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In structural geology, a fold is a stack of originally planar surfaces, such as sedimentary strata, that are bent or curved during permanent
deformation Deformation can refer to: * Deformation (engineering), changes in an object's shape or form due to the application of a force or forces. ** Deformation (physics), such changes considered and analyzed as displacements of continuum bodies. * Defo ...
. 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 Pore water pressure (sometimes abbreviated to pwp) refers to the pressure of groundwater held within a soil or rock, in gaps between particles ( pores). Pore water pressures below the phreatic level of the groundwater are measured with piezomete ...
, and
temperature gradient A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degree ...
, as evidenced by their presence in soft
sediments Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
, the full spectrum of
metamorphic rocks Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...
, 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 A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when magma (molten rock) rising through the Earth's crust begins to spread out horizontally, prying apar ...
.


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 In mathematics, curvature is any of several strongly related concepts in geometry. Intuitively, the curvature is the amount by which a curve deviates from being a straight line, or a surface deviates from being a plane. For curves, the canon ...
(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 In differential calculus and differential geometry, an inflection point, point of inflection, flex, or inflection (British English: inflexion) is a point on a smooth plane curve at which the curvature changes sign. In particular, in the case o ...
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 Strike and dip is a measurement convention used to describe the orientation, or attitude, of a planar geologic feature. A feature's strike is the azimuth of an imagined horizontal line across the plane, and its dip is the angle of inclination m ...
. 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 Chevron (often relating to V-shaped patterns) may refer to: Science and technology * Chevron (aerospace), sawtooth patterns on some jet engines * Chevron (anatomy), a bone * '' Eulithis testata'', a moth * Chevron (geology), a fold in rock la ...
, with planar limbs meeting at an angular axis, as ''cuspate'' with curved limbs, as
circular Circular may refer to: * The shape of a circle * ''Circular'' (album), a 2006 album by Spanish singer Vega * Circular letter (disambiguation) ** Flyer (pamphlet), a form of advertisement * Circular reasoning, a type of logical fallacy * 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 Asymmetry is the absence of, or a violation of, symmetry (the property of an object being invariant to a transformation, such as reflection). Symmetry is an important property of both physical and abstract systems and it may be displayed in pre ...
. 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 In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...
: 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 In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the l ...
: linear, strata dip away from the axial center, age unknown, or inverted. *
Synform In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimposed ...
: 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 Chevron (often relating to V-shaped patterns) may refer to: Science and technology * Chevron (aerospace), sawtooth patterns on some jet engines * Chevron (anatomy), a bone * '' Eulithis testata'', a moth * Chevron (geology), a fold in rock la ...
: 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 In structural geology, a homocline or homoclinal structure (from old el, homo = same, cline = inclination), is a geological structure in which the layers of a sequence of rock strata, either sedimentary or igneous, dip uni ...
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 Competence may refer to: *Competence (geology), the resistance of a rock against deformation or plastic flow. *Competence (human resources), a standardized requirement for an individual to properly perform a specific job *Competence (law), the me ...
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 fold A detachment fold, in geology, occurs as layer parallel thrusting along a decollement (or detachment) develops without upward propagation of a fault; the accommodation of the strain produced by continued displacement along the underlying thrust r ...
s 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 The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
evaporite An evaporite () is a water-soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean ...
s.


Folding in shear zones

Shear zones that approximate to
simple shear Simple shear is a deformation in which parallel planes in a material remain parallel and maintain a constant distance, while translating relative to each other. In fluid mechanics In fluid mechanics, simple shear is a special case of deformati ...
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 ''
sheath fold Sheath pronounced as , may refer to: * Scabbard, a sheath for holding a sword, knife, or other large blade, as well as guns, such as rifles. * The outer covering of a cable * Condom, a kind of contraception * Debye sheath, a layer of a plasma in ...
s''. Folds in shear zones can be inherited, formed due to the orientation of pre-shearing layering or formed due to instability within the shear flow.


Folding in sediments

Recently deposited sediments are normally mechanically weak and prone to remobilization before they become lithified, leading to folding. To distinguish them from folds of tectonic origin, such structures are called synsedimentary (formed during sedimentation). Slump folding: When slumps form in poorly consolidated sediments, they commonly undergo folding, particularly at their leading edges, during their emplacement. The asymmetry of the slump folds can be used to determine paleoslope directions in sequences of sedimentary rocks. Dewatering: Rapid dewatering of sandy sediments, possibly triggered by seismic activity, can cause convolute bedding. Compaction: Folds can be generated in a younger sequence by differential compaction over older structures such as fault blocks and reefs.


