Nappe

For other uses, see Nappe (disambiguation).

Schematic overview of an eroded thrust system. The shaded material is the nappe. The erosional hole is called a window or fenster. The klippe is the isolated block of the nappe overlying autochthonous material.

In geology, a nappe is a large sheetlike body of rock that has been moved more than 2 km (1.2 mi)^[1] or 5 km^[2] from its original position. Nappes form during continental plate collisions, when folds are sheared so much that they fold back over on themselves and break apart. The resulting structure is a large-scale recumbent fold. The term stems from the French word for tablecloth.

History

Nappes or nappe belts are a major feature of the European Alps, Carpathians and Balkans.^[3][4] Since the 19th century many geologists have uncovered areas with large-scale overthrusts. Some of these were substantiated with paleontological evidence. The concept was developed by M.A. Bertrand, who unraveled the complex tectonic history of the Alps and identified the feature as nappe de charriage. He reinterpreted earlier studies by Escher and Heim in the Glarus Alps.^[5] His work in Switzerland influenced A. Escher von der Linth and M. Lugeon. Several years later, nappe structure was investigated in northwestern Scotland by Ch. Lapworth. Lugeon later transferred the ideas of nappes to the Carpathians.

Structure

Klippe of Choč nappe, Vápeč, 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 folds and secondary thrusts 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 or detachment plane. 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.

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.
• A Fault inlier, fenster, or window is an area of the autochthonous basement uncovered by erosion, but continuously 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 nappes are composed generally of crystalline basement rocks, forming so-called thick-skinned style. Nappes of this type usually reach a large thickness and form independent superunits such as Penninic nappes.
• Cover nappes or so called Superficial nappes are composed generally of sedimentary rocks that form the upper part of crust, forming so-called 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

Coliiding tectonic plates and the orogenic wedge

Nappes are generally considered as compressional structures, however some exceptions could be found especially among the gravitational slides along low angle faults.^[6] Gravitational forces could be even important in certain cases during emplacement of compressional thrusts. It is assumed that the ability to move huge masses of rock may be influenced by several factors that could act together or change one into another.

At a shallower depths, low pressures and temperatures can't cause plastic and viscous behavior of solid rock which is 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 a tectonic lubricants. The process, which significantly reduces the frictional resistance is the fluid overpressure, which acts against the normal pressure, reducing high lithostatic pressures and allows fracturation, cataclasis and formation of tectonic breccia or fault gouge that could act as 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.^[7]

Behavior of thrust sheets is currently explained on the model of the orogenic wedge, which is dependent on the internal wedge taper θ.^[8] 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.^[9] Other mechanisms as push from behind, action of tangential compressive forces, shortening of the basement are essentially variations of the previous mechanisms.

References

1. ^ Howell, J.V. (Editor) 1960: Glossary of geology and related sciences. American Geological Institute, Washington D.C., 325 p.
2. ^ Marko, F., Jacko, S., 1999: Structural geology (General and systematic). ISBN 80-88896-36-3 Vydavateľstvo Harlequin, Košice, p. 81 - 93 (Slovak)
3. ^ 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 0012-9402
4. ^ 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
5. ^ Franks, S., Trümpy, R., 2005: The Sixth International Geological Congress: Zürich, 1894. Episodes, vol. 28, 3, p. 187-192
6. ^ Park, R. G., 2004: Foundation of Structural Geology. Taylor and Francis, Abingdon, 202 p.
7. ^ Davis, D.M., Engelder, T., 1985: The role of salt in fold-and-thrust belts. Tectonophysics, 119, p. 67-88
8. ^ Nemčok, M., Schamel, S., Gayer, R. A., 2005: Thrustbelts: structural architecture, thermal regimes and petroleum systems. Cambridge University Press, Cambridge, 554 p.
9. ^ 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.