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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 arc 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 alon ...
s and subduction zones, with many found in the western
Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contin ...
. 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 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 ...
, 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.


Structural characteristics

Back-arc basins are typically very long and relatively narrow, often thousands of kilometers long while only being a few hundred kilometers wide at most. For back-arc extension to form, a subduction zone is required, but not all subduction zones have a back-arc extension feature. Back-arc basins are found in areas where the subducting plate of oceanic crust is very old. The restricted width of back-arc basins is due to magmatic activity being reliant on water and induced mantle convection, limiting their formation to along subduction zones. Spreading rates vary from only a few centimeters per year (as in the Mariana Trough), to 15 cm/year in the Lau Basin. Spreading ridges within the basins erupt
basalt Basalt (; ) is an 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 surface of a rocky planet or moon. More than 90 ...
s that are similar to those erupted from the
mid-ocean ridges A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverg ...
; the main difference being back-arc basin basalts are often very rich in
magmatic water Magmatic water, also known as juvenile water, is an aqueous phase in equilibrium with minerals that have been dissolved by magma deep within the Earth's crust and is released to the atmosphere during a volcanic eruption. It plays a key role in as ...
(typically 1-1.5 weight % H2O), whereas mid-ocean ridge basalt magmas are very dry (typically <0.3 weight % H2O). The high water contents of back-arc basin basalt magmas is derived from water carried down the subduction zone and released into the overlying mantle wedge. Additional sources of water could be the eclogitization of
amphiboles Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol ...
and micas in the subducting slab. Similar to mid-ocean ridges, back-arc basins have hydrothermal vents and associated chemosynthetic communities.


Seafloor spreading

Evidence of
seafloor spreading Seafloor spreading or Seafloor spread is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. History of study Earlier theories by Alfred Wegener a ...
has been seen in cores of the basin floor. The thickness of
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
that collected in the basin decreased toward the center of the basin, indicating a younger surface. The idea that thickness and age of sediment on the sea floor is related to the age of the oceanic crust was proposed by Harry Hess.
Magnetic anomalies In geophysics, a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying ...
of the crust that had formed in back-arc basins deviated in form from the crust formed at mid-ocean ridges. In many areas the anomalies do not appear parallel, as well as the profiles of the magnetic anomalies in the basin lacking symmetry or a central anomaly as a traditional ocean basin does, indicating asymmetric seafloor spreading. This has prompted some to characterize the spreading in back-arc basins to be more diffused and less uniform than at mid-ocean ridges. The idea that back-arc basin spreading is inherently different from mid-ocean ridge spreading is controversial and has been debated through the years. Another argument put forward is that the process of seafloor spreading is the same in both cases, but the movement of seafloor spreading centers in the basin causes the asymmetry in the magnetic anomalies. This process can be seen in the Lau back-arc basin. Though the magnetic anomalies are more complex to decipher, the rocks sampled from back-arc basin spreading centers do not differ very much from those at mid-ocean ridges. In contrast, the volcanic rocks of the nearby island arc differ significantly from those in the basin. Back-arc basins are different from normal mid-ocean ridges because they are characterized by asymmetric seafloor spreading, but this is quite variable even within single basins. For example, in the central Mariana Trough, current spreading rates are 2–3 times greater on the western flank, whereas at the southern end of the Mariana Trough the position of the spreading center adjacent to the volcanic front suggests that overall crustal accretion has been nearly entirely asymmetric there. This situation is mirrored to the north where a large spreading asymmetry is also developed. Other back-arc basins such as the Lau Basin have undergone large rift jumps and propagation events (sudden changes in relative rift motion) that have transferred spreading centers from arc-distal to more arc-proximal positions. Conversely, study of recent spreading rates appear to be relatively symmetric with perhaps small rift jumps. The cause of asymmetric spreading in back-arc basins remains poorly understood. General ideas invoke asymmetries relative to the spreading axis in arc melt generation processes and heat flow, hydration gradients with distance from the slab, mantle wedge effects, and evolution from rifting to spreading.


Formation and tectonics

The extension of the crust behind volcanic arcs is believed to be caused by processes in association with subduction. As the subducting plate descends into the asthenosphere it begins to melt, causing the volcanism and formation of island arcs. Another result of this heating is a convection cell is formed. The rising magma and heat along with the outwards tension in the crust in contact with the convection cell cause a region of melt to form, resulting in a
rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...
. This process drives the island arc toward the subduction zone and the rest of the plate away from the subduction zone. The backward motion of the subduction zone relative to the motion of the plate which is being subducted is called trench rollback (also known as hinge rollback or hinge retreat). As the subduction zone and its associated trench pull backward, the overriding plate is stretched, thinning the crust and forming a back-arc basin. In some cases, extension is triggered by the entrance of a buoyant feature in the subduction zone, which locally slows down subduction and induces the subducting plate to rotate adjacent to it. This rotation is associated with trench retreat and overriding plate extension. The age of the subducting crust needed to establish back-arc spreading has been found to be 55 million years old or older. This is why they appear concentrated in the western pacific; where multiple back-arc spreading centers are located. The dip angle of the subducting slab may also be significant, as is shown to be greater than 30° in areas of back-arc spreading; this is most likely because as oceanic crust gets older it becomes denser, resulting in a steeper angle of descent. The thinning of the overriding plate from back-arc rifting can lead to the formation of new oceanic crust (i.e., back-arc spreading). As the lithosphere stretches, the asthenosphere below rises to shallow depths and partially melts as a result of adiabatic decompression melting. As this melt nears the surface, spreading begins.


