Editing Subduction (section)

===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]].