Rifting a post-orogenic continental crust: Insights from the South China Sea

• Structural inheritance distributes deformation and prolongs the rifting phase. • Four modes of continental breakup emerge from crustal and lithospheric boudinage. • Wide rifts can form under high extension rates and strong lower crustal strength. • Only warm mantle and thick lower crust reproduce the Southwest SCS margins. The South China Sea (SCS) is an oceanic marginal basin that transects continental lithosphere and comprises two sub-basins. Despite a nearly constant stretching rate along strike in both space and time, continental breakup in the Southwest Subbasin is delayed by 9 to 13 Myr relative to the East Subbasin. During this delay, a wide rift formed and accommodated fast extension at 4 cm/yr. This rift features synchronous detachments and basins, a non-planar Moho, and two crustal necking zones spaced 500 kilometers apart. These characteristics are not predicted by conventional wide-rift nor fast-rifting models. We test the hypothesis that structural inheritance, in particular preserved crustal nappe stacks formed in a forearc setting during post-orogenic processes linked to Pacific subduction, may explain these features. Using 2D numerical models, we show these inherited structures promote distributed deformation and systematically delay continental breakup. Variations in lower crustal thickness, initial dip angle, and mantle temperature control crust–mantle coupling and boudinage, producing three distinct Moho textures and two necking zones. One zone accommodates final breakup; the other fails at different maturity stages or develops into a secondary spreading center, defining four lithospheric architectures. Our results match the observed margin tectonic architecture only under conditions of a warm mantle, a thick viscous lower crust, and shallow-dipping upper-crustal structures.