THE GEOLOGICAL EVOLUTION AND TECTONIC DIVISIONS OF THE SOUTHWESTERN SECTION OF THE JIANGNAN OROGEN IN SOUTH CHINA

The geological evolution of the southwestern section of the Jiangnan Orogen was divided into three tectonic cycles according to the stratigraphic features, sedimentary facies, igneous rocks, tectonic patterns and tectonic movements. The ocean-continent transition occurred in the earlier two cycles: the Wuling tectonic cycle (during the Middle Qingbaikou period in the Neoproterozoic) and the Xuefeng Caledonian tectonic cycle (from the Late Qingbaikou period to the Early Paleozoic). The intraplate orogeny occurred in the Hercynian-Indosinian-Yanshanian tectonic cycle (from the Late Paleozoic to the Early Cretaceous).

The Wuling tectonic cycle was concentrated in the northern Fanjing Mountain in Guizhou, Dayong and Pingjiang in Hunan. These areas can be considered as the evidence of the shrinkage and disappearance of the narrow South China ocean basin and the formation of South China plate as the result of the convergence and collision between the Yangtze and Cathaysia blocks.

The Xuefeng Caledonian tectonic cycle was concentrated in the belt from Luocheng-Longsheng in Guangxi to Tongdao in Hunan. These areas provide evidence of the orogenic events including the shrinkage and disappearance of the South China oceanic rift, the formation of South China block by the convergence and collision between the Yangtze and Cathaysia blocks again.

During the Hercynian-Indosinian-Yanshanian tectonic cycle, the core of the orogenic belt formed by the Yanshanian movement was concentrated in the Shaoxing-Beihai region, lying east of Guizhou. These areas embody the intraplate orogeny in the Yangtze and Cathaysia blocks. These findings above indicated that the core zones of the sedimentary basin and orogenic belt became younger and moved westward laterally during the three tectonic cycles.

Based on these results, a tectonic divisions of South China was proposed. The Shizong-Jiujiang belt was advocated to be the boundary between the Yangtze block and the Jiangnan Orogen. The Shaoxing-Beihai belt was proposed to be the boundary between the Jiangnan Orogen and the Cathaysia block. The southeastern section of the Jiangnan Orogen was a multiple orogen composed of three subdivisions: the Wuling orogenic sub-belt, the Caledonian orogenic sub-belt, and the Yanshanian orogenic sub-belt.