Editing Obduction

{{Short description|Overthrusting of oceanic lithosphere onto continental lithosphere at a convergent plate boundary}}
{{distinguish|Abduction (disambiguation){{!}}Abduction}}
{{about|the geological process|the video game|Obduction (video game)|the medical procedure|autopsy}}
'''Obduction''' is a geological process whereby denser [[oceanic crust]] (and even upper [[mantle (geology)|mantle]]) is scraped off a descending ocean [[Plate tectonics|plate]] at a [[Convergent boundary|convergent plate boundary]] and [[thrust fault|thrust]] on top of an adjacent plate.<ref>{{Cite web|title=Obduction|url=https://www.dictionary.com/browse/obduction}}</ref><ref>{{Cite web|title=Plate Tectonics > Glossary > M – R|url=https://www.geolsoc.org.uk/Plate-Tectonics/Glossary/M-R}}</ref> When oceanic and continental plates converge, normally the denser oceanic crust sinks under the [[continental crust]] in the process of [[subduction]].<ref>{{Cite journal|title=Geodynamic models of continental subduction and obduction of overriding plate forearc oceanic lithosphere on top of continental crust|year=2015|doi=10.1002/2015TC003884|url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015TC003884|url-status=dead|last1=Edwards|first1=Sarah J.|last2=Schellart|first2=Wouter P.|last3=Duarte|first3=Joao C.|journal=Tectonics|volume=34|issue=7|pages=1494–1515|bibcode=2015Tecto..34.1494E|s2cid=129467525|access-date=2021-09-28|archive-date=2021-09-28|archive-url=https://web.archive.org/web/20210928074859/https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015TC003884|doi-access=free}}</ref> Obduction, which is less common, normally occurs in plate collisions at [[orogenic belts]] (some of the material from the subducting oceanic plate is emplaced onto the continental plate)<ref name="Dewey, J. F. 1975">Dewey, J. F., 1975. The role of ophiolite obduction in the evolution of the Appalachian/Caledonian orogenic belt. In: N. Bogdanov (editor), Ophiolites in the Earth’s Crust. Acad. Sci. U.S.S.R. (in press)</ref> or [[back-arc basins]] (regions where the edge of a continent is pulled away from the rest of the continent due to the stress of plate collision).<ref>{{cite journal |last1= Scliffke|first1= Nicholas|last2= van Hunen|first2= Jeroen|last3= Gueydan|first3= Frédéric|last4= Magni|first4= Valentina|last5= Allen|first5= Mark B.|date= 2021-08-12|title= Curved orogenic belts, back-arc basins, and obduction as consequences of collision at irregular continental margins|journal= Geology|volume= 49|issue= 12|pages= 1436–1440|doi= 10.1130/G48919.1|bibcode= 2021Geo....49.1436S|s2cid= 238718200|doi-access= free}}</ref>

Obduction of oceanic [[lithosphere]] produces a characteristic set of rock types called an [[ophiolite]]. This assemblage consists of deep-marine sedimentary rock ([[chert]], [[limestone]], [[Clastic rock|clastic]] sediments), volcanic rocks ([[pillow lava]]s, [[volcanic glass]], [[volcanic ash]], [[sheeted dyke complex|sheeted dykes]] and [[gabbro]]s) and [[peridotite]] (mantle rock).<ref name="Robinson et al. 2008">{{cite journal|last1=Robinson|first1=Paul T.|last2=Malpas|first2=John|last3=Dilek|first3=Yildirim|last4=Zhou|first4=Mei-fu|title=The significance of sheeted dike complexes in ophiolites|journal=GSA Today|date=2008|volume=18|issue=11|pages=4–10|doi=10.1130/GSATG22A.1|bibcode=2008GSAT...18k...4R |url=http://www.geosociety.org/gsatoday/archive/18/11/pdf/i1052-5173-18-11-4.pdf|doi-access=free}}</ref> John McPhee describes ophiolite formation by obduction as "where ocean crust slides into a trench and goes under a continent, [and] a part of the crust—i.e., an ophiolite—is shaved off the top and ends up on the lip of the continent."<ref>{{cite book |last= McPhee|first= John|date= 1998|title= Annals of the Former World|location= New York|publisher= Farmer, Strauss, and Giroux|page= 505}}</ref>

Obduction can occur where a fragment of continental [[crust (geology)|crust]] is caught in a subduction zone with resulting overthrusting of oceanic [[mafic]] and [[Ultramafic rock|ultramafic]] rocks from the mantle onto the continental crust. Obduction often occurs where a small [[Plate tectonics|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|date=August 2012}}

==Formation types==

===Upwedging in subduction zones===
This process is operative beneath and behind the inner walls of [[oceanic trench]]es (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.<ref name="Dewey">Dewey, J. F., 1976. Ophiolite Obduction. Tectonophysics, v. 31, p.93-120.</ref>

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.<ref name="Dewey" /> 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 (geology)|accretion]].

