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

Mature crustal fault zones are known to be zones of persistent weakness. Th is weakness is believed to result from microstructural modifications during deformation, such as grain-size reduction and foliation development. Aroun d the brittle-ductile transition. phyllosilicates are expected to have a si gnificant effect on fault strength, in particular under conditions favourin g pressure solution. To study such effects, we performed rotary shear exper iments 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 significa nt strain weakening, and a transition with progressive strain towards more rate-sensitive and less normal stress-sensitive behaviour. This was accompa nied by a microstructural evolution from a purely cataclastic microstructur e to a mylonitic microstructure consisting of elongate, asymmetric clasts i n a fine-grained, foliated matrix. The results demonstrate that strain weak ening and the development of a typical 'mylonitic' microstructure can occur as a consequence of grain-size reduction by cataclasis, and a transition t o pressure solution accommodated deformation, even in the absence of disloc ation creep. The data raise questions regarding the reliability of microstr uctures as rheology indicators, as well as on the use of low strain, monomi neralic flow laws for modelling crustal dynamics. (C) 2001 Elsevier Science Ltd. All rights reserved.