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We present new constraints on the upper mantle transition zone structure beneath eastern and southeastern Tibet based on P wave receiver functions for a large broadband data set from two very dense seismic arrays. A clear depression of both the 410 km and 660 km discontinuities is detected west of the Red River fault relative to the east. The correlated topographic variations across the Red River fault are indicative of temperature changes in the upper mantle above the transition zone, which suggests that the fault is a deep‐rooted structure that penetrates into the upper mantle and separates Indochina from South China. West of the Red River fault, the transition zone thickness under the Tengchong volcano is found to be normal compared to the global average. This strongly suggests that the intraplate volcano may originate from slab tearing of the eastward subducting Indian plate at shallow depths in the upper mantle rather than from dehydration of a flattened plate within the transition zone. Our results further show that the 660 km discontinuity is significantly depressed under the western Yangtze Craton and that the transition zone therefore thickens by up to 20 km. This thickening is suggestive of lowered temperatures associated with a remnant of detached lithosphere in response to overlying asthenospheric escape flow in and around the western Yangtze Craton. In addition, we find that the transition zone thickness beneath much of the Sichuan Basin is similar to the global average.

The eastern region of the Tibetan Plateau and surrounding regions have gentle to moderate topographic gradients, contrasting the steep margins in northern and southern Tibet. The mechanisms for plateau growth in eastern Tibet are uncertain so far. Here we present a new shear wave tomography model of the Tibetan Plateau derived from S wave traveltimes from teleseismic waveforms recorded by the dense ChinArray seismic network. The model reveals the deep structures beneath the eastern region of the Tibetan Plateau and its adjacent regions, showing clear velocity contrast boundaries roughly along the eastern margins of the Tibetan Plateau. We interpret the slow velocity anomalies beneath the Tibetan Plateau as soft lithosphere, which absorbed most of the northeastward push of the Indian Plate, while the fast velocity anomalies beneath the region surrounding the plateau represent craton‐like rigid block roots. The deformation of the soft/weak lithospheric mantle beneath the Tibetan Plateau is constrained by the convergence between the Indian and Eurasian plates, and the framework of the surrounding rigid blocks. We suggest that weak lithosphere deformation (horizontal shortening and vertical stretching) accounts for the plateau growth in eastern Tibet.

SUMMARY We imaged the crust and uppermost mantle structure beneath northeastern (NE) China with fundamental mode Rayleigh waves recorded by 125 broad‐band stations deployed in the region. Rayleigh wave phase and group velocities along more than 700 interstation paths were estimated using the wavelet transformation method and then these data were utilized to construct 2‐D phase and group velocity maps in the period range of 15–60 s. Owing to the dense ray coverage and short ray path, our results provide better lateral resolution in the NE China region compared with previous phase and group velocity studies. The regularized Rayleigh wave phase and group velocity dispersions at each cell were jointly inverted to determine 1‐D shear wave velocity structure using the linear inversion method and then assembled into a 3‐D model. Our results show that obvious low velocities exist in the uppermost mantle beneath the Changbaishan volcanic region, which may be due to asthenospheric upwelling. The thin lithosphere with fast S‐wave velocity in the lower crust of the Songliao Basin implies that the lithospheric mantle beneath NE China is partly removed.

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