Seismic-velocity structure of the lithosphere beneath the southeastern margin of the Tibetan Plateau: Insights into geodynamic processes and deformation mechanisms

The southeastern margin of the Tibetan Plateau is a tectonic transition zone between the Tibetan Plateau and Yangtze Craton of the South China Block. The lithosphere beneath the western corner of Yangtze Craton was not only modified by magma underplating and intraplating associated with the Emeishan plume during the Permian but has also been affected by Tibetan Plateau orogenesis since the early Cenozoic. Seismic-wave velocity structures of lithosphere can provide information about the mechanisms of plateau deformation and expansion, including analysis of P-wave receiver functions, surface-wave dispersion, and body-wave travel-time data. During the last decade, several models of lithospheric seismic velocity based on such data have been obtained for the southeastern Tibet. However, these models carry uncertainty because of the inherent lack of uniqueness in the data inversion process. In this study, we review published lithospheric seismic-velocity models of the southeastern margin of the Tibetan Plateau and discuss possible interpretations of the formation mechanism of large-scale low- or highvelocity zones. Most of the reviewed models reveal a large-scale NNE–SSW-trending high-velocity zone in the crust beneath the core of the Emeishan large igneous province that separates two large-scale lowvelocity zones and may act as a barrier to lower-crustal flow from central Tibet. We argue that this largescale high-velocity zone is located primarily in the middle–lower crust rather than in the upper or entire crust and that it represents the track of the Emeishan plume hotspot. The lateral extrusion of rigid blocks is inferred to be the dominant crustal-deformation mode in the southeastern margin of the Tibetan Plateau, whereas deformation induced by hypothesised lower-crustal flow may be limited to localized regions. However, both the lateral extrusion of rigid blocks and lower-crustal flow may be coexistent processes causing the observed crustal deformation pattern beneath the northern Sichuan–Yunnan diamond-shaped block.

Keywords: Seismic-velocity models; Emeishan mantle plume; Lower-crustal flow; Rigid-block lateral extrusion; Plume-strengthened lithosphere