The nature of the Rucheng Basin in southeastern Hunan and its significance to the tectonic setting of South China during the early Yanshanian

The Rucheng Basin in southeastern Hunan is believed to be an Early Jurassic-early Middle Jurassic continental basin.The Lower Jurassic Xintianmen Formation and Gaojiatian Formation are built up of the continental lacustrine-marsh clastic deposits intercalated with low-K and high-Na within-plate tholeiite contaminated by the Earth's crust,indicating that the basin is a synorogenic uplifted extensional rift basin during the Early Jurassic.Till the earliest Middle Jurassic,there existed a parallel unconformity between the Qianfoya Formation and its underlying Gaojiatian Formation.The molasse-like deposits occur in the lower part of the Qianfoya Formation,while the Gaojiatian Formation basaltic volcaniclastics appear locally in the upper part.These strata are west-leaning and monoclinal.The thrust faults are arranged along the western margin of the basin.This implies that the basin was once a compressional basin formed in an orogenic tectonic setting during the earliest Middle Jurassic.The integration of the Mesozoic tectonic framework and occurrence of the early Yanshanian post-orogenic granites shows that southeastern Hunan and HunanGuangdong-Jiangxi border areas were assigned to an intracontinental synorogenic tectonic setting during the Early Jurassic and earliest Middle Jurassic,and a post-orogenic tectonic setting during the early Middle Jurassic and Late Jurassic.The approaches of the nature of the basin will be significant to the research of the Mesozoic tectonic evolution in southern China.