Abstract:

Through anisotropy of magnetic susceptibility (AMS) analysis, it is investigated that the superposed structural characteristics and tectonic evolution of the southern margin of the Western Sichuan Plateau, including the Longmen Shan Fault Zone, the Songpan-Ganzi Block, and portions of the Qiangtang Block. The findings offer new constraints on the strain patterns and tectonic history of the study area. Focusing on the Triassic Xikang Group strata, six distinct types of magnetic fabrics were identified across 87 sampling sites, representing progressively increasing deformation intensity. These magnetic fabric types include: sedimentary fabric, incipient deformation fabric, pencil fabric, weak foliation fabric, strong foliation fabric, and stretching lineation fabric. Notably, the incipient deformation fabric is quantitatively dominant. Furthermore, a significant proportion of the magnetic fabrics exhibit superimposed characteristics, recording the overprinting of tectonic stresses and revealing two major phases of tectonic deformation: Late Triassic and Cenozoic. The magnetic fabrics record Late Triassic deformation characterized by widespread NE-SW oriented tectonic shortening (compression). Additionally, along the eastern margin of the Songpan- Garzê Terrane, this shortening exhibits a gradual decrease in intensity from SE to NW. The Cenozoic deformation recorded by the magnetic fabrics is superimposed upon the pre-existing Late Triassic fabrics and is primarily controlled by Cenozoic faultrelated shear deformation. Regionally, the Late Triassic deformation is pervasive, displaying marked characteristics of penetrative deformation. In contrast, the Cenozoic deformation is predominantly localized around major fault zones, exhibiting a nonpenetrative deformation pattern at the regional scale.

Key words: West Sichuan Plateau, magnetic fabric (AMS), superimposed deformation, Songpan-Garzê Block, finite strain