Testing the Pull-Apart vs. Subduction-Driven Rifting Debate with 3D Geodynamic Models

Tectonic structures associated with continental rifting depend on numerous factors: the nature and mechanical behaviour of the stretched lithosphere, geological inheritance, thermal conditions, and geodynamic forces. The South China Sea exhibits a complex geodynamic history, marked by pre-existing structures (granitoids, etc.). Oceanic accretion in the South China Sea began in the east around 32 Ma and propagated southwestward around 22 Ma, accompanied by a change in the extension direction. Spreading stopped around 16 Ma. The rifting phase lasted longer in the west, leading to the development of a wide rift, accompanied by core complexes and exhumed mantle.

Previous numerical modelling studies show that the formation of a wide rift requires a ductile lower crust and high temperatures at the base of the crust. Structural and thermal inheritance promotes distributed deformation. However, in 3D, an additional mechanism is required to slow down oceanic propagation in order to allow the formation of a wide rift. One possibility is the action of compressive stresses, which, in the case of the South China Sea, may be linked to the topography of the Indochinese block resisting rift propagation.

Here we explore another hypothesis for slowing an oceanic propagator: the transition from N–S extension to an N–S strike-slip system. The opening kinematics of the South China Sea remains debated, between (1) a pull-apart model linked to left-lateral motion along the Red River Fault associated with extrusion of the Indochinese block, and (2) a continental-rifting model induced by subduction of the proto–South China Sea. Modelling rift propagation toward a major transform fault allows us to assess how different kinematic scenarios influence the opening of the South China Sea, the formation of crustal structures, and topography. End-member models fail to reproduce a wide rift, whereas intermediate conditions better account for the slowing of rift propagation, the width of the rift, and the oblique localization of deformation in the southwest basin.