3.9.4 Fault (Forkastning)
3.9.4.1 A fault is a fracture separating two bodies of rock which have moved relative to one another. When the dip of the fault surface is less than 90°, the wall rock above the surface is termed the hanging wall (hengen) and that below it the footwall (liggen) (Fig. 18).
3.9.4.2 Fault is the fundamental term for a fracture surface in the crust on which displacement has occurred.
3.9.4.3 No limitations are placed on the dimensions of a fault as regards its geographical extent or displacement. The displacement need not necessarily be capable of demonstration on the map scale on which the fault is portrayed.
3.9.4.4 The structure is mappable with the help of geomorphological, geological and/or geophysical methods. On a map, a fault will normally be depicted as a line, but when large-scale mapping takes place many faults will be subdivisible into subsidiary faults and fault segments.
3.9.4.5 A fault may consist of (a) one boundary surface between two rock bodies which have been displaced relative to each other, or (b) a zone of bedrock between two fault blocks, which is characterized by a branching pattern of surfaces on which displacement has taken place. A zone of this nature will have a different structural character from the bedrock in the two fault blocks, and may consist of cataclastic (e.g. fault breccia) or mylonitic rocks. A fault may also (c) be developed partly like (a) and partly like (b).
3.9.4.6 A fault is given a formal or an informal name in accordance with the "general rules for naming and defining geological units" (Chap. 2) and the provisions of Section 3.9.1. The proper name (Section 2.2.2) is placed between the words "the ... Fault" (formal) in the compound name.
3.9.4.7 There are many types of faults. Faults may be classified according to (1) the relative displacement between the two adjacent bodies of rock and the slope of the fault surface, and (2) the geometrical relationship between the fault surface and the bedrock through which the fault passes (Dennis 1972). The descriptive part of the fault name will generally be related to the first classification method. Such faults comprise the following main types (Fig. 18):
3.9.4.7.a A normal fault, gravity fault, dip-slip fault or normal slip fault (normalforkastning) is one in which the hanging wall seems to have moved downwards relative to the footwall, chiefly along the dip of the fault surface. The dip of the fault surface is usually 45-90°. A distinction can be made between high angle (more than 45°) and low angle (less than 45°) normal faults.
Listric normal faults (listriske normalforkastninger) show decreasing dip towards depth (Fig. 18). They may be related to a sole fault (såleforkastning) or a floor fault (golvforkastning) formed by detachment (avglidning) of the hanging wall in relation to the footwall along a flat-lying fracture surface (Fig. 18). The hanging wall of a flat-lying fault of this kind may, in a basin formed by extension, consist of tilted, wedge-shaped blocks bounded laterally by listric normal faults (Bally et al. 1981, Wernicke & Burchfiel 1982, Gibbs 1984). Sole and floor faults are non-genetical designations for nearly flat-lying faults, and are also used for thrust faults (cf. Sections 3.9.9.7.a and 3.9.9.7.b).
3.9.4.7.b A reverse fault (reversforkastning) is one in which the hanging wall seems to have moved upwards relative to the footwall, chiefly along the dip of the fault surface. The dip of the fault surface is generally 45-90° (Fig. 19). When the angle of the fault surface decreases to less than 45°, a reverse fault becomes a thrust fault (Section 3.9.9).
3.9.4.7.c A strike-slip fault, lateral fault, wrench fault or transcurrent fault (sidelengsforkastning) is a fault where the relative displacement has chiefly taken place parallel to the strike direction of the fault surface. The fault surface usually has a steep dip. Vertical or nearly vertical strike-slip faults (transcurrent or wrench faults) may branch like a fan upwards or downwards in flower structures (blomsterstrukturer) (Harding 1985). There are two main varieties of strike-slip fault, c1 and c2 (Fig. 20).
3.9.4.7.c1 A dextral fault, right-slip fault, or right-lateral fault (høyrelengsforkastning, høyrehåndsforkastning, høyreglippforkastning) is one on which the displacement seems to have taken place towards the right.
3.9.4.7.c2 A sinistral fault, left-slip fault, or left-lateral fault (venstrelengsforkastning, venstrehåndsforkastning, venstreglippforkastning) is one on which the displacement seems to have taken place towards the left.
3.9.4.7.c3 An oblique-slip fault (skråforkastning) is one on which the displacement between the hanging wall and the footwall contains both a lateral component and one along the dip of the fault surface. Such faults can be normal oblique-slip faults and reverse oblique-slip faults of sinistral or dextral type.
3.9.4.7.c4 A transform fault (transformforkastning) is a variety of transcurrent fault found in a plate tectonic context (Wilson 1965). A transform fault connects two segments of a mid-oceanic ridge and therefore separates two lithospheric plates having opposite displacement directions. Away from the actively spreading ridge a transform fault can pass into an oceanic fracture zone (oseanisk bruddsone).
3.9.4.7.d Thrust or thrust fault (skyveforkastning) -- see Section 3.9.9.
3.9.4.7.e All the terms mentioned under a-d can be used as descriptive terms in formal and informal names of faults.
3.9.4.8 Examples: The Oslo district has many examples of normal faults that have been recognized for a long time, e.g. the Nesodden Fault and the Ekeberg Fault (Holtedahl & Dons 1952). The Rendalen Fault and the Engerdalen Fault (Schiøtz 1902) are examples of regional normal faults which can be divided into several subfaults and fault segments.
3.9.4.9 Key references: Anderson (1951), Price (1965), Dennis (1972), Hobbs, Means & Williams (1976), Bates & Jackson (1980), Davis (1984).
