cleavage and other tectonic foliations in rocks

cleavage and other tectonic foliations in rocks Although tectonic foliations are best displayed in thin sections, which enable the viewer to see through rock specimens, and in outcrops at the Earth's surface, these small-scale structures result primarily from tectonic deformation on a much larger scale. These structures generally occur as products of mountain-building.

A foliation is a pervasive set of parallel surfaces or zones in a rock. This broad definition encompasses features such as sedimentary bedding, igneous cumulate layering, and tectonic foliations (Fig. 1). Narrowing our focus, tectonic foliations develop from differential stress during deformation with metamorphism. The original grain arrangement or rock fabric changes by developing four fabric elements (Fig. 1): compositional banding such as alternating mafic and felsic layers in a gneiss, parallel discontinuities such as pressure-solution surfaces in a cleaved rock, elongate grains such as deformed quartz grains in a schist, or inequant grains such as aligned mica grains in a schist. The parallelism of these elements constitutes the foliation.

A cleaved rock has fabric domains of two types: cleavage zones, where the fabric elements and metamorphic minerals form the foliation; and microlithons, which mostly or entirely preserve the original mineralogy and fabric. Cleavage is either continuous (Fig. 2) or spaced. Continuously cleaved rocks have cleavage zones so close together that their spacing is not detectable, or all fabric elements are parallel to the foliation. A common rock with this type of foliation is slate, which splits along an almost innumerable number of parallel planes. Essentially, continuously cleaved rocks lack microlithons, whereas a rock with spaced cleavage has discernible microlithons.

Many workers view schistosity as a coarse continuous cleavage with visible grain size and all fabric elements parallel to the foliation. This description does not do justice to the appearance of these beautiful rocks. They have parallel bright shiny mica wrapping round large grains of pink garnet, brown staurolite, or blue kyanite. In contrast to cleaved rocks or schists, gneisses contain differentiated layers. The primary fabric element is compositional banding rather than parallel discontinuities or long axes of grains. The alternating bands of mafic and felsic minerals are a striking and distinctive feature of all gneisses.

A spectrum of foliations from cleavage to differential layering exists because the structures develop under different metamorphic conditions. The transition from cleavage to gneiss represents a progression in metamorphic intensity during deformation. If an unmetamorphosed mudstone is progressively metamorphosed and deformed, the grain size and mineral segregation in the evolving foliated rock increase until deformation ceases. Increases in conditions such as temperature, pressure, and fluid content drive this change.

Foliated rocks are common folded, and the folds and foliations will have simple geometric relationships if they were formed at the same time. The foliation is generally parallel or subparallel to the axial plane of the host fold, but it can also converge through the fold core. These differences in geometry indicate different sequences of deformation. For example, divergent cleavage geometry can indicate that shear planes parallel to the layers in the rock were rotated by the formation of cleavage during folding. Parallel cleavage geometry can indicate a fold that was modified by the later formation of foliation. Not all foliations and folds are, however, of the same age. Rocks that have been regionally metamorphosed can exhibit several phases of folds and foliations that were produced by successive phases of deformation during mountain-building. In such cases, younger folds will deflect an older foliation, or a younger cleavage will transect older folds without there being a consistent geometrical relationship between them.

William M. Dunne

Bibliography

Twiss, R. J. and and Moores, E. M. (1992) Structural geology. W. H. Freeman, New York.