Crustal S-wave 3D azimuthal anisotropy beneath the southern Sichuan-Yunnan block of SW China from multiple seismic arrays

Abstract

The southern Sichuan-Yunnan block (SYB) is intersected by the NW-striking Honghe faults (HHF) and the nearly NS-trending Xiaojiang faults (XJF), providing an excellent zone for exploring severe crustal deformation and complicated tectonic movement. However, the crustal-mantle deformation mechanisms are still controversial, partially due to the lack of detailed information. With ambient noise data from several temporary seismic arrays and regional permanent seismic stations, we applied the direct surface wave tomography to obtain S-wave velocity and azimuthal anisotropy simultaneously. The crustal S-wave structures show complex heterogeneity both horizontally and vertically, relating to geologic settings and large faults. In the mid-lower crust, there are two significant low-velocity anomalies with strong azimuthal anisotropy, with the NNW-SSE direction near the northwest end of HHF and the NE-SW direction around the mid-south segment of XJF, respectively. The fast axis within the SYB shows approximately in the N-S direction, which differs from those in the low-velocity zones on its east and west sides. Therefore, we consider the ductile deformation in the mid-lower crust is more likely restricted by large faults. At the end of the wedged intersection, the southward mid-lower crustal flow could be blocked by the HHF, resulting in the weak materials distributed along the faults rather than crossing over at large-scale. Combining other independent studies, we conclude that there may be different deformation between the crust and the lithospheric mantle. This 3-D model provides important constraints for the regional deformations and plate tectonics of the large boundary faults [supported by NSFC Projects 42074065 & 41730212].