Most obductions appear to have initiated at back-arc basins above the subduction zones during the closing of an ocean or an orogeny.
When two continental plates collide, obduction of the oceanic crust between them is often a part of the resulting orogeny. 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. 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. John McPhee describes ophiolite formation by obduction as "where ocean crust slides into a trench and goes under a continent, a part of the crust - i.e., an ophiolite - is shaved off the top and ends up on the lip of the continent." This assemblage consists of deep-marine sedimentary rock ( chert, limestone, clastic sediments), volcanic rocks ( pillow lavas, volcanic glass, volcanic ash, sheeted dykes and gabbros) and peridotite (mantle rock). Obduction of oceanic lithosphere produces a characteristic set of rock types called an ophiolite. Obduction, which is less common, normally occurs in plate collisions at orogenic belts (where an oceanic plate that is subducting scrapes some of its material onto the continental plate) or back-arc basins (places where the edge of a continent is pulled away from the rest of the continent due to the stress of plate collision). When oceanic and continental plates converge, normally the denser oceanic crust sinks under the continental crust in the process of subduction. Obduction is a geological process whereby denser oceanic crust (and even upper mantle) is scraped off a descending ocean plate at a convergent plate boundary and thrust on top of an adjacent plate. ( August 2019) ( Learn how and when to remove this template message) Please help improve it to make it understandable to non-experts, without removing the technical details.
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