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 zone. This consists of an overriding plate and the subduction plate. [1] Two plates move towards each other due to tectonic forces. [1] The overriding plate will be on the top of the subducting plate. [1] This type of tectonic interaction is found at many plate boundaries. [1]
However, some geologists propose that the roles of the overriding plate and subducting plate do not remain the same indefinitely. [2] Their roles will swap, which means the plate originally subducting beneath will become the overriding plate. [2] This phenomenon is called subduction switch, [3] the flipping of subduction polarity [4] or subduction polarity reversal. [2]
Examples of subduction systems with subduction polarity reversal are:
The phenomenon of subduction polarity reversal has been identified in the collision of an intra-oceanic subduction system, [12] which is the collision of two oceanic plates. [1] When two oceanic plates migrate towards each other, one subducts below the other. Generally, the oceanic plate with higher density subducts beneath and the other one overrides the down-going slab. [1] The process continues until a buoyant continental margin sitting on the top of the subducting plate is introduced into the down-going slab. [2] [4] The subduction of the slab becomes slower and may even cease. [2] [4] Geologists propose various possible models to predict what will be the next step for the intra-oceanic subduction system with the involvement of buoyant continental crust. [2] [4] One of the possible results is subduction polarity reversal. [4] [11] [12] [13] [14] [15]
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 model to represent subduction polarity reversal. How geologists develop the models depends on the parameters they focus on. [1] Some geologists attempt to construct models of subduction reversal through laboratory experiments [2] [12] [13] or observations. [4] [16] There are three common models: slab break-off, [4] double convergence [16] and lithospheric break-up. [2]
The models of slab-break up [4] and double convergence are based on observations by geologists, [16] and the lithosphere break-up model is based on experimental simulation. [2]
The criteria for having subduction polarity reversal are
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”. [17] 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.
Difference | Slab break-off | Double convergence | Lithospheric break-up |
---|---|---|---|
Reasons of slab break-off | Tensile force at the old slab | Lateral sliding by the new slab | Pre-existing fault leads to penetration of new slab |
Accommodation of new slab | Mantle window | A deep strike-slip movement | Penetration of new slab breaks off the old slab |
This model was developed by analyzing the geological cross section along the collision between Eurasian plate and the Philippine sea plate, which is the location of an ongoing flipping of subduction polarity. [4]
When two oceanic plates migrate towards each other, one plate overrides another forming a subduction system. Later, a light and buoyant passive continental margin introduced into this system will cause the cessation of subduction system. [4] On one hand, the buoyant plate resists subduction beneath the overriding plate. [4] On the other hand, the dense oceanic slab at the subducting plate prefers to move downward. [4] These opposite forces will generate a tensile force or gravitational instability on the downward slab and lead to the break-off of the slab. [18] The space where the break-off slab separates will form a mantle window. [4] Subsequently, the less dense continental margin forms the overriding plate, while the oceanic plate becomes the subducting slab. [4] The direction of the subduction system changes since the break-off of slab creates the space, which is the major parameter of this model. [4]
This model is developed based on the geological evolution of Alpine and Apennine subduction. [16]
Similarly, two oceanic plates move towards each other. The subduction 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 difference in density between the continental block and oceanic plate. [16] An orogenic wedge is built. [16] 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. [16] The development of co-existence of two opposite slabs is described as a double sided subduction [19] or doubly convergent wedge. [16] 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. [16] The direction of subduction system changes. [16]
The lithosphere break-up model is simulated by hydrocarbon experiments in the laboratory. [2] The researchers set up the setting of subduction zone which are analogized by hydrocarbons with different densities representing various layers in the subduction zone. [2]
The initial setting of the simulated 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. [2] More importantly, there is a relatively thin magmatic arc and pre-existing fault dipping towards the subducting plate at the overriding plate. [2] The detachment of the pre-existing fault occurs when buoyant continental margin is in contact with the overriding plate. [2] It is because the buoyant margin resists subduction and significantly increases the frictional force in the contact region. [2] The subduction then stops. Subsequently, the new subducting slab develops at an overriding plate with the continuous compression. [2] The new developing slab eventually penetrates and breaks the old slab. [2] A new subduction zone is formed with an opposite polarity to the previous one. [2]
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 [20] [21] and hot fluid. [22] [23] Pre-existing faults in this simulation are also common in the magmatic arc. [24] This experiment is a successful analogy to subduction polarity reversal happening at Kamchatka in early Eocene [7] [25] and the active example at Taiwan region [2] [11] as well as at Timor. [26] [27]
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, [28] while the southern part of Taiwan is concentrated with many high mountains like Yushan reaching about 3950m. This huge difference in topography is the consequence of the flipping of subduction polarity. [4] Most of models studying this phenomenon will focus on an active collision in Taiwan which appears to reveal the incipient stages of subduction reversal. [4] [11] [12] [13] [14] [15]
The collision of N- trending Luzon arc in Philippine Sea plate (PP) with E-trending Eurasian plate (EP) started at mid-Miocene [4] forming an intra-oceanic subduction system. [12] [29] 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 subducts below the Eurasian plate at south-west part of WEP (Western edge of north-dipping Philippine Sea Plate), [4] and the latter overrides the former at north east part of WEP. [4] 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. [30]
Oceanic trenches are prominent, long, narrow topographic depressions of the ocean floor. They are typically 50 to 100 kilometers wide and 3 to 4 km below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about 50,000 km (31,000 mi) of oceanic trenches worldwide, mostly around the Pacific Ocean, but also in the eastern Indian Ocean and a few other locations. The greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of 10,994 m (36,070 ft) below sea level.
Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.
Obduction is a geological process whereby denser oceanic crust is scraped off a descending ocean plate at a convergent plate boundary and thrust on top of an adjacent plate. When oceanic and continental plates converge, normally the denser oceanic crust sinks under the continental crust in the process of subduction. Obduction, which is less common, normally occurs in plate collisions at orogenic belts or back-arc basins.
A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.
Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle along the subduction zone. They are the principal way by which continental growth is achieved.
Forearc is a plate tectonic term referring to a region in a subduction zone between an oceanic trench and the associated volcanic arc. Forearc regions are present along convergent margins and eponymously form 'in front of' the volcanic arcs that are characteristic of convergent plate margins. A back-arc region is the companion region behind the volcanic arc.
A volcanic arc is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc located further from the subducting plate than the trench. The oceanic plate is saturated with water, mostly in the form of hydrous minerals such as micas, amphiboles, and serpentines. As the oceanic plate is subducted, it is subjected to increasing pressure and temperature with increasing depth. The heat and pressure break down the hydrous minerals in the plate, releasing water into the overlying mantle. Volatiles such as water drastically lower the melting point of the mantle, causing some of the mantle to melt and form magma at depth under the overriding plate. The magma ascends to form an arc of volcanoes parallel to the subduction zone.
The Sunda Plate is a minor tectonic plate straddling the Equator in the Eastern Hemisphere on which the majority of Southeast Asia is located.
The Farallon Trench was a subduction related tectonic formation located off the coast of the western California continental margin during the late to mid Cenozoic era, around 50 miles southeast of modern-day Monterey Bay. The time duration of subduction began from around 165 Ma when the Farallon Plate replaced the Mezcalera promontory, until the San Andreas Fault straightening around 35 Ma. As data accumulated over time, a common view developed that one large oceanic plate, the Farallon Plate, acted as a conveyor belt, conveying accreted terranes onto the North American west coast. As the continent overran the subducting Farallon Plate, the denser plate became subducted into the mantle below the continent. When the plates converged, the dense oceanic plate sank into the mantle to form a slab below the lighter continent. Rapid subduction under the southwestern North America continent began 40 to 60 million years ago (Ma), during the mid Paleocene to mid Eocene epochs. This convergent subduction margin created a distinctive geomorphologic feature called an oceanic trench, which occurs at a convergent plate boundaries as a heavy metal rich, lithospheric plate moves below a light silica rich continental plate. The trench marks the position at which the flexed subducting slab begins to descend beneath and deform the continental plate margin. By 43 Ma, during the Eocene, worldwide plate motions changed and the Pacific Plate began to move away from North America and subduction of the Farallon Plate slowed dramatically. By around 36 Ma, the easternmost part of the East Pacific Rise, located between the Pioneer and Murray fracture zones at that time, approached the trench and the young, hot, buoyant lithosphere appears to have clogged part of the subduction zone, resulting in widespread dramatic uplift on land. The eventual complete subduction of this plate, consequential contact of the Pacific Plate with the California continental margin, and creation of the Mendocino Triple Junction (MTJ), took place around 30 to 20 Ma. The partial complete subduction and division of the Farallon Plate by the Pacific Plate, created the Juan de Fuca Plate to the north and the Cocos Plate to the south. The final stages of the evolution of California's continental margin was the growth of the San Andreas transform fault system, which formed as the Pacific Plate came into contact with the continental margin and the MTJ was formed. As subduction of the Pacific Plate continued along this margin, and the contact zone grew, the San Andreas proportionally grew as well.
A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most of them result from tensional forces, caused by a process known as oceanic trench rollback, where a subduction zone moves towards the subducting plate. Back-arc basins were initially an unexpected phenomenon in plate tectonics, as convergent boundaries were expected to universally be zones of compression. However, in 1970, Dan Karig published a model of back-arc basins consistent with plate tectonics.
