Deep geodynamic processes involved in the continental collision between India and Asia are still controversial. To address this issue, phase velocities of 619 Rayleigh waves and 254 Love waves have been inverted by using an anisotropic tomographic technique. Such a technique enables us not only to retrieve phase velocity distributions but also to map azimuthal anisotropy on a large scale. Only phase velocity anomalies, in the period range 50--200 s, are dealt with here, but the azimuthal anisotropy has been taken into account in the inversion process. Moreover, because of the important influence of shallow layers on the inversion results, an inversion of phase velocities corrected for shallow layers has been performed. To correct raw phase velocities, the 3SMAC crust (heterogeneous reference Earth model) has been used. The comparison of phase velocity distributions for Love and Rayleigh waves with and without corrections of surficial layers implies that much of the low-velocity zone, visible at short periods, in central Tibet, can be explained by unusually thick crust. At periods longer than 100 s, Tibet is, overall, characterized by high-velocity anomalies. Such anomalies seem to converge toward the center of the plateau as period increases. We conclude that the phase velocity structure beneath Tibet results, at short periods, from thick crust and possible partial melting in the uppermost lithosphere and, at long periods, from the inward plunge of the cold lithospheric mantle surrounding the high plateau. ¿ 1998 American Geophysical Union