Shear zone geometry: A review

Abstract

Shear zones may be classified into brittle, brittle-ductile, and ductile shear zones. The geometry and displacement boundary conditions of these zones are established. The geometric characteristics of ductile shear zones relevant to geological studies are described: orientations and values of principal finite strains, rotation, and deformation features of pre-existing planar and linear structures. Ductile shear zones show a fabric (schistosity and lineation) related to the finite strain state. The methods for determining strains and displacements from field studies are described. Shear zones commonly occur in conjugate sets, but the two differently oriented sets do not seem to be able to operate synchronously. The angular relationships of conjugate ductile shear zones are different from those of brittle shear zones. The termination of all types of shear zones poses complex compatibility problems, some solutions are suggested. A synthesis of shear zone geometry in regions of crustal contraction and crustal extensions is made, and ideas on how deep level ductile shear zones relate to high level brittle shears and gliding nappes are put forward.