The eastern margin of the Tibetan Plateau, surrounded by the Yangtze Craton in the east and Ordos block in the northeast, has experienced complex evolution and deformation. Imaging mantle structure is important for understanding its formation. Here, we obtained the detailed upper mantle velocity structure from teleseismic tomography, and obtained the anisotropic distribution through SKS analysis. Our results reveal eastward and southeastward dipping high-velocity anomalies in the upper mantle beneath the Sichuan Basin and the Yangtze Block, which extend down to depths of ~250 km in the central basin, and more than 400 km to the east of the basin. These high-velocity anomalies may indicate that the Yangtze Block has subducted beneath the Cathaysia. The high-velocity anomaly area in SCB generally shows weak anisotropy, but there are some areas with strong anisotropy on the edges. A westward dipping high-velocity anomaly exists within the generally low-velocity upper mantle in eastern Tibetan Plateau. Taking into account the adjacent high-altitude, the Miocene granite emplacement, and the abnormal change of anisotropy, we interpret the high-velocity body as delaminated lithosphere. High-velocity anomalies and weak anisotropy are present beneath the Ordos block, which is consistent with the stable features of craton. In the western boundary zone of the Ordos block, there is a low velocity anomaly above the depth of 200 km, which is connected with the low velocity anomaly beneath the Tibetan Plateau. The fast wave direction obtained by SKS analysis shows that the anisotropy of this area is similar to that of the northeastern Tibetan Plateau, which indicates that the Ordos boundary zone has been affected by the expansion of the upper mantle in the Tibetan Plateau. This work was supported by the National Natural Science Foundation of China (Grant Nos. 41774102 and 41804062).

We construct a new 3-D P-Wave Velocity Structure model of the crust and uppermost from 30°N to 42°N and 103°E to 115°E , which includes the Ordos block and its adjacent structures. Benefit of the “China Seismological Science Array”, the distribution of seismic observation stations in Ordos region has been improved and the resolution of topography is enhanced. The seismic data we used in this work were recorded from November 2013 to October 2018, obtained by “China Seismological Science Array” II, III and permanent stations. After manual labeled and selected the seismic phase, about 59,000 phase datas (including P and Pn phase) recorded from 2357 local seismic events were participate in travel time tomography inversion.Our results reveal a low velocity anomaly is present in the upper crust of southern boundary zone of Ordos, coincides with the distribution of earthquakes. From lower crust to the upper mantle, Ordos block mainly show as a stable craton follow with high velocity anomaly. But the velocity is relatively lower from 37.5° south, which may be caused by the thicker crust in the south part of Ordos. At the west of Liupanshan fault zone, a low velocity anomaly enters beneath Ordos at the top of mantle. Broth of those results suggesting that the southern Ordos block has been affected by the eastward extrusion of the Tibetan Plateau, and has the feature of "activation". This work was supported by the National Natural Science Foundation of China (Grant Nos. 41804062 and 41774102).

High-resolution 3-D P and S wave velocity (Vp and Vs) models of the crust and uppermost mantle beneath the northeastern (NE) Tibetan Plateau and the surrounding areas were obtained by applying an improved double-difference seismic tomography method on abundant body wave travel-time data of local and regional earthquakes collected from 1985 to 2018. The improved method not only increases constraints from observed Moho depth information but also incorporates the later-arriving Pg and Sg phases to improve the ray coverage in the middle-lower crust and uppermost mantle. The results show significant low-velocity anomalies in the crust beneath the NE Tibetan Plateau and high-velocity features beneath the surrounding stable Alxa and Ordos blocks. The areas near the boundaries of the cratonic blocks show high velocity in the upper crust and uppermost mantle beneath the Hexi Corridor, and high velocity in the upper crust beneath the Liupan Shan belt. Both of those regions have significant low-velocity anomalies in the middle-lower crust similar to the Tibetan Plateau, which suggests they probably were parts of the Alxa and Ordos blocks respectively, and were cut off from the two cratonic blocks during the lateral expansion of the Tibetan Plateau because of middle-lower crustal ductile deformation and asthenospheric flow. We propose a new tectonic model controlled by middle-lower crustal ductile deformation and asthenospheric flow to illuminate the tectonic evolution and growth mechanism of the NE Tibetan Plateau.

