The effect of low-grade metamorphic epidote on fault stability in granitoids at shallow depth is not well understood. We present structured laboratory observations of epidote and simulated Pohang granodiorite (an EGS site) gouges and mixtures to evaluate their frictional responses. The experiments were performed on powdered rock gouges at a constant confining pressure of 110 MPa, pore fluid pressures of 42 and 63 MPa, temperatures of 100-250℃ and epidote percentages in the range of 0-100 vol.%. Results show that the simulated Pohang granodiorite gouge is frictionally strong (friction coefficient of ~0.71) but exhibits a transition from velocity-strengthening to velocity-weakening behavior at in-situ temperatures >150℃. Epidote gouge displays similar frictional strength to the simulated granodiorite gouge but a stronger velocity-weakening response at 150℃.  Increasing the epidote content in the homogeneously mixed gouges maintains the high frictional strength but increases velocity-weakening response approximately proportionately with epidote content. Modes of epidote precipitation likely control patch dimension and this in turn potentially changes the response of the 50:50 epidote-granodiorite mixed gouges in different geometric configurations. However, 50:50 mixtures that are variously homogeneously mixed, encapsulated and checkerboarded in their architectures are insensitive to their various geometries – all reflect the high frictional strength and strong velocity-weakening response of 100:0 pure epidote. This suggests that small volume percentages of epidote present as thin coatings on fractures and faults can impart velocity-weakening behavior, independent of individual patch size and can thereby support the potential seismic reactivation of faults. Considering the frictional and stability properties of epidote at conditions typical of shallow depths, the presence of low-grade metamorphism exerts a potentially important control on fault stability in granitoids with relevance as a marker mineral for susceptibility to injection-induced seismicity.

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The column headings for each file: 
Column 1 = The shear displacement
Column 2 = Coefficient of friction