Terrane
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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 the surrounding areas (thus the occasional term "exotic" terrane). The suture zone between a terrane and the crust it attaches to is usually represented by a fault.
In the lithospheric scheme of plate tectonics, a terrane is not an independent microplate, but a piece of crust which lies atop a larger plate which includes other, usually oceanic, crustal material. It can accrete to another piece of crust on another plate when the plate of which it is a part collides with the plate associated with the other crust. Typically, accreting terranes are portions of continental crust which have rifted off from another plate by a tectonic change, or they may represent old island arcs formed at some distant subduction zone. In some instances, this occurs with the interposition of a third plate, usually oceanic, which carries the terrane off some distance to be attached to another plate.
The transfer of a terrane from one plate to another typically occurs as the plate that carries it is subducted under the plate to which the terrane attaches. The terrane detaches from the denser subducting plate while being driven into the crust which lies on the other plate. This process may occur with such force that the terrane is driven on to the other plate, sometimes overriding crustal material already carried by the plate, and sometimes by compressing it. If the receiving plate is oceanic, these processes occur under the sea and involve the thinner oceanic crust. If the receiving plate is continental, these processes will occur on land, producing very complex topography. Typically, the receiving plate is part oceanic and part continental.
It is not yet clear whether or not terranes can actually change plate boundaries themselves. Since terranes are not microplates, they have no "plate of their own". Yet if a plate boundary changes as a result of terrane accretion, this implies that some terranes do in fact include a plate fragment as well, or else that a hypothesized process known as "plating" creates new plate material beneath accreted terranes, or that the plate which delivered the terrane starts a new subduction zone behind the terrane while the old subduction zone is closed off below the place where the terrane sutured on to the receiving plate. In this latter instance which seems to be favored, not only does the terrane suture on, but so does the "stranded" fragment of the plate which delivered it and remains beneath it. Research is ongoing.
The concept of terranes developed from studies in the 1970s of the complicated Pacific Cordilleran ("backbone") orogenic margin of North America, a virtual geological lasagna that had remained an inexplicable apparent "irreducible complexity", 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 by 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 ‘welded’ 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, with a richly complicated history and structure, they may be called superterranes.[1]
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[edit] See also
- Geology of Victoria
- Avalonia
- Smartville Block
- Sonomia Terrane
- Narryer Gneiss Terrane
- Salinian Block
- Wrangellia Terrane
- Yakutat Block
[edit] Notes
[edit] External links
- Terrane: a definition
- New techniques for modelling terranes in three dimensions
- West Antarctica terrane analysis
- Examples of accreted terrane in Idaho
- Alaskan Terranes
[edit] References
- J.N. Carney et al., Precambrian Rocks of England and Wales, GCReg. volume 20 (ISBN 978-1861074874)
- Basin and Range, by John McPhee, published 1981 by Farrar, Straus and Giroux, New York.
- In Suspect Terrain by John McPhee, published 1983 by Farrar, Straus and Giroux, New York.
- Assembling California, by John McPhee, published 1993 by Farrar, Straus and Giroux, New York.