Located in the western Pacific Ocean near Indonesia, the Molucca Sea Plate has been classified by scientists as a fully subducted microplate that is part of the Molucca Sea Collision Complex. The Molucca Sea Plate represents the only known example of divergent double subduction (DDS), which describes the subduction on both sides of a single oceanic plate.
The Nazca Ridge is a submarine ridge, located on the Nazca Plate off the west coast of South America. This plate and ridge are currently subducting under the South American Plate at a convergent boundary known as the Peru-Chile Trench at approximately 7.7 cm (3.0 in) per year. The Nazca Ridge began subducting obliquely to the collision margin at 11°S, approximately 11.2 Ma, and the current subduction location is 15°S. The ridge is composed of abnormally thick basaltic ocean crust, averaging 18 ±3 km thick. This crust is buoyant, resulting in flat slab subduction under Peru. This flat slab subduction has been associated with the uplift of Pisco Basin and the cessation of Andes volcanism and the uplift of the Fitzcarrald Arch on the South American continent approximately 4 Ma.
The back-arc region is the area behind a volcanic arc. In island volcanic arcs, it consists of back-arc basins of oceanic crust with abyssal depths, which may be separated by remnant arcs, similar to island arcs. In continental arcs, the back-arc region is part of the continental platform, either dry land (subaerial) or forming shallow marine basins.
In geology, the slab is a significant constituent of subduction zones.
Ultra-high-pressure metamorphism refers to metamorphic processes at pressures high enough to stabilize coesite, the high-pressure polymorph of SiO2. It is important because the processes that form and exhume ultra-high-pressure (UHP) metamorphic rocks may strongly affect plate tectonics, the composition and evolution of Earth's crust. The discovery of UHP metamorphic rocks in 1984 revolutionized our understanding of plate tectonics. Prior to 1984 there was little suspicion that continental rocks could reach such high pressures.
High pressure terranes along the ~1200 km long east-west trending Bangong-Nujiang suture zone (BNS) on the Tibetan Plateau have been extensively mapped and studied. Understanding the geodynamic processes in which these terranes are created is key to understanding the development and subsequent deformation of the BNS and Eurasian deformation as a whole.
Flat slab subduction is characterized by a low subduction angle beyond the seismogenic layer and a resumption of normal subduction far from the trench. A slab refers to the subducting lower plate. A broader definition of flat slab subduction includes any shallowly dipping lower plate, as in western Mexico. Flat slab subduction is associated with the pinching out of the asthenosphere, an inland migration of arc magmatism, and an eventual cessation of arc magmatism. The coupling of the flat slab to the upper plate is thought to change the style of deformation occurring on the upper plate's surface and form basement-cored uplifts like the Rocky Mountains. The flat slab also may hydrate the lower continental lithosphere and be involved in the formation of economically important ore deposits. During the subduction, a flat slab itself may deform or buckle, causing sedimentary hiatus in marine sediments on the slab. The failure of a flat slab is associated with ignimbritic volcanism and the reverse migration of arc volcanism. Multiple working hypotheses about the cause of flat slabs are subduction of thick, buoyant oceanic crust (15–20 km) and trench rollback accompanying a rapidly overriding upper plate and enhanced trench suction. The west coast of South America has two of the largest flat slab subduction zones. Flat slab subduction is occurring at 10% of subduction zones.
Divergent double subduction, also called outward dipping double-sided subduction, is a special type of subduction process in which two parallel subduction zones with different directions are developed on the same oceanic plate. In conventional plate tectonics theory, an oceanic plate subducts under another plate and new oceanic crust is generated somewhere else, commonly along the other side of the same plates However, in divergent double subduction, the oceanic plate subducts on two sides. This results in the closure of ocean and arc–arc collision.
In plate tectonics, slab detachment or slab break-off may occur during continent-continent or arc-continent collisions. When the continental margin of the subducting plate reaches the oceanic trench of the subduction zone, the more buoyant continental crust will in normal circumstances experience only a limited amount of subduction into the asthenosphere. The slab pull forces will, however, still be present and this normally leads to the breaking off or detachment of the descending slab from the rest of the plate. The isostatic response to the detachment of the downgoing slab is rapid uplift. Slab detachment is also followed by the upwelling of relatively hot asthenosphere to fill the gap created, leading in many cases to magmatism.
Oblique subduction is a form of subduction for which the convergence direction differs from 90° to the plate boundary. Most convergent boundaries involve oblique subduction, particularly in the Ring of Fire including the Ryukyu, Aleutian, Central America and Chile subduction zones. In general, the obliquity angle is between 15° and 30°. Subduction zones with high obliquity angles include Sunda trench and Ryukyu arc.
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