Kinematic flow and temperature variation within the Three Pagodas shear Zone, western Thailand

Abstract

The deformation of mineral grains in mylonitic rocks enables the direct calculation of strain patterns, vorticity analysis, and deformation temperatures in the shear zones. Quartzite mylonite rocks from the Three Pagodas shear zone, western Thailand, were analyzed for finite strain, flow vorticity, and estimate deformation temperatures. Our results indicate deformation temperatures ranging from 510 °C to 570 °C in the central zone, and from 320 °C to 450 °C in the western and eastern zones. The finite strain (R_s) varied from 1.86 to 2.63. Mean kinematic vorticity number (W_m) was determined using two different methods—the porphyroclast aspect ratio (PAR) and the rigid grain net (RGN). W_m values ranged from 0.80 to 0.90, with 59 % to 71 % simple shear and 29 % to 41 % pure shear. A comparison of these zones suggests a transition in strain variation from the eastern to the western zones, with a noticeable increase in strain approaching the central zone. The S values ranged from 0.57 to 0.65 in the western zone, 0.70 to 0.75 in the central zone, and 0.65 to 0.72 in the eastern zone. Our findings confirm that the internal structure of the Three Pagodas shear zone exhibits the heterogeneous distribution, demonstrating sinistral transpressional slip with strain partitioning of general shear and distributed simple shear components at the center of the shear zone. The exhumation and cooling of the Thabsila metamorphic complex were likely influenced by two distinct phases of the India-Eurasia collision during the Early Cenozoic: Early Eocene transpression followed by Late Eocene–Early Oligocene strike-slip deformation.