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'''Subduction polarity reversal''' is a geologic process in which two converging plates switch roles: The over-lying plate becomes the down-going plate, and vice versa. There are two basic units which make up a [[Subduction|subduction zone]]. This consists of an overriding plate and the subduction plate.<ref name=":142">{{Cite book|url=https://www.springer.com/gp/book/9783540885573|title=Arc-Continent Collision {{!}} Dennis Brown {{!}} Springer|isbn=9783540885573|publisher=Springer|year=2011|series=Frontiers in Earth Sciences}}</ref> Two plates move towards each other due to [[Tectonics|tectonic forces]].<ref name=":142" /> The overriding plate will be on the top of the subducting plate.<ref name=":142" /> This type of tectonic interaction is found at many [[Plate tectonics|plate boundaries]].<ref name=":142" />
However, some geologists propose that the roles of the overriding plate and subducting plate do not remain the same indefinitely.<ref name=":113">{{Cite journal|last1=Chemenda|first1=A. I.|last2=Yang|first2=R. -K.|last3=Stephan|first3=J. -F.|last4=Konstantinovskaya|first4=E. A.|last5=Ivanov|first5=G. M.|date=2001-04-10|title=New results from physical modelling of arc–continent collision in Taiwan: evolutionary model|journal=Tectonophysics|volume=333|issue=1–2|pages=159–178|doi=10.1016/S0040-1951(00)00273-0|bibcode=2001Tectp.333..159C}}</ref> Their roles will swap, which means the plate originally subducting beneath will become the overriding plate.<ref name=":113" /> This phenomenon is called '''subduction switch''',<ref>{{cite journal|last1=Willett|first1=S. D.|last2=Beaumont|first2=C.|title=Subduction of Asian lithospheric mantle beneath Tibet inferredfrom models of continental collision|journal=Nature|date=1994-06-23|volume=369|issue=6482|pages=642–645|doi=10.1038/369642a0|bibcode=1994Natur.369..642W|s2cid=4239739}}</ref> '''the flipping of subduction polarity'''<ref name=":132222">{{Cite journal|last1=Teng|first1=Louis S.|last2=Lee|first2=C. T.|last3=Tsai|first3=Y. B.|last4=Hsiao|first4=Li-Yuan|date=2000-02-01|title=Slab breakoff as a mechanism for flipping of subduction polarity in Taiwan|journal=Geology|language=en|volume=28|issue=2|pages=155–158|doi=10.1130/0091-7613(2000)28<155:sbaamf>2.0.co;2|bibcode=2000Geo....28..155T |issn=0091-7613|url=http://ntur.lib.ntu.edu.tw//handle/246246/172449 }}</ref> or '''subduction polarity reversal'''.<ref name=":113" />
Examples of subduction systems with subduction polarity reversal are:
* [[Caledonian orogeny|Caledonides]], Ireland<ref>{{Cite book|title=Arc-Continent Collision|last1=Ryan|first1=P. D.|last2=Dewey|first2=J. F.|chapter=Arc–Continent Collision in the Ordovician of Western Ireland: Stratigraphic, Structural and Metamorphic Evolution |date=2011-01-01|publisher=Springer Berlin Heidelberg|isbn=9783540885573|series=Frontiers in Earth Sciences|pages=373–401|language=en|doi=10.1007/978-3-540-88558-0_13}}</ref>
* [[Apennine Mountains|Alps-Apennines]], Italy<ref name=":43">{{Cite journal|last1=Molli|first1=G.|last2=Malavieille|first2=J.|date=2010-09-28|title=Orogenic processes and the Corsica/Apennines geodynamic evolution: insights from Taiwan|journal=International Journal of Earth Sciences|language=en|volume=100|issue=5|pages=1207–1224|doi=10.1007/s00531-010-0598-y|issn=1437-3254|bibcode=2011IJEaS.100.1207M|s2cid=129517282}}</ref>
* [[Kamchatka Peninsula|Kamchatka]], Russia<ref name=":54">{{Cite journal|last=Konstantinovskaia|first=E. A|date=2001-04-10|title=Arc–continent collision and subduction reversal in the Cenozoic evolution of the Northwest Pacific: an example from Kamchatka (NE Russia)|journal=Tectonophysics|volume=333|issue=1–2|pages=75–94|doi=10.1016/S0040-1951(00)00268-7|bibcode=2001Tectp.333...75K}}</ref>
