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ob·duc·tion

(ŏb-dŭk′shən)n.

A geologic process in which the oceanic crust of the edge of a tectonic plate is thrust over the continental crust of the edge of another, adjacent plate.

[Latin obductiō, obductiōn-, act of covering or enveloping, from obductus, past participle of obdūcere, to draw forward, cover over : ob-, toward, before; see ob- + dūcere, to lead, draw; see deuk- in Indo-European roots.]

American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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Obduction is the overthrusting of continental crust by oceanic crust or mantle rocks at a convergent plate boundary, such as closing of an ocean or a mountain building episode. This process is uncommon because the denser oceanic lithosphere usually subducts underneath the less dense continental plate.^{[citation needed]}

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.^{[citation needed]}

Most obductions appear to have initiated at back-arc basins above the subduction zones during the closing of an ocean or an orogeny.^{[citation needed]}

Characteristic rocks[edit]

The characteristic rocks of obducted oceanic lithosphere are the ophiolites. Ophiolites are an assemblage of oceanic lithosphere rocks that have been emplaced onto a continent. This assemblage consists of deep-marine sedimentary rock (chert, limestone, clastic sediments), volcanic rocks (pillow lavas, glass, ash, sheeted dykes and gabbros) and peridotite (mantle rock).^[1]

Types of obductions[edit]

Upwedging in subduction zones[edit]

This process is operative beneath and behind the inner walls of oceanic trenches (subduction zone) where slices of oceanic crust and mantle are ripped from the upper part of the descending plate and wedged and packed in high pressure assemblages against the leading edge of the other plate.^[2]

Weakening and cracking of oceanic crust and upper mantle is likely to occur in the tensional regime. This results in the incorporation of ophiolite slabs into the overriding plate.^[2]

Progressive packing of ophiolite slices and arc fragments against the leading edge of a continent may continue over a long period of time and lead to a form of continental accretion.

Abduction Movie

Compressional telescoping onto Atlantic-type continental margins[edit]

The simplest form of this type of obduction may follow from the development of a subduction zone near the continental margin. Above and behind the subduction zone, a welt of oceanic crust and mantle rides up over the descending plate. The ocean, intervening between the continental margin and the subduction zone is progressively swallowed until the continental margin arrives at the subduction zone and a giant wedge or slice (nappe) of oceanic crust and mantle is pushed across the continental margin.^{[citation needed]} Because the buoyancy of the relatively light continental crust is likely to prohibit its extensive subduction, a flip in subduction polarity will occur yielding an ophiolite sheet lying above a descending plate.^{[citation needed]}

If however, a large tract of ocean intervenes between the continental margin the subduction zone, a fully developed arc and back arc basin may eventually arrive and collide with the continental margin. Further convergence may lead to overthrusting of the volcanic arc assemblage and may be followed by flipping the subduction polarity.

According to the rock assemblage as well as the complexly deformed ophiolite basement and arc intrusions, the Coastal Complex of western Newfoundland may well have been formed by this mechanism.^[2]

Gravity sliding onto Atlantic-type continental margins[edit]

This concept involves the progressive uplift of an actively spreading oceanic ridge, the detachment of slices from the upper part of the lithosphere and the subsequent gravity sliding of these slices onto the continental margin as ophiolites. This concept was advocated by Reinhardt ^[3] for the emplacement of the Semail Ophiolite complex in Oman and argued by Church ^[4] and Church and Stevens ^[5] for the emplacement of the Bay of Islands sheet in western Newfoundland. This concept has subsequently been replaced by hypotheses that advocate subduction of the continental margin beneath oceanic lithosphere.

Transformation of a spreading ridge to a subduction zone[edit]

Many ophiolite complexes were emplaced as thin hot obducted sheets of oceanic lithosphere shortly after their generation by plate accretion.^[6] The change from a spreading plate boundary to a subduction plate boundary may result from rapid rearrangement of relative plate motion. A transform fault may also become a subduction zone, with the side with the higher, hotter, thinner lithosphere riding over the lower, colder lithosphere. This mechanism would lead to obduction of ophiolite complex if it occurred near a continental margin.^[2]

Interference of a spreading ridge and a subduction zone[edit]

In the situation where a spreading ridge approaches a subduction zone, the ridge collides with the subduction zone, at which time there will develop a complex interaction of subduction-related tectonic sedimentary, and spreading-related tectonic igneous activity. The left-over ridge may either subduct or ride upward across the trench onto arc trench gap and arc terranes as a hot ophiolite slice. These two mechanisms are shown in figure 2 B and C.^[2]Two examples of this interaction of a ridge colliding into a trench are well documented. The first one is the progressive diminution of the Farallon plate off California. Ophiolite obduction by the above proposed mechanism would not be expected as the two plates share a dextral transform boundary. However, the major collision of the Kula/Pacific plate with the Alaskan/Aleutian resulted in the initiation of subduction of the Pacific plate beneath Alaska, with no sign of either obduction or indeed any major manifestation of a ridge being “swallowed”.^[2]

Obduction from rear-arc basin[edit]

Dewey and Bird ^[7] sugg