Effects of epidote and chlorite on the frictional stability of granite faults: Implications for seismicity in deep geothermal reservoirs

Epidote and chlorite are both low-grade metamorphic minerals that are widely distributed in deep granite geothermal reservoirs. Hot fluid circulation promotes the precipitation of epidote and chlorite coatings on natural faults and fractures and this can in turn exert control on fault/fracture frictional stability. We use simulated epidote/chlorite gouges and conduct fault shear experiments at conditions typifying the ~4 km depth typical of deep geothermal reservoirs to explore the effect of epidote/chlorite content on frictional strength and stability. Results indicate that the frictional characteristics of epidote and chlorite gouges vastly differ. Epidote gouge is frictionally strong, with a coefficient of friction of ~0.73, nearly double that of the chlorite gouge (~0.35). In addition, the epidote gouge exhibits strong velocity-weakening and unstable frictional behavior under the test conditions, while the chlorite gouge is velocity-strengthening and stable. Fault frictional strength and velocity-weakening behavior are all enhanced for epidote-granite mixed gouges as epidote content increases. However, chlorite-granite mixed gouges show the opposite trend with increasing chlorite content. Our results have important implications in understanding the frictional stability of epidote/chlorite-filled granite faults and its influence on seismicity in deep geothermal reservoirs inhabiting the shallow crust.