A general framework for the occurrence and faulting of deformation bands in porous granular rocks

Deformation bands form in porous granular rocks by localized inelastic yielding that is well described by a modified Cam cap model. All five kinematic varieties of bands observed in the field, including dilation bands, dilation bands with shear, shear bands, compaction bands with shear, and compaction bands, can be explained by this unifying mechanical framework once their localization criteria are also specified. The growth of dilation bands with shear has been observed in the field and in the laboratory for low-pressure conditions. More commonly, however, strain hardening of compaction bands with shear will lead to faulting within, and eventually through, the resulting damage zone if the associated yield cap grows outward sufficiently to intersect the frictional strength surface. This sequence is best explained by replacing the critical state line with a frictional failure criterion in the general Cam cap approach. Given recent advances in localization theory and continuing refinement of field observations, the term “fracture” must now be expanded to include both weak and strong discontinuities, or deformation bands and displacement discontinuities (i.e., cracks, joints, veins, solution surfaces, anticracks, dikes, sills, faults), respectively. Shear zones and fault zones that accommodate both continuous and discontinuous changes in shear offset within them should also be considered as fractures according to this expanded definition.