Seismic Anisotropy and Mantle Flow beneath the Western Yangtze Craton

Previous seismic anisotropy studies suggest that the mantle azimuthal anisotropy in the central Chinese mainland has a dominantly E-W fast orientation which is consistent with the absolute plate motion direction of the area. However, seismic stations in the western Yangtze Craton east of the Sichuan Basin show NE-SW oriented azimuthal anisotropy and its origin is debated. In this study, we determine the fast orientation and splitting time using the SKS, SKKS, and PKS phases recorded at 17 seismic stations located on the east side of the Sichuan basin based on the transverse energy minimization technique. A total of 473 pairs of well-defined splitting measurements are obtained after manual checking. The splitting times in the study area are mostly smaller than the global average of 1.0 s, and the fast orientations at most stations are not parallel to the main structural features observed on the surface. The splitting parameters observed at some of the stations show strong piercing point dependent and are indicative of spatially varying anisotropic properties beneath the stations. The fast orientations obtained at one of the stations demonstrate a 90° periodic azimuthal variation, suggesting the existence of double-layered anisotropy. Spatial coherence analysis of the splitting parameters suggests that the observed azimuthal anisotropy is mainly from the transitional zone between the asthenosphere and the lithosphere. The observations can be explained by a model involving the flow of the asthenosphere beneath and around the thick root of the Sichuan Basin.