The effect of micas on the strength of experimental granitoid fault gouge

Sheet silicates play an important role in shaping crustal rheology and shear localization at shallow depths through their low strength. However, their effect at deeper levels (>3 km) remains unclear. We conducted hydrothermal ring shear experiments on three simulated gouges with similar quartz content but varying mica types and contents. Applied temperatures $T$ ranged from 20 to 650 °C, with sliding velocities $V$ between 0.03 and 1 μm/s, and an effective normal stress and pore water pressure of 100 MPa. Shear strains up to 30 were attained.

At 1 μm/s and 20 °C, granitoids exhibit a higher friction coefficient (μ = 0.81) than the muscovite-rich (μ = 0.47) and biotite-rich gouges (μ = 0.44). With increasing $T$ at a fixed $V$ of 1 μm/s, μ of granitoids decreases to 0.61 at 650 °C, whereas muscovite- and biotite-rich gouges strengthen (μ = 0.56–0.69) until T reaches 200–450 °C. At 650 °C, weakening is observed in granitoid and muscovite-rich gouges as $V$ decreases, while biotite-rich gouge shows no change. Granitoids are weaker than the mica-rich gouges once $V$ decreases to 0.03 μm/s and $T$ reaches 650 °C. All gouges at 650 °C exhibit μm-wide principal slip zones, featuring microstructures consistent with dissolution-precipitation creep (DPC). These include truncated grain contacts, mineral precipitates, submicrometer grain size and low porosity. Muscovite breakdown and biotite formation occurs in muscovite-rich gouge as they weaken. Our results imply that mica enrichment in granitoid faults can lead to a weaker upper crust and stronger middle crusts as they weaken less than granitoids. The onset of DPC may trigger a frictional-viscous transition at greenschist facies depths.