3.11.2 Nappe (Dekke, Skyvedekke)
3.11.2.1 A nappe is a sheet-, slice-, wedge-, or lens-shaped body of rock that has been displaced a large distance along a thrust fault (skyveforkastning). The thrust fault at the sole of the nappe is approximately horizontal or low-angled, or it may be assumed to have been so prior to subsequent deformation. A nappe consists of one or more nappe sheets or thrust sheets (dekkeflak, skyveflak) which are assumed to have undergone displacement together (piggy-back displacement) on the floor thrust of the nappe. The rocks in a nappe may differ from the autochthonous (stedegne) rocks in the geological region in which the nappe occurs with respect to the following properties: stratigraphy, composition, sedimentary facies, degree of metamorphism and deformation.
3.11.2.2 Nappe is the fundamental formal unit for tectonostratigraphical classification and nomenclature.
3.11.2.3 No limitations are placed on the size of a nappe. A nappe may, because of erosion, be divided up into several detached nappe remnants or klippen (dekkerester, klipper) and may envelop one or more windows (vinduer). The tectonostratigraphical units underlying the nappe are exposed in the window.
3.11.2.4 A nappe is mappable at the surface or traceable in the subsurface with the help of geological and/or geophysical methods.
3.11.2.5.a A nappe may consist of one or more rock types having a common or different origin, age, and degree of metamorphism and deformation. It is therefore possible to distinguish lithostratigraphical, lithodemic and biostratigraphical units within the nappe. The rocks in a nappe usually derive from one or a few plate tectonic environments in an orogen (terranes), e.g. a foreland basin on continental crust, a volcanic island arc, a deep ocean trench, or ocean-floor crust (Section 3.11.7).
3.11.2.5.b A nappe is delimited from underlying older or younger rocks by a thrust fault which is its floor thrust (golvforkastning). A nappe may in addition be bounded by later depositional surfaces, other tectonic surfaces (e.g. a roof thrust which may be a floor thrust to an overlying nappe), intrusive contacts and the erosion surface (Fig. 27).
3.11.2.5.c A nappe may be tectonically undivided or consist of imbricate fans and duplexes, and of large fold structures in which significant portions of the stratal sequence in the nappe may be preserved in an overturned state, as in fold nappes (foldedekker) (Fig. 27).
3.11.2.6 Nappes can be given formal or informal names, this being done in accordance with the "general rules for naming and defining geological units" (Chap. 2) and the provisions of Section 3.11.1.2. Nappes should preferably be defined formally. The type section or type locality of a nappe is situated within the area in which the thrust sheets forming the nappe are located. The first part of the nappe name must be the same geographical name as that of the floor thrust of the nappe, if that is given a name. A nappe must not be given the same geographical name as named lithological units in the nappe. The name of the nappe can be used formally or informally, also in those cases when the nappe unit is defined as a thrust sheet (skyveflak). The proper name (Section 2.2.2) is placed between the words "the ... Nappe" in formal names.
3.11.2.7.a The rocks in a nappe can be said to be allochthonous (alloktone, flyttet) if they differ significantly from the autochthonous (stedegne, autoktone) rocks of the foreland succession and its basement (underlag) in composition, stratigraphy, sedimentary facies, and degree of metamorphism and deformation, giving grounds for assuming that the nappe rocks have been transported over regional distances. Autochthonous rocks proper are only those rocks which have remained static in their relation to the continental crustal basement of the foreland. That part of the succession is said to be pinned (naglet), i.e. pinned to the basement.
3.11.2.7.b The rocks in a nappe can be said to be parautochthonous (nærstedegne, parautoktone) if the sedimentary succession in the nappe is readily correlatable with the stratigraphy and sedimentary facies in the autochthonous sequence, giving grounds for assuming a more moderate transport of the nappe.
3.11.2.7.c The term nappe is non-genetical and therefore says nothing about the mechanisms leading to the transport of the nappe rocks.
3.11.2.7.d In modern Anglo-American nomenclature relating to "thrust systems" (Elliott & Johnson 1980, Boyer & Elliott 1982, Butler 1982) the term "dominant thrust sheet" or perhaps "major thrust sheet" will correspond to "dekke". These may consist of "minor thrust sheets", but frequently the term "thrust sheet" is used for tectonostratigraphical units ranging in size from small thrust sheets to large nappe units.
3.11.2.8 Examples: Many nappes have been named in the Caledonian mountain chain of Scandinavia. Examples and references are found in Gee & Sturt (1985).
3.11.2.9 Key references: Tollmann (1968), McClay (1981), Boyer & Elliott (1982).
