The nucleation and evolution of Riedel shear zones as deformation bands inporous sandstone

The term Riedel shear zone refers to a geometric fracture pattern commonly associated with strike-slip fault systems. Using field mapping and three-di mensional digital modeling, the progressive temporal evolution of natural R iedel shear zones within exposures of the Jurassic Navajo Sandstone is inte rpreted from the spatial evolution of small-scale, incipient Proto-Riedel Z ones (PRZs) to more completely developed systems. PRZs nucleate as a tabula r zone of localized shearing marked by en echelon deformation bands, each o f which is no more than a Few mm wide and tens of cm long. These initial de formation bands have an opposite (antithetic) sense of slip with respect to the zone, and are oriented at an acute angle of 55-85 degrees to the trend of the zone. With increasing strain, deformation bands and sedimentary mar kers become sheared through granular Row across the zone and assume a sigmo idal form. Subsequent bands develop as conjugate shear fractures within the strain-hardened axis of the PRZ. These antithetic driven systems are not d irectly linked to pre-existing or external structural elements and are not compatible with traditional synthetic driven models of Riedel shear zones. (C) 2001 Elsevier Science Ltd. All rights reserved.