Dilation bands: A new form of localized failure in granular media. Xavier DU BERNARD*, Peter EICHHUBL, and Atilla AYDIN Rock Fracture Project, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305 *Present address: Laboratoire de Géophysique Interne et Tectonophysique (UMR 5559), BP 53X, 38041 Grenoble cedex 9, France. Abstract. We report a structure that is characterized by a localized increase in porosity or dilation within a tabular band of on average 1-2 mm thickness in porous, poorly consolidated sand. These dilation bands formed in predominantly opening-mode and provide a sharp contrast to planar opening-mode fractures or joints with two discrete surfaces in consolidated, low porosity brittle rock. Our observation of dilation bands, together with previously recognized shear and compaction bands, complete the spectrum of localized deformation modes along tabular bands in poorly consolidated sand and sandstone consistent with recent theoretical predictions of deformation localization in granular media.