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{{Short description|A large sheetlike body of rock that has been moved a considerable distance above a thrust fault}}
{{Other uses}}

[[Image:thrust system en.jpg|thumb|right|350px|Schematic overview of an eroded thrust system. The shaded material is the nappe. The erosional hole is called a [[window (geology)|window or fenster]]. The [[klippe]] is the isolated block of the nappe overlying [[Wiktionary:autochthonous|autochthonous]] material.]]
In [[geology]], a '''nappe''' or '''thrust sheet''' is a large sheetlike body of [[rock (geology)|rock]] that has been moved more than {{convert|2|km|mi|abbr=on}}<ref>Howell, J.V. (Editor) 1960: ''Glossary of geology and related sciences.'' American Geological Institute, Washington D.C., 325 p.</ref> or {{convert|5|km|abbr=on}}<ref>Marko, F., Jacko, S., 1999: [http://www.fns.uniba.sk/fileadmin/user_upload/editors/geol/kgp/skripta/Marko-Jacko-1999_Strukturna_geologia1.pdf ''Structural geology (General and systematic).''] {{webarchive|url=https://web.archive.org/web/20110719005730/http://www.fns.uniba.sk/fileadmin/user_upload/editors/geol/kgp/skripta/Marko-Jacko-1999_Strukturna_geologia1.pdf |date=2011-07-19 }} {{ISBN|80-88896-36-3}} Vydavateľstvo Harlequin, Košice, p. 81 - 93 {{in lang|sk}}</ref><ref>Dennis, J. G., 1967, International tectonic dictionary. AAPG, Tulsa, p. 112</ref> above a [[thrust fault]] from its original position. Nappes form in [[thrust tectonics|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 (or [[thrust fault|"thrust"]]) over another rock mass, typically on a low angle fault plane. The resulting structure may include large-scale recumbent [[Fold (geology)|folds]], shearing along the fault plane,<ref name=Twiss>Twiss, Robert J. and Eldridge M. Moores, ''Structural Geology,'' W. H. Freeman, 1992, p. 236 {{ISBN|978-0716722526}}</ref> [[Thrust fault#Thrust duplex|imbricate thrust stack]]s, [[Window (geology)|fensters]] and [[klippe]]s.

The term stems from the French word for ''[[tablecloth]]'' in allusion to a rumpled tablecloth being pushed across a table.<ref name=Twiss/>

== History ==
Nappes or nappe belts are a major feature of the [[Europe]]an [[Geology of the Alps|Alps]], [[Dinaric Alps|Dinarides]], [[Carpathians]] and [[Stara planina|Balkans]].<ref>Schmid, S. M., Fügenschuh, B., Kissling, E, and Schuster, R. 2004: [http://pages.unibas.ch/earth/tecto/research/Schmid_et_al_2004_Ecl.pdf ''Tectonic Map and Overall Architecture of the Alpine Orogen.''] {{webarchive|url=https://web.archive.org/web/20120112191120/http://pages.unibas.ch/earth/tecto/research/Schmid_et_al_2004_Ecl.pdf |date=2012-01-12 }} ''Eclogae geologicae Helvetiae'' v. 97, Basel: Birkhäuser Verlag, pp. 93–117, ISSN 0012-9402</ref><ref>Gamkrelidze, 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</ref> Since the 19th century many geologists have uncovered areas with large-scale overthrusts. Some of these were substantiated with [[paleontology|paleontological]] 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]].<ref>Franks, S., Trümpy, R., 2005: ''The Sixth International Geological Congress: Zürich, 1894.'' Episodes, vol. 28, 3, p. 187-192</ref> 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 ==
[[Image:Vápeč od Horní Poruby.jpg|thumb|right|Klippe of [[Hronicum|Hronic nappe]], Mt. [[Vápeč]], [[Strážovské vrchy|Strážovské vrchy Mts.]], Slovakia]]
''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 [[Fold (geology)|folds]] and secondary [[Thrust fault|thrusts]] and [[duplex (geology)|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 (geology)|suture]]''. A nappe whose root area is unknown, is called a ''rootless nappe''.

