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Fold (geology)
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- See also: folding
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.
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Encyclopedia
- See also: folding
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.
Describing folds
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 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 deformation of the layers, whereas similar folds usually form by some form of dislocation between the layers (sliding), with extension and contraction of the thickness of rock layers differently in the limb and hinge zones
Fold types
*Anticline: linear, strata dip away from axial center, oldest strata in center.
- Syncline: linear, strata dip toward axial center, youngest strata in center.
- 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 below the fold axis.
- 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.
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. The best analog 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 reaction, in which rocks shorten by dissolving constituents which move to 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
Folds may occur in rocks when they are at temperatures and pressures in which they deform ductily. Because of this, folding occurs at such depths in the crust that the minerals in the rock are allowed to deform ductily; if these rocks were brought to shallower depths, they would fault instead of fold.
The rheology also 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.
Response to Applied Stress
Folds are a deformational response (strain) to a compressive stress that is applied to a section of rock. These compressive stresses push on the rock. Because the rock is not able to deform like an ideal fluid in response to the stress by shortening and becoming thicker, it instead buckles, and forms folds. (These folds still, in essence, shorten the rock unit while making it thicker.) The axial plane of the fold forms perpendicular to the greatest compressive stress. This can easily be replicated by looking at how a stack of paper responds to compressional stress applied by one's hands as one pushes on the edges of the stack.
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.
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