Geologic History Of The Central And Northern Tibetan Plateau And The Role Of Wilson Cycles In Constructing The Tethyan Orogenic System

In order to better constrain the evolution of the Tethyan orogeny,we conducted an integrated investigation involving U-Pb dating of igneous and detrital zircon,geochemical analysis of igneous rocks,compositional analysis of sedimentary strata,and a synthesis of existing work across the Qilian Shan,Qaidam Basin,and the Eastern Kunlun Range of central and northern Tibet.This effort reveals five stages of arc magmatism at 1005–910 Ma,790–720 Ma,580–500 Ma,490–375 Ma,and290–195 Ma in Central-North Tibetan Plateau.Arc activities were interrupted by repeated continentcontinent collision followed by ocean opening along the older suture zones first created in the Neoproterozoic.This suggests that Wilson cycles have played a controlling role in constructing the southern Asian continent.The magmatic history and regional geologic constraints allow us to construct a coherent tectonic model that has the following key features.(1)The linked South Qilian suture in the west and North Qinling suture in the east formed the northern boundary of the coherent Kunlun–Qaidam–North Qinling Terrane in the early Paleozoic.(2)The Songpan-Ganzi Terrane has been the western part of the Yangtze craton since the Neoproterozoic.(3)Development of the wide(>700 km)Permian–Triassic arc across the Kunlun-Qaidam Terrane was induced by flat subduction and rapid slab rollback,which also caused extreme extension of the Songpan-Ganzi Terrane.(4)The formation of the Anymaqen-Kunlun-Muztagh Ocean(=the Neo–Kunlun Ocean)was created within Laurasia rather than being a preexisting ocean between Gondwana and Laurasia as postulated by most early studies.Existing work on the basement evolution of the Qilian Shan-Nan Shan region of the northeastern Tibetan Plateau has severe limitations because of the lack of systematic geologic mapping.Geochronology studies must be put in the context of Cenozoic strain that has accumulated across this region.The aim of this study was to first establish the structural framework of a?1500 km~2 region in the central Qilian-Nan Shan thrust belt through detailed geologic mapping and couple this mapping with a systematic U-Pb zircon geochronology.This study identifies two Cenozoic thrust systems in the region:(1)the Shule and(2)Tuo Lai thrust systems.We suggest that the Tuolai Group was deposited between 1178 and 897 Ma.Zircons from high-and ultrahigh-pressure eclogites from the northern central Qilian have an igneous protolith with an age of ca.916±22.8 Ma.Most zircons display recrystallized rims,which suggests that eclogite-faces metamorphism occurred at ca.485±22 Ma.We integrate our U-Pb zircon ages with stratigraphic and paleontological data to discuss the origin and tectonic evolution of the Qilian Shan Precambrian basement in northwestern China.Also,we provide further support for the idea that the Precambrian Qilian Shan basement was part of the North China Craton.Tectonically,the Eastern Kunlun Range is dominated by the Qimen Tagh thrust belt,a collection of dominantly north-and northeast-dipping thrusts that are responsible for uplift of the range.The thrust belt is oriented obliquely to and merges with the Kunlun fault.Left-slip is not entirely restricted to the Kunlun fault,however,and is distributed with the range on several distinctly linear sub-parallel strike-slip faults.Generally,the observed relationships between strike-slip and thrust faults in the range are consistent with transpressional orogenic belts described elsewhere.In a broader context,the Qimen Tagh thrust belt occupies the northwest quadrant of wall rock adjacent to the entire Kunlun fault.The southeast quadrant is also dominated by shortening,while the northeast and southwest quadrants are zones of extension,resulting in an overall antisymmetric distribution of strain along the Kunlun fault system.New fission-track thermochronologic results for the range suggesting rapid cooling after 20 Ma.