Uppermost Mantle Structure Beneath the Southeastern Tibetan Plateau and Its Implications

The southeastern Tibetan Plateau is a crucial region for understanding the relationship between crustal weakening and uppermost-mantle deformation during the outward growth of the Tibetan Plateau. Using 48,221 Pn arrivals and an improved Pn anisotropic tomography method with Moho depth correction, we imaged the velocity and anisotropic structure of the uppermost mantle beneath the southeastern Tibetan Plateau and adjacent regions. The model reveals pronounced lateral heterogeneity, including two prominent N–S trending low-velocity belts centered at approximately 98.5°E and 102°E. The Pn fast directions within these low-velocity belts are broadly parallel to their trends, suggesting coherent deformation fabrics in the uppermost mantle. We suggest that these features may represent uppermost-mantle weak zones influenced by mantle-derived thermal input. The stable blocks beneath the Sichuan Basin and western Chuan-Dian Block, together with lithospheric heterogeneity related to the Emeishan Large Igneous Province, appear to control the deflection, partitioning, and modulation of this low-velocity system. Our results provide new seismic constraints on the uppermost-mantle material redistribution of lithospheric weakening and its modulation by surrounding rigid blocks beneath the southeastern Tibetan Plateau.