Strong crustal heterogeneity of the Xianshuihe-Anninghe-Zemuhe fault system and its vicinity revealed by velocity and azimuthal anisotropy structures

Abstract

The Xianshuihe-Anninghe-Zemuhe (XAZ) fault system acts as the eastern boundary of the Chuan-Dian block, and it separates the Chuan-Dian block from the Songpan-Ganzi block (SGB) and the Yangtze block. Due to the SE-ward expansion of the Tibetan Plateau, strong crustal deformation has occurred in the XAZ fault system and its vicinity. Although many studies on crustal deformation have been conducted in the past twenty years, some key issues remain poorly resolved. To further investigate crustal deformation, we perform P-wave anisotropy tomography using an eikonal equation-based tomography method, and obtain high resolution 3D P-wave velocity and azimuthal anisotropy images beneath the XAZ fault system and its vicinity. Our tomographic results show two separate strong low-velocity bodies in the mid-lower crust of the XAZ fault system. In addition, our results do not present a consistent anisotropy pattern with fast velocity directions parallel to the strike of the XAZ fault system. Based on these features, we conclude that mid-lower crustal flow, which is employed to explain the expansion of the Tibetan Plateau, does not occur along the XAZ fault system. Furthermore, our tomographic results display a high-velocity body in the upper crust below the SGB, and the high-velocity body connects with the high-velocity anomaly beneath the Sichuan basin (SCB). According to these features, we suggest that the Yangtze crystalline basement penetrates into the eastern Tibetan Plateau across the Longmenshan fault.