The fate of the colliding Indian and Asian tectonic plates below the Tibetan high plateau may be visualized by, in addition to seismic tomography, mapping the deep seismic discontinuities, like the crust-mantle boundary (Moho), the lithosphere-asthenosphere boundary (LAB), or the discontinuities at 410 and 660 km depth. We herein present observations of seismic discontinuities with the P and S receiver function techniques beneath central and western Tibet along two new profiles and discuss the results in connection with results from earlier profiles, which did observe the LAB. The LAB of the Indian and Asian plates is well-imaged by several profiles and suggests a changing mode of India-Asia collision in the east-west direction. From eastern Himalayan syntaxis to the western edge of the Tarim Basin, the Indian lithosphere is underthrusting Tibet at an increasingly shallower angle and reaching progressively further to the north. A particular lithospheric region was formed in northern and eastern Tibet as a crush zone between the two colliding plates, the existence of which is marked by high temperature, low mantle seismic wavespeed (correlating with late arriving signals from the 410 discontinuity), poor Sn propagation, east and southeast oriented global positioning system displacements, and strikingly larger seismic (SKS) anisotropy.Tibetan lithosphere | receiver functions | anisotropy I t has long been recognised that the Tibetan plateau was created by the collision of the northward moving Indian plate and the relatively stationary Asian plate, which began about 50 million yr ago (1). However, the mode of deformation of the mantle lithospheres (2) remained largely unknown. A fundamental question is whether the postcollision convergence of India and Asia, estimated at >2;000 km (3, 4), was accommodated by homogeneous thickening or plate subduction (2). Global positioning systems (GPS) measurements have shown that at present an eastward motion dominates the surface deformation of northern and eastern Tibet (5). GPS and seismic anisotropy (6) indicate extrusion also of the deep Tibetan lithosphere to the east and southeast. Most surface wave studies revealed a thick lithosphere beneath much of the plateau (7-12), whereas body wave tomography observed the subducted Indian mantle lithosphere characterized by high wavespeed, in contrast to the Asian mantle lithosphere (13-15). Recently a high resolution P travel time tomographic study (15) imaged the high velocity Indian lithosphere in western Tibet below the entire plateau down to 300-400 km depth. In eastern Tibet, however, the front of the Indian plate is located south of the Yarlong-Zangbo Suture (YZS) (15). Relatively slow wave speeds are found in the upper mantle below the central and northeastern parts of the plateau. Modeling indicates that the Tibetan part of the lithosphere originated from the progressive accretion of a number of continental or island-arc type blocks before India came into direct contact with Asia (16) or stepwise subduction of the Asian pl...

We have obtained velocity images of the uppermost mantle beneath China by performing tomographic inversion of both Pn and Sn traveltimes. From the Annual Bulletin of Chinese Earthquakes, 99,139 Pn arrivals and 43,646 Sn arrivals were selected. Pn anisotropy was also obtained simultaneously with Pn velocity. Average Pn and Sn velocities are 8.05 and 4.55 km/s, respectively, and maximum velocity perturbations are about 3–4%. The Pn and Sn velocities are low in eastern China and high in western China. Particularly high velocities are associated with old basins (for example, Tarim, Junggar, Turpan‐Hami, Qaidam, and Sichuan) and stable craton (for example, Ordos). Low Sn velocities are found mainly throughout North China. In addition, velocities are relatively low beneath the central Tibetan Plateau and the North‐South Seismic Zone (along 103°E). In Tarim and central China where we observe strong anisotropy, the fast Pn velocity directions are consistent with the directions of maximum principal compressive stress as well as directions of crustal movement determined from Global Positioning System. Beneath the India‐Eurasia collision zone, the Pn anisotropy direction is parallel to the collision arc and nearly perpendicular to both the direction of maximum compression and crustal movement resulting from pure shear deformation. Both the velocity variations and anisotropy indicate that the Tibetan Plateau was extruded, and the mantle material beneath the plateau has flowed around the East Himalaya Syntax, while the remaining material has diverted northwestward beneath the Tarim Basin.