Mineralogical control on fault friction and stability: a systematic study on quartz, calcite and muscovite ternary mixtures.

During fault evolution different rock types are fractured and sheared within the fault core, producing fault gouges with heterogeneous mineralogical composition. Mineral composition exerts a primary control on fault frictional properties and hence on fault slip behaviour. Understanding the conditions that lead to seismic or aseismic fault slip is of great interest to earthquake hazard assessment both for natural and induced seismicity. Although the effect of single mineral phases is probably the most documented factor in laboratory tests, no clear link has been established to understand how systematic variation of different mineral phases in gouge mixtures influences the macroscopic frictional behaviour.

Here we present an experimental study designed to probe the control of mineral composition on fault friction and stability responses. We selected three representative mineral phases, commonly found in fault zones, that are known to have severely different frictional properties: muscovite (phyllosilicate), quartz (granular silicate) and calcite (granular carbonate). Thirty double direct shear experiments were performed using a biaxial rock deformation apparatus (BRAVA) on powders (with grain sizes < 125 µm) of pure minerals and their mixtures at normal stress of 50 and 100 MPa, at room temperature and water saturation conditions. After an initial sliding of 10 mm at 10 µm/s to develop a steady state shear fabric, slide‐hold‐slide sequences (30-1000 s) and velocity steps (0.3-300 µm/s) were employed to evaluate static healing and frictional stability, respectively.

Our experimental data indicate that the mineralogical composition of fault gouges significantly affects the frictional strength, healing, and stability with a non-trivial pattern. Increasing phyllosilicate (muscovite) content results in a decrease of the frictional strength, from 0.62 for pure calcite and 0.56 for pure quartz down to 0.33 for pure muscovite powders. This effect is more marked in calcite-rich mixtures rather than quartz-rich ones, possibly due to favourable conditions for fluid-assisted pressure-solution at grain contacts. Calcite-muscovite interaction also favours a reduction of frictional healing and a more marked velocity-strengthening behaviour (promoting stable sliding and fault creep) in comparison to quartz-muscovite mixtures.