- This article is about rock cleavage, for cleavage in minerals see Cleavage (crystal)
Cleavage, in structural geology and petrology, describes the tendency of a rock to break along preferred planes of weakness, caused by the development of a planar fabric as a result of deformation.
Rocks deformed under very low to low metamorphic grade often develop planes along which the rock can easily be split. Slates are an example of a rock with a penetrative cleavage caused partly by the realignement of phyllosilicate minerals with increasing flattening strain. The degree of development of cleavage appears to depend on the amount of rock strain, the rock-type and the deformation mechanisms involved.
Cleavage is generally regarded as a special case of foliation,[1][2] although some geologists keep the terms separate and take the view that cleavage grades into foliation with increasing metamorphic grade and new mineral growth.[3]
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Types of cleavage
Penetrative cleavage
A penetrative or continuous cleavage is present throughout the rock apparently down to the grain scale. Microscopic examination however, generally shows that such cleavage is domainal in nature with discrete cleavage lamellae in otherwise undeformed rock.[3]
Spaced cleavage
Spaced cleavage generally refers to discrete cleavage planes visible to the naked eye.[2]
Crenulation cleavage
Crenulation cleavage the fabric is dominated by the microfolding of a pre-existing planar fabric. This fabric may be primary depositional or diagenetic in type but is often an earlier deformation-related cleavage.[4]
Transposition cleavage
In transposition cleavage a fabric develops that produces compositional banding parallel to the cleavage, effectively obliterating bedding and other primary fabrics. In some cases it is clear that this type of cleavage has developed from a crenulation in which quartz has become concentrated in fold hinges with phyllosilicates concentrated along the limbs. This process occurs by pressure solution with quartz being dissolved along the limbs and being deposited in the hinges.[3]
Fracture cleavage
This form of cleavage consists of closely spaced fractures although both the terminology and interpretation of these structures remain matters of debate.[3]
Relationship to folds
When mixed sandstone and mudstone sequences are folded during very-low to low grade metamorphism, cleavage forms parallel to the fold axial plane, particularly in the clay-rich parts of the sequence. In folded alternations of sandstone and mudstone the cleavage has a fan-like arrangement, divergent in the mudstone layers and convergent in the sandstones. This is thought to be because the folding is controlled by buckling of the stronger sandstone beds with the weaker mudstones deforming to fill the intervening gaps.[3]
Formation
Two main mechanisms of cleavage formation have been proposed, rotation of existing phyllosilicate grains by increasing strain[5][6] and crystallization of new phyllosilicate minerals governed by the prevailing stress field.[7][8] Experimental data and observations of naturally deformed rocks have tended to support the former mechanism but the latter remains a possible contributor to fabric development.[9]
References
- ^ 1997. Specifications And Guidelines For Bedrock Mapping In British Columbia, Appendix VII: Glossary of Selected Geologic Terms, Resources Information Standards Committee, British Columbia Government
- ^ a b Report of IUGS sub-commission on the systematics of metamorphic rocks
- ^ a b c d e Price, N.J. & Cosgrove, J.W. 1990. Analysis of geological structures. Cambridge University Press, 520pp.
- ^ Ramsay, J.G. & Huber, M.I. 1987. The techniques of modern structural geology: Volume 2, Academic Press, 391pp.
- ^ Sorby, H.C. 1853. On the origin of Slaty-cleavage, New Philosophical Journal Edinburgh, 55, 137-148.
- ^ March 1932. Mathematishce Theorie der Regelung nach der Korngestalt bei affiner deformation. Zeit Krist., 81, 285-297.
- ^ Harker, A. 1884, On Slaty-cleavage and Allied Rock Structures, Report British Association of Science, 813-852.
- ^ Kamb 1959, Theory of preferred orientation developed by crystallization under stress, J. Geophys. Res., 67, 153-170
- ^ Passchier, C.W, & Trouw, R.A.J. 2005, Microtectonics, Springer, 366pp.
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