We address the problem of late Cenozoic uplift, erosion, and growth of northeastern Tibet by reconstructing, from isopach maps and drill holes, the volumes and masses of sediments deposited in the Qaidam and Hexi Corridor basins since ∼35 Myr ago. The mass budget is based on simple geometrical assumptions such as regional similarity of the thickness ratios between strata of different ages. In the Qaidam, where our record extends back to the Oligocene, the budget shows a huge rise of the accumulation rates after the beginning of the Pliocene (5.3 Ma). The early Pliocene seems to be the period of maximum deposition with accumulation rates in excess of 1 mm yr−1 (∼ 2.7 kg m−2 yr−1) of compacted rocks throughout the basin. There also seems to be a southeastward shift of the largest depocenters between the upper Pliocene (3.4–1.6 Ma) and the Quaternary. In the Hexi Corridor, sedimentation is confined to small foreland flexural depressions associated with the frontal thrusts of the Qilian Shan and occurs at an average rate one order of magnitude smaller than in the Qaidam basin. The accumulation rate is maximum in the Quaternary. The sedimentation history appears to support a plateau-building mechanism resulting from the combination of two geologicallly common processes: crustal-scale thrusting and sedimentary basin infilling. The time needed to completely fill the Qaidam basin and make its catchment a plateau closely resembling that of the highest part of Tibet (Qangtang) is of the order of 9 Myr. The mechanism now at work north of the Kunlun, which involves rapid infilling of broad, flat areas separated by relatively narrow mountain ranges, has thus probably been important in producing the high, smooth topography that characterizes much of central Tibet.