Origin of the Dongsha Event in the South China Sea

Post-rift tectonic activities have been widely observed in the northern continental margin of the South China Sea, especially during the late Miocene. Large numbers of faults became active. Unconformities, uplift of faulted blocks, sequence tilting, erosion along the Dongsha massif and canyon incision were also discriminated at this stage in the Pearl River Mouth Basin and the area to the east. This tectonism has been named Dongsha Event. A number of hypotheses have been put forward to explain the mechanism of the Dongsha Event, such as high-velocity lower crustal flow, magmatic underplating, and arc-continent collision. To investigate the tectonic dynamics, sequence contact relationships, fault activities, and magmatism were analyzed along large numbers of seismic profiles that cover the eastern Pearl River Mouth Basin and Southwest Taiwan Basin. Quantitative faults activity assessment show that post-rift fault activities can be divided into three stages: continuing fault activity (33.9-15.97 Ma), weak fault activity (15.97-10 Ma), fault reactivation (10-2.59 Ma). The timing, affected regions, and differences in the intensity of tectonic deformation were assessed, upon which the plate bending model was favored. Although post-rift magmatism was frequently detected within our study area, precise dating of drilling sites indicates that most of the volcanic zones formed around the early Miocene. No definite late Miocene magmatism was identified. The underplating model during the late Miocene was not well supported. In order to check the reasonableness of plate bending model, effective elastic thickness and other geodynamic parameters were calculated constrained by uplift area width and regarding the trench as sediment filling. A maximum Te value of 27 km and a minimum value of 4 km were obtained. Integrating with the former stress field calculation, we conclude that the Dongsha Event was mainly affected by subduction and collision of the South China Sea toward the Philippine Sea plate. This event commenced at about 10 Ma and peaked at around 3.6 Ma. Although the high effective elastic thickness required is a problem to be addressed, this research provides by far the most comprehensive evidences to the mechanism of the Dongsha Event.