The sedimentary record of the Sanshui Basin:Implication to the Late Cretaceous tectonic evolution in the northern margin of South China Sea

Whether the South China continental margin had shifted from active subduction to passive extension in the Late Cretaceous remains controversial. Located in the northernmost of the South China Sea continental margin, Sanshui Basin developed continuous stratigraphy from Lower Cretaceous to Eocene and provides precious outcrops to study the regional tectonic evolution during the Cretaceous. Therefore, we conducted field observations, petrology, clay mineralogy, geochemistry, and detrital zircon chronology analyses of sedimentary rocks from the Upper Cretaceous Sanshui Formation in Sanshui Basin. Results suggest that the Sanshui Basin is characterized as an intermoutane basin with multiple provenances, strong hydrodynamic environment, and proximal accumulation in the Late Cretaceous. An angular unconformity at the boundary between the Lower and Upper Cretaceous was observed in the basin. The sedimentary facies of the northern basin changed from lacustrine sedimentary environment in the Early Cretaceous to alluvial facies in the Late Cretaceous. The zircon U-Pb ages of granitic gravelly sandstone from Sanshui Formation prominently range from 100 Ma to 300 Ma, which is close to the deposition age of Sanshui Formation. The major and trace elements of the Late Cretaceous sedimentary samples show characteristics of active continental margin, and are different from the Paleogene rifting sequences. Hence, we propose that the northern South China Sea margin underwent an intense tectonic uplift at the turn of the Early and Late Cretaceous(around 100 Ma). Afterward, the northern South China Sea margin entered a wide extension stage in the Late Cretaceous(～100 to ～80 Ma). This extensional phase is related to the back-arc extension in the active continental margin environment, which is different from the later passive rifting in the Cenozoic. The transition from active subduction to passive extension in the northern South China Sea may occur between the late Late Cretaceous and the Paleogene.