Sedimentation

Sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the ...
is strongly asymmetric, with most of the sediment supplied from the active volcanic arc which regresses in step with the rollback of the trench. From cores collected during the Deep Sea Drilling Project (DSDP) nine sediment types were found in the back-arc basins of the western Pacific. Debris flows of thick to medium bedded massive conglomerates account for 1.2% of sediments collected by the DSDP. The average size of the sediments in the conglomerates are pebble sized but can range from granules to cobbles. Accessory materials include
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
fragments,
chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a ...
, shallow water fossils and
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...
clasts Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...
. Submarine fan systems of interbedded
turbidite A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing Turbidites wer ...
sandstone and mudstone made up 20% of the total thickness of sediment recovered by the DSDP. The fans can be divided into two sub-systems based on the differences in lithology,
texture Texture may refer to: Science and technology * Surface texture, the texture means smoothness, roughness, or bumpiness of the surface of an object * Texture (roads), road surface characteristics with waves shorter than road roughness * Texture ...
,
sedimentary structures Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different particl ...
, and
bedding Bedding, also known as bedclothes or bed linen, is the materials laid above the mattress of a bed for hygiene, warmth, protection of the mattress, and decorative effect. Bedding is the removable and washable portion of a human sleeping environm ...
style. These systems are inner and midfan subsystem and the outer fan subsystem. The inner and midfan system contains interbedded thin to medium bedded sandstones and mudstones. Structures that are found in these sandstones include load clasts, micro- faults, slump folds, convolute laminations, dewatering structures, graded bedding, and gradational tops of sandstone beds. Partial Bouma sequences can be found within the subsystem. The outer fan subsystem generally consists of finer sediments when compared to the inner and midfan system. Well sorted volcanoclastic sandstones,
siltstones Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...
and mudstones are found in this system. Sedimentary structures found in this system include parallel laminae, micro-cross laminae, and graded bedding. Partial Bouma sequences can be identified in this subsystem. Pelagic clays containing iron-manganese micronodules,
quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...
,
plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...
,
orthoclase Orthoclase, or orthoclase feldspar ( endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles ...
, magnetite, volcanic glass,
montmorillonite Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic crystals, known as clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite gro ...
,
illite Illite is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – alumina ...
,
smectite A smectite (from ancient Greek ''σμηκτός'' smektos 'lubricated'; ''σμηκτρίς'' smektris 'walker's earth', 'fuller's earth'; rubbing earth; earth that has the property of cleaning) is a mineral mixtures of various swelling sheet sil ...
, foraminiferal remains,
diatoms A diatom (New Latin, Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group com ...
, and sponge spicules made up the uppermost stratigraphic section at each site it was found. This sediment type consisted of 4.2% of the total thickness of sediment recovered by the DSDP. Biogenic pelagic silica sediments consist of radiolarian, diatomaceous, silicoflagellate oozes, and chert. It makes up 4.3% of the sediment thickness recovered. Biogenic pelagic
carbonates A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
is the most common sediment type recovered from the back-arc basins of the western Pacific. This sediment type made up 23.8% of the total thickness of sediment recovered by the DSDP. The pelagic carbonates consist of ooze, chalk, and limestone. Nanofossils and foraminifera make up the majority of the sediment. Resedimented carbonates made up 9.5% of the total thickness of sediment recovered by the DSDP. This sediment type had the same composition as the biogenic pelagic carbonated, but it had been reworked with well-developed sedimentary structures. Pyroclastics consisting of volcanic ash,
tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...
and a host of other constituents including nanofossils, pyrite, quartz, plant debris, and glass made up 9.5% of the sediment recovered. These volcanic sediments were sourced form the regional tectonic controlled volcanism and the nearby island arc sources.


Locations

Active back-arc basins are found in the Marianas, Kermadec-Tonga, South Scotia, Manus, North Fiji, and Tyrrhenian Sea regions, but most are found in the western Pacific. Not all subduction zones have back-arc basins, some like the central
Andes The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...
are associated with rear-arc compression. There are a number of extinct or fossil back-arc basins, such as the Parece Vela-Shikoku Basin, Sea of Japan, and Kurile Basin. Compressional back-arc basins are found, for example, in the
Pyrenees The Pyrenees (; es, Pirineos ; french: Pyrénées ; ca, Pirineu ; eu, Pirinioak ; oc, Pirenèus ; an, Pirineus) is a mountain range straddling the border of France and Spain. It extends nearly from its union with the Cantabrian Mountains to ...
and the Swiss Alps.


History of thought

With the development of plate tectonic theory, geologists thought that convergent plate margins were zones of compression, thus zones of strong extension above subduction zones (back-arc basins) were not expected. The hypothesis that some convergent plate margins were actively spreading was developed by Dan Karig in 1970, while a graduate student at the Scripps Institution of Oceanography. This was the result of several marine geologic expeditions to the western Pacific.


See also

*
Back-arc region 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- ...
* Forearc basin * Intra-arc basin


Notes


References

* * Taylor, Brian. (1995). ''Backarc Basins: Tectonics and Magmatism.'' New York: Plenum Press.
OCLC 32464941
* * * * * * * * * * *Hess, Henry H. (1962). "History of Ocean Basins". Petrological Studies: A volume to honor A .F. Buddington. 599-620 * * * * *


External links


Animation of subduction, trench rollback and back-arc basin expansion
in EGU GIFT2017: Shaping the Mediterranean from the inside out, YouTube. {{physical oceanography, expanded=other Marine geology Plate tectonics Sedimentology