SEVERAL models have been proposed for the geodynamical evolution of the Tibet-Himalayas collision zone(1-6). It is now generally recognized that the high elevations of the region have been caused by mechanical thickening of the crust and flow in the mantle, but there is debate as to whether the thickening has occurred by the underthrusting of Indian crust under Tibet, or by distributed shortening and thickening of the Tibetan crust as India has pushed northwards into it. Here we address this question using seismic measurements of heterogeneity and anisotropy at depth, obtained with a temporary teleseismic array spanning 500 km from the Lesser Himalayas to central Tibet (Fig. 1). We observe a significant change in seismic anisotropy across the Indus-Tsangpo suture (ITS), suggesting a change in mode or direction of deformation at depth. In the Himalayas, our results are consistent with the stacking of Indian and Tibetan lithospheres, whereas north of the ITS the data indicate ductile flow in the mantle and show no sign of the Indian lithosphere.