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Abstract Quartz deformation fabrics reflect stress and strain conditions in mylonites, and their interpretation has become a mainstay of kinematic and structural analysis. Quantification of grain size and shape and interpretation of textures reflecting deformation mechanisms can provide estimates of flow stress, strain rate, kinematic vorticity, and deformation temperatures. Empirical calibration and determination of quartz flow laws is based on laboratory experiments of pure samples; however, pure quartzite mylonites are relatively uncommon. In particular, phyllosilicates may localize and partition strain that can inhibit or enhance different deformation mechanisms. Experimental results demonstrate that even minor phyllosilicate content (<15 vol%) can dramatically alter the strain behavior of quartz; however, few field studies have demonstrated these effects in a natural setting. To investigate the role of phyllosilicates on quartz strain fabrics, we quantify phyllosilicate content and distribution in quartzite mylonites from the Miocene Raft River detachment shear zone (NW Utah, USA). We use microstructural analysis and electron backscatter diffraction to quantify quartz deformation fabrics and muscovite spatial distribution, and X-ray computed tomography to quantify muscovite content in samples with varying amounts of muscovite collected across the detachment shear zone. Phyllosilicate content has a direct control on quartz deformation mechanisms, and application of piezometers and flow laws based on quartz deformation fabrics yield strain rates and flow stresses that vary by up to two orders of magnitude across our samples. These findings have important implications for the application of flow laws in quartzite mylonites and strain localization mechanisms in mid-crustal shear zones. 

Abstract Oxygen and hydrogen stable isotope analyses of quartz and muscovite veins from the footwall of the Raft River detachment shear zone (Utah) provide insight into the hydrology and fluid‐rock interactions during ductile deformation. Samples were collected from veins containing 90%–100% quartz with orientations either at a high angle or sub‐parallel to the surrounding quartzite mylonite foliation. Stable isotope analysis was performed on 10 samples and compared with previous quartzite mylonite isotope data sets. The results indicate that the fluid present during deformation of the shear zone was meteoric in origin, with a δ²H value of approximately −100‰ and a δ¹⁸O value of approximately −13.7‰. Oxygen stable isotope O¹⁸O depletion correlates with the muscovite content of the analyzed rocks. Many of the analyzed samples in this and other studies show an apparent lack of equilibrium between the oxygen and hydrogen isotope systems, which can be explained by hydrogen and oxygen isotope exchange at varying fluid‐rock ratios. Our results suggest that the Raft River detachment shear zone had a low static fluid‐rock ratio (<0.1), yet experienced episodic influxes of fluids through semi‐brittle structures. This fluid was then expelled out into the surrounding mylonite following progressive shearing, causing further¹⁸O‐depletion and fluid‐related embrittlement. 

The southern Appalachian Inner Piedmont (IP) has been interpreted to represent a relict crustal escape flow system that was active during the Neoacadian (370-340 Ma) orogeny. Critical to the support of this hypothesis is the identification of both the high-temperature, rheologically weak “channel,” with crustal flow driven by relatively low differential stress, and the rheologically strong “buttress,” where deformation was driven by relatively high differential stress. Paleopiezometric analyses from southern Appalachian quartz mylonites, quartzites, and quartz-bearing pelitic rocks of the eastern Blue Ridge (EBR; buttress) and IP (channel) allow gradients of differential stress driving flow to be examined. Samples located along a transect in the EBR northwest of the Brevard fault zone (BFZ) are used to define gradients in differential stress and deformation temperatures in the proposed buttress. Preliminary results for these samples suggest that deformation temperature increased, and differential stress decreased during deformation approaching the BFZ from the northwest. Mechanisms of quartz recrystallization shift from minor grain boundary bulging and dominant subgrain rotation (SGR) in samples located away from the BFZ to dominant SGR and minor grain boundary migration (GBM) in samples in the immediate BFZ footwall. Recrystallized grain sizes in the EBR are, in almost all cases, less than 150 microns on the major axis of ellipses used to approximate grain size. In the proposed crustal channel, quartz-rich samples collected along a transect south of Rosman, NC in the Brevard and Brindle Creek thrust sheets of the IP show GBM and SGR as the dominant mechanisms of deformation, as well as a general increase in grain size into the “core” of the proposed crustal channel. Recrystallized grain sizes in the Piedmont away from the BFZ commonly exceed 500 microns. These preliminary results suggest increasing deformation temperatures and decreasing differential stresses from close to the BFZ into the Piedmont, which is consistent with increased cooling of the proposed channel proximal to the colder, stronger Blue Ridge. Ongoing piezometric analyses, combined with quartz c-axis thermometry and thermochronology, may provide additional evidence to better refine the hypothesized buttress-channel relationship.