EVOLUTION AND BASIC CHARACTERISTICS OF TETHYS OROGENIC ZONE,WESTERN YUNNAN
Luo Junlie (Yunnan Bureau of Geology and Mineral Resources)
The western Yunnan, on the basis of its position within the global tectonics and the dynamic field of Neoid orogenic zone, is part of the Tethys tectonic zone and borders on the intersection point with the circum-Pacific tectonic zone; in the structural framework of geological evolution, it localized in the collision domain between the southern margin on the Eurasia with Gondwana landmasses. During the collision, the Yangtze, Indosinian, Shan, southern Tibet and Indian Plates etc., respectively belonging to the above mentioned two superduty structure units, successively pieced together to make extremely complete geologic structures occur in W Yunnan region. In the course of the regional geological evolution of W Yunnan, the Tethys tectonics had acted as a major action. This paper points out that the Paleo-Tethys two branches of the east and west extended into the W Yunnan; although the Neo-Tethys didn’t pass along Yunnan, its aplice along the Yarlung-zangbojiang-Myikyina brought the most parts of W Yunnan into the Kimali Block and brought in strong intracontinental deformation. There are 11 structure units in W Yunnan which belonged respectively to the“metamorphic terrains”and“microplates”developed before the Paleo-Tethys and another Paleo-Tethys accretionary zone occurred in the Wilson cycle of the Paleo-Tethys. The geologic structures and correspondent mineralizations in W Yunnan are demonstrated by this paper in the light of the three stages——before, within and after the Paleo-Tethys. They can be summarized as the foloowing key points: 1. There are three types of Precambrian metamorphic basements(i.e., the Yangtze block, Indosinian block and Himalayan plate)in the periphery of W Yunnan. The Cangshan—Ailaoshan and Gaoligongshan terrenes were respectively the residual mountainous root of the nappe of Yangtze block basement and the decollement products of Himalayan plate-type basement, whereas the other metamorphic terrenes might be the island are group of circum-Indosinian and Yangtze Paleocontinent. 2. The three microplates are the passive sedimentary region extending from the Shan, Indosinian and Yangtze plates respectively. Judged by the sedimentary formation, paleontologic appearance and lithofacies etc., more specifically, they present each phase characters of the pan-African movement similar to the western Asia, southern Europe and northern Africa. 3. The Paleo-Tethys in W Yunnan had mainly opened since the Lower Carboniferous, closed in late Upper Permian, and taken place extension fracture in Middle Trlassle and an intense orogeny in Upper Triassic(i.e:, the Wilson cycle). The said cycle can be divided into two subcycles; and the lower one corresponded to the mainly developed stage for the Paleo-Tethys ocean, the upper one, to the island are stage transiting into the intense orogeny, it may be regarded as an independent Meso-Tethys. 4. The accretionary zone formed in W Yunnan by Paleo-Tethys had a suit of relatively special lithofacies, disordered ophiolite belts, volcanite association of complicaed types showing a certain evolutionary regularity and the biggest synorogenic granite belt in W Yunnan. 5. Because of the separation of the Paleo-Tethys, two times of glacial and interglacial periods appeared from middle-late Carboniferous to early Permian in W Yunnan and the glacioaqueous sediments displaying the affiliation with the Zhufeng and Baoshan types and different from the typical Gondwana facies. The western boundary of western branch in Paleo-Tethys localized at the Nandinghe fracture, the boundary with the Eurasia landmass, therefore, is not the Lan-cangjiang fracture but the Nandinghe fracture. 6. According to the time and space distribution of the ductile shear zone and high-pressure metamorphic minerals, this paper emphasized that in the formation of the high-pressure minerals and tin(rare earths)-bearing granite belt, and the localization for disordered ophiolite, the large scale nappe system of the Indosinian major cycle and the order structure levels from ductility, ductility—brittleness to brittleness as well as the obduction stage relative to the subduction one are more primary. 7. Conclusions in this paper are somewhat different from the past litera- tures based on the paleogeographic position map of each structure unit in W Yunnan preliminarily restored by the paleomagnetic data. 8. On the basis of the drawn red sedimentary basin formed by deformation which resulted from the Neo-Tethys superimposing the geologic structures of W Yunnan, strike-slip shear system transformed from the ductile nappe, plateau uplift, active structures and geologic hazard, combining with rather characteristic intraplate magmatism, the importance of late Yenshan and early Himalayan for metallization of W Yunnan are analysed and emphasized in this paper. 9. According to the geophysical prospecting results of the Moho depths and explosion seismic profiles etc. calculated from existing gravitational field and residual gravity anomaly, some key deep-seated structures are reexplained by the Tethys tectonic view. 10. There are 3 different types of mountain chains from east to west in W Yunnan: i.e., the mountain chains of Mt. Ailao. subduction type, Chong-shan-Lancang collision type and Gaoligongshan intracontinental type.