How mineralogy and fault structure influence frictional and acoustic properties of quartz-phyllosilicate mixtures

Fault zones are complex systems where mineralogy, fabric, and frictional properties interplay on fault strength and slip behavior. While prior investigations have focused on post-experimental microstructures to relate fault friction to deformation processes, the evolution of fault fabric coupled with Acoustic Emissions (AEs) during deformation remains elusive. In this study, we present experimental data integrating systematically microstructural, mineralogical, frictional, and AEs analysis coming from a suite of frictional experiments in a double direct shear configuration. These experiments aim to elucidate deformation micro-mechanisms and associated acoustic activity in heterogeneous fault systems. We performed a set of experiments in quartz-phyllosilicate mixtures and another set of tests where a quartz layer is sandwiched between two muscovite horizons in contact with the forcing blocks. The first set represents typical cataclastic rocks with random fabric while layered mixtures were designed to replicate the deformation behavior of block-and-matrix shear zones.

Pure quartz gouges exhibit cataclastic deformation (grain fragmentation and shear localization) which generates high AE rates and amplitudes. In contrast, muscovite-rich gouges deform through distributed sliding along anastomosed foliation and are characterized by low AE activity. In quartz-phyllosilicate mixtures, increasing muscovite content reduces friction, AE rate, and AE average amplitude inhibiting quartz grain interactions. Layered mixtures introduce additional complexity. While the two muscovite layers govern frictional strength and accommodate distributed deformation, cataclastic processes in the central quartz layer dominate AE activity. These layered systems combine the AE characteristics of quartz and muscovite, with high AE rates similar to pure quartz despite the overall weakening from muscovite. Microstructural observations support these findings, showing deformation concentrated at muscovite interfaces but also revealing localized shear bands in the quartz layer that significantly contribute to AE activity. Experiments performed at varying strain rates reveal that higher strain rates amplify AE rate and amplitude. In the layered mixtures, at high strain rate, AE rates and amplitudes are in the range of pure quartz gouge whereas at low strain rate, they are in the range of pure muscovite gouge.

The evolution of the frequency-magnitude distribution of AEs (b-values) with strain provides additional insights on micromechanical processes: quartz-dominated gouges show increasing b-values from yield to steady-state, suggesting a transition from distributed to localized deformation. In contrast, muscovite-dominated gouges maintain low b-values, reflecting consistent distributed deformation. Layered systems exhibit b-value evolution with strain similar to pure quartz, indicating the dominant role of quartz in the AE properties. Our findings emphasize that fault friction and acoustic behavior are controlled by mineralogical and structural heterogeneities and modulated by strain rate. Weak muscovite layers primarily control frictional strength, while strong quartz layers generate significant AEs, highlighting the potential for aseismic slip coupled with seismic activity in heterogeneous faults.