This paper presents a preliminary study on lake-level fluctuations since the Last Glaciation in Selin Co (lake), Central Tibet, by dating four groups of beach ridges using optically stimulated luminescence (OSL). The highest/oldest beach ridge group (>100 m higher than the current lake level) is dated back to 67.9 ± 2.4 ka BP, corresponding to the early stage of the Last Glaciation (marine isotope stage (MIS) 4). This date further supports that no plateau-scale ice sheet covered the Tibetan Plateau during the Last Glaciation. The other three groups produce OSL ages of 30.4 ± 2.9 to 18.6 ± 1.7, 12.5 ± 1.6 to 9.2 ± 0.5, and 6.9 ± 0.2 ka BP respectively, most likely corresponding to cold or wet climate periods of the late stage of the Last Glaciation (MIS 2), deglaciation, and Holocene Hypsithermal. On the plateau scale, these four beach ridge groups are almost synchronous with advances or standstills of Himalayan glaciers, indicating similar climate controls across the central and southern Tibetan Plateau, and being consistent with the conclusion, obtained from nearby ice core records, that this area is affected by the South Asia monsoon. Furthermore, beach ridges are also synchronous with fluvial terraces in the northern Tibetan Plateau, implying common driving forces during their formation. Therefore, some terraces may be formed as a result of climate events rather than being of tectonic origin.

The southeastern Tibetan Plateau is deeply incised by three parallel rivers, the Salween, the Mekong and the Yangtze. The river incision and surface uplift histories of this landscape are hotly debated. This study presents bedrock apatite (U-Th-Sm)/He data from a ~1800m vertical profile, located near the first bend of the Yangtze River. Ages range from 20 to 30Ma, indicating an Oligocene -early Miocene phase of moderate river incision at a rate of 0.10-0.18 mm/yr. This is considerably older than elsewhere in the region, but consistent with a previously proposed phase of Eocene surface uplift inferred from stable isotope geochemistry. We consider the implications of the new data under two different tectonic models. If the surface uplift and river incision resulted from lower crustal flow, the new results require such flow to have commenced at Oligocene -Early Miocene time rather than during the previously proposed Late Miocene. Alternatively, Oligocene to Early Miocene plateau growth might have resulted from transpressional deformation in the southeastern Tibetan Plateau. Highlights:Oligocene -early Miocene river incision Diachronous onset of river incision in the southeastern Tibetan Plateau Oligocene to Early Miocene lower crustal flow or transpressional deformation

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