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Abstract The Lhasa terrane is thought to be a key region for understanding the elevation history of the southern Tibetan Plateau after collision of continental India and Asia in early Paleocene. Stable isotopic studies consistently suggest that the Lhasa terrane had reached its near-present elevation before the Miocene. In this paper we report a plant megafossil assemblage from the Kailas Formation of the Kailas Basin in western part of the southern Lhasa terrane. Our U-Pb dating and magnetostratigraphic correlation show that the studied section of the Kailas Formation ranges from 25.1 Ma to 21.8 Ma in age, and that the fossil flora occurred at 23.3 Ma (latest Oligocene). The uppermost Oligocene Kailas fossil flora is low in diversity and dominated by Populus , Betulaceae and legume. It most likely represents a deciduous, broad-leaf vegetation and suggests a temperate, humid environment with a low to moderate palaeoelevation of 1500–2900 m in the Kailas Basin during the latest Oligocene, based on the co-existing range of the living analogues of the fossil plants. This conflicts with the existence of a continuous, high Gangdese Mountains stretching across the whole southern Lhasa terrane in the late Oligocene. Available evidence appears to suggest strong uplift of the southern Lhasa terrane after the latest Oligocene. Highlights • Plant fossils first reported from the uppermost Oligocene of Kailas Basin, Tibet • Fossils indicate temperate, humid environment and low to moderate paleoelevation. • Strong uplift of southern Lhasa terrane after the latest Oligocene [ABSTRACT FROM AUTHOR]

The mechanisms and controls of the Middle-Late Miocene paleoenvironmental evolution of the northeastern Tibetan Plateau remain elusive. Here, we present an integrated multiproxy study of total organic carbon isotopes, lipid biomarkers, petrographic components and micropaleontological records in the well-dated Xigou section of the Xunhua Basin. Our data reveal a large decrease (−1.1 ‰) in bulk organic carbon isotopes (δ13C org) at ∼12.7 Ma, a broad minimum at ∼12.7–8.0 Ma, and a large increase (+1.3 ‰) at ∼8.0 Ma. The decline in δ13C org at 12.7 Ma was accompanied by pronounced changes in n -alkane indices, clay minerals, palynology, and ostracods, recording a major rise of lake level of paleo-Lake Xunhua due to increased precipitation in the regional catchment. Our inference of sustained elevated humidity in the Xunhua Basin at 12.7–8.0 Ma is contrary to previously reported cooling and drying trends on the NE Tibetan Plateau for the Middle-Late Miocene (∼14–8 Ma), thus precluding the role of global climate change as the dominant driver. Rather, we attribute the paleoenvironmental evolution of the Xunhua Basin to changes in regional rainfall patterns related to orogenic uplift. The Jishi Shan mountain range reached a critical threshold elevation at ∼12.7 Ma, becoming an orographic barrier to Westerlies-transported moisture and thus enhancing intensified precipitation within the Xunhua Basin. By ∼8 Ma, further uplift caused this orogen to become a barrier to the East Asian monsoon, resulting in aridification of the Xunhua Basin. • The Xunhua Basin sensitively recorded environmental changes on NE Tibetan Plateau. • Uplift of Jishi Shan since ∼13 Ma caused moister Xunhua Basin and drier Lixia Basin. • Regional humidity changes due to trapping of Westerlies moisture to west, rain shadow effect to east. • This environmental change recorded by d13C org , n -alkanes, ostracods, and palynology. • Two key environmental transition periods—at ∼12.7 Ma and ∼8.0 Ma. [ABSTRACT FROM AUTHOR]