The type of deformation where rocks deform by developing marked discontinuities across which there is often a break in cohesion (fracture)
Under relatively high temperatures and pressures, rock deformation is accommodated by crystalline plasticity and diffusive mass transfer processes. In contrast, brittle failure occurs under low temperatures and pressures, usually with the development of dilatancy and shear localization very early in the deformation. Brittle faulting has been intensively studied in the laboratory under upper crustal conditions of pressure and temperature. As shear localization develops, there is a concomitant degradation of the rock samples overall strength, and the faulting process becomes catastrophic unless the surplus of elastic energy stored in the test machine is quickly relieved. Therefore details of the shear localization process cannot be observed unless the ``post-failure'' deformation is stabilized. The micro mechanical process involves the initially stable growth of stress-induced cracks sub parallel to the maximum compression direction, and the subsequent development of strain softening as the micro cracks interact and coalesce to form thoroughgoing shear band. The brittle-plastic transition in feldspar aggregates is similar to the quartz aggregate with two important differences. In feldspar aggregates, there is a broad regime of cataclastic flow which is facilitated by the ease of cleavage cracking and the difficulty of dislocation glide and climb. Furthermore, shear localization can develop in highly complex patterns. In many respects, the micro mechanical processes operative in the brittle-plastic transition in the quartzo-feldspathic aggregate are very similar those observed in Carrara marble. Significant advances have also been made in our understanding of the transition from brittle faulting to cataclastic flow in porous sediments and sedimentary rocks. In such aggregates, significant stress concentration can develop at points where grains impinge on one another. Once nucleated, the tensile cracks tend to grow unstably across the grain, resulting in grain crushing and pore collapse. The micro mechanical process can be modeled using elastic contact theory and linear elastic fracture mechanics. Such a pore collapse process promotes strain hardening and inhibits the onset of shear localization, and the cataclastic flow is characterized by a yield stress which decreases with increasing pressure and by a yield envelope which expands with porosity reduction . The phenomenology seems to applicable to sandstone, carbonates and shale and it has been widely used in the modeling of accretionary prism tectonics, reservoir compaction and borehole instability.