THE FORMATIONS IN THE BEISHAN INTRAEUGEOSYNCLINE AND THE BASIC CHARACTERISTICS OF THE STRUCTURAL EVOLUTION

The area involved in this paper is a low mountain-hill area lying south of Qiong Tagh-Mazongshan of eastern Xinjiang and western Gansu and north of the Hexi Corridor of Gansu. The views as to the nature of the structures in the area have long been quite divergent. On the basis of the results of regional surveys on a scale of 1:200000 in recent years, we consider the Beishan Mountains to be a Paleozoic polycyclic intraeugeosyncline; it does not belong to the Tianshan geosynclinal fold system, but is an independent tectonic unit. The formation of the Beishan intraeugeosyncline might be due to back- are spreading caused by subduction of oceanic crust of the Northern Tian- shan Mountains towards continental crust of Tarim which resulted in contin- uous transformation of the backare basin into oceanic crust. The process of its development is as follows. At the beginning of the Ordovician an oceanic rift. was formed between fractures at the southern and northern sides of the area, in which was deposited a thicker flysch-like formation, accompanied by submarine eruptions of intermediate-basic magmas. At the end of the Silurian the area was folded and metamorphosed under the influence of the Caledonian movement, and then the geosyncline closed. At the beginning of the middle Devonian, the rift spread gradually again owing to the influence of the early Variscan movement, and terrigenous clastic sediments began to be deposited. By the late Devonian, large amounts of submarine volcanic eruptions took place, thus giving rise to a considerable thickness of keratophyre formation. At the end of the Devonian, the rift closed once again. In the early Carboniferous it tgain began to spread gradually, and a thicker flysch-like formation was deposited, accompanied by local outpouring of basaltic lava flows. In the middle to late Carboniferous more intense spreading occurred, resulting in wide distribution of tholeiite in the western part of the Beishan eugeosyncline. At the end of the Carboniferous intense folding took place, causing the strata to undergo greenstone metamorphism universally, which was followed by the closing of the rift. The rift spreading reached a climax in the Permian, when a mid-oceanic ridge was formed in the main rift zone. In the mid-oceanic ridge there occurred an association of ultrabasic rocks, basic rocks, pillow basalts and siliceous rocks which are similar to those in modern oceanic ridges. Besides, a branch rift extending from Cihai northeastwards through the Hongliu River up to the Sino-Mongolia border began to develop; it and the main rift zone combined to form a triple junction structure. During the whole of the Permian Period there were two repetitions of the process from spreading to closing and then back to spreading in the Beishan rift zone, and in the early part of the late Permian the rift closed with the cessassion of subduction of the the oceanic crust of the Northern Tianshan Mountains. In the latest Permian the area entered the stage of development of a platform. The process of development of the Paleozoic structures is represented by a movement of spreading→closing→respreading→reclosing. Such a manner of movement was the result of continuous changes in relationships between the velocities of accretion and consumption of the oceanic crust plate of the Northern Tianshan. But the general trend of development is marked by the formation of faulting→rifting→microspreading→formating of mid-oceanic ridge