Mariana Trough
Encyclopedia
The Mariana Trough is an active back-arc basin
in the western Pacific Ocean
. It is an integral part of the Izu-Bonin-Mariana Arc
system.
or California
. The Trough is crudely crescent-shaped, opening on the south; it is bounded to the east by the active Mariana arc, to the west by the remnant arc of the West Mariana Ridge , and to the south by the Challenger Deep
, part of the Mariana Trench
. It narrows northward until the Mariana arc and West Mariana Ridge meet at about 24°N. It is widest in the middle, at 18°N, where it is about 240 km wide, and narrows to about half this at its southern, open end. Depths in the basin are distributed asymmetrically, being greater adjacent to the West Mariana Ridge than next to the active arc, due to a westward-thinning wedge of volcaniclastic sediments derived from the active arc, and also less thermal buoyancy of the mantle
. Where not covered by sediments, the seafloor is deeper and bathymetry
more rugged than normal. The most recent, "zero-age" seafloor of the Philippine Sea
, including the Mariana Trough, lies at a mean depth of 3200 m compared to normal zero-age seafloor depths of 2500 m .
and a northern third characterized by rifting. From as far north as 19°45'N south to 13°10’N, the spreading ridge has the typical morphology of a slow-spreading ridge, with an axial graben
that is sometimes occupied by a central ridge where volcanic activity is concentrated (neovolcanic zone); south of this the ridge more resembles a fast-spreading ridge, probably because magma
supply is enhanced by proximity to the arc . Spreading half-rates in the region between 16° and 18°N are estimated at 1.5 to 2.2 cm/year . The ridge becomes punctiform north of 18°30', and true seafloor spreading does not occur north of 19°45'N (but see the different conclusion of ). Rifting forms a series of amagmatic deeps between 19°45'N and 21°10'N called the 'Central Graben' . These basins have low heatflow, lack igneous activity, and are the deepest parts of the Mariana Trough (>5400 m). The deepest part of the Central Graben is also unique among active back-arc basin
s in exposing mantle peridotite
s along the extension axis . Extension north of the Central Graben occurs by combined tectonic and magmatic processes that are distinct from seafloor spreading, in a region known as the Volcano-Tectonic Zone (VTZ; ). The VTZ corresponds with a part of the Mariana Trough where the crust thickens from 6 to 15 km. The southern VTZ is dominated by fissure eruptions associated with a ridge-like feature, ~30 km long, which rises to less than 2800 m water depth and which is similar to the inflated segment at the southern terminus of the spreading ridge. The northern VTZ is dominated by point-source volcanism, with edifices spaced 50–60 km apart alternating with rift basins. There is no volcanic activity along the adjacent arc segment and it appears that the extension axis has captured the arc magma supply between the Kasuga cross-chain at 22°N and Nikko near 23°N, where the extension axis intersects the arc. North of this, incipient rifting is magmatically manifested by unusual, shoshonitic lavas
of the Hiyoshi complex, Fukutoku-oka-no-ba (or Sin Iwo Jima), and Iwo Jima
. Rifting and spreading are inferred to be propagating northward at a rate of 10 to 40 cm/year , so the variations in tectonic and magmatic style seen along-strike north of 18°N provide an example of the sequence of events that occur at any one section across the back-arc basin as the rift evolves from updoming through rifting to true spreading.
Back-arc basin
Back-arc basins are geologic features, submarine basins associated with island arcs and subduction zones.They are found at some convergent plate boundaries, presently concentrated in the Western Pacific ocean. Most of them result from tensional forces caused by oceanic trench rollback and the...
in the western Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
. It is an integral part of the Izu-Bonin-Mariana Arc
Izu-Bonin-Mariana Arc
The Izu-Bonin-Mariana arc system is an outstanding example of a plate tectonic convergent boundary. IBM extends over 2800 km south from Tokyo, Japan, to beyond Guam, and includes the Izu Islands, Bonin Islands, and Mariana Islands; much more of the IBM arc system is submerged below sealevel...
system.
Location and Bathymetry
The Mariana Trough stretches 1300 km from north to south, about the distance from Los Angeles CA to Portland OR, Tokyo, Japan to Seoul, Korea, or London, England to Rome, Italy. The Mariana Trough has roughly the dimensions and areal extent of JapanJapan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
or California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
. The Trough is crudely crescent-shaped, opening on the south; it is bounded to the east by the active Mariana arc, to the west by the remnant arc of the West Mariana Ridge , and to the south by the Challenger Deep
Challenger Deep
The Challenger Deep is the deepest known point in the oceans, with a depth of to by direct measurement from submersibles, and slightly more by sonar bathymetry . It is located at the southern end of the Mariana Trench near the Mariana Islands group...
