The Effects of Hydrolytic Weakening in Quartz-Rich Rocks from the Raft River Detachment Shear Zone

Abstract

Hydrolytic weakening – the reduction of mineral strength in the presence of water – has long been invoked as a key mechanism controlling quartz rheology, yet its role in naturally deformed rocks remains debated. This study investigates the relationship between water content, fluid distribution, and deformation mechanisms in quartz-rich rocks from the Miocene Raft River detachment shear zone (Utah, USA). We analyze a suite of quartzite mylonites and quartz veins spanning a range of strain intensities and degrees of fluid-rock interaction. A comprehensive analytical approach combining optical petrography, scanning electron microscopy cathodoluminescence (SEM-CL), electron backscatter diffraction (EBSD), and Fourier transform infrared spectroscopy (FTIR) is used to characterize microstructures, crystallographic fabrics, and spatial variations in water content. Microstructural observations indicate that quartz deformation is dominated by dislocation creep accommodated by subgrain rotation recrystallization across all sample types. FTIR data reveal systematic variations in water content, with the lowest concentrations in grain interiors and significantly higher concentrations along grain boundaries and within recrystallized domains. However, the most highly strained rocks (mylonites) exhibit overall lower water contents compared to less deformed vein samples. Cathodoluminescence imaging further demonstrates a strong correlation between luminescence intensity and deformation microstructures, with recrystallized grains consistently exhibiting lower CL intensity than relic grains. These results suggest that hydrolytic weakening in natural systems is not simply controlled by bulk water content, but rather by the distribution of water within the rock, particularly its concentration along grain boundaries. These findings also demonstrate that progressive deformation, accommodated by dislocation creep and dissolution-precipitation, leads to redistribution and homogenization of the water content across the detachment shear zone. This work highlights the importance of microstructural controls on fluid localization and provides new constraints on the role of fluids in weakening mid-crustal shear zones.