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We investigated velocity and anisotropic structure of the crust beneath the eastern margin of the Tibetan Plateau to better understand its deformation and evolution mechanism. We performed H‐κ and Pms anisotropy analyses to obtain crustal thickness, Vp/Vs ratio, fast polarization direction, and splitting time from 711 stations, and further conducted quality control using slowness, harmonic and statistical analyses. The Songpan‐Ganzi Block has a large splitting time and a fast polarization direction roughly parallel to the GPS motion and SKS fast direction. It also shows an overall high but complex distribution of Vp/Vs ratio, and large variations in crustal thickness, indicating that crustal deformation is likely caused by crustal shortening and lower crustal flow. The northern Sichuan‐Yunnan Rhombic Block (SYRB) is featured by a thick crust and high Vp/Vs ratio, suggesting that the crust is likely inflated by partial melting lower crustal rocks. The subblock also exhibits a strong azimuthal anisotropy with a splitting time greater than 0.6 s. The fast polarization direction aligns with the nearly N‐S extended direction and rotates clockwise in front of the Emeishan Large Igneous Province (ELIP). The observed anisotropy agrees with aligned amphibole minerals under a simple shear condition, supporting a southward lower crust flow being diverted by the ELIP. Anisotropy measurements on the southern SYRB are less robust and widely scattered, suggesting a deformation mechanism different from the northern SYRB. In addition, the southeastern margin of the Sichuan Basin shows a systematic pattern of crustal anisotropy consistent with a pure shear deformation mechanism. Plain Language Summary: We studied the crustal structure and deformation beneath the eastern edge of the Tibetan Plateau. By analyzing teleseismic data from 727 stations, we measured crust thickness, Vp/Vs ratio, and azimuthal anisotropy using receiver functions. Our findings show a complex crustal behavior pattern in the eastern Tibet margin. In the Songpan‐Ganzi Block, crustal shortening and lower crust flow seem to be causing deformation. The northern Sichuan‐Yunnan Rhombic Block, with its thick crust and high Vp/Vs ratio, appears to be influenced by partial melting of lower crust rocks. And a southward lower crust flow being diverted by the Emeishan Large Igneous Province. The eastern and southern part of this block shows a different pattern, indicating a unique deformation mechanism. Additionally, the southeastern margin of the Sichuan Basin appears to undergo a distinct type of crustal deformation. Key Points: A new crustal model with much more reliable and denser Pms anisotropic measurements is built in the eastern Tibetan MarginSongpan‐Ganzi Block is likely deformed by crustal shortening and lower crustal flowChannel flow may exist in the west low velocity zone (LVZ) while absent at the east LVZ [ABSTRACT FROM AUTHOR]

The convergence of India and Eurasia and the obstruction from the rigid Sichuan Basin cause the Longmenshan (LMS) to have the steepest topographic gradient at the eastern margin of the Tibetan Plateau. However, the mechanisms of surface uplift are still controversial. In this paper, we estimate the crustal structure and deformation under the LMS and its surroundings by analyzing a large amount of receiver function data recorded by regional seismic networks of the China Earthquake Administration. We apply a comprehensive splitting measurement technique on Ps conversion phase at the Moho (Moho Ps splitting) to calculate crustal anisotropy from azimuthal variations of receiver functions. Our results show that most of the seismic stations beneath the LMS area exhibit significant seismic anisotropy with the splitting time of 0.22–0.94 s and a fast polarization direction of NW–SE, while less or even no crustal anisotropy has been observed under the Sichuan Basin. Comparing the fast polarization directions of Moho Ps splitting with the indicators of lithospheric deformation (such as shear wave splitting, absolute plate motion, and global positioning system) imply a consistent tendency of deformation between the lower crust and upper mantle, but decoupling deformation in the crust beneath the LMS area. We further compare Moho Ps splitting time to that estimated from previous SKS splitting, indicating that crustal anisotropy is an important source of the SKS splitting time in this study area. In addition, a thick crust (>50 km) with high Vp / Vs values (1.74–1.86) is also observed using the H-κ stacking method. These seismic observations are consistent with the scenario that the LMS area has been built by the lower crustal flow. Combined with the seismic reflection/refraction profile and geology studies, we further suggest that the lower crustal flow may extrude upward into the upper crust along the steeply dipping strike faults under the LMS area, resulting in the surface uplift of the LMS. [ABSTRACT FROM AUTHOR]

