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[[File:Subduction_polarity_reversal.svg|thumb|471x471px|The concept of flipping of subduction polarity]]
'''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|
Examples of subduction systems with subduction polarity reversal are:
* [[Caledonian orogeny|Caledonides]], Ireland<ref>{{Cite book|title=Arc-Continent Collision|
* [[Apennine Mountains|Alps-Apennines]], Italy<ref name=":43">{{Cite journal|
* [[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>
* [[Wetar]], eastern Indonesia<ref>{{Cite book|url=https://books.google.com/books?id=bKg_AAAAIAAJ
* [[Timor]], east Savu Sea<ref>{{Cite journal|
* [[Mediterranean Sea|Mediterranean]]<ref>{{Cite journal|title=Structure and dynamics of subducted lithosphere in the Mediterranean region|url=http://cat.inist.fr/?aModele=afficheN&cpsidt=4305568|journal=Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen|volume=95|issue=3|issn=0924-8323}}</ref>
* [[Geology of Taiwan|Taiwan]]<ref name=":132222"/><ref name=":43" /><ref name=":03">{{Cite journal|
== Background ==
The phenomenon of subduction polarity reversal has been identified in the collision of an intra-oceanic subduction system,<ref name=":322">{{Cite journal|
== Models of subduction polarity reversal ==
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# Subduction system ceases with the involvement of continental plate
# Old slab breaks off<ref name=":113" /><ref name=":132222" />
In addition to the criteria for the occurrence of subduction polarity reversal, some geologists have attempted to define controls of this phenomenon’s initiation. Zhang proposes that “the plastic strength and age of the overriding oceanic plate in the arc-continent collision system control the initiation modes.” When the whole overriding oceanic plate has a small plastic strength and younger oceanic plate it prefers a “spontaneous subduction polarity reversal”.<ref>Zhang, S., & Leng, W. (2021). Subduction polarity reversal: Induced or spontaneous? Geophysical Research Letters, 48, e2021GL093201. https://doi {{Webarchive|url=https://web.archive.org/web/20130711094433/https://doi/ |date=2013-07-11 }} . org/10.1029/2021GL093201</ref> This is because the lack in plastic strength allows negative buoyancy to overcome and “spontaneously initiate” subduction. while the stronger plastic strength and older oceanic plate prefers an “induced subduction polarity reversal. This is because the stronger plastic strength in the oceanic plate, the more it will resist a “spontaneous subduction”, making it necessary for a compression induced subduction polarity reversal.
Different models representing the subduction polarity reversal depends highly on parameters the Geologists considered. Here is the summary table showing the comparison models.
{| class="wikitable"
!Slab break-off
!Double convergence
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This model was developed by analyzing the geological cross section along the collision between [[Eurasian Plate|Eurasian plate]] and the [[Philippine Sea Plate|Philippine sea plate]], which is the ___location of an ongoing flipping of subduction polarity.<ref name=":132222" />
When two [[Oceanic crust|oceanic plates]] migrate towards each other, one plate overrides another forming a [[Subduction|subduction system]]. Later, a light and buoyant [[Continental margin|passive continental margin]] introduced into this system will cause the cessation of [[Subduction|subduction system]].<ref name=":132222" /> On one hand, the buoyant plate resists subduction beneath the overriding plate.<ref name=":132222" /> On the other hand, the dense oceanic [[Slab (geology)|slab]] at the subducting plate prefers to move downward.<ref name=":132222" /> These opposite forces will generate a [[Tension (physics)|tensile force]] or gravitational instability on the downward [[slab (geology)|slab]] and lead to the break-off of the slab.<ref name=":822">{{Cite book|url=https://books.google.com/books?id=7l9KrXgOELwC
=== Double convergence model ===
This model is developed based on the geological evolution of Alpine and Apennine subduction.<ref name=":103">{{Cite journal|
Similarly,
=== Lithosphere break-up ===
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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|
[[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)
== 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|
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|
[[File:Taiwan_geology.jpg|thumb|center|526x526px
|'''1) Cross-section A-A’''' <ref name=":132222"/> (Post-collision): The passive continental margin of [[Eurasian Plate|Eurasian plate]], a buoyant continental crust, overrides the [[Philippine Sea Plate|Philippine sea plate]] .The Eurasian plate is undergoing lithospheric stretching, forming the [[Okinawa Trough]].<br/>
'''2) Cross-section B-B’''':<ref name=":132222" /> The [[Philippine Sea Plate]] subducts beneath the [[Eurasian Plate|Eurasian plate]], and [[Ryukyu Trench|Ryukyu trench]] [[Oceanic trench#Trench rollback|roll-back]] leads to the extensional collapse of Taiwan orogenic wedge.<ref name=":52222"/> The direction of subduction changes in cross-section C-C'.<br/>
'''3) Cross-section C-C’''':<ref name=":132222" /> Drastic collision between two plate creates an accretionary wedge and develops orogenic belt. Taiwan orogens reached the maximum height with an equal amount of erosion and growth rate.<ref name=":15222">{{Cite journal|last=Suppe, J. (1984)|title=Kinematics of arc-continent collision, flipping of subduction, and backarc spreading near Taiwan|url=http://twgeoref.moeacgs.gov.tw/GipOpenWeb/imgAction?f=/1984/19840281/0021.PDF|journal=Mem. Geol. Soc. China|issue=6 |pages=21–33}}</ref> The angle of the slab is almost 80 degrees dipping downward.<ref name=":6222">{{Cite journal|
'''4) Cross-section D-D’''':<ref name=":132222" /> The [[Eurasian Plate|Eurasian plate]] is actively subducting into the [[Philippine Sea Plate|Philippine Sea plate]] at 80mm/year along the Manila Trench.<ref name=":52222" /> The slab is penetrating into the mantle and the volume of melt in mantle wedge keeps increasing. Meanwhile, the angle of subduction slab is not as steep as in cross-section C-C'.<ref name=":6222" /> The accretionary wedge was just developed.<br/>
'''5) Cross-section E-E’''' <ref name=":132222" />(Pre-Collision): Slab penetrates beneath the [[Philippine Sea Plate|Philippine Sea plate]] and brings hydrous materials to generate a [[mantle wedge]]<ref name=":132222" /> and [[Luzon Volcanic Arc|Luzon volcanic arc]].
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