Structure and sediment budget of Yinggehai-Song Hong basin, South China Sea: Implications for Cenozoic tectonics and river basin reorganization in Southeast Asia

The temporal link between offshore stratigraphy and onshore topography is of key importance for understanding the long-term surface evolution of continental margins. Here we present a grid of regional, high-quality reflection seismic and well data to characterize the basin structure. We identify fast subsidence of the basin basement and a lack of brittle faulting of the offshore Red River fault in the Yinggehai-Song Hong basin since 5.5. Ma, despite dextral strike-slip movement on the onshore Red River fault. We calculate the upper-crustal, whole-crustal, and whole-lithospheric stretching factors for the Yinggehai-Song Hong basin, which show that the overall extension observed in the upper crust is substantially less than that observed for the whole crust or whole lithosphere. We suggest that fast basement subsidence after 5.5. Ma may arise from crustal to lithospheric stretching by the regional dynamic lower crustal/mantle flow originated by collision between India-Eurasia and Indian oceanic subduction below the Eurasian margin. In addition, we present a basin wide sediment budget in the Yinggehai-Song Hong basin to reconstruct the sedimentary flux from the Red River drainage constrained by high-resolution age and seismic stratigraphic data. The sediment accumulation rates show a sharp increase at 5.5. Ma, which suggests enhanced onshore erosion rates despite a slowing of tectonic processes. This high sediment supply filled the accommodation space produced by the fast subsidence since 5.5. Ma. Our data further highlight two prominent sharp decreases of the sediment accumulation at 23.3. Ma and 12.5. Ma, which could reflect a loss of drainage area following headwater capture from the Paleo-Red River. However, the low accumulation rate at 12.5. Ma also correlates with drier and therefore less erosive climatic conditions.