Faults and fluids activity controlled structural heterogeneity in the upper crust beneath the Xiaojiang fault system revealed from 2-D <i>Pg</i> seismic tomography

SUMMARY The Xiaojiang fault system (XJFS) is located in the southeastern margin of the Tibetan Plateau, which has been considered as an ideal site to explore the traces and effects of past tectonic activity. In this study, we obtain a high-resolution P-wave velocity and azimuthal anisotropy model of the upper crust beneath the XJFS, utilizing the 2-D Pg wave tomography method including both station and event depth corrections. The upper crust displays obvious heterogeneity of both azimuthal anisotropy and P-wave velocity underlying the XJFS. The large azimuthal anisotropy beneath the XJFS, especially the regions where several faults interact, suggests the cracks are widely distributed. Generally, the upper crust is featured by several high-velocity bodies separated by low-velocity anomalies. The high-velocity bodies are speculated to be related to the remnant magmatic rocks of the Emeishan large igneous province. The low-velocity anomalies are interpreted to represent fault damage zones which could be attributed to the strike-slip faulting/shearing along the faults and upwelling of deeply sourced partial melts and fluids. The present tectonic activity in the XJFS is characterized by rigid block extrusion along strike-slip faults in the upper crust, which is consistent with the rigid block extrusion model. We further propose a tectonic model to display the evolution of the upper crust beneath the XJFS, in which the faults and fluids activity plays an essential role.