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In structural geology, a suture is a joining together along a major fault zone, of separate terranes, tectonic units that have different plate tectonic, metamorphic and paleogeographic histories. The suture is often represented on the surface by an orogen or mountain range.  The term was borrowed from surgery where it describes the sewing together of two pieces of tissue, but the sutures of the skull, where separate plates of bone have fused, may be a better metaphor.
Outcrops of sutures can vary in width from a few hundred meters to a couple of kilometers. They can be networks of mylonitic shear zones or brittle fault zones, but are usually both. Sutures are usually associated with igneous intrusions and tectonic lenses with varying kinds of lithologies from plutonic rocks to ophiolitic fragments.
An example from Great Britain is the Iapetus Suture which, though now concealed beneath younger rocks, has been determined by geophysical means to run along a line roughly parallel with the Anglo-Scottish border and represents the joint between the former continent of Laurentia to the north and the former micro-continent of Avalonia to the south. Avalonia is in fact a plain which dips steeply northwestwards through the crust, underthrusting Laurentia.
When used in paleontology, suture can also refer to fossil exoskeletons, as in the suture line, a division on a trilobite between the free cheek and the fixed cheek; this suture line allowed the trilobite to perform ecdysis (the shedding of its skin).
- There is a "difference between the geophysical and geological definition of suture zones and terrane boundaries. The geophysical definitions are generally based upon the integration of a physical response over the thickness of the crust, whereas the geological definition is almost always as a line or zone at the Earth's surface." Harris, Anthony L. and Fettes, D. J. (1988) The Caledonian-Appalachian orogen published for the Geological Society by Blackwell Scientific Publications, Oxford, England, page 4, ISBN 978-0-632-01796-6
- Oliver, G. J. H.; Stone, P. and Bluck, B. J. (2002) "The Ballantrae Complex and Southern Uplands terrane" pp. 167–200 In Trewin, N. H. (editor) (2002) The Geology of Scotland The Geological Society, London, page 191, ISBN 978-1-86239-126-0
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