![Conventional Plate tectonics vs DDS](https://upload.wikimedia.org/wikipedia/commons/d/d2/Conventional_Plate_tectonics_vs_DDS.svg)
Divergent double subduction (abbreviated as DDS), also called outward dipping double-sided subduction, is a special type of
subduction
Subduction is a geological process in which the oceanic 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 ...
process in which two parallel subduction zones with different directions are developed on the same oceanic plate.
In conventional
plate tectonics
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
theory, an oceanic plate subducts under another plate and new
oceanic crust
Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumu ...
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.
This concept was first proposed and applied to the Lachlan Fold Belt in southern Australia.
Since then, geologists have applied this model to other regions such as the Solonker Suture Zone of the
Central Asian Orogenic Belt
The Central Asian Orogenic Belt (CAOB), also called the Altaids, is one of the world's largest Phanerozoic accretionary orogens, and thus a leading laboratory of geologically recent crustal growth. The orogenic belt is bounded by the East Europea ...
, the Jiangnan Orogen,
the
Lhasa
Lhasa (; Lhasa dialect: ; bo, text=ལྷ་ས, translation=Place of Gods) is the urban center of the prefecture-level city, prefecture-level Lhasa (prefecture-level city), Lhasa City and the administrative capital of Tibet Autonomous Regio ...
–
Qiangtang collision zone
and the Baker terrane boundary. Active examples of this system are (1) the
Molucca Sea Collision Zone in Indonesia, in which the
Molucca Sea Plate subducts below the
Eurasian Plate
The Eurasian Plate is a tectonic plate that includes most of the continent of Eurasia (a landmass consisting of the traditional continents of Europe and Asia), with the notable exceptions of the Indian subcontinent, the Arabian subcontinent and ...
and the
Philippine Sea Plate
The Philippine Sea Plate or the Philippine Plate is a tectonic plate comprising oceanic lithosphere that lies beneath the Philippine Sea, to the east of the Philippines. Most segments of the Philippines, including northern Luzon, are part of ...
on two sides,
and (2) the
Adriatic Plate
The Adriatic or Apulian Plate is a small tectonic plate carrying primarily continental crust that broke away from the African Plate along a large transform fault in the Cretaceous period. The name Adriatic Plate is usually used when referring ...
in the central
Mediterranean
The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the e ...
, subducting both on its western side (beneath the
Apennines
The Apennines or Apennine Mountains (; grc-gre, links=no, Ἀπέννινα ὄρη or Ἀπέννινον ὄρος; la, Appenninus or – a singular with plural meaning;''Apenninus'' (Greek or ) has the form of an adjective, which wou ...
and Calabria) and on its eastern side (beneath the
Dinarides).
Note that the term ''divergent'' is used to describe one oceanic plate subducting in different directions on two opposite sides. This sense should not be confused with the use of the same term in ''
divergent plate boundary
In plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent b ...
'', which refers to a
spreading center, where two separate plates move away from each other.
Evolution of divergent double subduction system
The complete evolution of a divergent double subduction system can be divided into four major stages.
![Stage 1](https://upload.wikimedia.org/wikipedia/commons/e/e5/Stage_1.svg)
Initial stage
As the central
oceanic plate subducts on both sides into the two overriding plates, the subducting oceanic slab brings fluids down and the fluids are released in the
mantle wedge
A mantle wedge is a triangular shaped piece of mantle that lies above a subducting tectonic plate and below the overriding plate. This piece of mantle can be identified using seismic velocity imaging as well as earthquake maps. Subducting oceanic ...
.
This initiates the partial melting of the
mantle wedge
A mantle wedge is a triangular shaped piece of mantle that lies above a subducting tectonic plate and below the overriding plate. This piece of mantle can be identified using seismic velocity imaging as well as earthquake maps. Subducting oceanic ...
and the magma eventually rise into the overriding plates, resulting in the formation of two
volcanic arc
A volcanic arc (also known as a magmatic 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 lo ...
s on the two overriding plates.
At the same time, sediment deposits on the two margins of the overriding plates, forming two
accretionary wedges.
As the plate subducts and rollback occurs, the ocean becomes narrower and the subduction rate reduces as the oceanic plate becomes closer to an inverted "U" shape.
![Stage2](https://upload.wikimedia.org/wikipedia/commons/6/69/Stage2.svg)
Second stage
The ocean is closed eventually as subduction continues. The two overriding plates meet, collide, and weld together by a "soft" collision.
