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In geology, a nappe or thrust sheet is a large sheetlike body of rock that has been moved more than Howell, J.V. (Editor) 1960: Glossary of geology and related sciences. American Geological Institute, Washington D.C., 325 p. or Marko, F., Jacko, S., 1999: Structural geology (General and systematic). Vydavateľstvo Harlequin, Košice, p. 81 - 93 Dennis, J. G., 1967, International tectonic dictionary. AAPG, Tulsa, p. 112 above a thrust fault from its original position. Nappes form in thrust tectonics like continental collision zones or on the overriding plate in active subduction zones. Nappes form when a mass of rock is forced (or thrust fault) 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,Twiss, Robert J. and Eldridge M. Moores, Structural Geology, W. H. Freeman, 1992, p. 236 imbricate thrust stacks, fensters and .
The term stems from the French word for tablecloth in allusion to a rumpled tablecloth being pushed across a table.
Areas with a nappe structure often contain two types of geological features:
At shallower depths, low and 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 rocks or evaporites, which can then act as tectonic . The process, which significantly reduces the resistance, is the fluid overpressure, which acts against the normal pressure, thereby reducing high lithostatic pressures and allowing fracturation, cataclasis and formation of Fault 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.Davis, D.M., Engelder, T., 1985: The role of salt in fold-and-thrust belts. Tectonophysics, 119, p. 67-88
Behavior of Thrust fault sheets is currently explained on the model of the orogenic wedge, which is dependent on the internal wedge taper θ.Nemčok, M., Schamel, S., Gayer, R. A., 2005: Thrustbelts: structural architecture, thermal regimes and petroleum systems. Cambridge University Press, Cambridge, 554 p. 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.Price, N.J., McClay, K.R., 1981: Introduction. p. 1-5 in Price, N.J., McClay, K.R. (Eds.), Thrust and Nappe Tectonics. Geological Society, Special Publications vol. 9, London, 528 p. Other mechanisms, such as push from behind, action of tangential compressive forces, and shortening of the basement, are essentially variations of the previous mechanisms.
History
Nappes or nappe belts are a major feature of the Alps, Dinaric Alps, Carpathians and Stara planina.Schmid, S. M., Fügenschuh, B., Kissling, E, and Schuster, R. 2004: Tectonic Map and Overall Architecture of the Alpine Orogen. Eclogae geologicae Helvetiae v. 97, Basel: Birkhäuser Verlag, pp. 93–117, ISSN 0012-9402Gamkrelidze, I.P. 1991: Tectonic nappes and horizontal layering of the Earth’s crust in the Mediterranean belt (Carpathians, Balkanides and Caucasus). Tectonophysics, 196, p. 385-396 Since the 19th century many geologists have uncovered areas with large-scale overthrusts. Some of these were substantiated with paleontology evidence. The concept was developed by Marcel Alexandre Bertrand, who unraveled the complex tectonic history of the Alps and identified the feature as nappe de charriage. He reinterpreted earlier studies by Arnold Escher von der Linth and Albert Heim in the Glarus Alps.Franks, S., Trümpy, R., 2005: The Sixth International Geological Congress: Zürich, 1894. Episodes, vol. 28, 3, p. 187-192 His work in Switzerland influenced Escher and Maurice Lugeon. Several years later, nappe structure was investigated in northwestern Scotland by Charles Lapworth. Lugeon later transferred the ideas of nappes to the Carpathians.
Structure
Nappe can be qualified in a number of ways to indicate various features of a formation. The frontal part in the direction of movement, is called the leading edge of a nappe; numerous folds and secondary Thrust fault and duplexes are common features here and are sometimes called digitations. The surface of a thrust fault which caused movement of a nappe is called a decollement, detachment plane or sole of thrust. The root area is an area where the nappe is completely separated from its substratum. It is often compressed and reduced, even underthrust below the surrounding tectonic units, resulting in a specific structure called a suture. A nappe whose root area is unknown, is called a rootless nappe.
Classification
According to petrographical composition, two basic types of nappes are known:
Mechanisms of emplacement
Nappes are generally considered as Thrust tectonics structures, however some exceptions could be found especially among the gravitational slides along low angle faults.Graham, R.H. (1979) "Gravity sliding in the Maritime Alps" pp. 335–352 In McClay, K. R. and Price, N.J. (editors) (1981) Thrust and Nappe Tectonics (Geological Society of London Special Publication 9) Blackwell Scientific, Oxford, England, Gravitational forces could even be 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.Rodrigues, S. W. O., Martins-Ferreira, M. A. C., Faleiros, F. M., Neto, M. D. C. C., & Yogi, M. T. A. G. (2019). Deformation conditions and quartz c-axis fabric development along nappe boundaries: The Andrelândia Nappe System, Southern Brasília Orogen (Brazil). Tectonophysics.
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