Areas with a nappe structure often contain two types of geological features:

* A ''nappe outlier'' or ''[[klippe]]'' is a small area isolated from the main body of the nappe by [[erosion]] that lies on the autochthonous base; the summit of [[Veľký Rozsutec]] in the [[Western Carpathians]] is a typical example, or [[Chief Mountain]] in [[Glacier National Park (U.S.)|Glacier National Park]], [[United States|USA]].<ref>Nevin, C. M., 1950: Principles of structural geology. 4th ed. John Willey & Sons, London</ref>
* A ''fault inlier'', ''fenster'', or '''[[Window (geology)|window]]''' is an area of the autochthonous basement uncovered by erosion, but entirely surrounded by the body of the nappe; the [[Hohe Tauern window]] in the Alps is a typical example.

== Classification ==
According to petrographical composition, two basic types of nappes are known:

* [[Basement nappe]]s are composed generally of [[Basement (geology)|crystalline basement]] rocks (but may contain basement sedimentary cover), forming so-called [[thick-skinned deformation|thick-skinned style]]. Nappes of this type usually reach a large thickness and form independent superunits such as [[Penninic nappes]].
* [[Cover nappe]]s or so called ''superficial nappes'' are composed generally of [[sedimentary rock]]s that form the upper part of [[Earth crust|crust]], forming so-called [[thin-skinned deformation|thin-skinned style]]. Therefore, nappes of this type form smaller units, such as the [[Hallstatt nappe]] in the [[Austroalpine nappes]] of the Alps.

== Mechanisms of emplacement ==
[[File:Orogenic wedge.jpg|thumb|right|Converging tectonic plates and the orogenic wedge]]
Nappes are generally considered as [[Thrust tectonics|compressional]] structures, however some exceptions could be found especially among the gravitational slides along low angle faults.<ref>Graham, R.H. (1979) "Gravity sliding in the Maritime Alps" pp. 335&ndash;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, {{ISBN|978-0-632-00614-4}}</ref><ref>{{Cite book|author=Park, R. G.|orig-year=1997|year=2004|title=Foundations of Structural Geology|edition=third|type=reprint of 1997 edition of Chapman & Hall|publisher=Taylor and Francis|location=Abingdon, England|pages= 131–132|isbn=978-0-7487-5802-9}}</ref> 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.<ref>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.</ref>

At shallower depths, low [[pressure]]s and [[temperature]]s can't cause the [[plastic]] and [[viscous]] behavior of solid [[Rock (geology)|rock]] necessary to move along low angle faults. It is considered that such characteristics may be achieved at significantly less extreme conditions in the [[clay]]ey rocks or [[evaporites]], which can then act as tectonic [[lubricant]]s. The process, which significantly reduces the [[friction]]al 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|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 (geology)|shear deformation]] and therefore preferred planes of detachment.<ref>Davis, D.M., Engelder, T., 1985: ''The role of salt in fold-and-thrust belts.'' Tectonophysics, 119, p. 67-88</ref>

Behavior of [[Thrust fault|thrust]] sheets is currently explained on the model of the [[orogenic wedge]], which is dependent on the internal wedge taper θ.<ref>Nemčok, M., Schamel, S., Gayer, R. A., 2005: ''Thrustbelts: structural architecture, thermal regimes and petroleum systems.'' Cambridge University Press, Cambridge, 554 p.</ref> 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 (Geology)|hinterland]].<ref>Price, N.J., McClay, K.R., 1981: [http://sp.lyellcollection.org/cgi/reprint/9/1/1 ''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.</ref> Other mechanisms, such as push from behind, action of tangential compressive forces, and shortening of the basement, are essentially variations of the previous mechanisms.

== References ==
{{wiktionary|nappe}}
{{Commons category|Nappes|lcfirst=yes}}
{{Reflist}}

{{Structural geology}}
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[[Category:Plate tectonics]]
[[Category:Structural geology]]