Obduction
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Obduction is the overthrusting of continental crust by oceanic crust or mantle rocks at a destructive plate boundary. It can occur during an orogeny.
Obduction occurs where a fragment of continental crust is caught 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, becoming attached as a new terraine to an adjacent continent. When two continental plates collide obduction of the oceanic crust between is often a part of the resulting orogeny, or mountain building episode. 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. The characteristic rocks of the 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.
It seems that most obductions are initiated at supra-subduction, back-arc basins. These basins are caused where the edge of the continent collapses seawards, and extension in the back-arc basin enhances volcanism and crustal accretion. Since while the continental crust collapses the upper part of the ductile lithosphere, namely the upper lithospheric mantle, is exposed, and the ophiolitic volcanism accretes on metamorphic lithologic series. As the subduction turns into mountain-building, the ophiolites and their metamorphic basement find their way to mountain tops.
