Abstract:

Continental marginal basins in southern and northern South China Sea (SCS) show not only distinct tectonic regimes, including divergence, transformextensional and flexuralextensional complex types, but also have different tectonic evolutionary stages, due to different boundary conditions. Discordance between the breakup of continental margins and South China Sea spreading processes are presented. The research results indicate that a series of rifting basins are developed in the southern and northern continental margins during the SCS spreading, which are resulted in the formation of main rifting stage. However, the time of breakup unconformity is quite different. The rifting terminates on the age of 23 Ma or 21 Ma in the northern margins of the SCS, while did not terminate until the age of 15.5 Ma in the southern margins of the SCS. Those features indicate that the age of the breakup unconformity in the continental margins show a clear shift southward, which means that in the northern SCS is earlier than in the southern SCS. There are clear differences in tectonic regimes after the termination of the SCS spreading. Extension is dominated in the northern SCS and rapid subsidence episode is present since the Late Miocene, but compress regimes are present in the southern SCS, where the age and intension of compressive stress in the Zengmu basin are earlier and stronger than in the Beikang and Nanweixi basins. Transformation and distinct tectonic evolution in the southern and northern continental marginal basins indicate the shift of the SCS spreading, and are used to suggest the shift southward in the different ending times of the spreading processes for the three oceanic subbasins of the SCS. Therefore, differences in formation and evolution of continental marginal basins in the northern and southern SCS provide a helpful evidence for the reasonable interpretation for the spreading processes of the SCS.

Key words: distinct tectonic evolution, South China Sea spreading, tectonic event, continental marginal basins of South China Sea