PLATE TECTONIC CHARACTERISTICS DURING THE EARLY PALEOZOIC IN BEISHAN NEAR THE SINO-MONGOLIAN BORDER REGION, CHINA

The tectonic units during the Early Paleozoic in the Beishan area comprise the northern margin of the eastern end of the Tarim Plate and the northern flank of the Mingshui-Hanshan microplate. These two paleoplates were separated by an ophiolitic melange belt. All of them underwent the tectonic evolution typical of the continental margins of Atlantic, Andean and west Pacific Ocean types. The continental margin was an open continental shelf and a continental slope from the Early Cambrian to the Early Ordovician. At the southern flank of the continental shelf an isolated shallow sea basin developed separated from the shelf by an upwarping zone. Its northern flank, Shibanjing-Xiaohuangshan, is an ocean basin. North of this ocean basin is the Mingshui-Hanshan microcontinent or microplate. This area is situated in a tensional tectonic environment, it shows the characteristics of a passive, Atlantic-type margin. During the middle and late parts of the Early Ordovician the continental crust on the southern fringe of the shallow sea basin was pulled open, and a narrow, ocean rift formed. In the mid-Ordovician, this area underwent a period of considerable development, in which extension and subduction existed simultaneously. The southern fringe of the continental margin came to the end of the passive margin. On its northern side, a marginal volcanic zone, with calc-alkali volcanism, formed. In the Late Ordovician-Silurian, the oceanic crust was folded and subducted southward, and the Hengluanshan-Xichanjing marginal basin developed into an interarc basin, where the Hongliuhe-Niujuazi backarc basin and the Baiyunshan-Dongqiyishan volcanic arc had formed, with associated granitic magma. At the beginning of the Devonian, the Tarim Plate finally came into collision with the Mingshui-Hanshan microplate and the ocean basin closed, forming the Mingshui-Shibanjing-Xiaohuangshan suture belt. There were volcanic molasse basins on both sides of this orogenic zone. From the Late Paleozoic the Beishan area was incorporated into an intraplate mobile zone.