We use ambient-noise tomography to map regional differences in crustal Rayleigh-wave group velocities with periods of 8-40 s across north Tibet using the International Deep Profiling of Tibet and the Himalaya phase IV arrays (132 stations, deployed for 10-24 months). For periods of 8-24 s (sensitive to midcrustal depths of ~5-30 km), we observe striking velocity changes across the Bangong-Nujiang and Jinsha suture zones as well as the Kunlun-Qaidam boundary. From south to north, we see higher velocities beneath the Lhasa terrane, lower velocities beneath the Qiangtang, higher velocities in the Songpan-Ganzi and Kunlun terranes, and the lowest velocities beneath the Qaidam Basin. Maps at periods of 34 and 40 s (sensitive to the middle and lower crust at depths of ~30-60 km) do not show evidence of changes across those boundaries. Any differences between the Tibetan terrane lower crusts that were present at accretion have been erased or displaced by Cenozoic processes and replaced almost ubiquitously by uniformly low velocities.