Abstract: We analyze a large amount of receiver function data recorded by regional seismic networks of the China Earthquake Administration to estimate crustal structure and deformation beneath the southeast margin of the Tibetan plateau and its surrounding areas. We develop a comprehensive analysis method that facilitates robust extraction of azimuthal seismic anisotropy from receiver function data. The method includes an estimate of fast polarization direction and splitting time by a joint analysis of radial and transverse receiver function data, and an evaluation of measurement reliability by statistical and harmonic analysis. We find significant seismic anisotropy with a splitting time of 0.5–0.9s beneath the SE margin of the Tibetan plateau. Both the splitting time and fast polarization direction are comparable to those estimated from SKS/SKKS data, suggesting that crustal anisotropy is the main cause of shear wave splitting of the SKS/SKKS wave. This also suggests that deformation in the upper mantle is either weak or predominantly vertical, and is obviously different from the one in the crust. A vertical flow in the upper mantle, combined with the observation of a thin lithosphere beneath the study area, leads to the inference that part of the mantle lithosphere may have been removed and is descending into deep mantle. Stations located in the surrounding areas, on the other hand, exhibit very little to no crustal anisotropy. The estimated Moho depth and Vp/Vs ratio also show a distinct difference between the SE Tibetan plateau and the surrounding regions. Stations on the Tibetan plateau have a Vp/Vs ratio of ∼1.79, which is substantially higher than those measured at the Yunnan–Guizhou (Yungui) plateau (∼1.69). Our observations here are consistent with the scenario that the SE Tibet has been built by lower crustal flow. They also suggest that the mantle lithosphere beneath the margin may have been mechanically decoupled from the upper crust. [Copyright &y& Elsevier]

The southeastern Tibetan Plateau (SE Tibet), located between the Eastern Himalayan Syntaxis and Sichuan Basin, is a heterogeneous structure combining different tectonic regimes. It encompasses a gently sloping long-wavelength margin, and is characterized by intense deformation and active seismicity. Mechanical strength of the lithosphere is one of the parameters that controls deformation and evolution of any region. In particular, anisotropy of the strength is important for understanding the relation between tectonic processes and lithosphere deformation. As a proxy, the effective elastic thickness of the lithosphere (T e ) can be used to study both spatial distribution and anisotropy of the lithospheric strength. Here we use the fan wavelet coherence method to determine variations of T e and its anisotropy based on the topography and Bouguer gravity anomaly data. The coherence analysis shows that low T e (5–30 km) and high T e anisotropy are widespread in SE Tibet, indicating a weak and highly mechanical anisotropic lithosphere. This also implies that the style of lithospheric deformation is continuous, and may be dominated by lower crustal flow. Compared with the crustal stress and lithospheric strain indicators, we find that the T e anisotropy is predominantly governed by the present-day tectonic stress in SE Tibet. This may indicate a post-collision compression tectonics within the plateau due to the India–Eurasia plate convergence and a resistance by the rigid Sichuan Basin. However, the anisotropy reflects extension stresses in approximately WNW–ESE direction in Yunnan and northern Indochina off the plateau, which may relate to the Burma plate subduction processes. We infer that the present-day tectonic stresses and mantle processes associated with the India–Eurasia collision continuously modify and rework the mechanical structure, and both govern the lithospheric deformation in SE Tibet. [ABSTRACT FROM AUTHOR]