Geological Term Glossary

Miscellaneous Geological Terms

Taken from: The Glossary of Geology (1980)

Basin = A low area in the Earth's crust, of tectonic origin, in which sediments have accumulated, e.g. a circular centrocline such as the Michigan Basin, a fault bordered intermontane features such as the Bighorn Basin of Wyoming, or a linear crustal downwarp such as the Appalachian Basin. Such features were basins at the time of sedimentation but are not necessarily so today. Syn: structural basin.

Craton = A part of the Earth's crust that has attained stability, and has been little deformed for a prolonged period. As originally defined, cratons included parts of both continents and ocean basins, but modern knowledge of the ocean basins indicates that the existence of cratons there is unlikely, so the term is now restricted to continental areas. The extensive central cratons of the continents, including both shields and platforms, have been called hedreocratons. Parts of the more mature Phanerozoic fold belts have now achieved, or are approaching, a cratonic condition. Also spelled: Kraton. See also: thalassocraton. Syn: kratogen.

Cratonic basin = A basin on top of a craton. Syn: intracratonic basin

Foredeep = An elongate depression bordering an island arc or other orogenic belt. Cf: trench.

Island arc = A chain of islands, e.g. the Aleutians, rising from the deep-sea floor and near to the continents; a primary arc expressed as a curved belt of islands. Its curve is generally toward the open ocean. According to Bucher (1965), an island-arc pattern results from shrinkage. This is a typical tension pattern that would result in the brittle crust by rotating a polar circumference into an equatorial circumference such as by sliding the Earth's crust on the interior. Syn: volcanic arc.

Orogen = A linear or arcuate region that has been subjected to folding and other deformation during an orogenic cycle. Orogenic belts are mobile belts during their formative stages, and most of them later became mountain belts by post-orogenic processes. Syn: fold belt; orogenic; belt orogene.

Platform = That part of a continent that is covered by flat-lying or gently tilted strata, mainly sedimentary, which are underlain at varying depths by a basement of rocks that were consolidated during earlier deformations. A platform is a part of the craton.

Rift = A long, narrow continental trough that is bounded by normal faults; a graben of regional extent. It marks a zone along which the entire thickness of the lithosphere has ruptured under extension. Cf: paar.

Continental margin (= Rifted margin ?) = The ocean floor that is between the shoreline and the abyssal ocean floor, including various provinces: the continental shelf, continental borderland, continental slope, and the continental rise.
Shield = A large area of exposed basement rocks in a craton, commonly with a very gently convex surface, surrounded by sediment-covered platforms.; e.g. Canadian Shield, Baltic Shield. The rocks of virtually all shield areas are Precambrian. Syn: continental shield; cratogene; continental nucleus.

Three terms in the scheme for crustal type classification are not found in The Glossary of Geology. Those are defined as follows:

Continental arc = A volcanic within the continental crust above a subducting slab.

Extended crust = A province of thinned crust (>50 %) due to extension. e.g. The Basin & Range, North America.

Forearc = All crust between the volcanic arc and the trench in a subduction zone, including the accretionary wedge.