Numerical modeling of the anomalous post-rift subsidence in the Baiyun Sag, Pearl River Mouth Basin

Abstract

The Baiyun Sag is the deepest sag in the Pearl River Mouth Basin in northern continental margin of South China Sea, with the maximum sediment thickness over 12.5 km above the basement including >6.5 km sediments above the 30 Ma breakup unconformity. According to the theoretical models for the rifted basins, the post-rift subsidence is driven solely by the thermal contraction and can be calculated as the function of the lithospheric stretching factor. A method combining the forward modeling and reverse backstripping was designed to estimate lithospheric stretching factor. Using the 2D forward modeling based on the flexural cantilever model, we simulated the multi-rifting process of the Baiyun Sag with constrain of the backstripped profiles. By doing this the lithospheric stretching factor was obtained, and then the theoretical post-rift subsidence was calculated. The calculated theoretical subsidence was much smaller than the observed subsidence given by backstripping. Along the 1530 line in the Baiyun Sag, the anomalous post-rift subsidence is over 2 km in the sag center, and varies slightly to the north and south edges of the sag. This suggests that the anomalous post-rift subsidence continues beyond the sag both in the continental shelf to the north and in the continental slope to the south. The sensitivity tests in the forward modeling process indicate that only the use of low-angle faults (⩽13°) can we simulate the shape of the backstripped profile.