3.9.9 Thrust Fault, Thrust (Skyveforkastning)
3.9.9.1 A thrust fault or thrust is a fracture surface, or set of fracture surfaces, separating two rock bodies one of which, the hanging wall (hengen) has been displaced upwards across the footwall (liggen). Thrusts have originally been nearly horizontal or low-angle (less than 45°). Along a thrust, older rocks may be displaced across younger, or younger across older. Thrusts are formed by compression and imply horizontal shortening of the part of the crust that is deformed by the thrust (Fig. 21).
3.9.9.2 Thrusts often occur together in a branching thrust system (skyveforkastningssystem) in which the individual thrusts have different rank or order, with major thrusts (hovedskyveforkastninger) and subordinate or minor thrusts (underordnete or mindre forkastninger) (Figs. 22, 27).
3.9.9.3 No limitations are placed on the dimensions of thrusts, either geographically or as regards displacement.
3.9.9.4 The structure is mappable with the help of geomorphological, geological and/or geophysical methods.
3.9.9.5 A thrust is generally distinguished by a zone of bedrock which differs structurally from neighbouring bedrock. Such a zone may consist of cataclastic or mylonitic rocks. When the thrust consists of a set of fracture surfaces, these will often delimit lens-shaped bodies of rock in the movement zone.
3.9.9.6 A thrust can be given a formal or an informal name in accordance with the "general rules for naming and defining geological units" (Chap. 2). A thrust which forms the floor thrust of a specific nappe or thrust sheet should be given the same geographical name as the nappe or sheet. When choosing the geographical name for a thrust, care should be taken to ensure that this name is also suitable for the tectonostratigraphical unit which overlies the thrust. If there is no danger of misunderstanding, the word fault alone can be used in the name of a thrust fault.
3.9.9.7 Modern literature dealing with thrusts (Elliott & Johnson 1980, Boyer & Elliott 1982, Butler 1982) employs several terms which have not yet been given Norwegian names. Thrust faults in many nappes, sheets, nappe complexes and nappe systems form a highly branching, often staircase (trappeformet) thrust system (skyveforkastningssystem). The different types of thrust system, or parts of thrust systems, are classified in relation to the tectonostratigraphical bodies of rock which they delimit, or in relation to the mutual geometrical relationships between the thrusts.
3.9.9.7.a A sole thrust (såleforkastning) is a thrust separating a nappe or major thrust sheet from underlying rock units. The term is often used for the lowermost regional thrust in a sequence of nappes (Figs. 22, 27). It is also used for a nearly flat-lying fault formed by extension (cf. Section 3.9.4.7.a).
3.9.9.7.b A floor thrust (golvforkastning) is a thrust marking the base of a nappe, thrust sheet or small thrust sheet (Figs. 22, 27). The term is also used for a nearly flat-lying fault formed by extension (cf. Section 3.9.4.7.a).
3.9.9.7.c A roof thrust (takforkastning) is a thrust at the top of a nappe, thrust sheet or small thrust sheet (Figs. 22, 27).
3.9.9.7.d A leading thrust (ledeforkastning) is a thrust forming the base and frontal edge of a small thrust sheet, thrust sheet, nappe, nappe complex or nappe system (Figs. 22, 27).
3.9.9.7.e A trailing thrust (slepeforkastning) is a thrust forming the upper and rear edge of a small thrust sheet, thrust sheet, nappe, nappe complex or nappe system (Figs. 22, 27).
3.9.9.7.f A flat (flateforkastning) is a thrust which was originally horizontal when the thrust was formed. A flat which is formed in an undeformed stratigraphical sequence will be parallel to bedding (Butler 1982) (Figs. 22, 27).
3.9.9.7.g A ramp (rampeforkastning or rampe) is a portion of a thrust which cuts stratigraphically upwards through the thrust-transported sequence in the direction of the foreland. Ramps consist of a front ramp (frontrampe), a lateral ramp (siderampe) and a diagonal or oblique ramp (diagonalrampe, skrårampe). These terms are used with reference to the direction of tectonic transport (Figs. 23, 27).
3.9.9.7.h A splay thrust (splittforkastning) is a subordinate thrust fault branching off a major thrust along a branch line or branching line. Splay thrusts can be further subdivided according to their geometrical relationship to the major thrust (Boyer & Elliott 1982) (Figs. 24, 27).
3.9.9.7.i Thrusts of the types mentioned in a-h can be given informal or formal names. Apart from particularly prominent thrusts, names should preferably be informal.
3.9.9.8 Examples: In the Caledonian mountain chain of Scandinavia there are many well-known examples of thrusts of different rank and order. They form floor thrusts beneath nappe units and various types of more subordinate thrusts within the nappe sequences. In Norway, it has not been common to name the thrusts themselves. From the Valdres area, Hossack et al. (1985) have named the thrusts beneath the previously defined Valdres and Synnfjell Nappes, the Valdres and Synnfjell thrusts, respectively (Valdresskyveforkastningen and Synnfjellskyveforkastningen).
3.9.9.9 Key references: McClay (1981), Boyer & Elliott (1982), Butler (1982).