Igneous intrusion

The emplacement of igneous intrusions tends to deform the surrounding
country rock Country rock is a genre of music which fuses rock and country. It was developed by rock musicians who began to record country-flavored records in the late 1960s and early 1970s. These musicians recorded rock records using country themes, vocal ...
. In the case of high-level intrusions, near the Earth's surface, this deformation is concentrated above the intrusion and often takes the form of folding, as with the upper surface of a
laccolith A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when magma (molten rock) rising through the Earth's crust begins to spread out horizontally, prying apar ...
.


Flow folding

The compliance of rock layers is referred to as ''competence'': a competent layer or bed of rock can withstand an applied load without collapsing and is relatively strong, while an incompetent layer is relatively weak. When rock behaves as a fluid, as in the case of very weak rock such as rock salt, or any rock that is buried deeply enough, it typically shows ''flow folding'' (also called ''passive folding'', because little resistance is offered): the strata appear shifted undistorted, assuming any shape impressed upon them by surrounding more rigid rocks. The strata simply serve as markers of the folding.; Such folding is also a feature of many igneous intrusions and glacier ice.


Folding mechanisms

Folding of rocks must balance the deformation of layers with the conservation of volume in a rock mass. This occurs by several mechanisms.


Flexural slip

Flexural slip allows folding by creating layer-parallel slip between the layers of the folded strata, which, altogether, result in deformation. A good analogy is bending a phone book, where volume preservation is accommodated by slip between the pages of the book. The fold formed by the compression of competent rock beds is called "flexure fold".


Buckling

Typically, folding is thought to occur by simple buckling of a planar surface and its confining volume. The volume change is accommodated by ''layer parallel shortening'' the volume, which grows in ''thickness''. Folding under this mechanism is typical of a similar fold style, as thinned limbs are shortened horizontally and thickened hinges do so vertically.


Mass displacement

If the folding deformation cannot be accommodated by a flexural slip or volume-change shortening (buckling), the rocks are generally removed from the path of the stress. This is achieved by pressure dissolution, a form of metamorphic process, in which rocks shorten by dissolving constituents in areas of high strain and redepositing them in areas of lower strain. Folds created in this way include examples in migmatites and areas with a strong axial planar cleavage.


Mechanics of folding

Folds in the rock are formed about the
stress field A stress field is the distribution of internal forces in a body that balance a given set of external forces. Stress fields are widely used in fluid dynamics and materials science. Consider that one can picture the stress fields as the stress cr ...
in which the rocks are located and the
rheology Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an appli ...
, or method of response to stress, of the rock at the time at which the stress is applied. The rheology of the layers being folded determines characteristic features of the folds that are measured in the field. Rocks that deform more easily form many short-wavelength, high-amplitude folds. Rocks that do not deform as easily form long-wavelength, low-amplitude folds.


Economic implications


Mining industry

Layers of rock that fold into a hinge need to accommodate large deformations in the hinge zone. This results in voids between the layers. These voids, and especially the fact that the water pressure is lower in the voids than outside of them, act as triggers for the deposition of minerals. Over millions of years, this process is capable of gathering large quantities of trace minerals from large expanses of rock and depositing them at very concentrated sites. This may be a mechanism that is responsible for the veins. To summarize, when searching for veins of valuable minerals, it might be wise to look for highly folded rock, and this is the reason why the mining industry is very interested in the theory of geological folding.


Oil industry

Anticlinal traps are formed by folding of rock. For example, if a porous sandstone unit covered with low permeability shale is folded into an anticline, it may contain hydrocarbons trapped in the crest of the fold. Most anticlinal traps are created as a result of sideways pressure, folding the layers of rock, but can also occur from sediments being compacted.


See also

*
3D fold evolution In geology, 3D fold evolution is the study of the full three dimensional structure of a Fold (geology), fold as it changes in time. A fold is a common three-dimensional geological structure that is associated with Deformation (mechanics), strain ...
* Orogeny *
Mountain building Mountain formation refers to the geological processes that underlie the formation of mountains. These processes are associated with large-scale movements of the Earth's crust (tectonic plates). Folding, faulting, volcanic activity, igneous intr ...
*
Rock mechanics Rock mechanics is a theoretical and applied science of the mechanical behavior of rock and rock masses; compared to geology, it is that branch of mechanics concerned with the response of rock and rock masses to the force fields of their physical env ...
*
Thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...


Notes


Further reading

* * * Ramsay, J.G., 1967, ''Folding and fracturing of rocks'': McGraw-Hill Book Company, New York, 560pp., ISBN 193066589X


External links


Mark Peletier

Oil and gas traps
{{Authority control Structural geology Geological processes Deformation (mechanics)