Fold (geology)
Bent rock layers from microscopic crinkles to mountain-sized structures.
In structural geology, a fold is a stack of originally planar surfaces, such as sedimentary strata, that are bent or curved during permanent deformation. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds and occur as single isolated folds or in periodic sets known as fold trains. They form under varied conditions of stress, pore pressure, and temperature gradient, appearing in soft sediments, metamorphic rocks, and even as primary flow structures in some igneous rocks. Folds are commonly produced by the shortening of existing layers, but can also result from displacement on a non-planar fault, at the tip of a propagating fault, by differential compaction, or above a high-level igneous intrusion such as a laccolith. When distributed on a regional scale, a set of folds constitutes a fold belt, a typical feature of orogenic zones. Synsedimentary folds are those that form during the deposition of sediment. The key geometric elements of a fold include the hinge, which is the line of maximum curvature on a folded surface, and the limbs, which are the flanks that converge at the hinge zone. The axial surface connects all hinge lines of stacked folded surfaces; if planar, it is called an axial plane. A fold axis is a straight line that, when moved parallel to itself, generates the form of the fold, and folds that can be so generated are termed cylindrical. Folds are described by their shape (chevron, cuspate, circular, or elliptical), tightness (based on the interlimb angle, ranging from gentle to isoclinal), and symmetry (symmetrical or asymmetrical). Concentric folds maintain uniform layer thickness, while similar folds show thinning of limbs and thickening of the hinge zone. Common linear fold types include anticlines, synclines, antiforms, synforms, monoclines, and recumbent folds; nonlinear types include domes and basins.
- field
- Structural geology
- known_for
- Bending of originally planar surfaces (sedimentary strata) during permanent deformation
- types
- Anticline, syncline, antiform, synform, monocline, recumbent, dome, basin, chevron, slump, ptygmatic, parasitic, disharmonic
- scale
- Microscopic crinkles to mountain-sized folds
- classification_basis
- Interlimb angle, symmetry, layer thickness, shape
Lore & Background
Folds are described using specific terminology. The fold hinge is the line joining points of maximum curvature on a folded surface, which may be straight or curved. A fold surface seen perpendicular to its shortening direction can be divided into hinge and limb portions; the limbs converge at the hinge zone. The axial surface connects all hinge lines of stacked folded surfaces; if planar, it is called an axial plane. A fold axis is the closest approximation to a straight line that, when moved parallel to itself, generates the form of the fold; folds generated this way are called cylindrical folds.
Reader's Guide
Folds in structural geology are bent or curved stacks of originally planar surfaces, such as sedimentary strata, formed during permanent deformation. They range from microscopic crinkles to mountain-sized structures and can occur as isolated folds or in periodic fold trains. Synsedimentary folds form during deposition. Folds develop under varied stress, pore pressure, and temperature conditions, appearing in soft sediments, metamorphic rocks, and even as primary flow structures in some igneous rocks. Regionally, sets of folds constitute a fold belt, common in orogenic zones. Folds are commonly created by shortening of existing layers, but also form through displacement on non-planar faults (fault bend folds), at propagating fault tips (fault propagation folds), by differential compaction, or above high-level igneous intrusions like laccoliths. Descriptive features include fold size, shape (chevron, cuspate, circular, elliptical), tightness measured by interlimb angle (gentle, open, close, tight, isoclinal), and symmetry. Deformation style classes are concentric folds, which maintain uniform layer thickness, and similar folds, which show limb thinning and hinge thickening. Linear fold types include anticlines, synclines, antiforms, synforms, monoclines, and recumbent folds; nonlinear types include domes and basins. Fold terminology involves the hinge, limbs, axial surface, and fold axis. Minor folds in outcrop often reveal the shape, closure direction, and axial plane attitude of related major folds.
Did You Know?
- Folds in rocks vary in size from microscopic crinkles to mountain-sized folds.
- The interlimb angle defines fold tightness, with isoclinal folds having an angle between 10° and zero.
- Concentric folds maintain uniform layer thickness, while similar folds display thinning of limbs and thickening of the hinge zone.
- Synsedimentary folds are those formed during sedimentary deposition.
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