The Mariana Trough is an active

back-arc basin A back-arc basin is a type of geologic Structural basin, basin, found at some convergent boundary, 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 . It is an integral part of the Izu–Bonin–Mariana Arc 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 Japan or

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

. 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 The Challenger Deep is the List of submarine topographical features#List of oceanic trenches, deepest known point of the seabed of Earth, located in the western Pacific Ocean at the southern end of the Mariana Trench, in the ocean territory o ...

, part of the

Mariana Trench The Mariana Trench is an oceanic trench located in the western Pacific Ocean, about east of the Mariana Islands; it is the deep sea, deepest oceanic trench on Earth. It is crescent-shaped and measures about in length and in width. The maxi ...

. 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 Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

, the seafloor is deeper and

bathymetry Bathymetry (; ) is the study of underwater depth of ocean floors ('' seabed topography''), river floors, or lake floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of wate ...

more rugged than normal. The most recent, "zero-age" seafloor of the

Philippine Sea The Philippine Sea is a List of seas#Marginal seas by ocean, marginal sea of the Pacific Ocean, Western Pacific Ocean east of the list of islands of the Philippines, Philippine Archipelago (hence the name) and the List of seas#Largest seas ...

, 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 Extension, extend or extended may refer to: Mathematics Logic or set theory * Axiom of extensionality * Extensible cardinal * Extension (model theory) * Extension (proof theory) * Extension (predicate logic), the set of tuples of values that ...

axis for the Mariana Trough can be subdivided along strike into a southern two-thirds characterized by slow

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 ...

and a northern third characterized by

rifting 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 Fault (geology), downfaulted depression, called a graben, or more commonly ...

. 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 In geology, a graben () is a depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...

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 () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

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

s in exposing mantle

peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high pr ...

s along the extension axis . Extension north of the Central Graben occurs by combined

tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

and

magmatic Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

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 is a submarine volcano that is part of the Volcano Islands in the Bonin Islands of Japan. It is located northeast of the island of South Iwo Jima. Geography The volcano is part of a larger elongated submarine volcano with two peaks and with ...

(or Sin Iwo Jima), and

Iwo Jima is one of the Japanese Volcano Islands, which lie south of the Bonin Islands and together with them make up the Ogasawara Subprefecture, Ogasawara Archipelago. Together with the Izu Islands, they make up Japan's Nanpō Islands. Although sout ...

. 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.

References

* * * * * * * * {{Cite journal, last=Yamazaki , first=T. , last2=Murakami , first2=F. , last3=Saito , first3=E. , year=1993 , title=Mode of seafloor spreading in the northern Mariana Trough , journal=Tectonophysics , volume=221 , pages=207–221 , doi=10.1016/0040-1951(93)90333-F , issue=2 , bibcode = 1993Tectp.221..207Y Geology of the Pacific Ocean Back-arc basins