Low‐Temperature Frictional Characteristics of Chlorite‐Epidote‐Amphibole Assemblages: Implications for Strength and Seismic Style of Retrograde Fault Zones

Abstract

In retrograde faults exhuming mafic rocks, shearing occurs in metamorphic and/or hydrothermally altered mineral assemblages whose frictional properties are not well known. Here, we present the results of laboratory shearing experiments on chlorite schist, epidotite, and hornblende-dominated amphibolite and mixtures of these rocks and evaluate their frictional properties and microstructures. The experiments were conducted on powdered rock samples with starting grain size of <125 μm, at room temperature, under fluid-saturated conditions and applied normal stress of 10 MPa. The results show that chlorite schist is relatively weak (friction coefficient of 0.36), whereas epidotite and amphibolite are strong (friction coefficients of 0.63 and 0.67, respectively). The friction of chlorite schist-epidotite and chlorite schist-amphibolite mixtures decreases nearly linearly with increasing chlorite content. Chlorite schist exhibits velocity-strengthening behavior, epidotite is velocity-weakening, and the amphibolite shows mostly velocity-weakening friction. Mixtures show intermediate strength and velocity dependence of friction. Well-developed striations formed on slip surfaces in samples with ≥50% chlorite schist. The epidotite slip surface exhibits a mixture of very fine particles and coarser crystals. Amphibolite slip surfaces have less very fine grains and are composed of subhedral to euheral needles. Few intragranular fractures are preserved, and we infer wear at contact asperities to be the likely cause of velocity-weakening in our epidote gouges. Addition of chlorite to epidotite and amphibolite produces a striated slip surface and disrupts contacts between harder grains. Therefore, retrograde chlorite growth is expected to facilitate frictional weakening and stable slip in higher-grade mineral assemblages exhumed to low-temperature conditions.

Frictional Stability of Metamorphic Epidote in Granitoid Faults Under Hydrothermal Conditions and Implications for Injection‐Induced Seismicity

“…The coefficient of friction of epidote gouge (∼0.73) is consistent with the reported friction coefficient of ∼0.72 of epidote gouge recovered from veins and tested under hydrothermal conditions (σ c = 165 MPa, P f = 60 MPa, and T = 140°C; Kolawole et al, 2019; Table S2 in Supporting Information S1). However, this is higher than the coefficient of friction of ∼0.63 of epidotite gouge (69% epidote, 20% amphibole, and 11% sphene) at a normal stress of 10 MPa and at room temperature (Fagereng & Ikari, 2020). Our results for the frictional stability of epidote gouge are in accordance with the results of previous studies (Fagereng & Ikari, 2020;Kolawole et al, 2019) that the epidote-rich gouges tend to exhibit velocity-weakening behavior under both hydrothermal conditions and dry and at room temperature.…”

“…However, this is higher than the coefficient of friction of ∼0.63 of epidotite gouge (69% epidote, 20% amphibole, and 11% sphene) at a normal stress of 10 MPa and at room temperature (Fagereng & Ikari, 2020). Our results for the frictional stability of epidote gouge are in accordance with the results of previous studies (Fagereng & Ikari, 2020;Kolawole et al, 2019) that the epidote-rich gouges tend to exhibit velocity-weakening behavior under both hydrothermal conditions and dry and at room temperature.…”

The Potential for Low‐Grade Metamorphism to Facilitate Fault Instability in a Geothermal Reservoir

“…In addition to being a major rock forming mineral, Ep is also abundant in veins and distributed within metamorphic, igneous and hydrothermally altered rocks—identifying that metamorphism and fluid‐rock interaction can contribute to its presence and abundance (Franz & Liebscher, 2004; Hamilton et al., 2021). At room temperature, the epidote‐rich gouges are frictionally strong and exhibit velocity‐weakening behavior (Fagereng & Ikari, 2020). Cl is a phyllosilicate mineral found commonly in natural fault zones (Lacroix et al., 2012; Schleicher et al., 2012).…”

“…To date, few laboratory studies have been carried out to explore the frictional properties of Ep or Cl gouges at hydrothermal conditions (Fagereng & Ikari, 2020; Okamoto et al., 2019). Ep is a nesosilicate mineral that is commonly found in active geothermal systems and volcanic rocks (Bird & Spieler, 2004; Evans, 1990).…”

Microstructures of epidote-prehnite bearing damaged granitoids (northern Victoria Land, Antarctica): clues for the interaction between faulting and hydrothermal fluids

“…We suggest that the relatively low amount of prehnite (and its texture) played a major role in this context, not favoring the localization of slip along UCTs horizons. As observed for chlorite-rich fault rocks (Fagereng and Ikari, 2020), a higher volume fraction of prehnite could have instead focused deformation inside prehnite-rich UCTs.…”