Orogeny, Formation And Evolution In The Bogeda Mountain Chains, Northwestern China

Although the study on the orogen has been continuing for about 100 years, it is still the hotspot in geology. Especially from the 1980's on, the mechanics of the orogens have become one of the key fields in the solid earth science. The Cenozoic Bogeda mountain chain taken as the representative intracontinental orogen is different from other intracontinental orogens by its especially erogenic process. In allusion to the characteristics of "strong orogeny" and "weak exhibition", the much systemic researches on the orogeny, evolution of Bogeda mountain have been carried out by means of the structural geology, petrology (sediment rocks, volcanic rocks, igneous rocks and metamorphic rocks) and chronology. The re-orogenic model of Bogeda Mountain is put forward. And the Cenozoic dynamics of Bogeda Mountain has been discussed. Based on these, the point of view that the far range effect of collision between Indian Plate and Eurasian Plate and deep structures of lithosphere together control the orogenic process is brought forward.Based on the systemic and comprehensive analysis of the data of many other workers, the tectonic setting of the northern part of Xinjiang (including eastern Tian shan) before Permian is analyzed synoptically. For resolving the many questions about the northern part of Xinjiang, the point of view that the many ophiolites outcropped in the northern part of Xinjiang maybe belong to one ocean basin is put forward, i.e. maybe there is only one ocean-Junggar-Tien Shan Ocean (part of paleo-Asian Ocean) developed in the northern part of Xinjiang. Based on this point, the tectonic units of northern part of Xinjiang are plotted out. There are two passive continental margins (Aletai passive continental margin and northern passive continental margin of Tarim) and three Arc-Basin belts (the eastern Junggar Arc-Basin belt in Devonian, the northern Tian Shan Arc-Basin belt in carboniferous and the southern Tian Shan Arc-Basin belt in Devonian). And in the premise of only one ocean, the tectonic evolution of northern part of Xinjiang is discussed, four evolution stages are put forward. They are (1) the extension of continent crust during Sino-Cambrian period; (2) the formation of oceanic basin during Ordovician -Silurian period; (3) the subduction and closure of oceanic basin during Devonian-Carboniferous period; (4) the intercontinental extension during Permian-Triassic period. And the ocean floor subducted symmetrically to the south and north respectively.There are many disputations about the geological setting of the Bogeda Mountain, such as small oceanic basin, aulacogen, rift or Island-Arc and so on. Based on the geochemical analysis combining other data, the author thinks that in Devonian time one aulacogen developed in the Bogeda Mountain region, and with the closure of the oceanic basin, the aulacogen developed into one Island-Arc.For the first time, the ductile shear zone has been found in the Bogeda Mountain, and this discloses the magic veiling of the initial orogeny. By the detailed analysis on the micro-scale andmacro-scale structures, the mechanics of the ductile shear zone, the phases of deformation and structural styles of the Bogeda Mountain are ail confirmed. The main deformation of the ductile shear zone was the north-south shortening, and later the extension and strike-slip movement were main ways of deformation, so the shear zone was a superimposed ductile one. Using the most advanced dating method-zircon SHRIMP dating, the author obtained the more reliable age data of the formation of the ductile shear zone, i.e. later Carboniferous. So the author thinks that the time of the initial orogeny of Bogeda Mountain was later Carboniferous, and the main mechanics of deformation was the north-south shear shortening. The formation of the ductile shear zone was the results of the closure of the oceanic basing accretion of plates and orogeny in Island-Arc zone.By studying the sediment sequence, the types of sediment material, tectonstratigraphy, paleocurrent and source region, the reconstr...