, part of the Mariana Trench
Mariana Trench
The Mariana Trench or Marianas Trench is the deepest part of the world's oceans. It is located in the western Pacific Ocean, to the east of the Mariana Islands. The trench is about long but has a mean width of only...
. It narrows northward until the Mariana arc and West Mariana Ridge meet at about 24°N. It is widest in the middle, at 18°N, where it is about 240 km wide, and narrows to about half this at its southern, open end. Depths in the basin are distributed asymmetrically, being greater adjacent to the West Mariana Ridge than next to the active arc, due to a westward-thinning wedge of volcaniclastic sediments derived from the active arc, and also less thermal buoyancy of the mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
. Where not covered by sediments, the seafloor is deeper and bathymetry
Bathymetry
Bathymetry is the study of underwater depth of lake or ocean floors. In other words, bathymetry is the underwater equivalent to hypsometry. The name comes from Greek βαθύς , "deep", and μέτρον , "measure"...
more rugged than normal. The most recent, "zero-age" seafloor of the Philippine Sea
Philippine Sea
The Philippine Sea is a marginal sea east and north of the Philippines occupying an estimated surface area of 2 million mi² on the western part of the North Pacific Ocean...
, including the Mariana Trough, lies at a mean depth of 3200 m compared to normal zero-age seafloor depths of 2500 m .
Extension style
The extension axis for the Mariana Trough can be subdivided along strike into a southern two-thirds characterized by slow seafloor spreadingSeafloor spreading
Seafloor spreading 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. Seafloor spreading helps explain continental drift in the theory of plate tectonics....
and a northern third characterized by rifting. From as far north as 19°45'N south to 13°10’N, the spreading ridge has the typical morphology of a slow-spreading ridge, with an axial graben
Graben
In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural....
that is sometimes occupied by a central ridge where volcanic activity is concentrated (neovolcanic zone); south of this the ridge more resembles a fast-spreading ridge, probably because magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
supply is enhanced by proximity to the arc . Spreading half-rates in the region between 16° and 18°N are estimated at 1.5 to 2.2 cm/year . The ridge becomes punctiform north of 18°30', and true seafloor spreading does not occur north of 19°45'N (but see the different conclusion of ). Rifting forms a series of amagmatic deeps between 19°45'N and 21°10'N called the 'Central Graben' . These basins have low heatflow, lack igneous activity, and are the deepest parts of the Mariana Trough (>5400 m). The deepest part of the Central Graben is also unique among active back-arc basin
Back-arc basin
Back-arc basins are geologic features, submarine basins associated with island arcs and subduction zones.They are found at some convergent plate boundaries, presently concentrated in the Western Pacific ocean. Most of them result from tensional forces caused by oceanic trench rollback and the...
s in exposing mantle peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
s along the extension axis . Extension north of the Central Graben occurs by combined tectonic and magmatic processes that are distinct from seafloor spreading, in a region known as the Volcano-Tectonic Zone (VTZ; ). The VTZ corresponds with a part of the Mariana Trough where the crust thickens from 6 to 15 km. The southern VTZ is dominated by fissure eruptions associated with a ridge-like feature, ~30 km long, which rises to less than 2800 m water depth and which is similar to the inflated segment at the southern terminus of the spreading ridge. The northern VTZ is dominated by point-source volcanism, with edifices spaced 50–60 km apart alternating with rift basins. There is no volcanic activity along the adjacent arc segment and it appears that the extension axis has captured the arc magma supply between the Kasuga cross-chain at 22°N and Nikko near 23°N, where the extension axis intersects the arc. North of this, incipient rifting is magmatically manifested by unusual, shoshonitic lavas
Shoshonite
Shoshonite is a basaltic rock, properly a potassic trachyandesite, composed of olivine, augite and plagioclase phenocrysts in a groundmass with calcic plagioclase and sanidine and some dark-colored volcanic glass. Shoshonite gives its name to the shoshonite series and grades into absarokite with...
of the Hiyoshi complex, Fukutoku-oka-no-ba (or Sin Iwo Jima), and Iwo Jima
Iwo Jima
Iwo Jima, officially , is an island of the Japanese Volcano Islands chain, which lie south of the Ogasawara Islands and together with them form the Ogasawara Archipelago. The island is located south of mainland Tokyo and administered as part of Ogasawara, one of eight villages of Tokyo...
. Rifting and spreading are inferred to be propagating northward at a rate of 10 to 40 cm/year , so the variations in tectonic and magmatic style seen along-strike north of 18°N provide an example of the sequence of events that occur at any one section across the back-arc basin as the rift evolves from updoming through rifting to true spreading.