The mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics, and thus the understanding of the processes at the lithosphere-asthenosphere boundary (LAB) is fundamental to understand how our planet works. It is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the LAB. Data from borehole broadband ocean bottom seismometers show that the LAB beneath the Pacific and Philippine Sea plates is sharp and age-dependent. The observed large shear wave velocity reduction at the LAB requires a partially molten asthenosphere consisting of horizontal melt-rich layers embedded in meltless mantle, which accounts for the large viscosity contrast at the LAB that facilitates horizontal plate motions.

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...

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A high‐resolution image of the base of the lithosphere from S‐to‐P converted seismic waves revealed the collision architecture of the Indian and Asian continental plates beneath the Tibetan Plateau. The base of the Indian lithosphere dips northward from a depth of 160 km beneath the Himalayas to a depth of 220 km just south of the Bangong suture. The base of the Asian lithosphere is nearly horizontal at the depth of 160–180 km from central to northern Tibet. There is a vertical gap of about 50 km between Indian and Asian lithospheres. Our observation of a well‐defined, thick lithosphere throughout the entire plateau is not consistent with models of wholesale convective instability of a thickened mantle lithosphere, which would predict a very thin Asian lithosphere. The hypothesized sequential southward subduction of Tibetan and Asian continental lithospheres leading to the growth of the Tibetan Plateau, if correct, cannot be occurring below ∼180 km depth. Our results, along with available geological and geophysical data, strongly support that the plateau is predominantly formed by a relatively coherent north dipping subducted Indian continental lithosphere in the south, which presently can be traced to the middle of the plateau, and a south dipping subducted Asian lithosphere in the north at a shallower depth.