Anomalous Radial Anisotropy and Its Implications for Upper Mantle Dynamics Beneath South China From Multimode Surface Wave Tomography.

The strength of radial anisotropy can effectively reveal vertical or horizontal material flow, which may provide significant dynamic implications. The South China Block (SCB) is a classic laboratory to study strong deformation under compression from the surroundings. We construct a radially anisotropic 3‐D shear‐wave model of the entire SCB using multimode surface wave tomography with a lateral resolution of 2.0°. Distinctive structural contrasts are revealed in the S‐wave velocity anomalies in the Cathaysia Block and the Yangtze Craton. In the Cathaysia Block, a low‐velocity anomaly becomes evident with increasing depth, particularly in the VSH model. The Yangtze Craton is, however, characterized by a high‐velocity anomaly, especially in the Sichuan Basin, where the high‐velocity feature can extend down to a depth of 300 km. Interestingly, an abrupt change in radial anisotropy is observed beneath the Sichuan Basin, with positive radial anisotropy (faster VSH) in the uppermost mantle and negative radial anisotropy (faster VSV) at depths from 80 to 250 km. The lower negative anisotropic layer may reflect the directional arrangement of constituent materials caused by eastward compression and erosion of mantle flow under the Tibetan Plateau. In addition, remarkable negative radial anisotropy is found in the asthenosphere beneath Hainan Island and its surroundings. The features beneath Hainan Island and its surroundings may be related to the Hainan mantle plume in the Cenozoic. Plain Language Summary: Seismic radial anisotropy represents that the vertically (VSV) and horizontally (VSH) polarized shear waves propagate at different wave velocities in the Earth's medium, which is useful to reflect the deformation characteristics and the deformation pattern of the medium, so as to constrain the mantle dynamics and tectonic processes of the crust and mantle. In this study, we measure multimode Rayleigh wave and Love wave phase velocity dispersion to construct a 3‐D shear‐wave model with radial anisotropy in the entire South China Block. Our results show that the shear‐wave velocity under the Yangtze Craton is anomalously fast and beneath the Cathaysia Block is anomalously slow. Moreover, we find different manifestations of radial anisotropy in the lithosphere of the Sichuan Basin, in which the negative radial anisotropy of 80–250 km is inferred to be affected by eastward compression and erosion of mantle flow under the Tibetan Plateau. A remarkable negative radial anisotropy beneath the Hainan Island and its surroundings in the asthenosphere is interpreted to reflect the Hainan mantle plume in the Cenozoic. Key Points: An anisotropic shear wave model beneath South China is constructed by multimode surface wave tomography with improved vertical resolutionSichuan basin shows negative radial anisotropy, reflecting the influence of eastward compression and mantle flow under the Tibetan PlateauThe low‐velocity anomaly and strong negative radial anisotropy of the Hainan Island may be related to the Hainan mantle plume [ABSTRACT FROM AUTHOR]