Abstract
Identifying the dominant slip systems in naturally deformed quartz is crucial for understanding the rheology of the continental crust because the rheological properties of quartz aggregates may differ for the different dominant slip systems. Although the basal <a> slip system has been considered the dominant slip system in quartz under upper crustal conditions, some studies argue its activity in deforming quartz. In this study, we analyzed the crystallographic- and shape-preferred orientation of quartz phenocrysts in a deformed granitic porphyry in the Ryoke belt, SW Japan, to clarify the dominant slip systems in the naturally deformed quartz under the upper crustal conditions based on the optical and electron backscatter diffraction (EBSD) observations. The quartz phenocrysts show microstructures indicative of crystal plasticity, such as elongation parallel to the stretching lineation, wavy extinction, and marginal dynamic recrystallization. The identified active slip systems include prism <a>, basal <a>, prism [c], and rhomb <a> through misorientation analyses via EBSD data. The aspect ratios of grains with dominant prism <a> and basal <a> slip systems are higher than prism [c] and rhomb <a> slip systems, implying that the similar strength of prism <a> and basal <a> slip systems and weaker than prism [c] and rhomb <a> slip systems under the upper crustal conditions. The c-axis orientations of quartz grains with basal <a> slip system inferred from the misorientation analysis distributed at the peripheral of pole figures, whereas those with prism <a> slip system located at the center of pole figures. The observation supports the traditional view that peripheral c-axis fabrics indicate basal <a> slip system activation rather than the recently proposed oriented nucleation and growth model.
