Tectonics of the North Qilian orogen, NW China

The Qilian Orogen at the northern margin of the Tibetan Plateau is a type suture zone that recorded a complete history from continental breakup to ocean basin evolution, and to the ultimate continental collision in the time period from the Neoproterozoic to the Paleozoic. The Qilian Ocean, often interpreted as representing the Proto-Tethyan Ocean, may actually be an eastern branch of the worldwide Iapetus Ocean between the two continents of Baltica and Laurentia, opened at >= 710 Ma as a consequence of breakup of supercontinent Rodinia. Initiation of the subduction in the Qilian Ocean probably occurred at similar to 520 Ma with the development of an Andean-type active continental margin represented by infant arc magmatism of similar to 517-490 Ma. In the beginning of Ordovician (similar to 490 Ma), part of the active margin was split from the continental Alashan block and the Andean-type active margin had thus evolved to western Pacific-type trench-arc-back-arc system represented by the MORB-like crust (i.e., SSZ-type ophiolite belt) formed in a back-arc basin setting in the time period of similar to 490-445 Ma. During this time, the subducting oceanic lithosphere underwent LT-HP metamorphism along a cold geotherm of similar to 6-7 degrees C/km. The Qilian Ocean was closed at the end of the Ordovician (similar to 445 Ma). Continental blocks started to collide and the northern edge of the Qilian-Qaidam block was underthrust/dragged beneath the Alashan block by the downgoing oceanic lithosphere to depths of similar to 100-200 km at about 435-420 Ma. Intensive orogenic activities occurred in the late Silurian and early Devonian in response to the exhumation of the subducted crustal materials. Briefly, the Qilian Orogen is conceptually a type example of the workings of plate tectonics from continental breakup to the development and evolution of an ocean basin, to the initiation of oceanic subduction and formation of arc and back-arc system, and to the final continental collision/subduction and exhumation. (C) 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.