Lower Permian formations of the Buqingshan Mountains in the A'nyemaqen ophiolitic zone (Eastern Kunlun, Qinghai Province, China): On the beginning of the Paleotethys closure

Abstract

Several Lower Permian formations are most completely represented within the Buqingshan Mountains (eastern portion of the Eastern Kunlun orogenic belt), a part of the Paleotethyan ophiolite suture commonly considered as the A'nyemaqen ophiolite zone. The formations were formed in the following paleogeodynamic environments (in a southward direction): (1) shelf and slope of a passive continental margin of a marginal sea; (2) partially Permian metamorphic rocks representing a subduction-accretion complex of an ensimatic island arc and volcanogenic rocks of an ensimatic island arc, of the age limited from above by the Asselian and Sakmarian stages; and (3) slope of an island arc and oceanic trench. The formations of the subduction-accretion complex and of the island arc volcanites are overlain with a sharp angular unconformity by a carbonate conglomerate sequence, which presents as a local molasse of the Lower Permian age. Based on fusulinids contained in the basal limestones, the age of the local molasse is first defined as the Yakhtashian-Bolorian, i.e. Artinskian-Kungurian (?). The sediments of the northern shelf and slope of the passive continental margin in the marginal sea representing at that time as the Paleotethys relict, are synchronous. The thorough investigations revealed that the initial closure of the eastern Paleotethys within the Eastern Kunlun corresponded to the Sakmarian-Yakhtashian (Artinskian) boundary, whereas in the western portion (Northern Pamirs) the closure occurred considerably earlier, prior to the Late Bashkirian time. Thus, the idea that the Paleotethys in the Eastern Kunlun reached its maximum width in the Permian, is highly questionable. During the Early Permian the A'nyemaqen branch of the Paleotethys intensely decreased. Beginning from the Bolorian (Kungurian) time and up to the end of the Permian this branch represented its relict in the form of a marginal sea depression. It may be suggested that the Paleotethys closure in the A'nyemaqen took place gradually from the west to the east and covered a long period from the Late Carboniferous to the terminal Early Permian. The final closure of the Paleotethys branches corresponded to the Permian-Triassic boundary and was accompanied by an intensive orogeny and deposition of a thick continental molasse. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts and inversion tectonics; 8140 Tectonophysics: Ophiolites; 9320 Geographic Location: Asia;