Editing Subduction (section)

==Structure of subduction zones==
===Arc-trench complex===
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The surface expressions of subduction zones are arc-trench complexes. On the ocean side of the complex, where the subducting plate first approaches the subduction zone, there is often an ''[[outer trench high]]'' or ''outer trench swell''. Here the plate shallows slightly before plunging downwards, as a consequence of the rigidity of the plate.<ref>{{cite journal |last1=Whitman |first1=Dean |title=The Isostatic Residual Gravity Anomaly of the Central Andes, 12° to 29° S: A Guide to Interpreting Crustal Structure and Deeper Lithospheric Processes |journal=International Geology Review |date=May 1999 |volume=41 |issue=5 |pages=457–475 |doi=10.1080/00206819909465152|bibcode=1999IGRv...41..457W |s2cid=129797807 }}</ref> The point where the slab begins to plunge downwards is marked by an ''[[oceanic trench]]''. Oceanic trenches are the deepest parts of the ocean floor.

Beyond the trench is the ''[[forearc]]'' portion of the overriding plate. Depending on sedimentation rates, the forearc may include an [[accretionary wedge]] of sediments scraped off the subducting slab and accreted to the overriding plate. However, not all arc-trench complexes have an accretionary wedge. Accretionary arcs have a well-developed forearc basin behind the accretionary wedge, while the forearc basin is poorly developed in non-accretionary arcs.{{sfn|Stern|2002|pp=25-26}}

Beyond the forearc basin, volcanoes are found in long chains called ''[[volcanic arc]]s''. The subducting basalt and sediment are normally rich in [[hydrous]] minerals and clays. Additionally, large quantities of water are introduced into cracks and fractures created as the subducting slab bends downward.<ref>{{cite journal |last1=Fujie |first1=Gou |display-authors=etal |year=2013 |title=Systematic changes in the incoming plate structure at the Kuril trench |journal=Geophysical Research Letters |volume=40 |issue=1 |pages=88–93 |doi=10.1029/2012GL054340 |bibcode=2013GeoRL..40...88F |doi-access=free }}</ref> During the transition from basalt to eclogite, these hydrous materials break down, producing copious quantities of water, which at such great pressure and temperature exists as a [[supercritical fluid]].{{sfn|Stern|2002|pp=6-10}} The supercritical water, which is hot and more buoyant than the surrounding rock, rises into the overlying mantle, where it lowers the melting temperature of the mantle rock, generating [[magma]] via [[flux melting]].{{sfn|Schmincke|2003|pp=18,113-126}} The magmas, in turn, rise as [[diapir]]s because they are less dense than the rocks of the mantle.{{sfn|Stern|2002|pp=19-22}} The mantle-derived magmas (which are initially basaltic in composition) can ultimately reach the Earth's surface, resulting in volcanic eruptions. The chemical composition of the erupting lava depends upon the degree to which the mantle-derived basalt interacts with (melts) Earth's crust or undergoes [[fractional crystallization (geology)|fractional crystallization]]. Arc volcanoes tend to produce dangerous eruptions because they are rich in water (from the slab and sediments) and tend to be extremely explosive.{{sfn|Stern|2002|p=27–28}} [[Krakatoa]], [[Nevado del Ruiz]], and [[Mount Vesuvius]] are all examples of arc volcanoes. Arcs are also associated with most [[ore]] deposits.{{sfn|Stern|2002|pp=19-22}}

Beyond the volcanic arc is a [[back-arc region]] whose character depends strongly on the angle of subduction of the subducting slab. Where this angle is shallow, the subducting slab drags the overlying continental crust partially with it, which produces a zone of shortening and crustal thickening in which there may be extensive [[Folding (geology)|folding]] and [[thrust fault]]ing. If the angle of subduction steepens or rolls back, the upper plate lithosphere will be put in [[Tension (geology)|tension]] instead, often producing a [[back-arc basin]].{{sfn|Stern|2002|p=31}}

