3.11.7 Tectonostratigraphical terrane (Tektonostratigrafisk terreng)
Tectonostratigraphical terrane, tectonic terrane or terrane are terms used in plate tectonic contexts. They characterize a block or portion of the Earth's crust entirely bounded by faults. A terrane is distinguished from adjacent portions of crust by having a distinctly different geological history.
The bedrock composition in a tectonostratigraphical terrane may be homogeneous, or complex and varied. The terms "suspect terrane" and "exotic terrane" are used to emphasize that the geological origin and composition of such a block of crust seem to be foreign relative to the surroundings.
Tectonostratigraphical terranes are formed by plate tectonic processes. Plate tectonic movements also accrete terranes during orogenies along the edges of older, tectonically stable cratons.
Terranes may be (a) large and small continental masses bounded by suture zones (collision boundaries) and newly-formed ocean-floor crust, (b) segments bounded by steep, regional, transform faults, (c) nappes bounded by thrust faults, or (d) portions of crust having complicated and structurally composite fault surfaces.
A tectonostratigraphical terrane is defined on the basis of its characteristic geological structure and evolutionary history, age, fauna and flora, palaeomagnetic history and bounding faults.
Tectonostratigraphical terranes can be distinguished informally using their assumed plate tectonic origin, e.g. "Mid-Ordovician island-arc terrane", "ophiolite terrane", "rift-basin terrane", "fore-arc basin terrane". Such terms can be used together with a geographical name from a type section or type area (see Section 2.4.5).
If a tectonostratigraphical terrane coincides with a previously defined and named nappe, nappe complex or nappe system, the geographical name in the proper name of that unit should be used in the proper name of the terrane in formal nomenclature (cf. rootless palaeobasins, Section 3.7.14.7.c1). If the terrane has a different delimitation from previously defined tectonostratigraphical units it should be defined and named in accordance with the rules in Chap. 2.
Examples: India is a huge tectonostratigraphical terrane. The mountain chain along the North American Pacific coast consists of many long, narrow terranes. The Caledonian mountain chain in Norway contains tectonostratigraphical terranes which largely coincide with nappe units.
Key reference: Howell (1985).