The spatial-temporal variations in dynamic uplift and deep mantle upwelling in the northwest South China Sea margin: Insights into continental rifting and magmatism

Abstract

Complex subduction system surrounding the South China Sea (SCS) region drove vigorous mantle upwelling, however, its time and pattern are not well constrained. Hot mantle upwelling induced broad OIB-type volcanisms, caused dynamic uplift, altered basin subsidence and was consecutively recorded by strata in the SCS region. The northwest SCS margin is located in the center of diffusive igneous provinces and featured by significant subsidence anomaly and along-strike variable rifting styles and magmatism, being an ideal place to explore the time-space variations in dynamic uplift and mantle upwelling, and reveal the effects on continental rifting and magmatism via seismic interpretation, strain rate calculation and residual subsidence analysis. Results show that the margin experienced two stages of extension during the syn-rifting period (45–23 Ma), a slow strain rate (1.5e-15 to 3.5e-15) in 45–28.4 Ma and a fast, eastward increasing strain rate (4e-15 to 7e-15) in 28.4–23 Ma. Subsidence analysis reveals that the entire margin initiated dynamic uplift and mantle upwelling at ca. 28.4 Ma, then increased to ca. 23 Ma, during which continental stretching and faulting were promoted, but the syn-rifting subsidence was inhibited. In the post-rifting period, mantle upwelling remained vigorous in 23–11.6 Ma, then decayed in 11.6–5.5 Ma to generate a fast post-rifting subsidence. Spatially, the dynamic uplift of more than 1 km has a long bar shape with the wider side in the east sub-basin, implying stronger mantle upwelling occurring in the east, which well explains why higher strain rates, more symmetric rifts and intense post-rifting magmatism (23–11.6 Ma) appeared in the east. Combined with plate reconstructions, it is inferred that the mantle return flow induced by slab subduction of the proto-SCS and the following SCS caused the mantle rising in the northwest SCS margin.