Anticline

In structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with "antiform", which is a purely descriptive term for any fold that is convex up. Therefore if age relationships (i.e. younging direction) between various strata are unknown, the term antiform must be used.

On a geologic map, anticlines are usually recognized by a sequence of rock layers that are progressively older toward the center of the fold because the uplifted core of the fold is preferentially eroded to a deeper stratigraphic level relative to the topographically lower flanks. The strata dip away from the center, or "crest", of the fold.

If an anticline plunges (i.e., is inclined to the earth's surface), the surface strata will form "V"s that point in the direction of plunge. Anticlines are typically flanked by synclines although faulting can complicate and obscure the relationship between the two. Folds typically form during crustal deformation as the result of compression that accompanies orogenic mountain building.

Anticline terminology

Any fold whose form is convex upward is an antiform. Antiforms containing progressively younger rocks from their core outwards are anticlines.

An anticline or antiform has a "crest", which is the highest point on a given stratum along the top of the fold. A "hinge" in an anticline is the locus of maximum curvature or bending in a given stratum in the fold. An "axis" is an imaginary line connecting the hinges in the different strata in a two-dimensional cross-section through the anticline. Connecting the hinges or points of maximum curvature in the different layers in three dimensions produces an "axial plane" or "axial surface." In a symmetrical anticline, a surface trace of the axial plane coincides with the crest. With an asymmetrical anticline, the surface trace of the axial plane or axis will be offset from the crest toward the steeper flank of the fold. An "overturned anticline" is an asymmetrical anticline with a flank or limb that has been tilted beyond perpendicular so that the beds in that limb are upside-down.

A structure that plunges in all directions to form a circular or elongate structure is a dome. Domes are generally formed from one main deformation event, or via diapirism from underlying magmatic intrusions or movement of upwardly mobile, mechanically ductile, material such as rock salt (salt dome) and shale (shale diapir).

An anticline which plunges at both ends is termed a "doubly-plunging anticline", and may be formed from multiple deformations, or superposition of two sets of folds, or be related to the geometry of the underlying detachment fault and the varying amount of displacement along the surface of that detachment fault. The highest point on a doubly-plunging anticline (or any geologic structure for that matter) is called the "culmination."

An elongate dome which developed as the sediments were being deposited is referred to as a "pericline".

An "anticlinorium" is a series of parallel anticlinal folds on a regional-scale anticline. Example include the Late Jurassic to Early Cretaceous Purcell Anticlinorium in British Columbia [http://www.litho.ucalgary.ca/atlas/sbc/purcells.html] and the Blue Ridge anticlinorium of northern Virginia in the Appalachians [http://www.gsajournals.org/perlserv/?request=get-abstract&doi=10.1130%2F0016-7606(1979)90%3C525:SBTTBR%3E2.0.CO%3B2] .

Economic significance

Doubly-plunging or faulted anticlines, culminations, and structural domes are favored locations for oil and natural gas drilling; the low density of petroleum causes it to buoyantly migrate upward to the highest parts of the fold, until stopped by a low-permeability barrier such as an impermeable stratum or fault zone. Examples of low-permeability seals that contain the hydrocarbons, oil and gas, in the ground include shale, limestone, sandstone, and even salt domes. The actual type of stratum does not matter as long as it has low-permeability.

Periclines are important focal points for pooling of hot, metal-laden formational brines, which can form manto ore deposits, Irish-type lead-zinc deposits and uranium deposits, amongst others.

Culminations in folded strata which are cut by shears and faults are favoured loci for deposition of saddle-reef style lode gold deposits.

See also

* Homocline
* Monocline
* Syncline
* Weald-Artois Anticline - linked Britain to continental Europe until 225,000 years ago
* Makhtesh

References

*Bates, Robert L., and Julia A. Jackson, editors. "Dictionary of Geological Terms": Prepared under the direction of the American Geological Institute, Anchor Press/Doubleday, Garden City, New York.
*Davis, George H., Reynolds, Stephen J., 1996. "Structural Geology of Rocks and Regions": John Wiley & Sons, Inc., New York. ISBN 0-471-52621-5
*Monroe, James S., and Reed Wicander. "The Changing Earth: Exploring Geology and Evolution." 2nd ed. Belmont: Wadsworth Publishing Company, 1997. ISBN 0-314-09577-2