Rifting Characteristic And Spreading Pattern Of The NW Subbasin

Located at the northwestern part of the South China Sea(SCS) among the east subbasin, the Zhongsha Islands(Maccelesfield Bank) and the Xisha Islands(Paracel) , the NW subbasin is the smallest ocean basin in all the three subbasins of SCS. It presents all major stages from continental rifting in the late Oligocene to initial seafloor spreading in the mid-Miocene, thus provides an ideal place to study therifting and initial seafloor spreading process in great detail. And because of its small scale and short distance between the northern and southern continental margin, theNW subbasin is a good place to study the continental rifting process, the conjugatenature and local geology events.Based on two high quality Ocean Bottom Seismometer(OBS) wide anglereflection/refraction profiles which were obtained in the northwest subbasin SCS,in 2006, through precious data processing, 2D ray tracing method and inversion technique, we calculated the velocity distribution under the NW subbasin and get the deep crust structure model. At the same time, cooperated with gravity data density model was calculated. Synthesize other geophysics data collected in the research area and read other scholars'research results for reference, analysis and contrast were done to the characteristic of the deep crust structure of the NW subbasin. The key analysis was conducted to the rifting characteristic, Moho surface geometry and crust nature of the NW subbasin in this paper, combined with the rifting evolution history of the northern continental margin and tectonic evolution history of the SCS, we have widely discussion on the continental margin nature, the morphologic shape, deep crust characteristic and tectonic evolution history,many beneficial results have been obtained which are mainly showed in the follow,1)For the 1st time the deep crust structure was built with OBS seismic data in the NW subbasin, detailed Moho geometry information and layered structure characteristic were got. Density model was calculated through gravity inversion in the NW subbasin.2)Data inversion and analysis show that, the crust thickness under the continental slope decreases from 21km to 11km, the crust thickness of the NW subbasin is about 7.7km, and the moho depth ascends from 21km to 11km. The tectonic geometry and velocity structure of the NW subbasin and its margins on both sides shows symmetrical and conjugate and indicates pure shear mode in continental margin rifting mechanism.3)We don't find clear seismic signals from high velocity layer under the lower crust of the continental margin in the northern of the NW subbasin, which supplies new evidence for the viewpoint that western part of the northern continental margin of South China Sea is non-volcano crust.4)The crust of the NW subbasin similar to the east subbasin in structure is confirmed to be oceanic crust, with thicker layer 1(sedimentary layer) and thinner layer 2 , which is some different from typical oceanic crust structure. In addition, because the seafloor spreading period of the NW subbasin was short, the layer 2 experienced asymmetric basalt magma flow, which blurred the magnetic anomaly belt of the NW subbasin.5)Analysis on the geomorphic feature, sediment and tectonic showed that the Xisha Trough was not simply the extension result of the NW subbasin. The Xisha Trough, NW subbasin and east subbasin had more difference in dynamic characteristic which implied different evolution history and potential relationship.