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The initial setting of the simulated [[Subduction|subduction zone]] model is confined by two pistons. The piston connected to the overriding plate is locked, while the piston linking to subducting plate is subjected to a constant rate of compression.<ref name=":113" /> More importantly, there is a relatively thin [[Volcanic arc|magmatic arc]] and pre-existing fault dipping towards the subducting plate at the overriding plate.<ref name=":113" /> The detachment of the pre-existing fault occurs when buoyant [[continental margin]] is in contact with the overriding plate.<ref name=":113" /> It is because the buoyant margin resists [[subduction]] and significantly increases the [[Friction|frictional force]] in the contact region.<ref name=":113" /> The subduction then stops. Subsequently, the new subducting slab develops at an overriding plate with the continuous compression.<ref name=":113" /> The new developing slab eventually penetrates and breaks the old slab.<ref name=":113" /> A new subduction zone is formed with an opposite polarity to the previous one.<ref name=":113" />
In reality, the magmatic arc is a relatively weak zone at the overriding plate because it has a thin lithosphere and is further weakened by high heat flow<ref>{{Cite journal|last1=Currie|first1=Claire A.|last2=Hyndman|first2=Roy D.|date=2006-08-01|title=The thermal structure of subduction zone back arcs|journal=Journal of Geophysical Research: Solid Earth|language=en|volume=111|issue=B8|pages=B08404|doi=10.1029/2005JB004024|issn=2156-2202|bibcode=2006JGRB..111.8404C|doi-access=free}}</ref><ref>{{Cite journal|last1=Currie|first1=C. A|last2=Wang|first2=K|last3=Hyndman|first3=Roy D|last4=He|first4=Jiangheng|date=2004-06-30|title=The thermal effects of steady-state slab-driven mantle flow above a subducting plate: the Cascadia subduction zone and backarc|journal=Earth and Planetary Science Letters|volume=223|issue=1–2|pages=35–48|doi=10.1016/j.epsl.2004.04.020|bibcode=2004E&PSL.223...35C}}</ref> and hot fluid.<ref>{{Cite journal|last1=Arcay|first1=D.|last2=Doin|first2=M.-P.|last3=Tric|first3=E.|last4=Bousquet|first4=R.|last5=de Capitani|first5=C.|date=2006-02-01|title=Overriding plate thinning in subduction zones: Localized convection induced by slab dehydration|journal=Geochemistry, Geophysics, Geosystems|language=en|volume=7|issue=2|pages=Q02007|doi=10.1029/2005GC001061|issn=1525-2027|bibcode=2006GGG.....7.2007A|s2cid=135199593 |url=https://hal.archives-ouvertes.fr/hal-00407579/file/Geochem%20Geophys%20Geosyst%20-%202006%20-%20Arcay%20-%20Overriding%20plate%20thinning%20in%20subduction%20zones%20Localized%20convection%20induced%20by.pdf }}</ref><ref>{{Cite journal|last1=Honda|first1=Satoru|last2=Yoshida|first2=Takeyoshi|date=2005-01-01|title=Application of the model of small-scale convection under the island arc to the NE Honshu subduction zone|journal=Geochemistry, Geophysics, Geosystems|language=en|volume=6|issue=1|pages=Q01002|doi=10.1029/2004GC000785|issn=1525-2027|bibcode=2005GGG.....6.1002H|s2cid=134357121 }}</ref> Pre-existing faults in this simulation are also common in the magmatic arc.<ref>{{Cite journal|last1=Toth|first1=John|last2=Gurnis|first2=Michael|date=1998-08-10|title=Dynamics of subduction initiation at preexisting fault zones|journal=Journal of Geophysical Research: Solid Earth|language=en|volume=103|issue=B8|pages=18053–18067|doi=10.1029/98JB01076|issn=2156-2202|bibcode=1998JGR...10318053T|url=https://authors.library.caltech.edu/38328/1/1998_TothGurnis_JGR.pdf|doi-access=free}}</ref> This experiment is a successful analogy to subduction polarity reversal happening at Kamchatka in early Eocene<ref name=":54" /><ref>{{Cite journal|last=Konstantinovskaia|first=Elena A|date=2000-10-15|title=Geodynamics of an Early Eocene arc–continent collision reconstructed from the Kamchatka Orogenic Belt, NE Russia|journal=Tectonophysics|volume=325|issue=1–2|pages=87–105|doi=10.1016/S0040-1951(00)00132-3|bibcode=2000Tectp.325...87K}}</ref> and the active example at Taiwan region<ref name=":113" /><ref name=":03" /> as well as at Timor.<ref>{{cite journal|last1=Silver|first1=Eli A.|last2=Reed|first2=Donald|last3=McCaffrey|first3=Robert|last4=Joyodiwiryo|first4=Yoko|title=Back arc thrusting in the Eastern Sunda Arc, Indonesia: A consequence of arc-continent collision|journal=Journal of Geophysical Research: Solid Earth|date=1983-09-10|volume=88|issue=B9|pages=7429–7448|doi=10.1029/JB088iB09p07429|bibcode=1983JGR....88.7429S}}</ref><ref>{{Cite journal|last1=Snyder|first1=D. B.|last2=Prasetyo|first2=H.|last3=Blundell|first3=D. J.|last4=Pigram|first4=C. J.|last5=Barber|first5=A. J.|last6=Richardson|first6=A.|last7=Tjokosaproetro|first7=S.|date=1996-02-01|title=A dual doubly vergent orogen in the Banda Arc continent-arc collision zone as observed on deep seismic reflection profiles|journal=Tectonics|language=en|volume=15|issue=1|pages=34–53|doi=10.1029/95TC02352|issn=1944-9194|bibcode=1996Tecto..15...34S}}</ref>
