Post-spreading Magmatism And The Velocity Structure Of A Fossil Spreading Center In The Southwest Sub-basin, South China Sea

The Southwest Sub-basin is a propagating spreading basin with active post-spreading magmatisms, which includes all processes of tectonic evolution of the marginal seas thus becomes a key field. Various studies of the post-spreading seamounts have been carried out in the Southwest Sub-basin, however the crustal structures of them were yet unkonw. Compared with the initial spreading and the seafloor spreading in the Southwest Sub-basin, the study of the final stage of seafloor spreading is relative rare, which leads to the lacking of important information between seafloor spreading and ridge extinction. Hence, in this paper, we planed to analysis the post-spreading magmatisms and the magmatic and tectonic processes on the spreading ridge at the final stage of spreading based on the velocity structure.We modeled the OBS data collected in the Southwest Sub-basin in 2010 for detail P-and S-wave velocity structures of the axial seamount and the fossil spreading center (NE) by using forward method. In order to check the robustness of the forward models, P-wave velocity structures of the fossil spreading center (NE) were simultaneously modeled by using inversion method. The following conclusions were drawn:(1) A new realization was proposed that the post-spreading axial seamount was mainly formed by extrusive processes with an extrusive/intrusive ratio of-1.92 based on the relationship between the shape of the iso-velocity contours and the emplacement mechanism.(2) The difference in crustal thickness in the Southwest Sub-basin indicates that the magma supply varied in time and space during or even after the seafloor spreading based on the relationship between the thickness and the magma supply. Between Chron C6n and C5En, the magma supply was constant along the ridge and relatively high, whereas the magma supply exhibited along-axis variation between Chron C5En and C5Cr and was low in the northeast SWSB. Reduced melt supply occurred during the waning stage of seafloor spreading, leading to a decreasing crustal thickness. After the cessation of seafloor spreading, the post-spreading magmatism became strong in the northeast SWSB and weak in the southwest SWSB.(3) An oceanic detachment fault was proposed at the NW side of the spreading center in the Southwest Sub-basin based on the analysis of the seismic characteristics and the tectonic environment which is quite important to the tectonic evolution at the waning stage of seafloor spreading. The fossil spreading ridge showed an anomalous structure on the NW side of the spreading center with the uplifting of the upper mantle beneath a thinned oceanic crust. The SWSB ridge (NE) studied in this paper was a slow-spreading fossil ridge with half-spreading rates of less than 40 mm/yr, which supports the existence of the oceanic detachment fault.Compared with all fossil spreading ridges, we divided the fossil spreading ridges into three groups based on the velocity structures and magmatisms:(1) ridges characterized by an increasing crustal thickness at the waning stage of spreading, (2) ridges characterized by a decreasing crustal thickness form the waning stage of spreading to ridge extinction and (3) ridges characterized by a decreasing crustal thickness at the waning stage of spreading, followed by an increasing crustal thickness after spreading.