olelog

Plates, microplates, blocks and terranes are large chunks of the Earth’s crust. There is no way of counting them because geologists disagree on the exact definitions and - of course - the boundaries. Some geologists stress that terranes are not microplates, other geologists say that they are microplates, and in a well-known text book they are called microplate terranes! Personally I may go as far as to say that terranes are not microplates, but have been microplates in the geological past. We are however in a country of some consensus and much disagreeing.

The concept of terranes developed from studies in the 1970s of the complicated Western or Canadian Cordilleran orogenic margin of North America. The terrane concept was introduced in a paper by Coney et al. in 1980 (Coney, P.J., Jones, D.L., and Monger, J.W.H., 1980, “Cordilleran suspect terranes”: Nature, v. 288, p. 329–333, doi: 10.1038/288329a0.) to describe the tectonically and stratigraphically composite nature of the North American plate. Terranes were defined as fault-bounded crustal blocks that preserved a geological record distinct from that of adjacent terranes. Later the concept was further developed, and controversy arose because of disagreement over the definition.

In a paper titled "Northern Cordilleran terranes and their interactions through time” published in GSA Today (April/May 2007) Maurice Colpron et al. revisits these terranes some 25 years later. It's a fine article with a nice map of the relevant (ca. 30) terranes in Canada and Alaska. I am not going into more details about that. Read it yourself if you want to. I think it can be downloaded from http://www.gsajournals.org/perlserv/?request=index-html&issn=1052-5173 .

Here I will rehearse some of the terms.

A terrane is a crustal block or fragment that preserves a distinctive geologic history that is different from the surrounding areas and that is usually bounded by faults.

Suspect terranes
This term is used if it is not clear where the terrane came from.

Native terranes
Terranes that may potentially have travelled far from their sites of origin are called 'suspect' terranes, whereas those that are clearly related to the continental margin against which they presently sit are called 'native' terranes.

Exotic terranes
An exotic terrane is a piece or fragment of continent that has broken off from its parent continent and become accreted to another continent. Thus it is exotic to the continent that it has become a member of, and not native to that continent.

Accreted terranes
Small crustal fragments, island arc, or seamount which are transported by a moving oceanic plate and are added to a continental mass at the subduction zone. This term came into use by geologists who wanted to stress that this was what they were talking about when they used the word terrane. I have illustrated the concept with a cartoon that shows the principle. The oceanic plate act as a conveyor belt that continually adds new terranes to the continental margin (e.g. the Pacific (oceanic) Plate adding terranes to the west of North America).

Displaced terranes
The same as exotic terranes, stressing that they have been displaced.

Allochthonous terranes
The same as exotic terranes. In an article in Science of 3 July 1981 titled "Continental Accretion: From Oceanic Plateaus to Allochthonous Terranes" Ben-Avraham et al. suggested that modern analogs of many allochthonous terranes may be found in the oceans, in the puzzling topographic ridges, rises, or plateaus present on the ocean floor ... some of the oceanic plateaus, which comprise about 10 percent of the ocean floor, are modern allochthonous terranes in migration, moving with the oceanic plates in which they are embedded and fated eventually to be accreted to continents adjacent to the subduction zones that ring the Pacific.

Composite Terranes
Terranes which are made up of two or more distinct terranes that were joined together before beginning their travels.

Superterranes
Amalgamation of many smaller terranes.

Disrupted Terranes
As their name implies, disrupted terranes contain rocks from different backgrounds and ages - oceanic crust, shallow-water limestone, deep-water chert, and greywacke - usually found in a matrix of shale or serpentinite.

Metamorphic Terranes
These terranes show signs of terrane-wide geologic changes, before or after collision, which has been sufficiently powerful to obscure original rock formation.

Contiguous terranes
Probably much the same as native terranes?

Tectonic terranes
Stressing the tectonic context like in plates and tectonic plates.

Tectonostratigraphic terranes
Stressing both the tectonic and the stratigraphic aspects.

My list is not complete, and I wouldn’t call the definitions authoritative, just attempts to clarify a complex matter. A quarter of a century research into terranes (whatever they have been called) is nevertheless extremely important research and of great value for reconstructions of plate configuatoions millions of years ago.