A Comparative Study of Anomalous Post-rift Subsidence in the East and South China seas

Following cessation of rifting, the continental margin basins of the East and South China sea entered a post-rift stage, during which basement subsidence has traditionally been attributed to lithospheric cooling and thermal contraction. However, quantitative analyses from multiple sub-basins indicate that rapid or accelerated subsidence persisted into the post-rift period and locally exceeds predictions of classical thermal subsidence models. This is anomalous subsidence compared to simple rift models. In this study, we conduct a comparative analysis of post-rift subsidence histories in several representative basins in the East and South China seas. Regional porosity-depth relationships were established based on drilling data or generic models, and backstripping analyses were performed to reconstruct tectonic subsidence histories after accounting for sediment loading. Thermal subsidence and stretching factors were further calculated and compared with theoretical extensional subsidence models. Our results show that pronounced anomalous subsidence has developed since ~5.3 Ma in the Xihu Sag of the East China Sea, since ~5 Ma in the Yinggehai-Song Hong Basin, since ~2–5.3 Ma in the Qiongdongnan Basin, and since ~18 Ma in the Zhu III Depression and Baiyun Sag of the Pearl River Mouth Basin. Although these accelerated subsidence events all occurred during the post-rift stage, they exhibit marked differences in timing and persistence between basins. The mismatch between observed subsidence and model-predictions suggests that post-rift subsidence cannot be explained solely by lithospheric cooling, but is likely influenced by additional processes such as deep-seated mantle up or downwelling, tectonic reactivation, or mid and lower crustal flow driven by sedimentary loading. These findings highlight the stage-dependent and diachronous nature of post-rift anomalous subsidence in the East and South China sea marginal basins and provide new quantitative constraints on post-rift basin dynamics and sedimentary responses.