3.9.2 Fracture Zone (Bruddsone)
3.9.2.1 A fracture zone is a composite structure consisting of several neighbouring fracture surfaces (joints and/or faults). These may be genetically related to one another, but this is not a prerequisite (Fig. 17).
3.9.2.2 Fracture zone ranks next above fracture (fracture surface).
3.9.2.3 A fracture zone crops out in a long, narrow geographical area. Its breadth is defined by the outermost fractures in the zone. No other limitations are placed on the extent of a fracture zone.
3.9.2.4 A fracture zone is mappable at the surface and/or traceable in the subsurface with the help of geomorphological, geological or geophysical methods. The degree of detail in the cartographical presentation of a fracture zone will depend on the map scale and the knowledge available.
3.9.2.5 In many places a fracture zone will be characterized by irregular landscape features. It may be marked by long, narrow ridges or depressions, depending upon the type of deformation within the fracture zone and the competency of adjacent rocks.
3.9.2.6 A fracture zone can be 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 ... Fracture Zone" (formal) in the compound name.
3.9.2.7.a Fracture zones can occur within continental and ocean-floor crust, as well as between lithospheric plates of identical or different composition and origin. Fracture zones can form geomorphological features on land and on the sea floor. In a plate tectonic context the term fracture zone (bruddsone) is used about the extension of a transform fault. When the term is used in this context the descriptive term "oceanic" can be used in the compound name.
3.9.2.7.b Fracture (brudd) is a general term for all kinds of fracturing caused by mechanical stresses in the bedrock, irrespective of whether dislocation has taken place along the fracture. Fractures include joints and cracks (sprekker), and faults (forkastninger) (Fig. 16).
3.9.2.7.c A fracture system (bruddsystem) is a group of closely spaced fractures that are parallel to, or intersect each other, and that are assumed to have formed during the same deformation episode. Fracture systems should preferably only be given informal names (Fig. 17).
3.9.2.8 Examples: The Mjøsa--Va"nern Fracture Zone (Oftedahl 1980) is a broad, geologically composite fracture zone in continental crust. The Jan Mayen Fracture Zone (Johnson & Heezen 1967) is one of several oceanic fracture zones which have originated by ocean-floor spreading in the Norwegian Sea. These are all characterized by being prominent, irregular, submarine form elements (Grønlie & Talwani 1977, Talwani & Eldholm 1977).
3.9.2.9 Key references: Anderson (1951), Price (1968), Bates & Jackson (1980).
