The northward extent of subducted Indian plate is a fundamental component of hypotheses explaining deformation and magmatism within the Tibetan Plateau. Yet, these aspects of the plate are debated in west Tibet. Here we report a new three‐dimensional lithospheric structure of seismic velocity and radial anisotropy under west Tibet constructed from Rayleigh and Love wave phase velocity maps at periods of 20–167 and 20–125 s, respectively. Our results show the Indian lithospheric mantle to be subhorizontally subducted under west Tibet across the Bangong‐Nujiang suture to the Qiangtang terrane, as indicated by a prominent fast velocity anomaly accompanied with positive radial anisotropy (Vsh > Vsv). We find a positive spatial correlation of this result with information on the distribution of late Cenozoic potassic‐adakitic rocks in western Tibet. Additionally, we show that the midcrust of west Tibet is characterized by an anomalously low shear wave velocity (3.2–3.4 km/s at ~30‐km depth) and positive anisotropy, which is consistent with an estimated ~3% fraction of partial melt. We suggest that the midcrust of this region is capable of flowing and that its three‐dimensional structure shows it to extend south of the Karakoram fault (KKF), a shear zone interpreted as a barrier to crustal flow. Instead, our results are consistent with the KKF embedded in weak middle crust along with several other structures that display a pattern of distributed deformation in the western portion of the Tibetan Plateau.
The northeastern (NE) Tibetan plateau has been a prime site to understand the dynamic processes responsible for the rise and lateral growth of the Tibetan plateau. Here we construct a high‐resolution lithospheric scale isotropic SH wave velocity model (down to the depth of 130 km) for the NE Tibetan plateau and its adjacent regions based on the measurements of fundamental‐mode Love wave dispersions (20–100 s) with seismic data recorded by ChinArray II and China Digital Seismic Array using two‐plane wave method. Prominent slow SH wave velocity (VSH) is observed in the midlower crust of the NE Tibetan plateau, specifically in the Qilian Orogenic Belt and the Songpan‐Ganzi Terrane regions, coincident with previously indicated significantly slow SV wave velocity (VSV), probably implying the existence of partial melts in the midlower crust. This low SH wave velocity anomaly could be traced down to the uppermost mantle, indicating a weak and warm lithosphere, which we interpret as a consequence of asthenosphere upwelling after lithospheric mantle removal in the NE Tibetan plateau. The asthenosphere upwelling and associated deep‐seated thermal buoyancy could explain the occurrence of partial melting in the mid‐lower crust and account for parts of high elevations in the NE Tibetan plateau. The western Qinling orogenic belt is characterized with a high velocity anomaly in most of lithosphere, which is inconsistent with the existence of channel flow within the lithosphere along the Qinling orogenic belt from the Tibetan plateau.
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