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There are two basic units consisting of subduction zone. They are overriding plate and the subduction plate. Two plates move towards each other due to tectonic forces. The overriding plate with lower density will be on the top of subducting plate. This type of tectonic interaction is commonly found in many plate boundaries.

However, some scientists propose that the roles of overriding plate and subducting plate are not stationary. Their roles will swap, which means the plate originally subducting beneath will become the overriding plate. This phenomenon is called subduction switched, the flipping of subduction polarity or subduction polarity reversal.

Examples of location of subdcution system have been identified by subduction polarity reversal:

- Caledonides, Ireland

- Alps-Apennines, Italy

- Kamchatka, Russian

- Wetar, eastern Indonesia

- Timor, east Savu Sea

- Mediterranean

- Taiwan

Background
The phenomenon of subduction polarity reversal have been identified in the collision of an intra-oceanic subduction system, which is the collision of two oceanic plates with density contrast. When two oceanic plates migrate towards each other, The oceanic plates with higher density subducts beneath and the other one overrides the down-warding plate. The process continues until a buoyant continental margin sitting on the top of subducting plate was introduced into the down-going slab. The subudction of slab will become slower and even cease. Scientists proposed various possible models to predict what will be the next step when the intra-oceanic subduction system with the involvement of buoyant continental crust. One of the possible consequences is the subduction polarity reversal.

Models of subduction polartiy reversal
Even through the scientists proposed the coming event after the involvement of buoyant continental crusts is subduction polarity reversal, they have different opinions towards the mechanisms leading to the change of subduction direction. Thus, there is no single models to represent subduction polarity reversal. How scientists developing the models highly depending on the parameters they focus .Some geologists attempt to construct the models of subduction reversal by laboratory experiments  or numerical modelling. There are three generally-accepted models which are slab break-off model, lithoshperic break-up model and.

The models of Slab- break up are based observations by geologists, and the lithospheric break-up is based on the experiment simulation.

Slab break-off
This model was developed by analyzing the geological cross section along the collision between Eurasian plate and the Philippine plate, where is the location of an ongoing fillping of subduction polarity.

When two oceanic plates migrate towards each other, one plate overrides another forming subduction system. Later, a less dense, buoyant passive continental margin being introduced into this system will therefore cause the cessation of subduction system. On one hand, the buoyant plate resists subduction beneath the overriding plate. On the other hand, the dense oceanic slab at the subducting plate prefers to move downward. This opposite forces will generate a tensile force or gravitational instability on the downward slab and lead to the break-off of the slab. The space where the break-off slab creates will form mantle window. Subsequently, the less dense continental margin forms the overriding plate, while the oceanic plate becomes the subducting slab. The direction of subduction system changes since the break-off of slab create the space, which is the major parameter of this model.

Double convergence model
This model is developed based on the geological evolution of Alpine and Apennine subduction

Similarly, two oceanic plates move towards each other. Subducting process ceases with the involvement of buoyant continental block. A new slab at the overriding plate owing to the regional compression and the density contrast of the continental block and oceanic plate. Orogenic wedge is being built. However, there is an obvious space problem that how to accommodate two slab. The solution is the newly- developing slab moves not only vertically but also laterally leading to a deep strike-slip movement. The development of co-existence of two opposite slabs is described as a double sided subduction or doubly convergent wedge. Eventually, the development of new slab grows and slides on the old slab. The old slab was broken and the orogenic wedge collapses. The new slab stops the lateral motion and subducts beneath. The direction of subduction system changes.

Lithospheric break-up
The lithospheric break-up model is simulated by hydrocarbon experiments in the laboratory. The reserachers set up the setting of subduction zone which are analogized hydrocarbon with different viscosity representing various layers in subdcution zone.

The initial setting of the simulated subdcution zone model is confined by two pistons. The piston connected to the overriding plate is locked, while the piston linking to subducting plate is applied a constant rate of compression. More importantly, there are a relative thin magmatic arc and pre-existing fault dipping towards the subducting plate at the overriding plate. The detachment of pre-existing fault occurs when buoyant continental margin is in contact with the overriding plate. It is because the buoyant margin resists to subduct and significantly increases the frictional force in the contact region. The subduction then stops. Subsequently, the new subducting slab develops at overriding plate with the continuous compression. The new developing slab eventually penetrates and breaks the old slab. A new subdcution is developed with an opposite direction of the previous one.

In reality, the magmatic arc is relatively weak zone at the overriding plate because it has s thin lithosphere and is further weaken by high heat flow and hydro thermal fluid. Pre-existing fault in this simulation is also common the magmatic arc. This experiment is successful analogy to subduction polarity reversal happened at Kamchatka in early Eocene and the active example at Taiwan region as well as at Timor ."" """"""""""""

Summary
The criteria of having subduction polarity reversal Different models represeting the subduction polarity reversal depends highly on parameters the scientists considered. Here is the summary table showing the comparison models.
 * 1) Intra-oceanic subduction system with a buoyant continental plate
 * 2) Subduction system ceases with the involvement of continental plate
 * 3) Old slab breaks off

Taiwan as an active example of subduction polarity reversal
A sharp contrast of landforms in Taiwan lures many people to investigate. The northern part of Taiwan has lots of flat plain such as Ilan Plain and Pingtung Plain, while the southern part of Taiwan is concentrated with many high mountains like Kushan reaching about 3950m. This huge difference in topographies is the consequence of the flipping of subduction polarity. Most of models studying this phenomenon will focus on an active collision in Taiwan which appears to reveal the incipient stages of subduction reversal.

The collision of N- trending Luzon arc in Philippine Sea plate (PP) with E-trending Eurasian plate (EP) started at mid-Miocene forming an intra-oceanic subduction system. Taiwan was formed by this process. The south-north dichotomy in Taiwan is like a story book telling the evolution in subduction zone. The Philippine Sea plate subducts into Eurasian plate at south-west part of WEP (Western edge of north-dipping Philippine Sea Plate), and the latter overrides the former at north east part of WEP .The collision between two plates started at the Northern Taiwan and propagated towards south with 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.