Quartz c-axis fabric characterization of the strike-slip ductile deformation within the Three Pagodas shear zone, western Thailand

Abstract

The Three Pagodas shear zone is associated with mainland Southeast Asia deformations. The evolution of the shear zone is divided into deformation phases D1a, D1b, and D2. The first phase (D1a) involved ductile sinistral shearing associated with metamorphism. Foliations are well developed in a NW–SE with variable dips from east to west. Stretching lineations show NW–SE trending strike and flatly plunge to the south. Based on deformation patterns exhibited by outcrop and microscopic analysis, D1a shows NW–SE trending of ductile sinistral shearing. Additionally, the quantitative analysis of mylonitic quartzite exhibits an oblique angle of quartz grain shape fabric and quartz c-axis fabric asymmetry. Sinistral simple shear was confirmed by kinematic vorticity (Wk). The spatial distribution of the kinematic velocity number (Wk) of D1a shows a range between 0.93 and 0.99 at the central and western parts of the shear zone, which is higher than the 0.82–0.85 range for the eastern part of the shear zone. The strain variation within the Three Pagodas shear zone derived from the kinematic vorticity number (Wk) shows that the central and the western parts have been deformed under simple shear more than the eastern part. D1b is represented by the exhumation of high-grade metamorphic complex from sinistral transpression. The last stage (D2) involved the inversion of this strike-slip system, which is represented by dextral strike-slip faults.