Limited distribution of mid-lower crustal flow in the SE Tibetan Plateau revealed by P-wave velocity and azimuthal anisotropy beneath the Lijiang–Xiaojinhe Fault and its vicinity

The Lijiang–Xiaojinhe fault (LXF) and its vicinity are located in a transition zone among the Tibetan Plateau, the South China block and the Indochina block. Researchers believe that this area acts as a key tectonic zone for the evolution of the Tibetan Plateau. Owing to the continuous growth and SE-ward expansion of the Tibetan Plateau, the LXF and its vicinity suffer from intense deformation. Although different models, such as the rigid block extrusion and mid-lower crustal flow models, have been proposed to interpret intense deformation, a consensus has not yet been achieved. To better understand the deformation of the LXF and its vicinity, a high-resolution image of the subsurface structure must be constructed. In this study, we construct images of wave velocity and azimuthal anisotropy structures by using an eikonal equation-based traveltime tomography method. We collect high-quality seismic data from 276 broadband seismic stations and manually pick a total of 48,037 first arrivals for the tomography study. Our tomographic results show a strong low-velocity body below the LXF and its vicinity. In addition, a strong azimuthal anisotropy structure with an NNW–SSE-oriented fast velocity direction distributes (FVDs) along the low-velocity body. These features indicate the existence of mid-lower crustal flow, which penetrates across the LXF and extends to the Dianzhong block (DZB). In addition, we find obvious low-velocity perturbations in the mid-lower crust and uppermost mantle beneath the DZB. The low-velocity may be attributed to the upwelling of hot materials from the upper mantle.