Inherited structures and their impact on rift evolution: A numerical study of the South China Sea

The South China Sea experienced Cenozoic rifting in a region that was previously part of a Mesozoic Andean-type orogeny, which presumably had resulted in structural, compositional, and thermal inheritance. Recent studies using seismic profiles, drill cores, and geochronological analysis have revealed evidence for such heterogeneous pre-rift lithosphere in the South China Sea (Fan et al., 2017). Here, we further investigate the impact of orogenic inheritance on rift evolution using a numerical forward model that integrates both geodynamic and landscape evolution software (Neuharth et al., 2022). By varying our velocity boundary conditions over time, the model encompasses first continental collision, followed by post-orogenic collapse, continental rifting, and final lithospheric breakup. The model is constrained by observed crustal thicknesses, cooling history, and lithosphere-asthenosphere boundary depth, and successfully reproduces realistic orogenic topography, thrust fault distribution, and rifted margin of the SCS.We find that during orogeny, crustal thickening leads to the development of inherited weaknesses in the modelled crust. From orogenic collapse to continental rifting, pre-existing thrust faults are reactivated and serve as nucleation sites for normal faults, which interact with later rift-related normal faults to modify the regional stress field. The modeling results demonstrate that pre-existing thrust faults and a ductile lower crust play a crucial role in shaping the wide rifted margin of the SCS. We infer from our results that the location of crustal breakup is often influenced by these inherited structures. These regions have typically undergone thermal weakening, which further facilitates the process of crustal breakup during rifting.