Folds (World of Earth Science)
Compositional or metamorphic layers of rocks may bend during ductile deformation to produce folds. Folds commonly form during regional horizontal shortening in orogenic (mountain building) belts at microscopic to regional scales in all rock types (given suitable deformation conditions). Even rocks that at Earth's surface may be brittle and shatter when rapidly deformed, may fold during the application of regional, tectonic stresses over a long period of time at depth. Such a change in rock rheology is due to elevated temperature and confining pressure and the presence of fluids at deeper levels of the crust.
Upright layers (where young beds overlie older beds) that are arched upward are called anticlines. If the direction of younging (facing) is not known, such folds are called antiforms. Layers that are bent downward are called synclines (where beds are upright) or synforms where facing is not known. Cylindrical folds show the same profile in sections normal to their axes at any position along the axis. Folds where profiles vary from section to section and layers describe part of a cone are called conical folds. Folds are also classified according to the orientation of their hinge line or fold axis (the axis of curvature) and of their axial surface (the surface that bisects fold limbs and passes through the fold axis). The angle the fold hinge makes with the horizontal is called the plunge of a fold. Folds plunge gently when this angle is 100°, moderately between 300°, steeply between 600°, and are vertical when axes plunge 90°. Folds are upright where the axial surface is steeply dipping, inclined where the axial surface is moderately dipping, overturned where the axial surface is shallowly dipping and one limb is inverted, and recumbent where the axial surface is horizontal. In parallel folds, the layer thickness measured normal to the layer is constant around the fold. In similar folds, layer thicknesses measured parallel to the axial plane are constant. In describing folds, it is also important to note the inter-limb angle and whether fold hinges are rounded or angular.
Strong (competent) layers interlayered with more ductile (incompetent) layers buckle during layer-parallel shortening. The wavelength of the resulting folds depends on both the layer thickness and the viscosity (competence) contrast between layers. Larger wavelength folds develop in thick or competent layers. Folds may also develop during ductile flow in high-grade metamorphic rocks and in incompetent, lower-grade rocks. Irregular and often highly contorted syn-sedimentary folds can form during deposition of sedimentary rocks within slumps (which may be triggered by earthquakes).
When rocks that have already been folded are subjected to further shortening, early-formed folds may be refolded. Different fold interference patterns develop depending on the relative orientations of axes and axial surfaces for both generations of folds. A "dome and basin" (or, "egg carton") pattern results from the interference between two sets of upright folds whose axial surfaces are at a large angle to each other. A mushroom-shaped interference pattern results where folds with horizontal or shallowly dipping axial surfaces are folded by upright folds. A "hook" interference pattern occurs where fold axes are of similar orientation, but where axial surfaces are at a high angle to each other.
Folds may also form during regional crustal extension, such as in sedimentary basins. Roll-over antiforms develop over curved extensional (normal) faults in the upper, brittle crust or ductile shear zones in the middle to lower crust. Synforms are formed above areas where the underlying fault or ductile shear zone changes from shallowly to steeply dipping. Folds may also form during back-rotation of layers between two extensional faults or ductile shear zones. In high-grade rocks, folds may also form in surrounding layers when a competent layer pinches and swells or separates into barrel-shaped fragments (boudins) during layer-parallel extension.
Folds control the formation and localization of some petroleum and mineral deposits. Many oil and gas traps are created by regional-scale antiforms or domes formed by fold superposition, in wrench zones, or on the margins of salt diapirs. Some gold deposits are also controlled by folds. Differences in fold style of adjacent beds may lead to parting of beds along fold hinges. Quartz and, if chemical conditions are favorable, gold, may be deposited from fluids that migrate to such dilatational sites forming saddle reefs. In higher-grade rocks, rare metal pegmatites may intrude dilatational sites along fold hinges. Folds also provide geologists with valuable information about the orientation of stresses in Earth's crust at the time of their formation, helping them to unravel regional geological history.
See also Industrial minerals; Orogeny; Plate tectonics