High shear strain behaviour of synthetic muscovite fault gouges under hydrothermal conditions

Major continental fault zones typically contain phyllosilicates and have long been recognised as zones of persistent weakness. To establish whether the presence of micas can explain this weakness, we studied the frictional behaviour of simulated muscovite fault gouge by performing rotary shear experiments in the temperature range 20–700 °C, under constant effective normal stresses of 20–100 MPa, a fixed fluid pressure of 100 MPa and at sliding velocities of 0.03–3.7 μm/s, reaching shear strains up to 100. Cataclasis causes substantial grain size reduction up to 600 °C. With increasing strain, both pervasive and localized cataclasis and related compaction result in strain hardening, until steady state is reached. This reflects the progressive development of a continuous network of fine grained, hardening bands. Coarse grained relict lenses between these bands show folded and kinked muscovite grains indicative of ductile mechanisms. Samples deformed at 700 °C show evidence for chemical alteration and partial melting. Since our data suggest that muscovite gouge strengthens with depth and strain, it is questionable whether its presence can contribute to the long-term weakness of major crustal fault zones, unless a substantial decrease in strength occurs at shear strain rates lower than attained in our study.