Cenozoic denudation history of southern Hainan Island, South China Sea: Constraints from low temperature thermochronology

As a large block of continental crust, Hainan Island located in the western South China Sea forms a rift shoulder to the adjacent offshore Yinggehai and Qiongdongnan Basins, and lies in the footwall of their main boundary faults. We present 11 apatite fission track (AFT) data and 16 (U–Th–Sm)/He ages to elucidate the island's Cenozoic morphotectonic evolution. Our data indicate that much of southern Hainan Island experienced a well-defined cooling episode commencing in Late Eocene–Oligocene time. During this time, most samples cooled from ≥ 110 °C to ~ 55 ± 15 °C at a rate of ≥4.0 °C/Ma, following which the cooling rate decreased to ≤ 2.5 °C/Ma. Assuming a constant paleogeothermal gradient of 23 °C/km (similar to that of the present day), since Oligocene time when most samples cooled into the base of the AFT partial annealing zone, the total eroded section is estimated at a minimum of ~ 3.5 km, about two-thirds of which was removed during the Oligocene rapid cooling phase. This implies that elevation of southern Hainan Island has decreased since the Oligocene and this is supported by independent paleobotanical evidence. Although a further cooling episode since ~ 5 Ma is also suggested by AFT thermal history models, cooling occurred from relatively low temperatures (~ 45–50 °C) where annealing kinetics are not well understood so this cannot be rigorously assessed, despite evidence for increased sedimentation in offshore basins at this time. Our data suggest that the No.1 Fault of the adjacent offshore Yinggehai Basin to the west, which is thought to be an offshore extension of the Red River Fault Zone, was mainly active from Late Eocene through much of the Oligocene, but declined rapidly after ~ 22 Ma. The marked reduction of potential sediment supply rate from Hainan Island into neighboring basins after that time suggests that Miocene to Recent sedimentation in the Yinggehai Basin was mainly supplied by the Red River.

Research Highlights
► AFT and AHe data were used to elucidate Hainan island's Cenozoic evolution.
► Southern Hainan experienced a well-defined cooling episode.
► Most samples cooled from ≥ 110 °C to ~ 55 ± 15 °C at a rate ≥ 4.0 °C/Ma, then at ≤ 2.5 °C/Ma.
► Since Oligocene, the erosion is >~ 3.5 km, ~ 2/3 of which happened in early phase.
► No.1 Fault mainly active in Late Eocene–Oligocene, declined rapidly after ~ 22 Ma.