Compression at low angles to well-developed planar anisotropy in rocks can cause instability that results in the formation of a kink band. The development of kink bands in experimentally deformed brittle anisotropic rocks, such as slate, reflects the operation of several mechanisms. These are gliding on cleavage accompanied by cataclasis, definition of kink planes along planes of high shear stress, and rotation of the foliation segments between the kink planes with slip, cataclasis, and dilatation all occurring within the kink band. Initially straight and parallel kink-band boundaries remain straight and parallel during continued compression, and maintain a constant inclination to the direction of maximum compression. In slate the mean value of this angle is 47.4 degrees for compression at 15 degrees to the anisotropy.
Rotation of foliation segments within the kink band is effected by slip on the anisotropy and continues until the orientation is such that slip is no longer possible. Further deformation causes faulting parallel to the kink-band boundaries, the development of secondary slip surfaces, or production of other kink bands. The shortening across a kink band is a function of the original separation of kink-band boundaries and of the amount of rotation of foliation segments within the kink band. If the boundaries are closely spaced initially, far less shortening is required to cause rotation to the limiting position than if the boundaries are widely spaced, and faulting or another event is initiated at much lower strains.
Kink-band relationships can be used to determine the sense of slip along the anisotropy, the amount of shortening that has occurred across the kink band, and the directions of the principal stress axes. The experimental data suggest that a relationship between pressure and kink-band width may exist that could be useful in determining pressure conditions in natural environments. Thus, the kink-band phenomenon may be a very valuable tool in the dynamic structural analysis of deformed rock sequences.