The inverted "U" shape of the oceanic plate inhibits the continued subduction of the plate because the mantle material below the plate is trapped.
![Stage3](https://upload.wikimedia.org/wikipedia/commons/a/a4/Stage3.svg)
Third stage
The dense oceanic plate has a high tendency to sink. As it sinks, it breaks along the oceanic plate and the welded crust above and a gap is created.
The extra space created leads to the decompression melting of
mantle wedge
A mantle wedge is a triangular shaped piece of mantle that lies above a subducting tectonic plate and below the overriding plate. This piece of mantle can be identified using seismic velocity imaging as well as earthquake maps. Subducting oceanic ...
materials.
The melts flow upward and fill the gap and intrude the oceanic plate and welded crust as mafic dykes intrusion.
Eventually, the oceanic plate completely breaks apart from the welded crust as it continues to sink.
Final stage
When the oceanic plate breaks apart from the crust and sinks into the mantle,
underplating continues to occur. At the same time, the sinking oceanic plate starts to dewater and release the fluids upward to aid the partial melting of mantle and the crust above.
It results in extensive magmatism and
bimodal volcanism
Bimodal volcanism is the eruption of both mafic and felsic lavas from a single volcanic centre with little or no lavas of intermediate composition. This type of volcanism is normally associated with areas of extensional tectonics, particularly r ...
.
Magmatic and metamorphic features
Arc magmatism
Unlike one sided subduction where only one magmatic arc is generated on the overriding plate, two parallel magmatic arcs are generated on both colliding overriding plates when the oceanic plate subducts on two sides. Volcanic rocks indicating arc volcanism can be found on both sides of the
suture zone
In structural geology, a suture is a joining together along a major fault zone, of separate terranes, tectonic units that have different plate tectonic, metamorphic and paleogeographic histories. The suture is often represented on the surface by ...
.
Typical rock types include calc-alkaline basalt, andesites, dacite and tuff.
These arc volcanic rocks are enriched in
large ion lithophile element (LILE) and
light rare earth element (LREE) but depleted in
niobium
Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has sim ...
,
hafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri M ...
and
titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
.
Extensive intrusions
Partial melting of mantle generate mafic
dyke
Dyke (UK) or dike (US) may refer to:
General uses
* Dyke (slang), a slang word meaning "lesbian"
* Dike (geology), a subvertical sheet-like intrusion of magma or sediment
* Dike (mythology), ''Dikē'', the Greek goddess of moral justice
* Dikes, ...
intrusion. Because the mantle is the primary source, these dykes record isotopic characteristics of the depleted mantle in which the
87Sr/86Sr ratio is near 0.703 and
samarium-neodymium dating is positive.
On the other hand, partial melting of the lower crust (accretionary complex) leads to
S-type granitoid intrusions with enriched
aluminium oxide throughout the evolution of divergent double subduction.
Bimodal volcanism
When the oceanic plate detaches from the overlying crust, intense decompressional melting of mantle is induced. Large amount of hot
basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
ic magma intrude and melt the crust which generate
rhyolitic
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...
melt.
This results in alternating eruption of basaltic and rhyolitic lava.
Low grade metamorphism
Without continental collision and deep subduction, high grade metamorphism is not common like other subduction zones. Most of the sedimentary strata and volcanics in the
accretionary wedge experience low to medium grade metamorphism up to
greenschist or
amphibolite
Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky ...
facies
In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...
only.
Structural features
![Molucca Sea Cross Section](https://upload.wikimedia.org/wikipedia/commons/e/e7/Molucca_Sea_Cross_Section.svg)
Thrusting and folding
When the two overriding plates converge, two
accretionary wedges will develop. The two accretionary wedges are in opposite direction. Thus, direction of thrust and vergence of the folds in the accretionary wedges are opposite also.
However, this proposed feature may not be observed because of the continuous deformation. For example, in the modern day example of Molucca Sea Collision Zone, the continuous active collision causes the
Sangihe Arc Sangir, Sangihe, Sangi or Sanghir may refer to:
* Sangir, Central Asia, in the Kimek Khanate
* Sangihe Islands (''Kepulauan Sangir''), an archipelago in Indonesia
* Sangihe Island (''Sangihe Besar''), the largest of the Sangir Islands
* Sangirese ...
to override the
Halmahera Arc
Halmahera Arc is the volcanic arc of the Halmahera region of eastern Indonesia. It is considered to belong to the Halmahera Plate within the Molucca Sea Collision Zone.