[[File:Lithospheric_break-up_model_setup.svg|thumb|444x444px|A. Chemenda's Experiment setup of lithosphere break-up model: White colour indicates the oceanic plate ( Higher density) ; Brown colour indicates the continental plate ( Lower density) ;Green colour shows the pre-existing fault ; The plates represented by hydrocarbons floats at the asthenosphere represented by water.|center]][[File:Lithoshperic_break-up.gif|thumb|444x444px|The evolution diagram showing how the subduction reversal initiated by a pre-existing fault at the overriding plate. 1: Compression pushing ; 2: New slab develops with the failure of the fault ; 3: New slab penetrates ; 4: New slab breaks the old slab|center]]
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== Taiwan as an active example of flipping of subduction reversal ==
[[File:Taiwan_222222.jpg|thumb|382x382px|Map of Taiwan shows the ___location of geological cross-section and the major subuductions]]
A sharp contrast of landforms in Taiwan lures many people to investigate. The northern part of Taiwan has many flat plains such as Ilan Plain and Pingtung Plain,<ref name=":52222">{{Cite journal|last1=Angelier|first1=Jacques|last2=Chang|first2=Tsui-Yü|last3=Hu|first3=Jyr-Ching|last4=Chang|first4=Chung-Pai|last5=Siame|first5=Lionel|last6=Lee|first6=Jian-Cheng|last7=Deffontaines|first7=Benoît|last8=Chu|first8=Hao-Tsu|last9=Lu|first9=Chia-Yü|date=2009-03-10|title=Does extrusion occur at both tips of the Taiwan collision belt? Insights from active deformation studies in the Ilan Plain and Pingtung Plain regions|journal=Tectonophysics|series=Geodynamics and active tectonics in East Asia|volume=466|issue=3–4|pages=356–376|doi=10.1016/j.tecto.2007.11.015|bibcode=2009Tectp.466..356A|url=http://ntur.lib.ntu.edu.tw//handle/246246/172548 }}</ref> while the southern part of Taiwan is concentrated with many high mountains like [[Yushan (mountain)|Yushan]] reaching about 3950m. This huge difference in topography is the consequence of '''the flipping of subduction polarity'''.<ref name=":132222"/> Most of models studying this phenomenon will focus on an active collision in Taiwan which appears to reveal the incipient stages of subduction reversal.<ref name=":132222"/><ref name=":03" /><ref name=":322"/><ref name=":23" /><ref name=":73" /><ref name=":93" />
The collision of N- trending Luzon arc in [[Philippine Sea Plate|Philippine Sea plate]] (PP) with E-trending [[Eurasian Plate|Eurasian plate]] (EP) started at mid-Miocene<ref name=":132222"/> forming an intra-oceanic subduction system.<ref name=":322" /><ref>{{Cite journal|last1=Leat|first1=P. T.|last2=Larter|first2=R. D.|date=2003-01-01|title=Intra-oceanic subduction systems: introduction|url=http://sp.lyellcollection.org/content/219/1/1|journal=Geological Society, London, Special Publications|language=en|volume=219|issue=1|pages=1–17|doi=10.1144/GSL.SP.2003.219.01.01|issn=0305-8719|bibcode=2003GSLSP.219....1L|s2cid=131046715}}</ref> Taiwan was formed by this process. The south–north topographic difference in Taiwan is like a story book telling the evolution in subduction zone. The [[Philippine Sea Plate|Philippine Sea plate]] subducts below the [[Eurasian Plate|Eurasian plate]] at south-west part of WEP (Western edge of north-dipping Philippine Sea Plate),<ref name=":132222"/> and the latter overrides the former at north east part of WEP.<ref name=":132222"/> The collision between two plates started at the Northern Taiwan and propagated south with the younger region at the southern part. Each incipient stage of subduction reversal process could be studied by correlating cross-sections in various parts of Taiwan.<ref>{{Cite journal|last1=Van Avendonk|first1=Harm J. A.|last2=McIntosh|first2=Kirk D.|last3=Kuo-Chen|first3=Hao|last4=Lavier|first4=Luc L.|last5=Okaya|first5=David A.|last6=Wu|first6=Francis T.|last7=Wang|first7=Chien-Ying|last8=Lee|first8=Chao-Shing|last9=Liu|first9=Char-Shine|date=2016-01-01|title=A lithospheric profile across northern Taiwan: from arc-continent collision to extension|journal=Geophysical Journal International|language=en|volume=204|issue=1|pages=331–346|doi=10.1093/gji/ggv468|issn=0956-540X|bibcode=2016GeoJI.204..331V|doi-access=free}}</ref>
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