===Deep structure===
The arc-trench complex is the surface expression of a much deeper structure. Though not directly accessible, the deeper portions can be studied using [[geophysics]] and [[geochemistry]]. Subduction zones are defined by an inclined zone of [[earthquake]]s, the [[Wadati–Benioff zone]], that dips away from the trench and extends down below the volcanic arc to the [[Transition zone (Earth)|660-kilometer discontinuity]]. Subduction zone earthquakes occur at greater depths (up to {{Convert|600|km|mi|abbr=on}}) than elsewhere on Earth (typically less than {{Convert|20|km|mi|abbr=on}} depth); such deep earthquakes may be driven by deep [[phase transformation]]s, [[thermal runaway]], or dehydration [[embrittlement]].<ref>{{cite journal | last1 = Frolich | first1 = C. | year = 1989 | title = The Nature of Deep Focus Earthquakes | journal = Annual Review of Earth and Planetary Sciences | volume =  17| pages =  227–254| doi = 10.1146/annurev.ea.17.050189.001303 | bibcode = 1989AREPS..17..227F}}</ref><ref>{{cite journal |last1=Hacker |first1=B. |display-authors=etal |year=2003 |title=Subduction factory 2. Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions? |url=http://www.geol.ucsb.edu/faculty/hacker/viz/Hacker03_SubFac2_Intermediate_depth_earthquakes.pdf |journal=Journal of Geophysical Research |volume=108 |issue=B1 |pages=2030 |doi=10.1029/2001JB001129 |bibcode=2003JGRB..108.2030H }}</ref> [[Seismic tomography]] shows that some slabs can penetrate the [[lower mantle]]<ref name=":0">{{cite journal |last1=Domeier |first1=Mathew |last2=Doubrovine |first2=Pavel V. |last3=Torsvik |first3=Trond H. |last4=Spakman |first4=Wim |last5=Bull |first5=Abigail L. |title=Global correlation of lower mantle structure and past subduction |journal=Geophysical Research Letters |date=28 May 2016 |volume=43 |issue=10 |pages=4945–4953 |doi=10.1002/2016GL068827 |pmid=31413424 |pmc=6686211 |bibcode=2016GeoRL..43.4945D |doi-access=free }}</ref><ref>{{cite journal |last1=Faccenna |first1=Claudio |last2=Oncken |first2=Onno |last3=Holt |first3=Adam F. |last4=Becker |first4=Thorsten W. |title=Initiation of the Andean orogeny by lower mantle subduction |journal=Earth and Planetary Science Letters |year=2017 |volume=463 |pages=189–201 |doi=10.1016/j.epsl.2017.01.041 |bibcode=2017E&PSL.463..189F |hdl=11590/315613 |hdl-access=free }}</ref> and sink clear to the [[core–mantle boundary]].<ref>{{cite journal |last1=Hutko |first1=Alexander R. |last2=Lay |first2=Thorne |last3=Garnero |first3=Edward J. |last4=Revenaugh |first4=Justin |title=Seismic detection of folded, subducted lithosphere at the core–mantle boundary |journal=Nature |date=2006 |volume=441 |issue=7091 |pages=333–336 |doi=10.1038/nature04757 |pmid=16710418 |bibcode=2006Natur.441..333H |s2cid=4408681 }}</ref> Here the residue of the slabs may eventually heat enough to rise back to the surface as [[mantle plume]]s.<ref>{{cite journal |last1=Li |first1=Mingming |last2=McNamara |first2=Allen K. |title=The difficulty for subducted oceanic crust to accumulate at the Earth's core-mantle boundary |journal=Journal of Geophysical Research: Solid Earth |date=2013 |volume=118 |issue=4 |pages=1807–1816 |doi=10.1002/jgrb.50156 |bibcode=2013JGRB..118.1807L |doi-access=free }}</ref>{{sfn|Stern|2002|p=1}}

===Subduction angle===
Subduction typically occurs at a moderately steep angle by the time it is beneath the volcanic arc. However, anomalous shallower angles of subduction are known to exist as well as some that are extremely steep.<ref>{{cite journal |last1=Zheng |first1=YF |last2=Chen |first2=RX |last3=Xu |first3=Z |last4=Zhang |first4=SB |year=2016 |title=The transport of water in subduction zones |journal=Science China Earth Sciences |volume=59 |issue=4 |pages=651–682 |doi=10.1007/s11430-015-5258-4 |bibcode=2016ScChD..59..651Z |s2cid=130912355 }}</ref>
* [[Flat slab subduction]] (subducting angle less than 30°) occurs when the slab subducts nearly horizontally. The relatively flat slab can extend for hundreds of kilometers under the upper plate. This geometry is commonly caused by the subduction of buoyant lithosphere due to thickened crust or warmer lithosphere. Recent studies have also shown a strong correlation that older and wider subduction zones are related to flatter subduction dips. This provides an explanation as to why flat subduction only presently occur in the eastern pacific as only these regions were old and wide enough to support flat slab subduction and why the Laramide flat slab subduction and South China flat slab subduction were possible.<ref>Schellart WP (2020) Control of Subduction Zone Age and Size on Flat Slab Subduction. ''Front. Earth Sci.'' 8:26. {{doi|10.3389/feart.2020.00026}}</ref> Hu ultimately proposes that a combination of subduction age and slab characteristics provide the strongest controls over subduction dips.<ref>Hu, J., & Gurnis, M. (2020). Subduction duration and slab dip. Geochemistry, Geophysics, Geosystems, 21, e2019GC008862. https://doi.org/  10.1029/2019GC008862</ref> Because subduction of slabs to depth is necessary to drive subduction zone volcanism, flat-slab subduction can be invoked to explain [[volcanic gap]]s.

