Uppermost mantle structure beneath the southeastern Tibetan Plateau and its implications

The southeastern Tibetan Plateau region is crucial for understanding the relationship between crustal weakening and uppermost-mantle deformation during the outward expansion of the Tibetan Plateau. We imaged the Pn-wave velocity and anisotropy structure of the uppermost mantle beneath the southeastern Tibetan Plateau and adjacent regions using 48,221 Pn arrivals and an improved anisotropic Pn tomography method with Moho depth correction. The model revealed pronounced lateral heterogeneity, including two prominent N–S-trending low-velocity belts respectively centered at approximately 98.5°E and 102°E. The Pn fast directions within these low-velocity belts are broadly parallel to their spatial trends, suggesting a coherent deformation fabric in the uppermost mantle. We suggest that these features represent weak uppermost-mantle zones influenced by mantle-derived thermal inputs. The stable blocks beneath the Sichuan Basin and western Chuan-Dian Block, together with the 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 novel seismic constraints on the redistribution of lithospheric weakening in the upper mantle and its modulation by the surrounding rigid blocks beneath the southeastern Tibetan Plateau.