Subduction polarity reversal: Difference between revisions

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== Models of subduction polarity reversal ==

Even though many geologists agree that after the involvement of buoyant [[continental crust]], subduction polarity reversal may occur, they have different opinions towards the mechanisms leading to the change of subduction direction. Thus, there is no single modelsmodel to represent subduction polarity reversal. How geologists develop the models depends on the parameters they focus on.<ref name=":142" /> Some geologists attempt to construct the models of subduction reversal by laboratory experiments<ref name=":113" /><ref name=":322"/><ref name=":23" /> or observations.<ref name=":132222"/><ref name=":103" /> There are three common models: [[#Slab break-off|slab break-off]], [[#Double convergence model|double convergence]] and [[#Lithosphere break-up|lithospheric break-up]].

The models of slab-break up and double convergence are based on observations by geologists, and the lithosphere break-up model is based on the experimental [[simulation]].

=== Slab break-off ===

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]] being introduced into this system will therefore 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]] and lead to the break-off of the [[Slab (geology)|slab]].<ref name=":822">{{Cite book|url=https://books.google.com/books?hl=en&lr=&id=7l9KrXgOELwC&oi=fnd&pg=PR9&dq=subduction:+Insights+from+Physical+Mod-+eling.+Kluwer,+Dordrecht&ots=CrVyim6ABr&sig=5Oe0Gge72kTdliFQ6md_m065c9I&redir_esc=y#v=onepage&q=subduction:%20Insights%20from%20Physical%20Mod-%20eling.%20Kluwer,%20Dordrecht&f=false|title=Subduction: Insights from Physical Modeling|last=Shemenda|first=Alexander I.|date=1994-09-30|publisher=Springer Science & Business Media|isbn=9780792330424|language=en}}</ref> The space where the break-off [[Slab (geology)|slab]] createsseparates will form a mantle window.<ref name=":132222"/> Subsequently, the less dense continental margin forms the overriding plate, while the oceanic plate becomes the subducting slab.<ref name=":132222"/> The direction of subduction system changes since the break-off of [[Slab (geology)|slab]] creates the space, which is the major parameter of this model.<ref name=":132222"/>

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This model is developed based on the geological evolution of Alpine and Apennine subduction<ref name=":103">{{Cite journal|last=Vignaroli|first=Gianluca|last2=Faccenna|first2=Claudio|last3=Jolivet|first3=Laurent|last4=Piromallo|first4=Claudia|last5=Rossetti|first5=Federico|date=2008-04-01|title=Subduction polarity reversal at the junction between the Western Alps and the Northern Apennines, Italy|url=http://www.sciencedirect.com/science/article/pii/S004019510700457X|journal=Tectonophysics|volume=450|issue=1–4|pages=34–50|doi=10.1016/j.tecto.2007.12.012}}</ref>

Similarly, two oceanic plates move towards each other. SubductingThe subducting process ceases with the involvement of buoyant continental block. A new slab is formed at the overriding plate owing to the regional compression and the density contrast of the continental block and oceanic plate.<ref name=":103" /> An [[orogenic wedge]] is built.<ref name=":103" /> However, there is an obvious space problem about how to accommodate two slabs. The solution is the new developing slab moves not only vertically, but also laterally leading to a deep strike-slip movement.<ref name=":103" /> The development of co-existence of two opposite slabs is described as a double sided subduction<ref>{{Cite journal|last=Tao|first=Winston C.|last2=O'connell|first2=Richard J.|date=1992-06-10|title=Ablative subduction: A two-sided alternative to the conventional subduction model|url=http://onlinelibrary.wiley.com/doi/10.1029/91JB02422/abstract|journal=Journal of Geophysical Research: Solid Earth|language=en|volume=97|issue=B6|pages=8877–8904|doi=10.1029/91JB02422|issn=2156-2202}}</ref> or doubly convergent wedge.<ref name=":103" /> Eventually, the development of new slab grows and slides onto the old slab. The old slab breaks off and the orogenic wedge collapses. The new slab stops the lateral motion and subducts beneath.<ref name=":103" /> The direction of subduction system changes.

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The lithosphere break-up model is simulated by hydrocarbon experiments in the laboratory.<ref name=":113" /> The researchers set up the setting of subduction zone which are analogized by hydrocarbons with different densities representing various layers in the [[Subduction|subduction zone]].<ref name=":113" />

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 appliedsubjected 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 is developed with an opposite direction of 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|last=Currie|first=Claire A.|last2=Hyndman|first2=Roy D.|date=2006-08-01|title=The thermal structure of subduction zone back arcs|url=http://onlinelibrary.wiley.com/doi/10.1029/2005JB004024/abstract|journal=Journal of Geophysical Research: Solid Earth|language=en|volume=111|issue=B8|pages=B08404|doi=10.1029/2005JB004024|issn=2156-2202}}</ref><ref>{{Cite journal|last=Currie|first=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|url=http://www.sciencedirect.com/science/article/pii/S0012821X04002687|journal=Earth and Planetary Science Letters|volume=223|issue=1–2|pages=35–48|doi=10.1016/j.epsl.2004.04.020}}</ref> and hot fluid.<ref>{{Cite journal|last=Arcay|first=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|url=http://onlinelibrary.wiley.com/doi/10.1029/2005GC001061/abstract|journal=Geochemistry, Geophysics, Geosystems|language=en|volume=7|issue=2|pages=Q02007|doi=10.1029/2005GC001061|issn=1525-2027}}</ref><ref>{{Cite journal|last=Honda|first=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|url=http://onlinelibrary.wiley.com/doi/10.1029/2004GC000785/abstract|journal=Geochemistry, Geophysics, Geosystems|language=en|volume=6|issue=1|pages=Q01002|doi=10.1029/2004GC000785|issn=1525-2027}}</ref> Pre-existing faults in this simulation are also common in the magmatic arc.<ref>{{Cite journal|last=Toth|first=John|last2=Gurnis|first2=Michael|date=1998-08-10|title=Dynamics of subduction initiation at preexisting fault zones|url=http://onlinelibrary.wiley.com/doi/10.1029/98JB01076/abstract|journal=Journal of Geophysical Research: Solid Earth|language=en|volume=103|issue=B8|pages=18053–18067|doi=10.1029/98JB01076|issn=2156-2202}}</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|url=http://www.sciencedirect.com/science/article/pii/S0040195100001323|journal=Tectonophysics|volume=325|issue=1–2|pages=87–105|doi=10.1016/S0040-1951(00)00132-3}}</ref> and the active example at Taiwan region<ref name=":113" /><ref name=":03"/> as well as at Timor.<ref>Silver, E. A., Reed, D., McCaffrey, R., & Joyodiwiryo, Y. (1983). Back arc thrusting in the eastern Sunda arc, Indonesia- A consequence of arc-continent collision. ''Journal of Geophysical Research'', ''88''(B9), 7429-7448.</ref><ref>{{Cite journal|last=Snyder|first=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|url=http://onlinelibrary.wiley.com/doi/10.1029/95TC02352/abstract|journal=Tectonics|language=en|volume=15|issue=1|pages=34–53|doi=10.1029/95TC02352|issn=1944-9194}}</ref>