Impact of structural inheritance and mantle potential temperature on wide asymmetric rifts

Continental rifting, as the initial and critical phase of the Wilson Cycle, has been extensively observed and studied using 2D and 3D analogue and numerical models. These studies have effectively reproduced the characteristics of wide and narrow, symmetric and asymmetric rift structures, through horizontal multi-layered models with variable strength parameters. One prominent natural case is the South China Sea (SCS), which began its rifting and subsequent seafloor spreading at ~32 Ma. The SCS then underwent a shift in its spreading direction from a north-south to a northwest-southeast orientation at ~23 Ma. Despite resembling a typical wide rift, the SCS poses specific challenges in explaining its complex synchronous basins with detachments , non-flat Moho surface, inland hyper-thinned continental crust extending over 500 km from the continent-ocean boundary (COB), and the wide asymmetric geometry with narrow OCT (ocean continent transition) observed in the Southwest Subbasin (SWSB). Former 1D and 2D simulations have shown that wide continental rift can be produced either by rift migration, i.e. sequential basins associated with non-flat Moho, or in post orogenic context, i.e. synchronous basins form over a flat Moho due to the weakness of the lower crust.  They equally fail at capturing the synchronous basins and non-flat Moho of the SCS. Considering the SCS's pre-rift fore-arc environment with thrusts featuring strong and weak crust due to tectonic events such as the Pacific subduction and the Proto South China Sea (PSCS) plate subduction, we applied 2D numerical models to replicate these features. Our models incorporate a dipping layered continental crust structure composed of strong and weak layers with varying dipping angles and thickness of lower crust, alongside temperature variations at the lithosphere-asthenosphere boundary (T_LAB). This setup successfully reproduced the margin style observed in the SWSB. Our models show four distinct rifting styles: pure single spreading center, single spreading center with hyper-thinned continental crust, single spreading center with exhumed mantle, and double spreading centers style, and three styles of Moho surface, including flat Moho, hummocky Moho and wavy Moho. Model with synchronous basins with detachments, single spreading center with inland hyper-thinned crust/exhumed mantle and hummocky Moho style fits well with observations in SWSB. This suggests that by incorporating structural, compositional and thermal variations from surrounding tectonic events, new insights into the diverse rifting features seen in SCS and a robust framework to understand wide asymmetric margins across complex geologic settings can be expected.