Lithospheric deformation of the Tibetan plateau is caused by subduction of the Indian (northward) and Asian (southward) plates. The effects of this interaction on inner reaches of the Asian continent, between the Tibetan plateau and the North China craton (NCC) for example, remain uncertain due to poor geophysical data coverage in northeastern Tibet (NE Tibet). We provide here detailed knowledge of the lithospheric structure beneath NE Tibet as determined from a dense broadband seismic profile traversing NE Tibet to the westernmost NCC. Receiver function imaging reveals several significant features, including a north-dipping intracrustal converter (NC), Moho offset/overlap beneath major fault zones, and a low velocity layer (LVL) in the middle-lower crust. The lithosphere–asthenosphere boundary (LAB) is clearly defined and appears as a south-dipping interface that runs continuously from the Alxa interior to the Qilian orogen. Interpretation of these observations, combined with other seismic evidence, implies that the NCC lithospheric mantle has been persistently underthrust beneath the Qilian orogen. This process forms the thick-skinned crustal accretionary wedges, which develop above a middle-lower intracrustal decollement. Our results provide further deep-geophysical constraints on the Cenozoic post-collisional evolution of the convergent boundary between NE Tibet and the NCC and help clarify the mechanism of plateau growth in this boundary area

Detailed seismic structure in the crust beneath the northeastern margin of Tibetan Plateau was revealed by receiver functions of a regional permanent seismic network. At most stations, negative P‐to‐S converted phases can be detected in the radial receiver functions, prior to the Moho phases, indicating low velocities in the midlower crust. Prominent azimuthal variations in the transverse receiver functions with polarity reversal suggest azimuthal anisotropy in the crust. We used time variations of the P‐to‐S converted phases at the Moho in the radial receiver functions and the azimuth‐weighted stacking of transverse receiver functions to determine the fast direction and magnitude of anisotropy. The low‐velocity midlower crust with the coherent azimuthal anisotropy in the northeastern margin of Tibetan Plateau is consistent with the lower crustal channel flow model.

Global stacks of receiver functions clearly exhibit the upper mantle stratification. Besides the most prominent seismic discontinuities, such as the Moho and the 410 and 660 km discontinuities, a negative discontinuity is detected at a depth of ~600 km, indicating a low‐velocity layer at the base of the mantle transition zone. The slant‐slack technique helps to identify the primary conversions from the multiple reverberations. Presence of the negative 600 km discontinuity underneath both continent and ocean island stations, where the crustal thickness significantly differs, also precludes the possible cause of crustal reverberations. We conclude that the negative 600 km discontinuity could be a global feature, possibly resulted from accumulation of ancient subducted oceanic crust. The X‐discontinuity at ~300 km depth is also observed in our global stacks, which can be explained by the coesite‐stishovite phase transformation.