PROBLEMS ON THE GEOLOGICAL STRUCTURE OF ALXA

The Alxa area is located in the juncture of the geosynclinal region of northwestern China and the North China platform and occupies an important position in terms of regional geological structure. As medium- and high-grade metamorphic rocks outcrop over large tracts in the area, it has been considered for many years that the area belongs to a platform region and is a part of the North China platform. The data obtained in the 1:200, 000-scale regional geological surveys in the Alxa area since the 1960’s show that there exists a large patch of late Paleozoic geosynclinal low-grade metamorphic strata containing fossils, thus suggesting that it is a part of the late Paleozoic Tianshan-Nei Monggol-Khingan geosynclinal system. Its western part adjoins the Beishan geosyncline of Gansu, its eastern part is adjacent to the Nei Monggol geosyncline, and its southern part is separated from the early Paleozoic Qilian geosyncline by the Longshoushan regional fault. The Alxa geosyncline occurs as an arc convex towards the south, which is consistent with the entire Mon- golian arc. The North China platform should be bounded on the west by the western slopes of the Helan and Liupan mountains. A correct understanding of the geological structure of Alxa is of certain significance for a further know- ledge of the history of geological structure prior to the. Paleozoic in northern China.