the collision of India-Eurasiawas mainly accommodated by SE trending Indochinese escape, the effect on SCSdecreased. We suggested that the India-Eurasia collision mainly result in NW-SEtrending extension during an initial stage of rifting, and do not the leading element forthe opening of SCS. The Neogene opening of SCS is suggested as a consequence of a slab-pullinduced by subduction of the Proto-SCS underneath Borneo, the latter can generateeffective N-S trending extension for the opening of SCS.The results of cross-section models show that the location of the position ofcontinental break-up is predetermined by a large lithosphere fracture, which is theMesozoic subduction zone in eastern of SCS. In center of SCS, the position ofcontinental break-up is predetermined by the abnormal lower crust, which could act asa weak zone. Under extensional stress field, strain concentrate on abnormal lowercrust zone and break-up in the end.In a word, according the characteristic of crust structure, basin and fractures,SCS has experienced the evolution from onset of rifting, break to seafloor spreading.The driving forces mainly came from the slab pull of the proto SCS, assistant with thesouthward and southeastward mantle flow in rift stage. Location of continentalbreak-up is determined by the proto-subduction and abnormal lower crust. Theseshow the importance of inherited structures and the pre-rift evolution of a margin.