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See also: folding
Very tight folds. Formation near Moruya, New South Wales, Australia
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. Moruya For a topic outline on this subject see List of basic Australia topics. Geology (from Greek γη gê, "earth" and λόγος Logos, "speech" lit Sedimentary rock is one of the three main rock types (the others being igneous and Metamorphic rock) In Geology and related fields a stratum (plural strata) is a layer of rock or Soil with internally consistent characteristics that distinguishes e. permanent) deformation. In Materials science, deformation is a change in the shape or size of an object due to an applied force. Synsedimentary folds are those due to slumping of material before deformation. 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. Stress is a measure of the average amount of Force exerted per unit Area. Fluid statics (also called hydrostatics) is the Science of Fluids at rest and is a sub-field within Fluid mechanics. Pore water pressure refers to the pressure of Groundwater held within a Soil or rock, in gaps between particles ( Pores) Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Sedimentary rock is one of the three main rock types (the others being igneous and Metamorphic rock) Metamorphic rock is the result of the transformation of an existing rock type the protolith, in a process called Metamorphism, which means "change Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock A set of folds distributed on a regional scale constitutes a fold belt, a common feature of orogenic zones. Orogeny (Greek for "mountain generating" is the process of natural Mountain building and may be studied as a tectonic structural event as a geographical event and

Contents

Describing folds

A fold in Slichowice nature reserve in Kielce (Variscan orogeny)
A fold in Slichowice nature reserve in Kielce (Variscan orogeny)
Rainbow Basin Syncline near Barstow, California
Rainbow Basin Syncline near Barstow, California

Folds are classified by their size, fold shape, tightness, dip of the axial plane. Kielce is a city in central Poland with 202609 inhabitants (2006 Orogeny (Greek for "mountain generating" is the process of natural Mountain building and may be studied as a tectonic structural event as a geographical event and In Structural geology, a syncline is a downward-curving fold, with layers that dip toward the center of the structure Barstow is a city in San Bernardino County, California, United States.

2D fold terms

Looking at a fold surface in profile the fold can be divided into a hinge portion and the limbs. A hinge is a type of bearing that connects two solid objects typically allowing only a limited angle of Rotation between them 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. In Mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry The crest of the fold is the highest point of the fold surface, and the trough is the lowest point. A crest is the point on a Wave with the greatest positive value or upward displacement in a Cycle. 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. In Differential calculus, an inflection point, or point of inflection (or inflexion) is a point on a Curve at which the Curvature

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 transverse or axial plane (also often transaxial plane) is an imaginary plane that divides the body into superior and inferior parts Strike and dip refer to the orientation or attitude of a Geologic feature 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. This article is about chevron-shaped folds of sediment For the large deposits hypothesised to be formed by tsunamis see Chevron (land form. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency.

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. Symmetry generally conveys two primary meanings The first is an imprecise sense of harmonious or aesthetically-pleasing proportionality and balance such that it reflects beauty or Asymmetry is the absence of or a violation of a Symmetry. In organisms Due to how cells divide in Organisms asymmetry in organisms is 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. Concentric objects share the same center, axis or origin with one inside the other 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 - USGS
Anticline - USGS
Monocline at Colorado National Monument
Monocline at Colorado National Monument

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.
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 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. Pressure solution or pressure dissolution in Structural geology and Diagenesis is a Deformation mechanism that involves the dissolution Folds created in this way include examples in migmatites, and areas with a strong axial planar cleavage. Migmatite is a rock at the frontier between igneous and Metamorphic rocks They can also be known as diatexite.

References

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