3-D Crustal Azimuthal Anisotropy Reveals Multi-Stage Deformation Processes of the Sichuan Basin and Its Adjacent Area, SW China

The collision of the South China Block and the North China Block and the eastward extrusion of the Tibetan Plateau materials have caused complicated tectonic deformation and strong seismicity around the Sichuan Basin (SCB). In order to reveal regional stress distribution and deformation mechanisms, we invert for a 3-D high-resolution shear wave velocity and azimuthal anisotropy model from ambient noise tomography with data from 483 broadband seismographs deployed around the SCB. Our results reveal strong lateral contrasts of azimuthal anisotropy in the crust. The western Sichuan depression shows weak deformation. The central Sichuan uplift, used to be a stable microcontinent, shows relatively strong and coherent fossil anisotropy. In contrast, the eastern Sichuan folds shows strong anisotropy, indicating intense regional deformation. The fast axis direction is bifurcated when reaching the Longmenshan fault, which indicates the obstruction of the SCB to the crustal low velocity materials beneath the Songpan-Ganzi Block. In the east, the rigid SCB and strike-slip faults may transform the compressive stress to shear deformation in adjacent areas and cause a ring-shaped anisotropy pattern below 15 km depth. Furthermore, anisotropy patterns display strong contrast at shallow depths in the Weiyuan and Changning regions, which may facilitate the accumulation of strain and more likely induce earthquakes during the shale gas exploration stage. Our new model provides important constraints for understanding the multi-stage deformation processes in and around the SCB.

Automated summary

The collision of the South China Block and the North China Block, along with the eastward extrusion of the Tibetan Plateau materials, have resulted in complex tectonic deformation and high seismic activity in the Sichuan Basin. In this study, we used data from 483 instruments to create a 3-D high-resolution model of shear wave velocity and azimuthal anisotropy through ambient noise tomography. Our findings reveal significant lateral variations in azimuthal anisotropy within the crust, with different regions displaying different levels of deformation. The results provide valuable insights into the multi-stage deformation processes in and around the Sichuan Basin.