The onset of seafloor spreading at the northeastern continent-ocean boundary of the South China Sea

Abstract

The transition from continental to oceanic crust and the initial formation of typical oceanic crust at passive rifted margin are still poorly understood. In this study, deep crustal structures in the transitional domain of the northeastern South China Sea (SCS) margin are investigated and studied according to newly-acquired high-resolution reflection seismic data and gravity modeling. Three strong reflectors (R1, R2 and R3) are imaged in the narrow transitional domain. About 1 s in the two-way travel time (TWTT) beneath the top basement, R1 shows a similar morphology to the top basement, and quite likely represents a partial serpentinization interface. The presence of R1 suggests a tectonically dominated extensional stage prior to the final lithospheric rupture of the SCS. R2 is from magmatic intrusions or plutons (sills or dikes) from decompression melting. R3 is observed at ~2.5–2.2 s TWTT beneath the top basement but shoals seawards. R3 likely resulted from magmatic relicts in the paleo-melt transfer channels and/or from frozen molten materials in a shear zone of the lithospheric and asthenospheric mantle. The occurrence of R3 indicates the onset of a robust seafloor spreading system. The gradual seaward increase in magmatic activity highlights that, towards the final lithospheric breakup, tectonic extension was overtaken by magmatism. Increased magmatism and partial serpentinization formed a narrow hybrid transitional crust. Nevertheless, the breakup was not an instantaneous event. The last-stage transient magmatism occurred in a very narrow zone and does not rule out that the margin was predominantly amagmatic during the rifting and breakup. Based on reflection seismic and gravity modeling, we propose that the onset of typical seafloor spreading occurred at about 30 Ma around the study area, and the continent-ocean boundary (COB) should be located further seawards in the northeastern SCS margin.