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

The term Riedel shear zone refers to a geometric fracture pattern commonly associated with strike–slip fault systems. Using field mapping and three-dimensional digital modeling, the progressive temporal evolution of natural Riedel shear zones within exposures of the Jurassic Navajo Sandstone is interpreted from the spatial evolution of small-scale, incipient Proto-Riedel Zones (PRZs) to more completely developed systems. PRZs nucleate as a tabular zone of localized shearing marked by en échelon deformation bands, each of which is no more than a few mm wide and tens of cm long. These initial deformation bands have an opposite (antithetic) sense of slip with respect to the zone, and are oriented at an acute angle of 55–85° to the trend of the zone. With increasing strain, deformation bands and sedimentary markers become sheared through granular flow across the zone and assume a sigmoidal form. Subsequent bands develop as conjugate shear fractures within the strain-hardened axis of the PRZ. These antithetic driven systems are not directly linked to pre-existing or external structural elements and are not compatible with traditional synthetic driven models of Riedel shear zones.