Mechanics of shear zones in isotropic dilatant materials

Abstract For the present paper it is assumed that rocks have linear isotropic viscous or elastic deformational properties, but their moduli may vary with mean stress. It is argued that the rock in shear zones should be weaker than the surrounding rock. Models of shear zones in which they are represented as sheets of rock with lower elasticity or viscosity modulus are set up. The stresses within the shear zones are calculated for different orientations of the shear zones relative to the regional compression direction. For brittle failure it is found that the modification of the stress in the shear zone can favour further cracking and that the deformation is unstable. The model is used to provide an explanation for brittle failure in shear, second-order faults and en echelon arrays of tension fractures. The mean stress in a weak zone is different from that in the matrix, and it is proposed that shear zones in rocks deformed at high metamorphic grade are caused by ductility enhanced as a result of reactions promoted by changes in mean stress.