The collision of the South China Block and the North China Block and the eastward extrusion of the Tibetan Plateau material cause complicated tectonic deformation and strong seismicity around the Sichuan Basin (SCB). Seismic anisotropy is closely related to regional stress fields and faults movements thus helping us reveal the stress distribution and deformation mechanisms. We invert for a 3-D high-resolution shear wave velocity and azimuthal anisotropy model from 483 broadband seismographs deployed around the SCB and eastern margin of the Tibetan Plateau. Our results reveal strong horizontal contrasts of azimuthal anisotropy above 40 km depth. The western Sichuan depression and central Sichuan uplift show weak deformation. In contrast, the eastern Sichuan folds experienced intense deformation from the northwestward extension of the Xuefeng intracontinental tectonic system and northeastward extrusion of the Daliang mountain. In the east, strike-slip faults may transform the compressive stress to shear deformation and cause distinct anisotropy patterns inside and outside the SCB in the crust. Below 40 km depth, the collision of the South China Block and the North China Block formed E-W trending anisotropy. The different anisotropy patterns with depth in the east may indicate this area was dominated by N-S trending compressive stress in the uppermost mantle, but NW-SE and NE-SW trending compressive stresses in the crust. Furthermore, anisotropy patterns display strong contrast at shallow depths in the Weiyuan and Changning area, which may facilitate the accumulation of strain and more likely to induce earthquakes during the shale gas exploration stage.

Study on the deep tectonic environment and physical characteristics of the Tibet Plateau will be helpful to explore the deep dynamic effect and strengthen the understanding of the role of anisotropy and tectonic deformation. We have obtained the three-dimensional P wave velocity structure of the crust and upper mantle under the southeastern margin of the Tibet Plateau based on the observational data of 224 fixed seismic stations in the regional digital seismic network of Yunnan and Sichuan and the observational data of 356 mobile seismic arrays of “Chinese seismic array detection--the southern section of North-South seismic belt by using the method of joint inversion of regional earthquake and teleseismic. The results indicate that the spatial distribution of P wave velocity anomalies in the shallow upper crust is closely related to the surface geological structure, terrain and lithology, Baoxing and Kangding with basic volcanic rocks and volcanic clastic rocks presents obvious high-speed anomaly, Chengdu basin shows low-velocity anomalies associated with the quaternary sedimentary area, Xichang mesozoic basin and Butuo basin are characterized by low-velocity anomalies related to very thick sedimentary layer. Upper and middle crust beneath the Chuandian and Songpan-Garze block has apparent lateral heterogeneities, including low velocity zones at different depth. There are a large range of low velocity layers in the Songpan-Garze block and the sub-block in the northwest of Sichuan, show that the middle and lower crust is relatively weak, Sichuan basin, which located in western margin of the Yangtze platform, shows high velocity characteristics. Our results also revealed that the anomalous distribution of high wave velocity exists inside the crust of Panxi region, It is concluded that this may be related to the late Paleozoic mantle plume activity which leads to a large number of mafic and ultra mafic intrusions into the crust.

