The vast proto-Tibetan Plateau: New constraints from Paleogene Hoh Xil Basin

The surface uplift of the Tibetan Plateau is the key boundary condition in many Cenozoic geological events ranging from global cooling to changes of Asian environments during Cenozoic. However, poorly constrained timing for the uplift of Tibetan Plateau makes these interpretations highly debatable. Here we report results from sedimentology, detrital zircon U–Pb and Lu–Hf isotopic compositions, and paleomagnetic signatures from both the eastern and western Hoh Xil basins of north-central Tibetan Plateau. Sedimentary lithofacies and facies associations analyzed in the western Hoh Xil basin indicate they were deposited in a braided fluvial system and alluvial fan, similar with the Fenghuoshan Group, eastern Hoh Xil basin. Provenance analyses from conglomerate clast compositions, paleocurrent orientations, and detrital zircon U–Pb and Lu–Hf isotopic compositions document sediments in both western and eastern basins were derived from the Qiangtang and Lhasa blocks. These observations, in combination with comparative paleomagnetic results, imply that Hoh Xil basin was a single, wide basin during Paleogene. The period of Hoh Xil basin deposition was coeval with significant period of the early Cenozoic uplift and erosion of the Qiangtang and Lhasa blocks. These observations not only reinforce the suggestion that the Qiangtang and Lhasa blocks were uplifted during Eocene to form a proto-Tibetan Plateau, but also imply that the proto-Tibet Plateau is vast in areal extent. The large dimension and high elevation of the proto-Tibetan Plateau probably contributed to the global cooling during the early Eocene.