Lithospheric stretching-style variations and anomalous post-rift subsidence in the deep water sub-basins of the Pearl River Mouth Basin, northern South China Sea

Abstract The stretching and subsidence characteristics of passive continental margins have been investigated for decades but are still controversial. We focused our study on the deep water sub-basins of the Pearl River Mouth Basin in the northern South China Sea, the only well-preserved passive margin in the South China Sea. Inverse modeling and forward modeling based on a flexural cantilever model were employed. The stretching factors of the upper crust, lower crust, whole crust, and mantle lithosphere were mapped, and the anomalous post-rift subsidence was isolated from the McKenzie model. The Liwan Sub-basin showed highly depth-dependent stretching that significantly increased with depth. The stretching factor of the lower crust in the center of the Baiyun Sub-basin was higher than that of the mantle lithosphere. Two significant anomalous rapid subsidence events were revealed. The 23.03-19.8 Ma event was characterized by anomalous subsidence at the center, located in the southern Baiyun Sub-basin, with a maximum value in excess of 1200 m. Strong negative P-wave abnormal and magma addition may indicated an upwelling mantle under the Baiyun Sub-basin around 23 Ma. We suggest that the 23.6 Ma southward ridge jump in the South China Sea could have weakened or eliminated the mantle upwelling under the Baiyun Sub-basin and caused rapid subsidence at 23.03-19.8 Ma. The 14.3-11.9 Ma event was characterized by a subsidence zone orientated northeast along the Baiyun Sub-basin, with a maximum value in excess of 600 m. We propose that rapid depositional sediment during this period would have driven the lower crustal flow away from the Baiyun Sub-basin center, causing rapid vertical subsidence and leading to the stretching factor of the lower crust being higher than that of the mantle lithosphere.