The Effect of Micas on the Strength of Experimental Granitoid Fault Gouge

Sheet silicates play an important role in shaping crustal rheology and causing strain localization at shallow depths through their low strength. However, their effect on crustal rheology at deeper levels (>3 km) remains unclear. We conducted hydrothermal ring shear experiments on three simulated gouges with comparable quartz content but varying mica types (biotite/muscovite) and contents (8-61 wt.%). Applied temperatures (T) ranged from 20-650°C, with sliding velocities (V) between 0.03-1 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, granitoid gouge exhibits a higher friction coefficient (μ=0.81) than the muscovite-rich (μ=0.47) and biotite-rich gouges (μ=0.44). With increasing T and decreasing V, granitoid gouge firstly remains its strength, and then exhibits substantial weakening when T reaches 450°C and V is lower than 1 μm/s. In contrast, muscovite-rich gouge hardens and then levels off at μ=0.68 as T reaches 450°C across all V tested, and finally weakens once T reaches 650°C and V is lower than 0.1 μm/s. Biotite-rich gouge hardens and reaches μ=0.56 at 450°C, with little further changes as T and V continue to change. Overall, the two mica-rich gouges become stronger than granitoid gouge at ≤ 0.01 μm/s and at least T=650°C.

For all post-mortem gouges, mainly samples with substantial weakening exhibit both principal slip zones constituting <6% width of the entire layer, and mineral reactions. Microstructures within the principal slip zones are consistent with dissolution-precipitation creep, including truncated grain contacts, mineral precipitates, submicrometer grain size and low porosity. Mineral reactions are often observed at 650°C and 0.1 μm/s under FEG-SEM, including Ca-rich feldspar rims of albite grains in granitoids, and muscovite breakdown plus biotite formation in muscovite-rich one. By fitting the shear strain rate to shear stress obtained from tests run at 650°C, the apparent stress exponent for granitoids is 2.2 ± 1.8, and for muscovite-rich gouge is 6.8 ± 2.2. Our results imply that mica enrichment in crustal faults (mainly granitoid composition) can lead to a stronger crust at deep levels when temperatures are high and strain rates are low. Multiple similarities between experimental and natural microstructures suggest that the interpreted mechanisms dissolution-precipitation creep and mineral reactions may trigger a frictional-viscous transition at a depth range corresponding to greenschist metamorphic facies under natural conditions.