Fibrous clay minerals concentrate in highly deformed areas of the active Galera Fault Zone, in the Betic Cordillera (Spain).
Authigenesis of fibrous clay minerals implies an open chemical system where the fault acts as a conduit for Mg-rich fluids.
Variations in fluid-rock interaction can cause heterogeneities in the mineralogy of the fault gouge.
The properties of the new mineral associations have implications in fault strength and potential for earthquake nucleation.