Late Eocene tectonic-geomorphic transition and its dynamic mechanism: Case study of the Beibuwan Basin, northern South China Sea

Abstract

The continuous sedimentary records of the Beibuwan Basin preserve vital information on the interaction between Tethys and Pacific tectonic domains. The elusive interaction between the lithosphere and mantle significantly impacts the differential evolution of basins. However, the understanding of surface-mantle dynamic coupling beneath the Beibuwan Basin remains unclear. In this study, we analyze Cenozoic tectonic deformation, sedimentation, and mantle dynamic processes in the Beibuwan Basin in the northern South China Sea to clarify the nature of the tectonic-geomorphic transition during the late Eocene. Results show that the basin underwent late Eocene stratigraphic flexural folding, syn-rift transition from NE to EW orientated, and depocenter migration. The orientation of the primary controlling fault system during this period changed from NE-SW to NEE-SWW, the number of syn-depositional faults decreased from 368 to 172. The thickness of depocenters decreased from 5000 m to 1800 m, and the lacustrine Beibuwan Basin became shallower and wider. A comparison of the tectonic-structural history of the Beibuwan Basin with the history of plate subduction reconstructed from global geodynamic models shows that the late Eocene tectonic transition was closely related to changes in the stress field and the mantle wind related to oceanic plate subduction. We conclude that this enduring surface-mantle interaction associated with subduction of the Izanagi-Pacific mid-ocean ridge resulted in the late Eocene tectonic transition and geomorphic changes in the Beibuwan Basin.