Wholesale Retreat of the Continental Shelf, Shelf‐Edge and Slope: Triggered by Southward Jump of South China Sea Spreading Ridge?

Abstract This study utilizes seismic, borehole, biostratigraphic, backstripped tectonic subsidence and U‐Pb age data to investigate margin architectures and formative mechanisms of previously undocumented, wholesale retreat of the continental shelf, shelf‐edge and slope. Basin‐scale backstepping shelf‐edge trajectories recognized in the Baiyun Sag of the northern South China Sea are 35–40 times longer than their well‐documented counterparts created by relative sea‐level rise, and are located between nannofossil zones N4 and N3 (P22) and calcareous nannofossil zones NN1 and NP25 (ca. 24.80 Ma). They witnessed a reverse transition in depositional environments from shallow‐to deep‐marine environments and a prominent increase in tectonic subsidence rate from 27 to 73 m/My to 110–211 m/My across 24.80 Ma timeline. This rapid tectonic subsidence and resultant backstepping shelf‐edge trajectories immediately follow the termination of magmatic activity, as recorded by the uplift of Archean to Paleoproterozoic igneous basement (reported as zircon 207 Pb/ 235 U ages of 1971–3795 Ma) by South China Sea mantle upwelling underneath the hyperextended Baiyun Sag. Wholesale retreat of the entire Baiyun margin is, therefore, considered as geodynamic consequences of the elimination of asthenospheric mantle upwelling induced most likely by the southward jump of South China Sea spreading ridge. The spatiotemporal synchronicity of the termination of asthenospheric mantle upwelling to the forming age of backstepping shelf‐edge trajectories suggests that the terminal age of South China Sea mantle upwelling can be better placed at 24.80 Ma, helping to eliminate the debate on the evolutionary timing of South China Sea mantle upwelling.