Study On The Early Paleozoic Tectonic Evolution And Cenozoic Intracontinental Development Of The Qilian Orogen, Northern Tibet

The Qilian Orogen,located along the northeastern margin of the Tibetan Plateau,has experienced multiple phases of orogeny throughout the Phanerozoic,culminating in fold-thrust and strike-slip deformation associated with the Cenozoic India-Asia collision.Earlier phases of deformation in the Qilian Shan have been severely reactivated or transformed by the most recent Cenozoic strain.To better constrain the tectonic evolution of this complex region,we conducted an integrated investigation of field observations,geologic mapping,U-Pb dating of igneous and detrital zircon,Electron backscatter diffraction(EBSD),zircon-and apatite-fission track thermochronology,analogue modelling and a synthesis of previously published data to investigate the tectonic development of Qilian Orogen over a range of timescales,from the early Paleozoic orogeny to the active intraconteninental deformation that is shaping the northern margin of the Tibetan Plateau,and the role of early framework in controlling the growth of the northern Tibetan Plateau.This work reveals five major age populations important to the history of this region: 2550-2350 Ma,1850-1750 Ma,1050-950 Ma,500-435 Ma,and 320-240 Ma.Using this dataset,we identified three major depositional shifts and/or variations in drainage patterns that affected sediment provenance of the Qilian Orogen.Observed regional geologic constraints,the magmatic history,and new geophysical observations of the deep structure allow us to propose a coherent tectonic model for the pre-Cenozoic evolution of the northeastern margin of the Tibetan Plateau.(1)Late Neoproterozoic to Cambrian rifting opened the Qilian Ocean.(2)Cambrian subduction initiation along the margins of Qaidam and North China-Tarim continents resulted in Cambro-Ordovician bivergent subductions,arc magmatism within these two continents,and closure of the Qilian Ocean.(3)Final ocean closure and continental collision occurred at ~440 Ma and was associated with synand post-orogenic magmatism,and low-medium temperature(400-500 °C)metamorphism.(4)Collisional orogeny variably eroded the basement rocks,reconstructed erosional drainage networks,and altered sedimentary provenance from the early Paleozoic sediments to the late Paleozoic deposits.(5)Mesozoic extension leads to the development of thick Jurassic-Cretaceous terrestrial basins.Based on the field observations and zircon-and apatite-fission track thermochronology analyses across the central segment of left-slip Haiyuan fault,the central and northern Qilian Shan and northern Qaidam basin,the Qilian Shan underwent a multi-phase cooling history,including:(1)broad cooling during the Jurrassic to Cretaceous as a result of a far-field tectonic event;(2)a period of thrust-related cooling occurred along the western of Qilian Shan and northern Qaidam thrust belt in the Eocene-Oligocene,and long-term tectonic quiescence from late Cretaceous to middle Miocene in the eastern Qilian Shan;(3)Miocene cooling and denudation drove the final acceleration to the surface,the reactivation of the proximal thrust faults and initiation of the central and western segment of the Haiyuan fault occurred at ~15-10 Ma.As indicated by the analogue model,we argue that the Qilian Shan thrust belt has persisted as the stationary and internally deformed northern boundary of the Himalayan-Tibetan orogen since the early Cenozoic,involved overprinting out-of-sequence development starting by Eocene related to initiation of India-Asia collision,and the basins and ranges across the northern Tibetan Plateau have since experienced multi-phase of growth.This pre-Cenozoic history and early frameworks resulted in pre-existing weaknesses and/or low-friction detachment horizons that played a decisive role in controlling the pattern,distribution,and timing of Cenozoic brittle deformation within the upper crust across northern Tibetan Plateau.