Experimental investigation into the microstructural and mechanical evolution of phyllosilicate-bearing fault rock under conditions favouring pressure solution

Mature crustal fault zones are known to be zones of persistent weakness. This weakness is believed to result from microstructural modifications during deformation, such as grain-size reduction and foliation development. Around the brittle–ductile transition, phyllosilicates are expected to have a significant effect on fault strength, in particular under conditions favouring pressure solution. To study such effects, we performed rotary shear experiments on brine-saturated halite/kaolinite mixtures, aimed at investigating the relation between microstructural and mechanical evolution in a system where pressure solution and cataclasis dominate. The results show significant strain weakening, and a transition with progressive strain towards more rate-sensitive and less normal stress-sensitive behaviour. This was accompanied by a microstructural evolution from a purely cataclastic microstructure to a mylonitic microstructure consisting of elongate, asymmetric clasts in a fine-grained, foliated matrix. The results demonstrate that strain weakening and the development of a typical ‘mylonitic’ microstructure can occur as a consequence of grain-size reduction by cataclasis, and a transition to pressure solution accommodated deformation, even in the absence of dislocation creep. The data raise questions regarding the reliability of microstructures as rheology indicators, as well as on the use of low strain, monomineralic flow laws for modelling crustal dynamics.