Fold

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See also: folding

Very tight folds. Formation near Moruya, New South Wales, Australia

The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of plastic (i.e. permanent) deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales. Folds form under varied conditions of stress, hydrostatic pressure, pore pressure, and temperature - hydrothermal 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.

Describing folds

A fold in Slichowice nature reserve in Kielce (Variscan orogeny)

Rainbow Basin Syncline near Barstow, California

Folds are classified by their size, fold shape, tightness, dip of the axial plane.

2D fold terms

Looking at a fold surface in profile the fold can be divided into a hinge portion and the limbs. The limbs are the flanks of the fold and the hinge is where the flanks join together. The hinge point is the point of minimum radius of curvature for a fold. The crest of the fold is the highest point of the fold surface, and 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 mid-point of the limb.

3D fold terms

The hinge points along an entire folded surface form a hinge line. The trend and plunge of a linear hinge line gives you information about the orientation of the fold. To completely describe the orientation of a fold, one must use the axial surface. The axial surface is the surface defined by connecting all the hinge lines of stacked folding surfaces. If the axial surface is a planar surface then it is called the axial plane and can be described by the strike and dip of the plane. The axial trace is the line of intersection of the axial surface with any other surface (ground, side of mountain, geological cross-section). 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.

Fold shape

It is necessary to convey a sense of the shape of the fold. 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 angle between the fold's limbs, called the interlimb angle. Gentle folds have an interlimb angle of between 170° and 180° , open folds range from 170° to 90°, tight folds from 90° to 10°, and 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 will generally have an axis which is at an angle to the original, unfolded surface which they formed upon.

Deformation style classes

Folds which maintain uniform layer thickness are classed as concentric folds; those which 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 which results from active buckling of the layers, whereas similar folds usually form by some form of shear flow where the layers are not mechanically active.

Fold types

Anticline - USGS

Monocline at Colorado National Monument

Recumbent fold, King Oscar Fjord

• Anticline: linear, strata normally dip away from axial center, oldest strata in center.
• Syncline: linear, strata normally dip toward axial center, youngest strata in center.
• Antiform: linear, strata dip away from axial center, age unknown, or inverted.
• Synform: linear, strata dip toward axial centre, age unkown, 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.
• Ptygmatic: Folds are chaotic, random and disconnected. Typical of sedimentary slump folding, migmatites and decollement detachment zones.

Causes of folding

Folds appear on all scales, in all rock types, at all levels in the crust and 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 will 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.^[1]

Rollover anticline

Ramp anticline

Fault-propagation fold

Fault-related folding

Many folds are directly related to faults, associate 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.^[2] In thrusting, ramp anticlines are formed whenever a thrust fault cuts up section from one detachment level to another. Displacement over this higher-angle ramp generates the folding.^[3]

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.^[4]

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.^[5]

Compaction

Folds can be generated in a younger sequence by differential compaction over older structures such as fault blocks and reefs.^[6]

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.^[7]

Sedimentary folding

Recently deposited sediments are normally mechanically weak and prone to remobilisation.

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.^[8]

Dewatering

Rapid dewatering of sandy sediments, possibly triggered by seismic activity can cause convolute bedding.^[9]

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.^[10]

Flow folding

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, they typically show flow folding of 'similar' style.^[11] Such folding is also a feature of many igneous intrusions and glacier ice.^[12]

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.

Example of a large-scale crenulation, Glengarry Basin, W.A., an example of chevron-type flexural-slip folds.

Flexural slip

Flexural slip allows folding by creating layer-parallel slip between the layers of the folded strata which, altogether, result in deformation. The best analogy is bending a phone book, where volume preservation is accommodated by slip between the pages of the book.

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.

Mass displacement

If the folding deformation cannot be accommodated by 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 rock are formed in relation to the stress field in which the rocks are located and the rheology, or method of response to stress, of the rock at the time at which the stress is applied.

Rheology

Weathered marble anticline at General Carrera Lake, Chile

The rheology of the layers being folded determines characteristic features of the folds that are measured in the field. Rocks which deform more easily will form many short-wavelength, high-amplitude folds. Rocks which do not deform as easily will form long-wavelength, low-amplitude folds.

Relationship between folds and applied stress field

Understanding the relationship between the stress regime in which a fold forms and what structures one would expect is important in geology. Using these relationships, geologists are able to use the observed fold geometries to understand the physical forces that made them.

References

• Davis, George H.; Reynolds, Stephen J. (1996). "Folds". Structural Geology of Rocks and Regions. New York, John Wiley & Sons. pp. 372–424. ISBN 0-471-52621-5. 
• Donath, F.A., and Parker, R.B., 1964, Folds and Folding: Geological Society of America Bulletin, v. 75, p.45-62
• McKnight, Tom L; Hess, Darrel (2000). "The Internal Processes: Folding". Physical Geography: A Landscape Appreciation. Upper Saddle River, NJ: Prentice Hall. pp. 409–14. ISBN 0-13-020263-0. 
• Ramsay, J.G., 1967, Folding and fracturing of rocks: McGraw-Hill Book Company, New York, 560p.

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