Evolution of damage zone geometry and intensity in porous sandstone: insight gained from strain energy density

Abstract

In porous geological materials such as sandstone or limestone, fault-related damage zones form arrays of deformation bands, which are planar discontinuities characterized by localized shear and porosity change. We show that the geometry and intensity of fault-related deformation band damage zones is systematic and predictable using standard strain energy density-based criteria. These criteria are used to successfully predict the tendencies for the nucleation and for the propagation of deformation bands as observed in a classic outcrop of fault-related damage zones within the brittly deformed Jurassic Wingate Sandstone exposed in the Laramide-aged Uncompahgre fold, in western Colorado, USA. The separate distributions of volumetric and distortional strain energy density are calculated for the interpreted geometry and stress state of the causative Laramide-aged thrust fault displacements from boundary element calculations of the attendant slip-induced local stresses. Volumetric strain energy density predicts the tendency for deformation band nucleation, the growth stage at which the deformation bands are defined by pore space dilatancy or collapse. Deformation band propagation, where shear occurs along the band, is predicted by distortional strain energy density. Further deformation bands at the Uncompahgre are predicted and observed to be characterized by shear-enhanced dilation.