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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 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, 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. Image File history File linksMetadata Download high-resolution version (480x640, 109 KB) Folded rock formation near Moruya, New South Wales, Australia Photo: D.M. Vernon Date: 2006 I, the creator of this work, hereby release it into the public domain. ...
Image File history File linksMetadata Download high-resolution version (480x640, 109 KB) Folded rock formation near Moruya, New South Wales, Australia Photo: D.M. Vernon Date: 2006 I, the creator of this work, hereby release it into the public domain. ...
Dolphin Beach, Moruya South Head Moruya is a small regional centre in New South Wales, Australia, situated on the Moruya River, approximately 300 kilometers south of Sydney. ...
Capital Sydney Government Constitutional monarchy Governor Professor Marie Bashir Premier Morris Iemma (ALP) Federal representation - House seats 50 - Senate seats 12 Gross State Product (2004-05) - Product ($m) $305,437 (1st) - Product per capita $45,153/person (4th) Population (End of March 2006) - Population 6,817,100 (1st) - Density 8. ...
This article includes a list of works cited but its sources remain unclear because it lacks in-text citations. ...
Two types of sedimentary rock: limey shale overlaid by limestone. ...
For other uses, see strata (novel) and strata title. ...
In engineering mechanics, deformation is a change in shape due to an applied force. ...
Stress is the internal distribution of force per unit area that balances and reacts to external loads applied to a body. ...
Hydrostatic pressure is the pressure exerted by a fluid due to its weight. ...
Fig. ...
Two types of sedimentary rock: limey shale overlaid by limestone. ...
Metamorphic rock is the result of the transformation of a pre-existing rock type, the protolith, in a process called metamorphism, which means change in form, derived from the Greek words meta, change, and morphe, form. The protolith is subjected to extreme heat (>150 degrees Celsius) and pressure causing profound...
Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. ...
// Orogeny (Greek for mountain generating) is the process of mountain building, and may be studied as a tectonic structural event, as a geographical event and a chronological event, in that orogenic events cause distinctive structural phenomena and related tectonic activity, affect certain regions of rocks and crust and happen within...
Describing folds
Folds are classified by their size, fold shape, tightness, dip of the axial plane. Image File history File linksMetadata Download high resolution version (1300x1005, 389 KB) Slichowice nature reserve in Kielce in a former quarry, showing folding of Variscan orogeny period. ...
Image File history File linksMetadata Download high resolution version (1300x1005, 389 KB) Slichowice nature reserve in Kielce in a former quarry, showing folding of Variscan orogeny period. ...
Map of the centre of Kielce Monastery Exbud headquarters-symbol of todays Kielce City The monument to commemorate of tragedy in New York 11 September 2001 Bishops Palace Building of Stefan Żeromski Theatre The new stadium in Kielce Bus Station in Kielce of characterisic shape of alien saucer Kielce...
// Orogeny (Greek for mountain generating) is the process of mountain building, and may be studied as a tectonic structural event, as a geographical event and a chronological event, in that orogenic events cause distinctive structural phenomena and related tectonic activity, affect certain regions of rocks and crust and happen within...
2D Fold Terms Looking at a fold surface in profile the fold can be divided in to 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. A hinge is a mechanical device that connects two solid objects, allowing rotation between them. ...
A limb (from the Old English lim) is a jointed appendage of the human or animal body; a large or main branch of a tree; a representative, branch or member of a group or organization. ...
CREST (although written like a acronym it does not stand for anything) is the Central Securities Depository for the U.K., Republic of Ireland, Isle of Man and Jersey equities and UK gilts. ...
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Plot of y = x3 with inflection point of (0,0). ...
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. Diagram showing the three major planes of the body. ...
Strike and dip refer to the orientation or attitude of a geologic feature. ...
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. ...
A circular may be: the adjective form of circle an advertisement which is circulated a Pastoral letter, Encyclic, or Papal bull that is circulated between churches a circular argument is a term for a type of logical fallacy where the very thing that is trying to be proved is assumed...
The wavelength is the distance between repeating units of a wave pattern. ...
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. Sphere symmetry group o. ...
Symmetry is a characteristic of geometrical shapes, equations and other objects; we say that such an object is symmetric with respect to a given operation if this operation, when applied to the object, does not appear to change it. ...
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 Concentric objects share the same center, axis or origin with one inside the other. ...
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.
- Recumbent: linear, fold axis 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.
 Monocline at Colorado National Monument Image File history File links Anticline. ...
Image File history File links Anticline. ...
Anticline with syncline visible at far right- USGS In structural geology, an anticline is a Fold (geology) that is convex to the youngest beds—youngest sediments are on back of hand, older under the palm. ...
Road Cut near Ft. ...
In geology, a dome is a deformational feature consisting of symmetrically-dipping anticlines; their general outline on a geologic map is circular or oval. ...
A basin is the inverse of a dome: a symmetrically-dipping syncline that appears on a geologic map as roughly circular or elliptical, with concentric layers. ...
Ptygmatic folding in migmatite Migmatite on the coast of Saaremaa. ...
Image File history File linksMetadata No higher resolution available. ...
Image File history File linksMetadata No higher resolution available. ...
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. Image File history File linksMetadata No higher resolution available. ...
Image File history File linksMetadata No higher resolution available. ...
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. Ptygmatic folding in migmatite Migmatite on the coast of Saaremaa. ...
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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