Most theoretical models of crustal deformation assume that rocks either fracture according to some yield criterion (usually taken as a simple linear Mohr-Coulomb dependence on pressure) or flow in a viscous manner due to crystal plasticity or diffusion. However, direct field observation shows a much more intimate interplay between fracture and flow, with precursor fractures localizing subsequent ductile shear zones that may in turn be overprinted by discrete fractures, implying multiple cycles of brittle-ductile behaviour. Fracture and flow may be coeval over small distances, with the perturbation flow surrounding active fractures producing characteristic flanking structures. Rheological models must include this linked brittle-ductile behaviour if they are to realistically model rock deformation.

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