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Abstract: The timing of uplift of the Tibetan Plateau remains controversial, with estimates varying from Eocene to more recent than Pliocene. In particular, the paleoaltimetry of the Qiangtang terrane, on the central Tibetan Plateau, is completely unknown. Here, we present new stable isotope results of fluvial/lacustrine carbonate cement, pedogenic carbonate and marl from the Kangtuo and Suonahu formations deposited between ∼51 and ∼28Ma in the Heihuling–Bamaoqiongzong area of the northern Qiangtang terrane. The lithofacies associations indicate that the Kangtuo formation was deposited in alluvial fan and fluvial floodplain environments, and the Suonahu formation was deposited in near shore lacustrine, playa-lake and channelized fluvial environments. Carbon and oxygen isotope values, coupled with the sedimentary facies interpretations, point to evaporation and low respiration rates in the Eocene–Oligocene paleosols, suggesting an arid climate in the high Qiangtang area at the time. The δ 18Opsw values of paleo-surface water reconstructed from the lowest (i.e. the least evaporated) δ 18Oc (PDB) values of the unaltered authigenic carbonates are used to make the minimum estimates of the average paleoelevation of the drainage basin. The paleoelevation of the northern Qiangtang terrane is reconstructed as above 5000m by at least the middle Oligocene (28Ma), similar to the present elevation in this area. The aridity and the positive shift in oxygen isotope values of surface waters in our study area may suggest that the high Lhasa terrane established by the middle Oligocene blocked the northward transport of tropical moisture. Maintenance of high elevation (>5000m) from at least the Oligocene to the present suggests that the Qiangtang crust was already thickened by that time, and that underthrusting of India beneath Asia since then has continued to provide additional material while at the same time driving Asian lower crust eastward by crustal flow. [Copyright &y& Elsevier]

Abstract: We present the results of a seismic wide-angle reflection/refraction profile across the central Qaidam basin, the largest basin within the Qinghai-Tibetan plateau. The 350-km‐long profile extends from the northern margin of the East-Kunlun Shan to the southern margin of the Qilian Shan. The P- and S-wave velocity structure and Poisson''s ratio data provide constraints on composition. The crust here consists of a near-surface sedimentary layer and a four-layered crystalline crust having several significant features. (1) The sedimentary fill of the Qaidam basin reaches a maximum thickness of 8km, and the basin shape mirrors the uplifted Moho. (2) Within the four layers of the crystalline crust, P- (S-) wave velocities increase with depth and fall within the following velocity ranges: 5.9–6.3km/s (3.45–3.65km/s), 6.45–6.55km/s (3.7km/s), 6.65km/s (3.8km/s), and 6.7–6.9km/s (3.8–3.9km/s), respectively; (3) low-velocity zones with a 3–5% reduction in seismic velocity are detected in the lower half of the crust beneath the Qaidam basin and its transition to the Qilian Shan. (4) The crystalline crust is thickest beneath the northern margin of the basin towards the Qilian Shan (58–62km) and thinnest beneath the center of the basin (52km). Variations in crustal thickness are caused most pronouncedly by thickness variations in the lowermost layer of the crust. (5) Poisson''s ratio and P-wave velocity values suggest that the Qaidam crust has an essentially felsic composition with an intermediate layer at its base. Based on the crustal structure reported here, we suggest that late Cenozoic convergence is accommodated by thick-skinned tectonic deformation with thickening involving the entire crust across the Kunlun–Qaidam–Qilian system. [Copyright &y& Elsevier]