Post-rift faulting controlled by different geodynamics in the Pearl River Mouth Basin, northern South China Sea margin

The South China Sea (SCS), located at the junction of the Eurasian Plate, the Philippine Sea Plate and the IndoAustralian Plate, developed as the result of intra-continental rifting and seafloor spreading on the South China margin in the Cenozoic. Faults were developed extensively in the Pearl River Mouth Basin (PRMB), northern SCS margin, during the post-rift stage, but spatiotemporal evolution characteristics and geodynamics of the faulting were not resolved effectively. In this study, three stages of faulting evolution controlled by different geodynamics since the continental breakup were systematically analyzed and revealed based on the latest high resolution 2D/ 3D seismic data, the drilling data, the regional tectonic evolution, and the comparison with the previous studies. During Stage I, faulting showed tendency of migration from south to north (Zhu IV Depression to Baiyun Sag) and weak-intensive activities, which was mainly controlled by the southeastward mantle flow stemming from IndoEurasian collision. During Stage II, faulting showed tendency of migration from south to north (Baiyun Sag to Zhu I Depression) once again and intensive-weak activities, which was mainly controlled by the continental-ward lower crust flow stemming from the Baiyun Sag due to sedimentary loading. During Stage III, faulting showed tendency of migration from east to west and weak-intensive-weak activities, which was mainly controlled by the collision between the Luzon Arc and the Eurasian continental margin. From Stage I to Stage III, quantities of faults increased from nearly 400 to over 1000, but major lengths showed declined tendency from 3 to 15 km to 0-6 km. Striking of faults showed clockwise rotation from approximately E-W trending with azimuth of N90-100 degrees to NW-SE trending with azimuth of N110-125 degrees, and fault dip angles increased significantly from 35 to 45 degrees to 55-65 degrees and even 70-80 degrees. During Stage I and Stage II, faults were mainly developed in the extension stress field, showing roughly parallel distribution on plan view and were arranged in steps on the sections. During Stage III, faults were mainly developed in the trans-tensional stress field. Many en echelon arrangement, x-conjugatejoint and horsetail-type fault systems were developed on plan view and Y- or composite Y-type, as well as flowerlike structure fault systems on the seismic sections. Various geodynamics of the post-rift faulting in the northern SCS margin show transition from the interaction between plates to the west (the Indo-Eurasian Plates) during Stage I and Stage II, to the interaction between plates to the east (Philippine Sea Plate and the Eurasian Plate) during Stage III.