Origin
Potassium-argon ages of Neogene to Recent igneous rocks from the Halmah ...
and the back arc of Halmahera Arc to overthrust itself.
In this case, complex fold thrust belt including the accretionary complex is formed. In the future, the Sangihe Arc will override the Halmahera Arc and the rock records in Halmahera will disappear.
Unconformity
When the two overriding plates collide and the ocean basin is closed, sedimentation ceases. Sinking of the oceanic plate drag down the welded crust to form a basin that allows continued sedimentation.
After the oceanic plate completely detaches from the crust above, isostatic rebound occurs, leaving a significant unconformity in the sedimentary sections.
Factors controlling the evolution of divergent double subduction system
In nature, the inverted "U" shape of the oceanic plate in divergent double subduction should not be always perfectly symmetrical like the idealized model. An asymmetrical form is preferred like the real example in Molucca Sea where the length of the subducted slab is longer on its western side beneath the Sangihe Arc while a shorter slab on its eastern side beneath the
Halmahera Arc
Halmahera Arc is the volcanic arc of the Halmahera region of eastern Indonesia. It is considered to belong to the Halmahera Plate within the Molucca Sea Collision Zone.
Origin
Potassium-argon ages of Neogene to Recent igneous rocks from the Halmah ...
.
3D numerical modelling had been done to simulate divergent double subduction, to evaluate different factors that can affect the evolution and geometry of the system discerned below.
Width of the oceanic plate
![Toroidal flow](https://upload.wikimedia.org/wikipedia/commons/4/45/Toroidal_flow.svg)
The width of the plate determines whether the divergent double subduction can be sustained.
The inverted "U" shape of the oceanic plate is not an effective geometry for it to sink because of the mantle materials beneath.
Those mantle materials need to escape by toroidal flow at the edge of the subducted oceanic plate.
With a narrow oceanic plate (width < 2000 km), the trapped mantle beneath the oceanic plate can effectively escape by toroidal flow.
In contrast, for a persistent oceanic plate (width > 2000 km), the trapped mantle beneath the oceanic plate cannot escape effectively by toroidal flow and the system cannot be sustained.
Therefore, divergent double subduction can only occur in small narrow oceanic plate but not in large width oceanic plate.
This also explains why it is rare in nature and most subduction zones are single sided.
Order of subduction
Order of subduction control the geometry of divergent doubled subduction.
The side that begins to subduct earlier enters the eclogitization level earlier. The density contrast between the plate and the mantle increases which makes the sinking of the plate faster, creating a positive feedback. It results in an asymmetrical geometry where the slab length is longer on the side which subducts earlier.
The slab pull, amount of poloidal flow and the rate of convergence on the side with shorter length will be reduced.
It remains unclear how initiation occurs for both sides of a single plate if subduction is in form of divergent double subduction, even though this subduction type has been clearly observed . This is because it's difficult to break a moving oceanic plate (i.e., acting as a trailing edge, which moving in the reverse direction of the ongoing, earlier-initiated subduction) due to lack of compression required for forced (induced) subduction initiation. Therefore, self-consistent initiation of divergent double subduction, together with other forms of double subduction, requires further studies of structural and magmatic records.
State of motion of the overriding plates
The state of motion of overriding plates control the geometry of divergent doubled subduction and the position of collision.
The length of the subducting slab beneath a stagnant overriding plate is shorter because the mantle flow is weaker and the subduction is slower.
In contrast, the length of the subducting slab beneath a free moving plate is longer.
Additionally, the position of collision is shifted more to the side with stagnant plate as the rollback is faster on the free moving side.
Thickness of the overriding plates
Thickness of the overriding plates have similar effect as state of motion of overriding plates to control the geometry of divergent doubled subduction and the position of collision.
A thicker overriding plate hinders subduction because of the larger friction. It results in a shorter slab.
Vice versa, a thinner overriding plate have a longer slab.
Density contrast between oceanic plate and mantle
Larger density contrast between oceanic plate and mantle create a larger negative buoyancy of the oceanic plate.
It results in a faster subduction and a stronger rollback.
Therefore, the mantle flow induced by the rollback (poloidal flow) is also enhanced. The convergence rate is increased, resulting in a faster and more vigorous collision between the two overriding plates.
References
{{Reflist
Subduction