shear zones, deformation bands, and kink bands

shear zones, deformation bands, and kink bands Under certain circumstances, deformation in rocks produces a localized zone within which the rock material undergoes a rotation with respect to the walls of the zone. This process is known as simple shear. Such structures are analogous to faults in the sense that they accommodate a displacement of one wall with respect to the other, but differ in that the displacement is distributed through the zone rather than being confined to a planar fracture. Structures formed in this way can occur on all scales from fractions of a millimetre to many tens of kilometres across.

Deformation bands

Zones of simple shear strain within single crystals on a microscopic scale are termed ‘deformation bands’. These are tabular zones whose crystallographic orientation differs from that in the adjoining parts of the crystal. They are commonly observed in quartz crystals, where they show up in polarized light as alternating dark and light bands, forming a feature known as ‘undulatory extinction’.

Shear zones

Shear zones are localized zones of mainly ductile deformation within rocks at scales of between millimetres and kilometres across. The smallest shear zones, although only a few millimetres across, are usually many centimetres or even metres in length, and traverse many crystals, so that they represent structures at least an order of magnitude larger than deformation bands. The largest shear zones, found typically in regions of Precambrian basement, are tens of kilometres across and hundreds of kilometres long.

The simplest type of shear zone is illustrated in Fig. 1a. It is a tabular zone of ductile deformation enclosed between two opposing blocks of undeformed rock, and is produced by a lateral displacement of one wall of the shear zone relative to the other (that is, simple shear). In this case the displacement is distributed uniformly across the zone. Such zones are ductile analogues of faults, and, like faults, may have vary-ing orientations and sense of movement. For example, ver-tical zones exhibit strike-slip (transcurrent) displacement, or shear, and inclined zones can exhibit either a normal or a reverse sense of movement, produced by extensional or compressional stresses, respectively.

In Fig. 1, the deformation (strain) within the zone is distributed evenly across the zone but exhibits an abrupt change at its margins. In most shear zones, however, especially those affecting previously undeformed rocks, the strain increases gradually through the zone, reaching a maximum in the central part of the zone (Fig. 1b). With large displacements, and consequently large strains, the deformed rock in the central part of the shear zone possesses a strongly developed planar and linear fabric that has been rotated into near parallelism with the walls of the shear zone, and can be used to indicate the shear direction, that is, the direction of displacement along the zone.

Shear zones are often more complex than the simple models of Fig. 1. They can, for example, incorporate an element of compression or extension across the zone, or the wall rock may also be deformed, either before the development of the shear zone or contemporaneously with it. In each of these cases, the geometry of the strained rock within the zone will be different from that of Fig. 1, and more difficult to interpret.

Movements on shear zones represent the characteristic method of accommodating displacements at intermediate to deep levels of the crust and uppermost mantle. Major fault displacements at upper crustal levels are thought to transfer downwards into shear zones (Fig. 2). For example, the San Andreas fault zone of the western USA, which comprises a network of branching sub-parallel faults, between 100 and 400 km wide, with a total displacement of perhaps 1000 km, is probably replaced at depth by a broad shear zone in which the displacement is taken up by ductile deformation. Evidence for the nature of deep-level displacements comes from the study of Precambrian gneiss terrains, such as those of southern Greenland and north-west Scotland, where shear zones on every scale are exposed. In south-west Greenland, a thrust-sense shear zone 50 km wide forms the boundary between the Nagssugtoqidian orogenic belt to the north and the Archaean craton to the south. In the Lewisian complex of north-west Scotland, several large transcurrent shear zones occur. Among these is the Canisp shear zone in the Assynt area, which is about 1 km wide and has accommodated right-lateral displacement.

Kink bands

Kink bands are a special type of shear zone, occurring in rocks that already possessed a strongly layered or laminated structure before the development of the kink band (Fig. 3). They are small-scale structures, typically of the order of millimetres to centimetres in width, and are defined by sharp boundaries where the orientation of the pre-existing laminar structure changes abruptly. By analogy with faults, kink bands may display either a normal or a reverse sense of movement on inclined bands and transcurrent movement on steeply dipping bands.

Kink bands are commonly observed in conjugate pairs (as illustrated in Fig. 3), and can lead with increasing strain to the development of special types of folds known as box folds and chevron folds. Such folds form when two or more kink bands merge to form continuous zones of deformation. However, although they are in some ways analogous to folds, kink bands are formed by a simple shear mechanism and represent an alternative shortening mechanism to continuous folding. They are formed under special conditions where the deforming material consists of a set of strong thin layers separated by weak zones of easy slip. These slip horizons are essential for rotation to occur within the kink bands. Rocks with slaty cleavage or well-developed schistosity are the most likely to be affected by kink bands.

R. G. Park

Bibliography

Park, R. G. (1997) Foundations of structural geology (3rd edn), chapters 3, 10. Chapman and Hall, London.