Mechanism for plate-edge rifting of the South China Sea continental margin: Initial hydrous mantle induces rapid continental-oceanic transition

Abstract:

Recent investigations of IODP Expeditions 367-368-368X have revealed that the South China Sea (SCS) continental margin is neither “magma-poor” nor “magma-rich” type of rifted margin. It is hypothesized to be a new type of “plate-edge” rifting. However, dynamic mechanism for rapid continental-oceanic transition is still enigmatic. Here we directly connect the surface plate motion constraints with state-of-the-art 3D geodynamic models, quantifying the evolution of mantle flow, melt generation, and crustal accretion beneath the SCS since 50 Ma. Modeling results revealed that the presence of hydrous mantle can significantly accelerate the melt production rate during initial rifting-to-spreading stage of the SCS; however, such effect of accelerated melting rate become insignificantly after a few million years of quasi-steady-state seafloor spreading. It is further hypothesized that the presence of the hydrous mantle could be related to recycling of surrounding subducted plates. Particle tracers in the mantle domain revealed time-dependent 3D path, temperature, and pressure of mantle materials, suggesting that the eastern sub-basin has experienced a significantly greater initial melting depth than that of the southwest sub-basin of the SCS, which is consistent with the geochemical observations of the mid-ocean ridge basalts obtained at the IODP Expedition 349 drilling sites. Our models provide new insights into the initiation mechanism of marginal seas at the western Pacific Ocean.