EVIDENCE OF POLYPHASE DEFORMATION IN THE RUSHAN-PSHART ARC, A PRELIMINARY PETROGRAPHIC ANALYSIS

The Rushan-Pshart thrust fault marks the Late Triassic to Early Jurassic suture between the Central and Southern Pamir terranes. Located in the hanging wall of this established suture zone, the enigmatic Rushan-Pshart arc is isolated by antithetic thrust faults. Lenses of plutonic outcroppings have been thoroughly mapped in the Rushan-Pshart arc, but their tectonic significance and emplacement timing is poorly constrained. Preliminary petrographic analysis of igneous plutons, as well as surrounding metamorphic rocks, provides evidence of polyphase deformation in the Rushan-Pshart arc.

Five rock samples from the hanging wall and one rock sample from the footwall of the Rushan-Pshart suture zone were petrographically investigated to refine the geologic framework of the igneous activity. Hanging wall plutonic samples exhibit multiple regimes of dynamic recrystallization in both quartz and feldspar crystals. Overprinting of third-regime grain boundary migration by both second-regime subgrain rotation and first-regime bulging recrystallization indicates an initial, high-temperature phase of deformation followed by low-temperature phases. Additionally, undulose extinction in orthoclase coexisting with brittle deformations of plagioclase provides evidence of high- and low-temperature conditions, respectively. Similarly, hanging wall metamorphic samples exhibit multiple dynamic recrystallization regimes and contain relatively high-temperature minerals such as sillimanite, andalusite, and high-Ti biotite. All hanging wall samples show evidence of extensive hydrothermal alteration through various phases of sericitization. In contrast to the samples collected from the hanging wall, the low-grade quartzite collected from the footwall exhibits intense chloritization throughout in conjunction with sericitization. Imminent geochemical and geochronologic analyses will further illuminate the temporal and petrogenetic framework of the Rushan-Pshart arc, but foundational petrographic analyses is impertinent to understanding the polyphase deformation of the Rushan-Pshart suture zone.