Terrane

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A terrane in geology, in full a tectonostratigraphic terrane, is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "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.

Older usage of terrane simply described a series of related rock formations or an area having a preponderance of a particular rock or rock groups.

Overview

A tectonostratigraphic terrane is not necessarily an independent microplate in origin, since it may not contain the full thickness of the lithosphere. It is 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 the overriding plate. Therefore, the terrane transferred 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.[clarification needed]

A tectonostratigraphic terrane is a fault-bounded package of rocks of at least regional extent characterized by a geologic history which differs from that of neighboring terranes. The basic characteristics of these terranes is that the present spatial relations are not compatible with the inferred geologic histories. Where terranes which lie next to each other processes strata of the same age, it must be demonstrable that the geologic evolutions are different and incompatible, and there must be an absence of intermediate lithofacies which could link the strata

The concept of tectonostratigraphic terrane 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.[1]

When terranes are composed of repeated accretionary events, and hence are composed of subunits with distinct history and structure, they may be called superterranes.[2]

Tectonostratigraphic terranes

References

Notes

  1. ^ Carney et al.
  2. ^ "Terranes" University of British Columbia website
  3. ^ a b c d e f Aitchison, J. C., Ali, J. R., and Davis, A. M. (2007) "When and where did India and Asia collide?" Journal of Geophysical Research, v.112, pp.1-19
  4. ^ Pharao, et al. (1996) Tectonic map of Britain, Ireland & adjacent areas UK:British Geological Survey
  5. ^ a b c d Viola, G.; Henderson, I.H.C.; Bingen, B.; Hendriks, B.W.H. (2011). "The Grenvillian–Sveconorwegian orogeny in Fennoscandia: Back-thrusting and extensional shearing along the 'Mylonite Zone'". Precambrian Research. 189 (3-4): 368–88. doi:10.1016/j.precamres.2011.06.005. Retrieved 22 August 2015. 
  6. ^ Cuthberta, S.J.; Carswellb, D.A.; Krogh-Ravnac, E.J.; Waind, A. (2000). "Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides". Lithos. 52 (1–4): 165–195. 
  7. ^ a b c Hild, Martha; Barr, Sandra (2015). Geology of Nova Scotia. Portugal Cove: Boulder Publications. p. 18. ISBN 9781927099438. 
  8. ^ a b c d Miller, Brent (1997). Geology, Geochronology, and Tectonic Significance of the Blair River Inlier, Northern Cape Breton Island, Nova Scotia. Halifax: Dalhousie University. p. 260. 

Bibliography

  • Carney. J. N. et al. (2000) Precambrian Rocks of England and Wales, (Geological Conservation Review Series, v.20) UK: Joint Nature Conservation Committee (ISBN 978-1861074874)
  • McPhee, John (1981) Basin and Range, New York: Farrar, Straus and Giroux.
  • McPhee, John (1983) In Suspect Terrain. New York: Farrar, Straus and Giroux.
  • McPhee, John (1993) Assembling California, New York: Farrar, Straus and Giroux.

External links

  • West Antarctica terrane analysis
  • Examples of accreted terrane in Idaho
  • Alaskan Terranes
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