Excessive subsidence of oceanic basins caused by recycled oceanic crust in the mantle source: A new perspective on the oceanic topography within Southeast Asia

Additional subsidence of oceanic basins compared to the half-space cooling prediction is thought to be a dynamic response to the underlying mantle convection induced by slab sinking. Here, we identified a significantly elevated proportion (10%−20%) of recycled oceanic crust (ROC) in the source of mid-ocean-ridge basalts (MORBs) from the South China Sea (SCS), illustrating the ROC accumulation within a confined upper-mantle range, possibly as a result of the long-term surrounding subduction events. The presence of this lithological heterogeneity in the ROC leads to an increase in mantle density by 0.45%−0.90% at most, thereby contributing to the observed excessive subsidence in the SCS basin when compared to other small basins affected by dynamic topography, while open oceans experience subsidence primarily due to thermal cooling. We propose that the ROC-induced density change in the upper mantle is crucial for generating the topographic anomalies in oceanic basins influenced by subducted slabs, alongside dynamic topography caused by mantle flow.