Three-stage tectonic subsidence and its implications for the evolution of conjugate margins of the southwest subbasin, South China Sea

To reveal the tectonic characteristics of the continental margins in the southwest subbasin (SWB) of the South China Sea, a long high-resolution seismic profile was studied using empty basin subsidence. We find that tectonic subsidence features on both margins are uniformly divided into three stages: (1) slow subsidence from Tg to 18.5 Ma (synrift stage); (2) extremely slow subsidence/uplift from 18.5 to 16 Ma (spreading stage); and (3) accelerated subsidence from 16 to 0 Ma (post-spreading stage). This feature differs from the classic tectonic subsidence pattern of rifted basins, which exhibits fast subsidence during synrift stage and slow subsidence during the post-rift stage. The tectonic uplift occurred during the spreading stage and the magnitude increased from the continent to the ocean, which is likely related to mantle flow during seafloor spreading. We propose that lower crustal flow played a significant role in the tectonic evolution of the continental margins of the SWB. The lower crust of the SWB margins was warmer and therefore weaker, and more prone to flow beneath the faulting center, which compensated for the upper crustal thinning caused by brittle faulting during the synrift period and thus reduced the tectonic subsidence rate. During the spreading stage, faulting attenuated rapidly, and a necking zone appeared at the continent-ocean transition formed by lithospheric extension. With upwelling asthenosphere, small-scale secondary mantle convection occurred under the necking zone, which raised the continental margin isotherms and increased the buoyancy. Simultaneously, secondary mantle convection lifted the overriding crust, thus the overall subsidence rate decreased sharply or even reversed to uplift. After seafloor spreading, the effect of mantle convection faded away, and sediment loading drove the lower crust to flow landward. Thermal relaxation, lower crust flow, and vanish of secondary mantle convection together led to rapid subsidence in this stage.