Evolution of basin-range coupling in the Yunkai Dashan-Shiwan Dashan area, Guangdong and Guangxi:with a discussion of several tectonic problems of South China

The Yunkai Dashan (Dashan means Great Mountains in Chinese) area has become a fold belt or a relative uplift since the pan-Cathaysian orogeny, and meanwhte the present Shiwan Dashan area and its predecessor have become a front depression or a foreland basin. They combine to form a basin-range coupling system. The tectonic evolution of the whole basin-range system may fall into the following stages: the first phase (Yunan movement) of the Early Paleozoic orogenic event gave rise to the Yunkai anticlinorium, with the paleo-Bobai front depression formed at its southeast side; the second phase (Beiliu movement) resulted in northwestward advance of the Yunkai anticlinorium and the foreland basin also migrated toward the northwest side; and the third phase (Guangxi movement) caused the Yunkai Dashan area to continue folding and uplift and the early Indosinian movement (Dongwu movement) led the Qinfang sea trough to be folded into a mountain, with a foreland formed at its northwest side. In the late Indosinian movement the Darongshan area was folded and uplifted and then the foreland had migrated to the southeast margin of the present Shiwan Dashan area, thus laying a foundation of the present tectonic framework of the Shiwan Dashan basin. In the Yanshanian period, with continuous NW-directed propagation of the front of the southeastern Guangxi thrust nappe structure, the Jurassic-Cretaceous foreland basin also migrated continuously in a NW direction. In the Cenozoic Himalayan period, great changes took place in the tectonic framework in the study area, i.e. the original NW-SE compressional stress field changed into a NE-SW one.