Improved Half‐Space Cooling Model Reconciles Bathymetry and Thermal Evolution of the Southwest Subbasin, South China Sea

The bathymetry of marginal sea basins commonly deviates from depths calculated by classical lithospheric cooling models, yet the controlling mechanisms remain debated. Here, using 19 seismic profiles and corrected local heat‐flow data, we apply an improved half‐space cooling model within a two‐stage reconstruction to reassess thermal subsidence of the Southwest Subbasin (SWSB), South China Sea (SCS). By correcting for sediment and crustal loading, dynamic topography, sea‐level change, and post‐spreading cooling, we reconstruct paleobathymetry at seafloor spreading cessation (∼16 Ma) and isolate water‐loaded subsidence during active spreading. Paleobathymetric reconstructions yield mean depths of 2.70–2.86 km at ∼16 Ma, compared with a present‐day sediment‐corrected mean of 4.55 km, implying 1.69–1.85 km of integrated post‐spreading subsidence. We interpret this deepening primarily as cooling and contraction following the rapid cessation of magmatic heat input, potentially amplified by dense recycled mantle components. Removing post‐spreading effects restores cessation‐age bathymetry comparable to normal oceanic lithosphere. Syn‐spreading age–depth coefficients are ∼188–287 m/Ma 1/2 lower than open‐ocean reference values, indicating a pronounced subsidence deficit. We attribute this to enhanced magma supply linked to subduction‐driven mantle flow, which maintained thermal and dynamic support during spreading. Along‐strike variations reflect heterogeneous magma supply influenced by southern slab breakoff and westward spreading propagation over heterogeneous lithosphere. Our results reveal a two‐stage evolution linking magma‐supported shallow bathymetry during spreading to substantial post‐spreading subsidence. This framework provides new constraints on the coupled roles of mantle dynamics, magma supply, and lithospheric cooling in shaping SWSB bathymetry.