page_title: Nappe
section_title: Mechanisms of emplacement
hierarchical_section_title: Nappe / Mechanisms of emplacement
caption_attribution_description: English: An illustration of the collision between two tectonic plates. One plate goes under the other, and the result is the buildup of a critical wedge of material. This orogenic wedge, or mountain-building wedge, maintains its slope by a stress balance between failures along the décollment, or main basal fault, and the faults inside the wedge.
context_page_description: In geology, a nappe or thrust sheet is a large sheetlike body of rock that has been moved more than 2 km or 5 km above a thrust fault from its original position. Nappes form in compressional tectonic settings like continental collision zones or on the overriding plate in active subduction zones. Nappes form when a mass of rock is forced over another rock mass, typically on a low angle fault plane. The resulting structure may include large-scale recumbent folds, shearing along the fault plane, imbricate thrust stacks, fensters and klippe.
  The term stems from the French word for tablecloth in allusion to a rumpled tablecloth being pushed across a table.
context_section_description: Nappes are generally considered as compressional structures, however some exceptions could be found especially among the gravitational slides along low angle faults. Gravitational forces could be even important in certain cases during emplacement of compressional thrusts. The movement of huge masses of rock may be influenced by several forces, forces that may act together or sequentially. These forces frequently result in high temperature and pressure metamorphism and strong deformation of nappe rocks.
  At shallower depths, low pressures and temperatures can't cause the plastic and viscous behavior of solid rock necessary to move along low angle faults. It is considered that such characteristics may be achieved at significantly less extreme conditions in the clayey rocks or evaporites, which can then act as tectonic lubricants. The process, which significantly reduces the frictional resistance, is the fluid overpressure, which acts against the normal pressure, thereby reducing high lithostatic pressures and allowing fracturation, cataclasis and formation of tectonic breccia or fault gouge that could act as a decollement plane. Evaporites are also often related the decollement and thrust planes. Evaporites are strongly prone to shear deformation and therefore preferred planes of detachment.
  Behavior of thrust sheets is currently explained on the model of the orogenic wedge, which is dependent on the internal wedge taper θ. Gravitational sliding is movement generated by the movement down an inclined plane under the action of gravity. Gravitational spreading, possibly accompanied by an initial phase of diapirism, is generated by large heat flow that causes detachment in a hinterland. Other mechanisms, such as push from behind, action of tangential compressive forces, and shortening of the basement, are essentially variations of the previous mechanisms.
caption: Converging tectonic plates and the orogenic wedge
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