Research On The Crustal Velocity Structure In The Southern Yellow Sea

The South Yellow Sea is located in the lower Yangtze area near the Sulu orogenic belt, which is a superimposed basin developed above the MesozoicPaleozoic marine strata and reformed by the Mesozoic- Cenozoic tectonic movement with complicated geological structure. The thesis based on the South Yellow Sea OBS2013 deep seismic exploration line and used the OBS laid out at the bottom of the sea to receive the wide angle reflection/refraction seismic signal from the large volume air-gun to get the seismic wave travel time data in the deep crust to understand the characteristics of deep structure, the shape of Moho and internal structure in the area of South Yellow Sea, which made up for the deficiency of the multichannel seismic and filled the blank of deep seismic sounding in the research area. The main conclusions are as follows:Based on the geological background of the research area and the phase analysis and combined with the shallow constraint from the multichannel seismic in the line, by means of the structure structure simulation methods such as ray tracing and travel time fitting, we obtained the 2D velocity structure model of line OBS2013. The simulation results show that, in the model,we had traced many wide angle refraction /reflection seismic phase from different layers with different depth with rich information. A large number of Pm P seismic phases and other reflection seismic phases could be identified in the model, which shows that there are significant differences between the continental crust structure in the Southern Yellow Sea area and the oceanic crust structure where the refraction seismic phases was well developed, meanwhile, there are many unconformity surfaces produced by multi-period tectonic movement in the research area.By the means of gravity forward fitting, the results show that the calculated gravity value of the 2D velocity model and the real measured value have the same trend, which means that the velocity structure model is reliable.Combined with the analysis of the magnetic data, we believe that there has high-speed buried hills of compressive fold type in the marine strata and there has the development of Mesozoic and Cenozoic magmatite intrusion in the northern margin of central uplift belt where the marine sedimentary layer is thick and the sedimentary basement didn’t uplift. The differences of velocity distribution and interface fluctuation state between the upper and lower marine strata shows that the low velocity layer of Silurian shale caused by Caledonian movement could work as the structural decollement surface.The fluctuation on the reflection interface in the sedimentation and crust shows that the Indosinian movement with the most significant effect in the lower Yangtze area has made the sedimentary layer from Nanhua period to the Late Triassic in South Yellow Sea area involved in the tectonic deformation zone. There is strong extrusion deformation in South Yellow Sea area during the Indosinian epoch which has the sharp contrast with the extensiona during the late Mesozoic basin from the deep seismic profile.From the crustal velocity model of South Yellow Sea, We can see that there is no high-speed large-scale upwelling of mantle material.Combined with the regional geological background analysis, it means that the basin pattern of two depression with a uplift in South Yellow Sea was formed by the tensile stress field of Pacific structural domain. The two different dynamic mechanisms of early extrusion and later tension since Mesozoic can be identified in the 2D velocity mode from the perspective of deep seismic profile.