Abstract: The widely distributed onshore Cenozoic basins of the southern margin of the South China Block (SCB) commonly exhibit an unconformity between the Paleogene and Quaternary systems. Restoration of the erosion at this unconformity provides critical constraints for reconstructing the complete Cenozoic tectonic evolution history of the southern margin of the SCB. Focusing on the Nanxiong Basin, this study uses the vitrinite reflectance method of paleothermal indicator to restore the erosion thickness of the unconformity between the Guchengcun Formation (top age ~56 Ma) and overlying Quaternary, then constrain the initial time of erosion by combining low-temperature thermochronology information around the basin. The results show that the erosion thickness of Paleogene top strata in the Nanxiong Basin is approximately 2700±300 m, and the erosion started from the Early Oligocene and lasted until the Early Quaternary. Strong erosion caused the source rocks with high maturity to be denudated to a shallow burial depth. The analysis shows that the basin faulting development did not end after the depositional period of Guchengcun Formation in Nanxiong Basin but continued until the early Oligocene; subsequently, the tectonic environment changed from extension to compression, and the basin began to uplift and erode until the early Quaternary, during which intense erosion caused the basin to miss the sedimentary record of the late Paleocene−early Oligocene. The erosion process of the southern margin of the SCB since late Paleogene was affected by the southeast extrusion caused by the Indo-Eurasian plate collision and the subduction of the Pacific plate underneath the Eurasia plate. The uplift amplitude tended to gradually decrease from the inland to the coastal area, resulting in differential evolution between uplift onshore and subsidence offshore.

Key words: Vitrinite reflectance, erosion restoration, Southern margin of the South China Block, Nanxiong Basin