A region, narrow compared to its length, within which rocks have undergone intense deformation. There are two end-members. Brittle shear zones (faults), marked by a surface of rupture, are a common feature of higher crustal levels. In ductile shear zones deformation is continuous and characterized by high ductile strains due to the rocks having formed under high temperatures and pressures at deeper crustal levels. Both brittle and ductile shear zones may occur as subparallel or conjugate sets.