Abstract Experimental tests on simulated clay gouges and data from shear zones developed in pelitic media at convergent plate margins provide contrasting evidence regarding the hydraulic characteristics and, in consequence, the frictional properties of sheared clays. The natural shear zone analysed in this work indicates that shear strain can induce mineralogical changes in smectite-bearing sediments that imply loss of water from the smectite minerals and their replacement with anhydrous illite minerals. The extreme localization of the illitization process and its geometric characteristics allow us to argue that the reaction is initiated by stress concentration along the shear zone and, once discrete shears develop, it is accelerated by both cataclasis and the frictional dehydration of smectites. This process would generate fresh water from within the shear zone, leading to fluid overpressure build up, and can account for the observed hydraulic circulation and salinity anomalies in modern accretionary prisms.