Fold (geology)

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. They occur as single isolated folds or in periodic sets (known as ''fold trains''). Synsedimentary folds are those formed during sedimentary deposition.

Folds form under varied conditions of stress, pore pressure, and temperature gradient, as evidenced by their presence in soft sediments, the full spectrum of metamorphic rocks, and even as primary flow structures in some igneous rocks. A set of folds distributed on a regional scale constitutes a fold belt, a common feature of orogenic zones. Folds are commonly formed by shortening of existing layers, but may also be formed as a result of displacement on a non-planar fault (''fault bend fold''), at the tip of a propagating fault (''fault propagation fold''), by differential compaction or due to the effects of a high-level igneous intrusion e.g. above a laccolith.

Fold terminology

The ''fold hinge'' is the line joining points of maximum curvature on a folded surface. This line may be either straight or curved. The term ''hinge line'' has also been used for this feature.
A fold surface seen perpendicular to its shortening direction can be divided into ''hinge'' and ''limb'' portions, the limbs are the flanks of the fold and the hinge zone is where the limbs converge. Within the hinge zone lies the hinge point, which is the point of minimum radius of curvature (maximum curvature) of the fold. The crest of the fold represents the highest point of the fold surface whereas the trough is the lowest point. The inflection point of a fold is the point on a limb at which the concavity reverses; on regular folds, this is the midpoint of the limb.
The ''axial surface'' is defined as a plane connecting all the hinge lines of stacked folded surfaces. If the axial surface is planar then it is called an ''axial plane'' and can be described in terms of strike and dip.

Folds can have a ''fold axis''. A fold axis, “is the closest approximation to a straight line that when moved parallel to itself, generates the form of the fold.” Ramsay 1967). A fold that can be generated by a fold axis is called a ''cylindrical fold''. This term has been broadened to include near-cylindrical folds. Often, the fold axis is the same as the hinge line.

Descriptive features

Fold size

Minor folds are quite frequently seen in outcrop; major folds seldom are except in the more arid countries. Minor folds can, however, often provide the key to the major folds they are related to. They reflect the same shape and style, the direction in which the closures of the major folds lie, and their cleavage indicates the attitude of the axial planes of the major folds and their direction of overturning

Fold shape

A fold can be shaped like a chevron, with planar limbs meeting at an angular axis, as ''cuspate'' with curved limbs, as circular with a curved axis, or as elliptical with unequal wavelength.

Fold tightness

Fold tightness is defined by the size of the angle between the fold's limbs (as measured tangential to the folded surface at the inflection line of each limb), called the interlimb angle. Gentle folds have an interlimb angle of between 180° and 120°, open folds range from 120° to 70°, close folds from 70° to 30°, and tight folds from 30° to 0°. ''Isoclines'', or ''isoclinal folds'', have an interlimb angle of between 10° and zero, with essentially parallel limbs.

Fold symmetry

Not all folds are equal on both sides of the axis of the fold. Those with limbs of relatively equal length are termed symmetrical, and those with highly unequal limbs are asymmetrical. Asymmetrical folds generally have an axis at an angle to the original unfolded surface they formed on.

Facing and vergence

Vergence is calculated in a direction perpendicular to the fold axis.

Deformation style classes

Folds that maintain uniform layer thickness are classed as ''concentric'' folds. Those that do not are called ''similar folds''. Similar folds tend to display thinning of the limbs and thickening of the hinge zone. Concentric folds are caused by warping from active buckling of the layers, whereas similar folds usually form by some form of shear flow where the layers are not mechanically active. Ramsay has proposed a classification scheme for folds that often is used to describe folds in profile based upon the curvature of the inner and outer lines of a fold and the behavior of '' dip isogons''. that is, lines connecting points of equal dip on adjacent folded surfaces:

See, for example,

Types of fold

Linear

* Anticline: linear, strata normally dip away from the axial center, ''oldest'' strata in center irrespective of orientation.
* Syncline: linear, strata normally dip toward the axial center, ''youngest'' strata in center irrespective of orientation.
* Antiform: linear, strata dip away from the axial center, age unknown, or inverted.
* Synform: linear, strata dip toward the axial center, age unknown, or inverted.
* Monocline: linear, strata dip in one direction between horizontal layers on each side.
* Recumbent: linear, fold axial plane oriented at a low angle resulting in overturned strata in one limb of the fold.

Other

* Dome: nonlinear, strata dip away from center in all directions, ''oldest'' strata in center.
* Basin: nonlinear, strata dip toward center in all directions, ''youngest'' strata in center.
* Chevron: angular fold with straight limbs and small hinges
*
* Slump: typically monoclinal, the result of differential compaction or dissolution during sedimentation and lithification.
* Ptygmatic: Folds are chaotic, random and disconnected. Typical of sedimentary slump folding, migmatites and decollement detachment zones.
* Parasitic: short-wavelength folds formed within a larger wavelength fold structure - normally associated with differences in bed thickness
* Disharmonic: Folds in adjacent layers with different wavelengths and shapes
(A homocline involves strata dipping in the same direction, though not necessarily any folding.)

Causes of folding

Folds appear on all scales, in all rock types, at all levels in the crust. They arise from a variety of causes.

Layer-parallel shortening

When a sequence of layered rocks is shortened parallel to its layering, this deformation may be accommodated in a number of ways, homogeneous shortening, reverse faulting or folding. The response depends on the thickness of the mechanical layering and the contrast in properties between the layers. If the layering does begin to fold, the fold style is also dependent on these properties. Isolated thick competent layers in a less competent matrix control the folding and typically generate classic rounded buckle folds accommodated by deformation in the matrix. In the case of regular alternations of layers of contrasting properties, such as sandstone-shale sequences, kink-bands, box-folds and chevron folds are normally produced.

Fault-related folding

Many folds are directly related to faults, associated with their propagation, displacement and the accommodation of strains between neighboring faults.

Fault bend folding

Fault-bend folds are caused by displacement along a non-planar fault. In non-vertical faults, the hanging-wall deforms to accommodate the mismatch across the fault as displacement progresses. Fault bend folds occur in both extensional and thrust faulting. In extension, listric faults form rollover anticlines in their hanging walls. In thrusting, ''ramp anticlines'' form whenever a thrust fault cuts up section from one detachment level to another. Displacement over this higher-angle ramp generates the folding.

Fault propagation folding

Fault propagation folds or ''tip-line folds'' are caused when displacement occurs on an existing fault without further propagation. In both reverse and normal faults this leads to folding of the overlying sequence, often in the form of a monocline.

Detachment folding

When a thrust fault continues to displace above a planar detachment without further fault propagation, detachment folds may form, typically of box-fold style. These generally occur above a good detachment such as in the Jura Mountains, where the detachment occurs on middle Triassic evaporites.

Folding in shear zones

Shear zones that approximate to simple shear typically contain minor asymmetric folds, with the direction of overturning consistent with the overall shear sense. Some of these folds have highly curved hinge-lines and are referred to as ''