Forward and Inverse Modeling of Anomalous Post-Rifting Subsidence in Extensional Sedimentary Basins

Anomalous post-rifting subsidence, i.e. the post-rifting subsidence larger than the thermal subsidence predicted by the McKenzie model, observed in many extensional sedimentary basins including those in the northern South China Sea. The characterization of anomalous post-rifting subsidence is the first step in understanding its origin and mechanism and its impact on the formation of oil/gas reservoirs. This paper introduces three methods for characterizing anomalous post-rifting subsidence, namely, the paleo-water depth comparison, the strain-rate inversion, and the 2D forward-backward modeling. The conditions, advantages and disadvantages of these methods are discussed. The application of the third method, which is proposed by the authors, in the Pearl River Mouth basin of the northern South China Sea is presented. Taking into account the lithospheric flexural rigidity, the method calculates the lithosphere stretch factor and theoretical thermal subsidence through 2D forward modeling, and then compares the theoretic subsidence with the observed one given by 2D backstripping. Necessary improvements of the method are also discussed.