The Continent-ocean Transition Zone At The Northern Margin Of The South China Sea

The northern margin of the South China Sea(SCS) has particular structural and stratigraphic characteristics that are somewhat different from those described in typical passive margin models. The differences are attributable to poly-phase tectonic movements and magmatic activity resulting from the interaction among the Eurasian, Philippine Sea and Indo-Australian plates. Based on several high quality crustal-scale multi-channel seismic reflection profiles and latest satellite gravity data across the northern SCS margin, this paper analyses the structures, igneous rock and deep crust of the continent-ocean transition zone(COT) at the northern margin of the SCS to study the patterns and model of extension there.The structural evolution of the northern margin of the SCS can be divided into the rifting and post-rifting stages obviously. During the Late Oligocene, continued high or even higher deposition rate in some depressions, Early Miocene organic reef and shallow water platform carbonates indicated a delay of rifting in the whole northern margin. Magmatic activities have occurred across the northern margin during the evolution of the SCS, and the volcanic activities intensely happened at/after the end of seafloor spreading. Compared with the very weak magmatic activities occurred in the slope of the northwestern margin of the SCS, the intruded/extruded magma, which chiefly happened in the 29.3-23.3Ma, was well developed in the Northwest sub-sea basin, aborted rift, Xisha, and Zhongsha area to the south.The results indicate that the COT is limited landward by basin-bounding faults near break in slope or between the upper slope and lower slope and is often bounded by seaward-dipping normal faults near the oceanic basin, whereas it is bounded by volcano in some area(e.g. Line07 seismic profile of the west segment). Generally, the COT extended from NE to SW with a gradually shallowing from east to west. The shallow anatomy of the COT at the northeastern margin with an extent in excess of 200-km is characterized by rift depression infected by magma and volcanic zone. And it is characterized by rift depression, structural highs bounded by listric normal faults and/or a zone of tilted faults blocks, whereas it is marked by rift depressions, a structural high affected by lava flow, a volcanic zone and a zone of tilted faults blocks instead to the west in the mid-northern margin with a width of 220-265-km. Further to the west, the shallow anatomy of the COT at the northwestern margin with a narrower width of 69-km is characterized by rift depression only.Gravity modeling revealed that a high velocity layer(HVL), with a 0.8-5-km thickness, is frequently present in the lower slope below the lower crust, and can be up to 6-10 km below the crust of the Dongsha area, and then gradually thins to both sides below the lower slope and finally terminates at the Baiyun sag and subducts below the Luzon islands with the transitional crust. Our study shows that the HVL is widely located in the shelf and slope of the eastern portion of the northern SCS margin based on the available geophysical data, indicating that it is distributed essentially within the COT but is clearly not present everywhere in the COT. Thus, the HVL should not be considered as a necessary sign of the COT at the northern margin of the SCS. The magmatic rock located in the shelf and slope and HVL may be related to partial melting caused by the decompression of a passive, upwelling asthenosphere, which resulted primarily in post-rifting underplating and magmatic emplacement or modification of the crust.Compared with the typical magma-poor margins and volcanic margins, we propose that an intermediate mode of rifting was active in the mid-northern margin of the SCS with characteristics that are closer to those of the magma-poor margins than those of volcanic margins. Moreover, compared with the characteristics of the SCS conjugate passive margins, the large basin-bounding faults are well developed in the COT of the northern margin whereas the listric normal faults may merge into a main detachment fault in the deep crust or even the Moho in the COT of the southern margin. This structural difference may be controlled by the depth dependent lithospheric differential extensional deformation during the Cenozoic evolution of the SCS, which may result from the rifting mechanism of mantle lithosphere break-up before continental crust.