3.11.3 Thrust sheet (Flak, Skyveflak)
3.11.3.1 A thrust sheet is a sheet-, slice-, wedge- or lens-shaped body of rock that has been displaced upwards along a thrust fault (skyveforkastning). The thrust fault may be approximately horizontal, low-angled or listric (concave upwards), or it should be conceivable that it had one of those attitudes prior to subsequent deformation.
3.11.3.2 Thrust sheet ranks next after nappe in areas where a nappe has been defined as a formal tectonostratigraphical unit. A thrust sheet that forms part of a nappe unit may be called a nappe sheet (dekkeflak). Thrust sheet can also be used as a non-ranking tectonostratigraphical term (see Section 3.11.3.7).
3.11.3.3 No limitations are placed on the dimensions or length of transport of a thrust sheet. Like a nappe, a sheet may be eroded and divided up into klippen, and it may surround one or more windows.
3.11.3.4 A thrust sheet is mappable at the surface or traceable in the subsurface with the help of geological and/or geophysical methods.
3.11.3.5.a A thrust sheet can consist of one or more rock types having a common or different origin and degree of metamorphism and deformation. It is therefore possible to distinguish lithostratigraphical, lithodemic and biostratigraphical units in a thrust sheet.
3.11.3.5.b A thrust sheet is delimited from the underlying older or younger rocks by a thrust fault which is the leading thrust (ledeforkastning) or floor thrust (golvforkastning) of the thrust sheet. It may also be delimited from an overlying thrust sheet by a thrust fault which is its trailing thrust (slepeforkastning) or roof thrust (takforkastning). The thrust sheet may, in addition, be delimited by other tectonic surfaces, later depositional surfaces, intrusive contacts and the erosion surface (Figs. 22, 24, 27).
3.11.3.5.c A thrust sheet may be tectonically undivided, have a feather-like imbricate structure (imbricate fan) or duplex geometry (Fig. 28).
3.11.3.6 Thrust sheets can be given formal or informal names in accordance with the "general rules for naming and defining geological units" (Chap. 2) and the provisions of Section
3.11.1.2. If the term "thrust sheet" (flak, skyveflak, dekkeflak) is used for a tectonostratigraphical unit that has already been defined as a nappe, the geographical name of the nappe must also be used together with the term thrust sheet in the formal version of the name. A thrust sheet that has been defined and given a formal name can have its status changed to nappe if investigations show this to be desirable. If the floor thrust beneath the thrust sheet is given a name, the sheet must have the same name, formally or informally, as the thrust. (If the floor thrust beneath the thrust sheet is the floor thrust of a nappe of which the thrust sheet forms an integral part, the nappe and the floor thrust should have the same geographical name (see Sections 3.11.1.2 and 3.11.2.6).) A thrust sheet must not be given a formal or an informal name containing the same geographical name as one of the lithological units in the nappe or thrust sheet. The proper name (Section 2.2.2) is placed between the words "the ... Thrust Sheet" (formal) in the compound name.
3.11.3.7 In modern Anglo-American nomenclature relating to thrust systems (Boyer & Elliott 1982) the term thrust sheet is used broadly for all displaced bodies of rock that are bounded by one or more thrust faults, irrespective of the size of the unit. Subject to terms specified in this Code (see Sections 3.11.1.2, 3.11.2.7 and 3.11.3.2), a corresponding use of thrust sheet is permitted.
3.11.3.8 Example: An example of a named thrust sheet is the Mistra Thrust Sheet (Mistraflaket) (Holmsen & Oftedahl 1956).
3.11.3.9 Key references: McClay (1981), Boyer & Elliott (1982).
