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the process in which rock strata are bent into folds as a result of tectonic deformations. Sets of folds vary in shape, kinematic conditions of formation, and origin.
According to morphological characteristics, folding is said to be continuous, discontinuous, or intermediate. Continuous (holomorphic, linear, alpinotype) folding consists of long and narrow convex or concave folds that are continuous through the zone of folding; convex folds are known as anticlines, and concave folds are called synclines. Discontinuous (idiomorphic) folding is represented by groups of distinct, separate, and primarily anticlinal folds of varying shape (ridges, domes, irregularly shaped uplifts) separated by areas of undisturbed bedding. Intermediate (germanotype) folding consists either of broad, gentle synclines alternating with narrow, steep (crest-shaped) anticlines or box-shaped anticlinal folds (with steep limbs and flat tops) alternating with slit-shaped synclines.
Based on the kinematic conditions of formation, folding is classified as block (injection, reflected), pressure, general-compressional, or abyssal (metamorphogenetic) folding. Block folding occurs when the strata of the cover above certain uplifted or subsided blocks of the more ancient metamorphic (crystalline) basement are bent; morphologically, this is discontinuous folding. Pressure folding typically involves a differential (disharmonious) deformation of strata differing in density and plasticity: in series of strata that are deeply submerged and have low density (such as salt) or great plasticity (such as clay), material begins to flow, being forced out of some places and into others. Pressure (diapiric) cores form and lift or break through overlying strata in the form of domes or crests. Morphologically, pressure folding is classified in part as discontinuous folding (for example, diapiric domes with salt cores) and in part as the crest-shaped variation of the intermediate type.
General-compressional folding results from the action of longitudinal compression, that is, compression parallel to the strata. Because the strata are originally bedded horizontally, the compression is also horizontal. Morphologically, this folding is classed as continuous (linear). Abyssal (metamorphogenetic) folding is characterized by extreme complexity of outline, in which one may see the result of the imposition of folds of different order, shape, and direction on one another. Abyssal folding could apparently have occurred when volumetric forces caused flowage of rocks having high plasticity.
The origin of folding is still largely unclear. It is generally thought that pressure folding relates primarily to an inversion of densities in a sedimentary rock mass, that is, to the bedding of less dense rocks below more dense rocks. Abyssal folding appears to be related to pressure folding as regards conditions of formation. Unequal heating in metamorphic rocks causes the strata to become intricately deformed, and abyssal diapirs form, especially granite-gneiss domes. Rocks decrease in density and increase in fluidity during metamorphism, when re-crystallization occurs and constitutional and adsorbed water are released from the minerals into the rock pores. Relative displacements of crustal blocks, which lead to the formation of block folding, are of unknown cause.
There are two points of view concerning the origin of general-compressional folding. One holds that this type of folding occurs under the influence of horizontal compressive forces when certain blocks (platforms) of the lithosphere are thrust over or under others. The other interpretation assigns to the force of gravity the principal role in general-compressional folding. Strata are warped into folds along the slopes of mountain ranges formed by vertical movements of the crust as the result of slumping under the weight of uplifted crustal blocks as they spread laterally or the outward force of abyssal diapirs as they push into the sediments.
A number of regular patterns in the distribution of different types of folding have been established. Block folding is found primarily in relatively undisturbed regions of the earth’s crust (cratons) and on the margins of the mobile zones (geosynclines). Pressure folding is characteristic of the margins of geosynclines, primarily foredeeps, and the most deeply down-warped parts of cratons. General-compressional folding and abyssal folding are typical only of geosynclines, and only for a certain stage of their development (the stage of inversion), when mountain ranges begin to grow within the geosyncline in place of the deep troughs. A geosynclinal system changes into a fold system as a result of folding.
In the course of earth history there have been certain epochs of intensified general-compressional and abyssal folding. These epochs of folding coincide with times of increased intensity for all tectonic processes.
The study of folding is interesting for both theoretical and practical purposes because fold deformations influence minerals. One of the modern techniques used to study folding is the technique of tectonic modeling according to the principle of physical similarity.
REFERENCESBelousov, V. V. Osnovy geotektoniki. Moscow, 1975.
Khain, V. E. Obshchaia geotektonika, 2nd ed. Moscow, 1973.
V. V. BELOUSOV
in printing, the sequential bending of printed paper sheets to form signatures, which are then used to produce books and magazines. Sheets may be folded from one to four times; successive folds may be perpendicular or at right angles to each other, or a combination of arrangements may be used. Three-and four-fold processes, which produce 16- and 32-page signatures, respectively, are the most commonly used. If the press run is small, folding is done manually; otherwise, folding machines are used. When roll-fed rotary printing presses are used, folding operations are carried out by special folders on the printing press.