The Xianshuihe-Xiaojiang fault system extending from eastern Tibet to central Yunnan, China, is a major left-shear structural boundary, accommodating the clockwise rotation of crustal rocks between the Eastern Himalayan syntaxis and the South China Block. Zircon and apatite fission track (FT) data are reported for 111 samples of basement collected from both sides of the northern part of this fault and spanning 300 km across the eastern margin of the Tibetan Plateau, where its mean elevation drops from 3500 to 1500 m above sea level. The zircon FT ages define two fossil partial annealing zones at different elevations, one fossilised at circa 130 Ma, and the other at circa 21 Ma as a result of cooling probably via regional denudation. The apatite FT ages are mostly less than 25 Ma, but a few granitoids higher up on the plateau retain late Cretaceous apparent ages. Within the apatite FT data with Neogene ages, there may be several partial annealing zones, with mixture modeling identifying age components suggestive of discrete cooling phases at circa 22, 7 and 2 Ma. The first-order pattern of the minimum amount of Neogene denudation for a section across the plateau margin immediately northeast of Xianshuihe Fault suggests that a relatively uniform 4--6 km has been eroded from the inner part of the plateau margin, increasing to 7--10 km at Kangding, where the oldest rocks (Precambrian) are exposed, and then decreasing markedly into Sichuan Basin. These new FT data combined with published FT data suggest that the present extent of the Tibetan Plateau was defined during the early Miocene. ¿ 2000 American Geophysical Union