User:Ekgeo/Fold (geology)

Folds

Folds occur when one or a stack of planar surfaces such as: bedding, clivage or layering are bent as a result of permanent plastic and continuous deformation. Folds range in size from microscopic crinkles to mountain-sized folds, they occur singly as isolated folds and in extensive fold trains commonly as part of a fold belt in orogenic process.

Folds form under varied conditions of stress, hydrostatic pressure, pore pressure, and temperature gradient, as evidenced by their presence in unlitified sediments, the full spectrum of metamorphic rocks, and even as primary flow structures in some igneous rocks. Folds are commonly formed by shortening of existing layers parallel to the layering of rocks, but may also be formed by differential compaction, soil liquefaction triggered by seismic activity, due to the effects of a high-level igneous intrusion or diapiric intrusions, impact of meteorites, and as a result of displacement on a non-planar fault (fault bend fold), at the tip of a propagating fault (fault propagation fold).

Fold terminology
A fold surface seen parallel to its shortening can be divided mainly into hinge and limb portions, the limbs are the flanks of the fold and the hinge zone is where the flanks join together (Fig. 1). In this last portion lies the hinge point which is the point of minimum radius of curvature (maximum curvature) for a fold. The description parallel to shortening direction is completed by the crest of the fold which represents the highest point of the fold surface whereas the trough is the lowest point. Finally, 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.

However, folds are 3D structures that encompass more components, such as the hinge line which is the extrapolation of the hinge points along the crest and the axial trace difined as a plane connecting all the hinge lines of stacked foldind surfaces. These pararemeter are quite useful in reconstruction of fold geometry and paleostress determination. If the axial surface is a planar surface then it is called as axial plane and can be described in terms of strike and dip.

Finally, folds can have, but don't necessarily 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.” (Davis and Reynolds, 1996 after Donath and Parker, 1964; 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.

Describing folds
Due to the number of variations into fold morphology, folds are classified on the basis of several geometric component:

Fold shape
A fold can be shaped as 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.

(Figure in construction)

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.

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 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:

Fold types



 * Anticline: linear, strata normally dip away from axial center, oldest strata in center irrespective of orientation.
 * Syncline: linear, strata normally dip toward axial center, youngest strata in center irrespective of orientation.
 * Antiform: linear, strata dip away from axial center, age unknown, or inverted.
 * Synform: linear, strata dip toward axial center, age unknown, or inverted.
 * 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.
 * Monocline: linear, strata dip in one direction between horizontal layers on each side.
 * Chevron: angular fold with straight limbs and small hinges
 * Recumbent: linear, fold axial plane oriented at low angle resulting in overturned strata in one limb of the fold.
 * Slump: typically monoclinal, result of differential compaction or dissolution during sedimentation and lithification.
 * Collapse:
 * 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.)

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.


 * Bending
 * Active folding or buckling
 * Orthogonal flexure
 * Flexural slip
 * Flexural flow
 * Passive folding

Active folding or 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 typically of the similar fold style, as thinned limbs are shortened horizontally and thickened hinges do so vertically.

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.

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".

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.

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 neighbouring 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 sheath folds. 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 remobilisation 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. 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.

General references

 * Donath, F.A., and Parker, R.B., 1964, Folds and Folding: Geological Society of America Bulletin, v. 75, p. 45-62
 * Ramsay, J.G., 1967, Folding and fracturing of rocks: McGraw-Hill Book Company, New York, 560p.
 * Donath, F.A., and Parker, R.B., 1964, Folds and Folding: Geological Society of America Bulletin, v. 75, p. 45-62
 * Ramsay, J.G., 1967, Folding and fracturing of rocks: McGraw-Hill Book Company, New York, 560p.
 * Ramsay, J.G., 1967, Folding and fracturing of rocks: McGraw-Hill Book Company, New York, 560p.