EVOLUTION OF A HERCYNIAN-INDOSINIAN FORELAND BASIN ON THE SOUTHEASTERN MARGIN OF THE YANGZI PLATE AND TECTONICS IN SOUTH CHINA

South China consisted of the Yangzi and South China plates whose margins were irregular in general. Both the northern and southern margins of the Yangzi plate were the passive continental margins during the Early Palaeozoic, whereas northwestern and southeastern margins of the South China plate were the active or passive continental margins, respectively. The South China and Yangzi plates collided with the eastern part of the Honghe River region along the Chenzhou-Wuzhou zone during the late Caledonian, and during the Hercynian and Indosinian, the southeastern margin of the Yangzi plate passed into a foreland basin. The remnant Qinfang trough was closed in the Early Permian. Since then, a united foreland basin came into being on the southeastern margin of the Yangzi plate. On the basis of the detailed tectonic and sedimentological studies, five stages have been recognized for the evolution of the foreland basin:(1) collision-crustal upwarping stage; (2) subsidence-starved basin stage; (3)turbidite-filling stage; (4) migration-enlargement stage, and (5) continental molasse stage.
The Indosinian plate was once part of the South China plate were prior to the Early Permian. The suture of the Yangzi and South China plates were finished in the late Early Permian during which the Indosinian plate began to separate from the South China plate and prograded southwards along the Honghe River fault, resulting in the development of the Tethys. Nevertheless, the Indosinian plate failed to enter into the South China plate in the end.
From the Late Permian to the early Early Triassic, the southern Guizhou-western Guangxi area belonged to the southwestern margin of the Yangzi plate and existed as the passive continental margin. Later at the end of the Early Triassic, the collision between the Indosinian and Yangzi plates occurred. At last in the Middle and Late Triassic, this area began to develop into the foreland basin and received flysch and continental molasse sediments.