Terrane

A terrane in geology is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted — "sutured" — to crust lying on another plate. The crustal block or fragment preserves its own distinctive geologic history, which is different from that of the surrounding areas (hence the term "exotic" terrane). The suture zone between a terrane and the crust it attaches to is usually identifiable as a fault.

Overview

A terrane is not necessarily an independent microplate in origin, since it may not contain the full thickness of the lithosphere. It a piece of crust which has been transported laterally, usually as part of a larger plate, and is relatively buoyant due to thickness or low density. When the plate of which it was a part subducted under another plate, the terrane failed to subduct, detached from its transporting plate, and accreted onto he overriding plate. Therefore, the terrane transfered from one plate to the other. Typically, accreting terranes are portions of continental crust which have rifted off another continental mass and been transported surrounded by oceanic crust, or old island arcs formed at some distant subduction zone.

The concept of "terranes" developed from studies in the 1970s of the complicated Pacific Cordilleran ("backbone") orogenic margin of North America, a complex and diverse geological potpourri that was difficult to explain until the new science of plate tectonics illuminated the ability of crustal fragments to "drift" thousands of miles from their origin and fetch up, crumpled, against an exotic shore. Such terranes were dubbed "accreted terranes" by geologists.

:"It was soon determined that these exotic crustal slices had in fact originated as "suspect terranes" in regions at some considerable remove, frequently thousands of kilometers, from the orogenic belt where they had eventually ended up. It followed that the present orogenic belt was itself an accretionary collage, composed of numerous terranes derived from around the circum-Pacific region and now sutured together along major faults. These concepts were soon applied to other, older orogenic belts, e.g. the Appalachian belt of North America.... Support for the new hypothesis came not only from structural and lithological studies, but also from studies of faunal biodiversity and palaeomagnetism." (Carney "et al.")

When terranes are composed of repeated accretionary events, and hence are composed of subunits with distinct history and structure, they may be called superterranes. [ [http://www.geop.ubc.ca/Lithoprobe/transect/terrane.html University of British Columbia website: Terranes] ]

ee also

* Geology of Victoria
* Avalonia
* Chilenia
* Smartville Block
* Sonomia Terrane
* Narryer Gneiss Terrane
* Salinian Block
* Wrangellia Terrane
* Yakutat Block

Notes

External links

* [http://www.geop.ubc.ca/Lithoprobe/transect/terrane.html Terrane: a definition]
* [http://www.glassearth.com/terranepages/terranes.htm New techniques for modelling terranes in three dimensions]
* [http://www.antarctica.ac.uk/Key_Topics/Geological_Evolution/terrane_analysis/ West Antarctica terrane analysis]
* [http://imnh.isu.edu/digitalatlas/geo/accreted/attext/atmain.htm Examples of accreted terrane in Idaho]
* [http://www.alaskageography.com/essays/geologic_history.htm Alaskan Terranes]

References

*J.N. Carney et al., " Precambrian Rocks of England and Wales", GCReg. volume 20 (ISBN 978-1861074874)
*John McPhee, "Basin and Range", 1981 (Farrar, Straus and Giroux, New York).
*John McPhee, "In Suspect Terrain" 1983 (Farrar, Straus and Giroux, New York).
*John McPhee, "Assembling California", 1993 (Farrar, Straus and Giroux, New York).