Pre-cenozoic evolution of the northern Qilian Orogen from zircon geochronology: Framework for early growth of the northern Tibetan Plateau

The northern Qilian Shan, located along the northeastern margin of the Tibetan Plateau, 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 understand the tectonic evolution of this complex region, we conducted an integrated investigation of field observations, U-Pb dating of igneous and detrital zircons, and a synthesis of previously published data to constrain and reconstruct the pre-Cenozoic history of the northern Qilian Shan. This effort reveals five major age populations important to the history of this region: 2550-2350, 1850-1750, 1050-950, 500-435, 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 northern 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) Early Cambrian subduction initiation along the margins of Qaidam and North China-Tarim continents resulted in Cambrian-Ordovician bivergent subduction, arc magmatism within these two continents, and consumption of the Qilian Ocean. (3) Final ocean closure and continental collision occurred at ca. 440 Ma and was associated with syn- and post-orogenic magmatism. (4) Collisional orogeny variably eroded the basement rocks, reorganized drainage networks, and altered sedimentary provenance for the lower Paleozoic to upper Paleozoic deposits. (5) Mesozoic extension led to the development of thick Jurassic-Cretaceous terrestrial basins. This pre-Cenozoic history resulted in preexisting weaknesses and/or low-friction detachment horizons that played a decisive role in controlling the pattern, distribution, and timing of Cenozoic deformation across the northern Tibetan Plateau.