Obduction

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Obduction is the overthrusting of continental crust by oceanic crust or mantle rocks at a convergent plate boundary. It can occur during an orogeny, or mountain-building episode.

Obduction occurs where a fragment of continental crust is caught in a subduction zone with resulting overthrusting of oceanic mafic and ultramafic rocks from the mantle onto the continental crust. Obduction often occurs where a small tectonic plate is caught between two larger plates, with the crust (both island arc and oceanic) welding onto an adjacent continent as a new terrane. When two continental plates collide, obduction of the oceanic crust between them is often a part of the resulting orogeny.

The characteristic rocks of obducted oceanic crust are the ophiolites; consisting of basalt, gabbro, peridotite, dunite, and eclogite. There are many examples of oceanic crustal rocks and deeper mantle rocks that have been obducted and exposed at the surface worldwide. New Caledonia is one example of recent obduction. The Klamath Mountains of northern California contain several obducted oceanic slabs. Obducted fragments also are found in Oman, Cyprus, Newfoundland, New Zealand, the Alps of Europe, and the Appalachians of eastern North America.

Most obductions appear to have initiated at back-arc basins above subduction zones[citation needed]. These basins occur where the edge of the continent collapses seawards, and extension in the back-arc basin enhances volcanism and crustal accretion. While the continental crust is being subducted, the upper lithosphere is exposed, and ophiolitic volcanism accretes metamorphic rock series. As orogeny succeeds subduction, the ophiolites and their metamorphic basement end up atop mountain ranges.

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