The continental collision between the Indian and Eurasian plates caused large-scale uplift of the Tibetan Plateau and resulted in the extrusion of lithospheric materials through its southeast and northeast margins (Molnar & Tapponnier, 1975; Tapponnier et al., 2001). However, mechanisms of crustal material escape in the southeastern margin of the Tibetan Plateau remain under debate. Both seismic Pg and Lg wave attenuations are sensitive to the crustal material properties and status, such as the temperature, partial melting, and fractures. Lg waves propagate within the entire crust, whereas the Pg waves are mainly sensitive to the upper crust properties. Therefore, Pg and Lg attenuations can be useful indicators of potential crustal material escape, particularly, combining two attenuation patterns may provide constraints on the depth distribution of attenuations. In this study, based on the high-density ChinArray data, we constructed high resolution (approximately 0.5°×0.5° ) Pg and Lg attenuation models for southeast Tibetan Plateau (Zhao & Mousavi, 2018; Zhao & Xie, 2016; Zhao et al., 2013; Zhao et al., 2010). In the study region, most areas of the Songpan-Ganzi block west of the Longmenshan fault, the Western Sichuan block and the southeastern part of the Central Yunnan block are characterized by strong Pg and Lg attenuations. Along with previous studies (Bai et al., 2010; Bao et al., 2020; Shapiro et al., 2004), our results show that there are relatively weak regions in the middle-to-lower crust in southeast Tibet, where there may be small-scale randomly distributed ductile materials. The main fault systems of the upper crust and the ductile materials of the middle-to-lower crust have a synergistic effect on the deformation of southeast Tibetan crust. This research was funded by the China Earthquake Science Experimental Field Foundation (2019CSES0103, 2016CESE0203) and the National Natural Science Foundation of China (41630210, 41674060, 41974054, 41974061).

We used an azimuth-dependent dispersion curve inversion (ADDCI) method to extract 3D velocity and azimuthal anisotropy of the eastern Tibetan Plateau and western Yangtze Craton. The 3D anisotropic model gives us a unique opportunity to investigate the geodynamic process that is responsible for the uplift of the eastern Tibetan plateau, the creation of big faults, the deposits of minerals as well as the accumulation of oil and gas. Our results show large variations in fast propagation directions (FPD) and magnitude of anisotropy (MOAs) with depth and blocks; strong contrasts are observed across major faults. The average MOA in the crust is approximately 3%. The FPDs are positively correlated with GPS data and strikes of regional faults. The low-velocity zones (LVZs) in middle to lower crust are widely distributed in the Songpan Ganze Terrence, the north Chuan-Dian block. Surprisingly, the crust of the Huayingshan thrust and fold belt (HTFB) is dominated by LVZ extending from the surface all the way to the uppermost mantle. The coexistence of LVZs in middle-lower crust and uppermost mantle surrounding the Sichuan basin is dramatic and it may suggest that large-scale deformation is coupled vertically from surface to uppermost mantle. This observation may further suggest that the pure shearing crust shortening model, which involves the thrusting and folding of the upper crust and the lateral extrusion of blocks, may be the major mechanism causing the growth of the eastern Tibetan Plateau. On the other hand, the deep-rooted LVZs below the HTFB may suggest that the faults bounding the unit may be deep-rooted. This is contrary to the traditional view of faults.

The mechanism of crustal deformation and the development of large offset strike-slip faults during continental collision, such as the Himalayan-Tibetan orogeny, remains poorly understood. Previous mechanical models were simplified which are either (quasi-)2D approximations or made the a-priori assumption that the rheology of the lithosphere was either purely viscous (distributed deformation), or purely localized. Here we present three-dimensional visco-elasto-plastic thermo-mechanical simulations, which can produce both distributed and highly localized deformation simultaneously during continental collision. Our results show that large-scale shear zones develop as a result of frictional plasticity, which have many similarities with observed shear zones such as the Altyn-Tagh, the Kunlun, the Longmenshan, the Xianshuihe-Xiaojiang, the Red-River, the Sagaing and the Jiali faults. Yet, localized brittle deformation requires both a strong upper crust (>10²² Pa·s) and a moderately weak middle/lower crust (~10²⁰ Pa·s) in Tibet. The weak mid-lower crustal flow and strong terranes facilitate shear zones development. These models indicate that the localized large-scale strike-slip faults primarily accommodate the continental deformation. Different from the asthenospheric flow, rigid continental indentation alone explains the observed surface deformation.