The timing of formation of the low‐gradient, internally drained landscape of the Tibetan Plateau is fundamental to understanding the evolution of the plateau as a whole. Well‐dated sedimentary records of internal drainage of rivers into lakes are used to reveal the timing of this evolution. Here we redate the youngest continental sedimentary successions of central Tibet in the Lunpola Basin and propose a new age range of ca. 35 to 9 Ma, significantly younger than previously thought. We demonstrate long‐standing internal drainage in central Tibet since the late Eocene and stable sedimentary environments, source regions, and low topographic relief since at least the early Miocene. We suggest that sediment aggradation of internal drainage and reduction of hillslope gradients by erosion dominate the formation of low‐relief landscapes and that the late Cenozoic drainage basins in central Tibet developed in response to flow in the lower crust and/or mantle lithosphere.

Along with intensification of global warming, severe desertification has already impaired human sustainable development. In a near-future greenhouse world, the total area of desert will increase, and new types of desert may emerge. During the “greenhouse” Cretaceous, conventional large paleo-ergs developed in broad topographic basins, and many possible ergs developed in small-scale intermountain basins, which are unusual in near-modern times and less studied. A comprehensive study of their sedimentary architecture and mechanisms would refine our interpretation of desertification in a near-future “greenhouse” world. The Xinjiang Basin is a typical small-scale intermountain basin in Southeast China that formed >300 m of successive aeolian deposits during the early Late Cretaceous. In this study, we applied detailed facies and architecture analyses to the Tangbian Formation (K2t) in 16 outcrops throughout the Xinjiang Basin and reconstructed a three-dimensional sedimentary model for the intermountain ergs. We confirmed that the Tangbian Formation formed in a typical intermountain paleo-erg and summarized in detail the differences in sedimentary architecture between intermountain ergs and broad topographic ergs. We noticed that the “greenhouse” state during the Late Cretaceous seems to have been suitable for the development of ergs in intermountain basins due to the hot, arid climate conditions and penetrating winds with sufficient transport capacity. Therefore, we suggest that in addition to the ongoing expansion of broad topographic ergs, the emergence and development of intermountain ergs in a near-future “greenhouse” world would also contribute to global desert expansion and massive desertification.

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Although there is increasing evidence for wet, monsoonal conditions in Southeast Asia during the late Eocene, it has not been clear when this environment became established. Cenozoic sedimentary sequences constrained by radiometrically dated igneous rocks from the Jianchuan Basin in the southeast flank of Tibetan Plateau now provide a section whose facies and climatic proxies determine this evolution. Semi-arid conditions had dominated the region since Paleocene controlled by the northern sub-tropical high pressure system, culminating in mid Eocene when desert dunes developed. From 36 Ma, the basin began to accumulate swamp sediments with coals, together with synchronous braided river deposits and diversified pollen assemblages, indicating significant increase in precipitation. This remarkable transition from dry to wet conditions precedes the Eocene/Oligocene boundary at 34 Ma, thus excluding general global cooling as the prime driver. We propose that uplift of Tibetan Plateau might have reached a threshold elevation by that time, operating through thermal and dynamic forcing, causing the inception or significant intensification of monsoonal rains to penetrate into this downwind locality.