Shear-zone development in nominally anhydrous single-crystals of quartz

Earth is unique in that it exhibits plate tectonics, where weak localised zones of deformation enable the relative movement of rigid plates. At depth, shear deformation predominantly occurs on narrow zones of fine-grained ultramylonites thought to be both a product of, and contributor to, localisation. In natural rocks, most shear localisation is linked to grain-size sensitive creep, which is facilitated by grain-size reduction and phase mixing. However, the mechanisms of strain localization in large crystals or monomineralic materials (e.g., glaciers, quartz veins, the mantle) are less clear. We deformed nominally dry, synthetic single-crystals of quartz, a major component of the continental crust, at a pressure of 1.5 GPa and temperature of 900°C using a solid salt assembly (SSA) Griggs apparatus at Texas A&M University to examine the mechanisms of strain localization in monomineralic materials. Quartz crystals were cored parallel to <m₁> to promote slip of <a> on {m}, that is, prism-<a> slip. Slices of quartz were then cut at 45° to the long axis of the cylinder and deformed in general shear at an approximate shear strain rate of 10⁻⁵ s⁻¹. To explore how strain varied both throughout the sample and with progressive deformation, a gold foil was inserted into the centre of the quartz slice perpendicular to the shear direction prior to sample assembly to act as a passive strain marker. We stopped experiments at a variety of macroscopic shear strains ranging from 1.5 to 5.4. Despite being nominally dry, samples deformed pervasively by dislocation creep with extensive recrystallisation. After a critical strain threshold (Ɣ = ~ 1), deformation progressively localised to the central region of the sample with increasing strain. The highest strain experiments (Ɣ ≥ 4.4)  display local variations in strain of over two orders of magnitude. Although yield stress varied greatly between experiments, the sample fabric consistently evolved with increasing strain, with more deformed samples evolving ever finer grain sizes and a fabric orientation (defined by elongate grains or ribbons of quartz) which progressively rotated towards the shear plane. Interestingly, grain size seems to evolve as a function of strain rather than stress in these experiments. Our study provides a critical new dataset for exploring shear strain evolution, demonstrating that strain rates are non uniform in general shear experiments (like natural shear zones) following a critical strain threshold. Studies which assume a single steady-state experimental strain rate (e.g., flow laws, experimental studies of microstructural development in rocks) may need to be re-evaluated.