===Compressional telescoping onto Atlantic-type continental margins===
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|date=August 2012}} 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|date=February 2014}}

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 [[Subduction polarity reversal|flipping of the subduction polarity]]. According to the rock assemblage as well as the complexly deformed ophiolite [[Basement (geology)|basement]] and arc intrusions, the Coastal Complex of western Newfoundland may well have been formed by this mechanism.<ref name="Dewey" />

===Gravity sliding onto Atlantic-type continental margins===
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<ref name="Reinhardt">Reinhardt, B.M., 1969. On the genesis and emplacement of ophiolites in the Oman Mountains geosyncline. Schweiz. Mineral. Petrog. Mitt., 49:1-30</ref> for the emplacement of the [[Semail Ophiolite]] complex in [[Oman]] and argued by Church<ref>Church, W. R., 1972. Ophiolite: its definition, origin as oceanic crust, and mode of emplacement in orogenic belts, with special reference to the Appalachians. Dep. Energy, Mines Resourc. Can., Publ., 42:71-85.</ref> and Church and Stevens<ref>Church, W.R., and Stevens, R.K., 1971. Early Paleozoic ophiolite complexes of the Newfoundland Appalachians as mantle-oceanic crust sequences. J. Geophys. Res., 76:1460-1466.</ref> for the emplacement of the [[Bay of Islands, Newfoundland and Labrador|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===
Many ophiolite complexes were emplaced as thin, hot obducted sheets of oceanic lithosphere shortly after their generation by plate accretion.<ref name="Dewey, J. F. 1975"/> 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.<ref name="Dewey" />

===Interference of a spreading ridge and a subduction zone===
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 rock 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.<ref name="Dewey" />

A potential example is the progressive diminution of the [[Farallon plate]] off California. Ophiolite obduction would not be expected as the two plates share a dextral transform boundary. However, the major collision of the [[Kula plate|Kula]]/[[Pacific plate|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".<ref name="Dewey" />

===Rear-arc basin===
Dewey and Bird<ref name="Dewey_a">Dewey, J. F. and Bird, J.M., 1971. Origin and emplacement of the ophiolite suite: Appalachian ophiolites in Newfoundland. J. Geophys. Res., 76:3179-3206.</ref> suggest that a common form of ophiolite obduction is related to the closure of rear-arc marginal basins and that, during such closure by subduction, slices of oceanic crust and mantle may be expelled onto adjacent continental forelands and emplaced as ophiolite sheets. In the high heat-flow region of a volcanic arc and rear-arc basin the lithosphere is particularly thin.

This thin lithosphere may preferentially fail along gently dipping thrust surface if a compressional stress is applied to the region. Under these circumstances, a thin sheet of lithosphere may become detached and begin to ride over adjacent lithosphere to finally become emplaced as a thin ophiolite sheet on the adjacent continental foreland.<ref name="Dewey" /> This mechanism is a form of plate convergence where a thin, hot layer of oceanic lithosphere is obducted over cooler and thicker lithosphere.

===Continental collision===
As an ocean is progressively trapped in between two colliding continental lithospheres, the rising wedges of oceanic crust and mantle rise are caught between the continental plates and detach and begin to move up the advancing continental rise. Continued convergence may lead to the overthrusting of the arc-trench gap and eventually overthrusting of the metamorphic [[Igneous intrusion|plutonic]] and volcanic rocks of the volcanic arc.

Following total subduction of an oceanic tract, continuing convergence may lead to a further sequence of intra-continental mechanisms of crustal shortening. This mechanism is thought to be responsible for the various ocean basins of the Mediterranean region. The [[Alpide belt]] is believed to register a complex history of plate interactions during the general convergence of the [[Eurasian plate|Eurasian]] and [[African plate|African]] plates.<ref name="Dewey" />

==Examples==
There are few continental plates being obducted under an oceanic plate known today, but in the past it appears to have happened a number of times. Thus there are examples of oceanic crustal rocks and deeper mantle rocks that have been obducted and are now exposed at the surface, worldwide. [[Geology of New Caledonia|New Caledonia]] is one example of recent obduction. The [[Klamath Mountains]] of northern California contain several obducted oceanic slabs, most famously the [[Coast Range Ophiolite]]. Obducted fragments also are found in the [[Semail Ophiolite|Hajar Mountains]] of Oman,<ref name="Reinhardt" /> the [[Troodos Mountains#Geology|Troodos Mountains]] of [[Cyprus]], [[Newfoundland]],<ref name="Dewey_a" /> [[New Zealand]], the [[Alps]] of Europe, the [[Shetland]] islands of [[Unst#Geography and geology|Unst]] and Fetlar, [[Leka (island)|Leka]] island in Norway, and the [[Blue Ridge Ophiolite]] in the [[Appalachian Mountains]] of eastern North America.

==See also==
* {{annotated link|List of tectonic plate interactions}}

==References==
{{reflist}}

{{Structural geology}}

[[Category:Plate tectonics]]
[[Category:Geological processes]]