THE SICHUAN OROGENY IN WESTERN YUNNAN AND SICHUAN

Recent data clearly indicate that the Sichuan orogeny did take place in western Yunnan and Sichuan and comprises two episodes. The early episode took place between the Late Cretaceous and Paleocene. According to a synthetic analysis of the characteristics and distribution of unconformities, stratigraphic breaks and magmatic intrusion, the tectonic activity in this episode was mainly expressed by large-scale fold-downwarping, uplifting and faulting. The metamorphism and fold movement took place only along the fracture zone. he late episode took place between the early-mid Eocene and late Eocene. All characteristics indicate that the tectonic activity had a large scale and influenced broad areas. It was represented by remarkable fold and fault movements, followed by large-scale uplift of the earth’s crust. Magmatie intrusion occurred in both episodes. The tectonic activity became weak and uplifting gradually predominated over folding from south to north and from west to east. It had a tendency to become weak from the center of the region to the edges. Obviously, the Sichuan orogeny is closely related to the Tethyan tectogenesis. Undoubtedly, the driving force must come from the margins of the Tethys. The Sichuan orogeny represents a short period of relative stability after the strong Yanshanian orogeny and the eve of the collision between the Indian plate and the Eurasian plate as well as a transition period. So the Sichuan orogeny does not belong to the Yanshanian subcycle and was the beginning of the Himalayan subcycle.It was not a continuation of the Yanshanian orogeny, but the prelude of the Himalayan orogeny. Its tectonic significance lies in its forming a connecting link between two subcycles, i.e.the preceding and the following.