Massive eruptions of OIB-type volcanism in the Leizhou-Hainan area, Indochina and South China Sea oceanic basin around the northwestern South China Sea margin indicate occurrence of strong deep hot mantle upwelling. However, when it started and how it influenced the northwestern South China margin is still unclear. The mantle upwelling not only caused rising of lithospheric temperature, but also produced basement dynamic uplift. Thus, we conduct detailed analysis of the Cenozoic time-varying residual subsidence by subtracting the predicted subsidence from the backstripped subsidence along a new seismic reflection line in the Qiongdongnan Basin in the northwestern South China Sea margin to study the dynamic uplift and deep mantle upwelling. For the first time, we give a method to calculate the subsidence-independent, time-varying strain rates constrained by the varying faults growth rates. Then we forward predict the basement subsidence with a basin- and lithosphere-scale coupled finite extension model, and accurately recover the backstripped subsidence with a modified technique of backstripping to eliminate the effects of later episodes of rifting on earlier sediment thickness. Results show no residual subsidence in 45-28.4 Ma. But after 28.4 Ma, negative residual subsidence occurred, reached and remained ca. -1000 m during 23-11.6 Ma, and reduced dramatically after 11.6 Ma. In the syn-rift period (45-23 Ma), the residual subsidence is ca. -1000 m indicating a significant subsidence deficit, however in the post-rift period (23-0 Ma), it is positive of ca. 300 to 1300 m increasing southeastwards denoting considerable subsidence excess. These results suggest that the syn-rift subsidence deficit commenced at 28.4 Ma, while the post-rift excess subsidence occurred after 11.6 Ma. Combined with previous studies, it is shown that the subsidence anomalies cannot be explained only by lithospheric deformation, such as depth-dependent lithospheric stretching, post-rifting crustal thinning, lower crust flow and magmatic intrusion. We infer that the opposite residual subsidence in the syn- and post-rift periods with similar large wavelengths (>102 km) and km-scale amplitudes are results of transient dynamic topography induced by deep mantle upwelling beneath the central Qiongdongnan Basin, which started to influence the margin at ca. 28.4 Ma, continued into the Middle Miocene, and decayed at ca. 11.6 Ma. The initial mantle upwelling had precipitated considerable continental extension and faulting in the Late Oligocene (28.4-23 Ma), however, prohibited the syn-rift basement subsidence. After ca. 11.6 Ma, vanish of mantle upwelling in the Qiongdongnan Basin yielded rapid post-rift subsidence, and meanwhile, the strong mantle upwelling probably migrated beneath the Leizhou-Hainan area to form huge basaltic lava flow.

The transition from active to passive continental margin of the South China Sea (SCS) is usually inferred to occur in the Late Mesozoic to Early Cenozoic. However, it is less known about the tectonic characteristics of active continental margins before the Late Mesozoic, which hampers the recognition of integral evolution of the SCS. The International Ocean Discovery Program (IODP) site U1504 has sampled greenschist facies mylonite from the basement in the Outer Margin High of the northern SCS continental margin, which potentially record the Mesozoic and Cenozoic tectonic evolution of the SCS region. The microstructure has identified two episodes of deformation in the mylonite, namely early ductile and late brittle deformation, but without age constraints. Here, we further identify three episodes of carbonate veins (pre-mylonite, syn-mylonite and post-mylonite) in the greenschist facies mylonite according to the intersecting relationship between the veins and the mylonite foliation. Then we select 10 carbonate samples for in situ U-Pb dating, and obtain three accurate ages. The pre-mylonite carbonate veins are dated to 210 ± 20 Ma and 195 ± 32 Ma, respectively, which might denote the age of the protolith clast. The age of the syn-mylonite carbonate vein is 135 ± 12 Ma. But for the post-mylonite carbonate veins, no effective age was obtained using U-Pb dating method. Post-mylonite carbonate veins and late brittle fractures were formed at the same time, and the formation environment is similar to the overlying Late Eocene bioclastic limestone. Therefore, combining the microstructure, geochemistry and seismic profile, we speculate that the post-mylonite carbonate veins and brittle fractures may be formed during the Early Cenozoic rifting. These dating ages of the three episodes of carbonate veins suggest that the mylonite records at least two main periods of continental extension in the SCS region since the Early Cretaceous. In reference to the Mesozoic tectonic settings, we infer that, due to the slab rollback of the subducting paleo-Pacific, the SCS continental margin started significant extension during the Early Cretaceous as shown by the ductile deformation of the mylonite. In the Early Cenozoic, the mylonite was exhumated to the seafloor along with further continental extension, and weak brittle deformation occurred in the mylonite. Therefore, the Early Cretaceous extension of the SCS active continental margin may have a certain promotion effect on the rupture of the passive continental margin in the Cenozoic. Keywords: Greenschist facies mylonite; Carbonate U-Pb dating; Continental margin of the SCS; Early Cretaceous; IODP 368