Reconciling bathymetric anomalies of marginal sea basins through magmatic and cooling processes

       Bathymetry of marginal sea basins is commonly deeper than the half-space cooling prediction for large oceans, but what controls this pattern is poorly understood. Here, based on abundant seismic sections with increasingly available databases, we perform an enhanced approach that specifically corrects for post-spreading cooling to reassess thermal subsidence across the Southwest Subbasin (SWSB) and the broader South China Sea (SCS) basin. We attribute the current excessive subsidence of the SCS basin primarily as a response to the post-spreading cooling process, which has global applicability to other marginal sea basins and accounts for at least 86% of the observed depth anomaly. Additionally, the mode of magma supply during seafloor spreading plays a crucial role in shaping reconstructed shallower bathymetry of the SCS basin relative to predictions from the half-space cooling model. A stronger magma supply deriving from the regional subduction system can explain the relatively shallow depth developed during the opening of the SCS compared to large oceans. In contrast, a westward decayed magma supply, driven by localized rift propagation induced by the inherited pre-Cenozoic heterogeneous lithospheric structure of South China, attributes to subsidence discrepancies among sub-basins and within the SWSB.

        The sediment-corrected depth of most marginal seas is, on average, more than 500 m deeper than that of large oceans, with maximum anomalies ranging from -0.95 to -2.70 km (in 0.5° bins). The sediment-corrected depths exhibit statistically poor correlations with the spreading rate, indicating that the thermal evolution of marginal seas is not primarily controlled by the spreading rate, unlike large oceans. Neither can this anomaly be fully explained by dynamic topography driven by large-scale mantle convection or by localized variations in the degrees and patterns of subduction systems, although the latter may be an important factor influencing the bathymetry of still-active marginal seas. We interpret at least 44.5% of these anomalies as a result of long-term post-spreading thermal subsidence in inactive marginal seas, with magmatic processes influencing bathymetry during oceanic plate formation. We propose that the post-spreading secular cooling, together with the variable mode of magma supply and potential dynamic subsidence processes driven by subducting slabs, play pivotal roles in the formation of the topographic anomalies within the oceanic basins of marginal seas.