Localized failure modes in a compactant porous rock

Since dilatancy is generally observed as a precursor to brittle faulting an d the development of shear localization, attention has focused on how local ized failure develops in a dilatant rock. However, recent geologic observat ions and reassessment of bifurcation theory have indicated that strain loca lization may be pervasive in a compactant porous rock. The localized bands can be in shear or in compaction, and oriented at relatively high angles (u p to 90 degrees) to the maximum compression direction. Here we report micro structural characterization of the spatial distribution of damage in failed samples which confirms that compaction bands and high-angle conjugate shea rs can develop in sandstones with porosities ranging from 13% to 28%. These failure modes are generally associated with stress states in the transitio nal regime from brittle faulting to cataclastic ductile flow. The laborator y results suggest that these complex localized features can be pervasive in sandstone formations, not just limited to aeolian sandstone in which they were first documented. They may significantly impact the stress field, stra in partitioning and fluid transport in sedimentary formations and accretion ary prisms. While bifuraction theory provides an useful framework for analy zing the inception of localization, our data rule out a constitutive model that does not account for the activation of multiple damage mechanisms in t he transitional regime.