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 degrees. 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 V-SH 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 V-SH) in the uppermost mantle and negative radial anisotropy (faster V-SV) 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.