3.11.4 Small thrust sheet (Skjell)
3.11.4.1 A small thrust sheet is a thrust sheet that is (a) bounded on all sides by thrust faults, or (b) bounded by thrust faults and the erosion surface. A small thrust sheet is wedge- or lens-shaped and is not divided into smaller tectonostratigraphical units. The thrust faults beneath and above the small thrust sheet, floor and roof thrusts respectively, have or can be assumed to have had, listric or sigmoidal (S-shaped) geometry.
3.11.4.2 Small thrust sheet holds the lowest rank among tectonostratigraphical units, and in the form of imbricate fans and duplexes it can be an integral part of larger thrust sheets and nappes (Fig. 28).
3.11.4.3 No lower limit is placed on the size of a small thrust sheet. The dimensions will otherwise be limited by the size of the thrust sheet or nappe of which the small thrust sheet forms a part.
3.11.4.4 A small thrust sheet is mappable on the surface or traceable in the subsurface with the help of geological and/or geophysical methods; small ones are not necessarily mappable on the scale of available base maps.
3.11.4.5 A small thrust sheet 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 in a small thrust sheet.
3.11.4.6 Small 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, but should preferably be named informally.
3.11.4.7 Small thrust sheets are the smallest tectonic units in imbricate fans and duplexes (Fig. 28).
3.11.4.7.a An imbricate fan (Boyer & Elliott 1982) (skjelldelt flak) consists of small thrust sheets piled one on top of the other and having an imbricate, feather-shaped geometry when viewed in vertical section transverse to the thrust sheet structure. Each small thrust sheet is bounded by a leading thrust (ledeforkastning) and a trailing thrust (slepeforkastning) which have listric geometry and meet the floor thrust (golvforkastning) of the sheet along branching (or branch) lines (forgreiningslinjer). The imbricate fan is bounded uppermost by either the erosion surface or a succession of strata in which the thrust faults separating the small thrust sheets die out as blind thrusts (blindforkastninger) along tip lines (tupplinjer) beneath the present erosion surface (Figs. 22, 24, 28).
3.11.4.7.b Duplexes (duplekser) (Boyer & Elliott 1982) are thrust sheets comprised of sigmoidal (S-shaped), smaller thrust sheets called horses (skjell), piled one on top of the other in an imbricate pattern, and bounded at the base by a floor thrust (golvforkastning) and at the top by a roof thrust (takforkastning). The thrust faults separating the individual horses pass asymptotically into the floor and roof thrusts of the sheet to which they belong. An antiformal stack (antiformstakk) is a duplex in which each higher horse is folded around the underlying ones (Fig. 28).
3.11.4.7.c Lenses, megalenses, boudins and megaboudins (linser, megalinser) are small thrust sheets or thrust sheets bounded by thrust faults and located in broad movement zones between or within nappes (Fig. 27).
3.11.4.7.d Imbricate fans, duplexes, lenses and megalenses should preferably be given informal names. They should not be given the same name as the major tectonostratigraphical unit of which they form subordinate parts.
3.11.4.8 Examples: Small thrust sheets have been described from the Osen Nappe along the northern part of Mjøsa where they occur in imbricate fans (Skjeseth 1963, Bjørlykke 1979). Duplexes have been described from the Osen and Synnfjell Nappes in the Valdres district, being called, respectively, the Aurdal duplex and the Synnfjell duplex (Hossack et al. 1985). The Turtbakktjørna lens in Trøndelag (Kautsky 1978) and the Brakfjellet lens in Nordland (Ramberg 1981) are examples of megalenses that are assumed to have been formed by tectonic pinch and swell (uttynning og fortykning) of nappe units.
3.11.4.9 Key references: McClay (1981), Elliott & Johnson (1980), Boyer & Elliott (1982), Butler (1982).
