Frictional-viscous flow of simulated fault gouge caused by the combined effects of phyllosilicates and pressure solution

It is widely believed that conventional brittle-plastic strength envelopes may significantly overestimate crustal strength, in part because they fail to account for the effects of fluid-assisted deformation mechanisms. In particular, pressure solution has been suggested to allow fault creep behaviour at a shear stress well below that predicted by conventional strength envelopes. In addition, many natural fault zones contain significant amounts of phyllosilicates, often defining a foliation. Phyllosilicates are believed to inhibit grain contact healing and increase the rate of pressure solution in quartzose rocks. Hence, the interaction between phyllosilicates and pressure solution may strongly influence fault rheology under the hydrothermal conditions pertaining around the brittle-ductile transition. The aim of this research is to assess the combined effects of pressure solution and phyllosilicates on fault slip behaviour. To this end, we performed high strain, rotary shear experiments on experimental faults containing brine-saturated mixtures of halite and kaolinite as simulated gouge. The experiments were done under conditions when pressure solution and cataclasis dominate over dislocation creep, thus simulating the brittle-ductile transition. Halite was chosen as a rock analogue because of its well-constrained pressure solution kinetics. Experiments were done at room temperature and atmospheric pressure, under drained conditions. In the experiments we explored the effect of varying sliding velocity, normal stress and clay content on fault strength. The results showed frictional-viscous behaviour, i.e. shear strength depending on both normal stress and shear strain rate, in brine-saturated halite/kaolinite mixtures. In contrast, purely frictional (i.e. normal stress dependent and shear strain rate insensitive) behaviour was observed when an inert pore fluid (silicone oil instead of brine) or an inert solid (quartz instead of halite) was used. Monomineralic halite and kaolinite gouges showed purely frictional behaviour as well. This demonstrates that the observed frictional-viscous behaviour was caused by the combined effects of pressure solution and phyllosilicates. The microstructures, which strongly resemble natural mylonites in many respects, suggest that deformation of the gouge involved sliding along kaolinite-rich foliation planes, accommodated by pressure solution and dilatation/cataclasis, the relative amounts of which varied with sliding velocity. If similar behaviour occurs in natural phyllosilicate-rich fault zones, such zones are expected to be significantly weaker than predicted by traditional brittle-ductile strength envelopes. In addition, our microstructures suggest that pressure solution may play an important role next to dislocation creep in microstructural evolution in mylonitic rocks. (C) 2000 Elsevier Science B.V. All rights reserved.