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Crustal structure of the northern South China Sea simulated by jointing gravity and seismic data

LIU Jun1,2,LIU Jian-hua1,GAO Jin-yao1,ZHAO Li-hong1,3,FANG Yin-xia1,SHANG Ji-hong1 (1.Second Institute of Oceanography,SOA,Laboratory of Submarine Geosciences,SOA,Hangzhou 310012,China; 2.Qingdao Institute of Marine Geology,Key Laboratory of Marine Hydrocarbon Resource and Environmental Geology,Qingdao 266071,China; 3.Shandong University of Science and Technology,Qingdao 266510,China)  
The northern continental margin of the South China Sea has been affected significantly by the Pacific tectonic domain and Tethys tectonic domain in the Mesozoic,and related to the interaction of the Eurasian plate,the Indian plate and the Pacific plate in Cenozoic. Thus,different kinds of crust have been formed in this region. For the purpose of deciding the property of the crust,it is necessary to well understand its structure. According to the seismic and bathymetric data of the northern segment of SO49-18 surveying line which was obtained from the cooperate investigation of Germany and China,the paper studies its crustal features combining with the information of sonobuoy,OBS and two-boat refraction along this line. We first established an initial 2.5D geophysical model using gravity data and seismic profile based on the LCT joint inversion software. Second,we computed the theoretical free-air gravity anomaly from the assumed model. And then we separated anomalies of different depth by a matched filtering method from total anomalies and computed their corresponding theoretical free-air gravity anomalies which were compared with their observed free-air gravity anomalies. Finally,according to the difference between the theoretical anomalies and the observed anomalies we could modify the model layer by layer using some certain reasonable physical parameters to make the theoretical anomaly curve identical with the observed anomaly curve. The crust structure model couldn't be best until the difference is up to minimum. The simulated result of the profile shows that the crustal thickness of the northern continental margin of the South China Sea is thinning from the continental shelf to the continental slope,the thickness of the upper crust is thinner than the lower crust because of the influence of tensional stress induced by tectonic movements in the early Cenozoic,and the Moho depth decreases sharply from the continental margin to the deep-sea basin; the crustal thickness is thinner in the northwest deep-sea basin,and thicker in the sea plateau of Zhongsha Islands.
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