We report low average crustal P-wave velocities (5.9--6.1 km/s, Poisson's ratio 0.23--0.27, thickness 68--76 km) in southern Tibet from modelling regional Pnl waveforms recorded by the 1991--1992 Tibetan Plateau Experiment. We also find that the mantle lithosphere beneath the Indus-Tsangpo Suture and the Lhasa Terrane is shield-like (Pn velocity 8.20--8.25 km/s, lid thickness 80--140 km, positive velocity gradient 0.0015--0.0025 s-1). Analysis of relative Pn travel time residuals requires a decrease in the mantle velocities beneath the northern Lhasa Terrane, the Banggong-Nujiang Suture and the southern Qiangtang Terrane. Tectonic and petrologic considerations suggest that low bulk crustal velocities could result from a thick (50--60 km) felsic upper crust with vertically limited and laterally pervasive partial melt. These results are consistent with underthrusting of Indian Shield lithosphere beneath the Tibetan Plateau to at least the central Lhasa Terrane.¿ 1997 American Geophysical Union