Flat-slab subduction is ongoing beneath part of the [[Andes]], causing segmentation of the [[Andean Volcanic Belt]] into four zones. The flat-slab subduction in northern Peru and the [[Norte Chico, Chile|Norte Chico]] region of Chile is believed to be the result of the subduction of two buoyant aseismic ridges, the [[Nazca Ridge]] and the [[Juan Fernández Ridge]], respectively. Around [[Taitao Peninsula]] flat-slab subduction is attributed to the subduction of the [[Chile Rise]], a [[mid-ocean ridge|spreading ridge]].<ref>{{cite journal |last1=Sillitoe |first1=Richard H. |title=Tectonic segmentation of the Andes: implications for magmatism and metallogeny |journal=Nature |date=August 1974 |volume=250 |issue=5467 |pages=542–545 |doi=10.1038/250542a0 |bibcode=1974Natur.250..542S |s2cid=4173349 }}</ref><ref>{{cite journal |last1=Jordan |first1=Teresa E. |last2=Isacks |first2=Bryan L. |last3=Allmendinger |first3=Richard W. |last4=Brewer |first4=Jon A. |last5=Ramos |first5=Victor A. |last6=Ando |first6=Clifford J. |title=Andean tectonics related to geometry of subducted Nazca plate |journal=GSA Bulletin |date=1 March 1983 |volume=94 |issue=3 |pages=341–361 |doi=10.1130/0016-7606(1983)94<341:ATRTGO>2.0.CO;2 |bibcode=1983GSAB...94..341J }}</ref>

The [[Laramide Orogeny]] in the [[Rocky Mountains]] of the United States is attributed to flat-slab subduction.<ref>{{Cite journal |author1=W. P. Schellart |author2=D. R. Stegman |author3=R. J. Farrington |author4=J. Freeman |author5=L. Moresi |name-list-style=amp |title=Cenozoic Tectonics of Western North America Controlled by Evolving Width of Farallon Slab |journal=Science |date=16 July 2010 |volume=329 |issue=5989 |pages=316–319 |doi=10.1126/science.1190366 |pmid=20647465 |bibcode=2010Sci...329..316S |s2cid=12044269 }}
</ref> During this orogeny, a broad volcanic gap appeared at the southwestern margin of North America, and deformation occurred much farther inland; it was during this time that the [[basement (geology)|basement]]-cored mountain ranges of Colorado, Utah, Wyoming, South Dakota, and New Mexico came into being. The most massive subduction zone earthquakes, so-called "megaquakes", have been found to occur in flat-slab subduction zones.<ref>{{Cite journal|url=https://www.eurekalert.org/pub_releases/2016-11/uoo-fcm112216.php |title=Fault curvature may control where big quakes occur, Eurekalert 24-NOV-2016 |journal=Science |volume=354 |issue=6315 |pages=1027–1031 |doi=10.1126/science.aag0482 |pmid=27885027 |date=2016-11-24 |access-date=2018-06-05|bibcode=2016Sci...354.1027B |last1=Bletery |first1=Quentin |last2=Thomas |first2=Amanda M. |last3=Rempel |first3=Alan W. |last4=Karlstrom |first4=Leif |last5=Sladen |first5=Anthony |last6=De Barros |first6=Louis |doi-access=free }}</ref>
* Steep-angle subduction (subducting angle greater than 70°) occurs in subduction zones where Earth's [[oceanic crust]] and lithosphere are cold and thick and have, therefore, lost buoyancy. Recent studies have also correlated steep angled subduction zones with younger and less extensive subduction zones. This would explain why most modern subduction zones are relatively steep. The steepest dipping subduction zone lies in the [[Mariana Trench]], which is also where the oceanic lithosphere of [[Jurassic]] age is the oldest on Earth exempting [[ophiolite]]s. Steep-angle subduction is, in contrast to flat-slab subduction, associated with [[back-arc]] extension<ref>{{Cite journal |author1=Lallemand, Serge |author2=Heuret, Arnauld |author3=Boutelier, David |date=8 September 2005 |url=http://perso-sdt.univ-brest.fr/~jacdev/pdf/1.pdf |title=On the relationships between slab dip, back-arc stress, upper plate absolute motion, and crustal nature in subduction zones |journal=Geochemistry, Geophysics, Geosystems |volume=6 |issue=9 |page=Q09006 |doi=10.1029/2005GC000917 |bibcode= 2005GGG.....6.9006L|doi-access=free }}</ref> of the upper plate, creating volcanic arcs and pulling fragments of continental crust away from continents to leave behind a [[marginal sea]].