Pure path phase velocities of fundamental mode Rayleigh waves, in the period range 30-80s, are obtained for different paths across the Tibet Plateau, using a ''two-event'' method, with events located in Tibet and aligned with World-Wide Standard Seismograph Network stations at teleseismic distance. Evidence is found for significant lateral variations of structure of the crust and uppermost mantle in Tibet. In particular, phase velocities obtained on paths contained in the Chang Thang platform are higher than average in Tibet, at periods shorter than 60s, while remaining very low at longer periods. When interpreting these phase velocities in terms of simple layered S velocity models, it is found that, contrary to other areas in Tibet, they are incompatible with the existence of a high velocity lid. S velocities of 4.4-4.5 km/s are found immediately beneath the Moho, and the crust in somewhat thinner (50-60 km) than average in Tibet. This indicates a possible upwelling of hot asthenospheric material, compatible with Landsat evidence for subrecent basaltic volcanism, and independent seismic evidence such as inefficient transmission of Sn waves. On the other hand, a 70 km thick crust and very high upper mantle S velocities are found on the western border of Tibet (Karakorum, Hindu Kush) compatible with previous finer-scale results concerning the associated zone of intermediate depth seismicity.