The Agulhas Basin is an

oceanic basin In hydrology, an oceanic basin (or ocean basin) is anywhere on Earth that is covered by seawater. Geologically, ocean basins are large  geologic basins that are below sea level. Most commonly the ocean is divided into basins fol ...

located south of

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring coun ...

where the

South Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Africa, Europe an ...

and south-western

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by ...

meet. Part of the

African Plate The African Plate is a major tectonic plate that includes much of the continent of Africa (except for its easternmost part) and the adjacent oceanic crust to the west and south. It is bounded by the North American Plate and South American Plate ...

, it is bounded by the

Agulhas Ridge Agulhas (''needles'' in the Portuguese language — or ) may refer to: * Cape Agulhas, the southernmost point of Africa * L'Agulhas, a town near the Cape * Cape Agulhas Lighthouse, located at the Cape * Cape Agulhas Local Municipality, the municipa ...

(part of the Agulhas-Falkland Fracture Zone) to the north and the Southwest Indian Ridge to the south; by the

Meteor Rise A meteoroid () is a small rocky or metallic body in outer space. Meteoroids are defined as objects significantly smaller than asteroids, ranging in size from grains to objects up to a meter wide. Objects smaller than this are classified as micr ...

to the west and the Agulhas Plateau to the east. Numerous bathymetric anomalies hint at the basin's dynamic tectonic history.

Geology

In a Late Paleocene (59-56 Ma) reconstruction of the opening of the South Atlantic Ocean (i.e. the separation of

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sou ...

and

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

during the Gondwana break-up) the Meteor Rise lies conjugate to the Islas Orcadas Rise (east of the

Falkland Plateau Falkland may refer to: * Falkland, British Columbia, a community in Canada * Falkland, Nova Scotia, a community in Canada * Falkland Islands, an archipelago in the south Atlantic Ocean * Falkland, Fife, a former burgh in Fife, Scotland ** Falklan ...

). The separation of the Meteor Rise and the Islas Orcadas Rise marks the beginning of the formation of the Agulhas Basin. The Agulhas Ridge extends from the northern tip of the Meteor Rise towards the

Agulhas Bank The Agulhas Bank (, from Portuguese for Cape Agulhas, ''Cabo das Agulhas'', "Cape of Needles") is a broad, shallow part of the southern African continental shelf which extends up to south of Cape Agulhas before falling steeply to the abyssal p ...

south of

. The ridge, however, ends abruptly in a small plateau at where it intersects a northeastward-trending spreading centre (the Agulhas Rift) that was abandoned during the Early Paleocene (61 Ma). The presence of a short-lived tectonic plate between these structures was first proposed by . They named it the Malvinas Plate and proposed that it was active from 90 Ma until the spreading ceased in the Agulhas Basin at 65 Ma. The plate is located at a proto- Bouvet Triple Junction. found that

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

(100-66 Ma) fracture zones generated on the Agulhas Rift do not align with those north of the Agulhas Fracture Zone and cannot therefore have been formed by the spreading of South America and Africa. Furthermore, magnetic anomalies on the Malvinas Plate do not align with their conjugates on the African Plate if the spreading rates and directions of South America and Africa are used as a guide. They also noted that the Agulhas Fracture Zone do not lay perpendicular to traces of the South America-Africa spreading north of it and cannot, therefore, have been generate by this spreading. The Agulhas Rift is the abandoned Malvinas-Africa ridge crest. 97 Ma the plate boundary in the Agulhas Basin was reorganised when the

Mid-Atlantic Ridge The Mid-Atlantic Ridge is a mid-ocean ridge (a divergent or constructive plate boundary) located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. In the North Atlantic, the ridge separates the North A ...

made an eastward jump. This brought the boundary towards the Agulhas Plateau where excessive volcanism was building a

large igneous province A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive (sills, dikes) and extrusive (lava flows, tephra deposits), arising when magma travels through the crust towards the surface. The formation ...

. The inception of the Malvinas Plate accompanied this shortening of the Agulas Fracture Zone. 61 Ma, the Malvinas Plate was finally incorporated into the African Plate when the Malvinas-Africa ridge was abandoned as the result of a westward ridge jump along the Agulhas-Falkland Fracture Zone. This second ridge jump reduced one of the most spectacular fracture zones in Earth's history — in length — to .

Oceanography

The

Agulhas Current The Agulhas Current () is the western boundary current of the southwest Indian Ocean. It flows south along the east coast of Africa from 27°S to 40°S. It is narrow, swift and strong. It is suggested that it is the largest western boundary curren ...

flows south along the African east coast. When it reaches the southern tip of Africa, it retroflects back into the Indian Ocean. At this retroflection it leaks warm core

eddies In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object. Fluid ...

known as Agulhas rings into the South Atlantic. This mesoscale anti-cyclonic rings feed the

Atlantic meridional overturning circulation The Atlantic meridional overturning circulation (AMOC) is part of a global thermohaline circulation in the oceans and is the zonally integrated component of surface and deep currents in the Atlantic Ocean. It is characterized by a northward fl ...

(AMOC) and are therefore believed to affect the global climate, although the connection between the leakage, AMOC, and climate change is still poorly understood. In the Agulhas basin, half of these rings are subdivided one or several times. A majority of the subdivided rings split at or near the bathymetric obstacles on the western side of the Agulhas Basin but almost a quarter of them also merge there either because of the obstacles or because of intense interaction with other rings. Antarctic Bottom Water (AABW) originated in the Oligocene with the opening of the

Drake Passage The Drake Passage (referred to as Mar de Hoces Hoces Sea"in Spanish-speaking countries) is the body of water between South America's Cape Horn, Chile and the South Shetland Islands of Antarctica. It connects the southwestern part of the Atla ...

and the Tasmanian Seaway and resulted in the thermal insulation of Antarctica. AABW mixes with other masses in the

Antarctic Circumpolar Current The Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise (as seen from the South Pole) from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feat ...

(ACC) to form the

Circumpolar Deep Water Circumpolar Deep Water (CDW) is a designation given to the water mass in the Pacific and Indian oceans that is a mixing of other water masses in the region. It is characteristically warmer and saltier than the surrounding water masses, causing CD ...

(CDW). In the Agulhas Basin CDW flows northward and is deflect mostly westward by the Agulhas Ridge. A branch of CDW, however, enters the

Cape Basin A cape is a clothing accessory or a sleeveless outer garment which drapes the wearer's back, arms, and chest, and connects at the neck. History Capes were common in medieval Europe, especially when combined with a hood in the chaperon. T ...

west of the ridge from where it flows west along the northern side of the ridge before being deflected north-east (along the

Walvis Ridge The Walvis Ridge (''walvis'' means whale in Dutch and Afrikaans) is an aseismic ocean ridge in the southern Atlantic Ocean. More than in length, it extends from the Mid-Atlantic Ridge, near Tristan da Cunha and the Gough Islands, to the Africa ...

) at the eastern end of the ridge. This long detour in the Cape Basin and mixing with

North Atlantic Deep Water North Atlantic Deep Water (NADW) is a deep water mass formed in the North Atlantic Ocean. Thermohaline circulation (properly described as meridional overturning circulation) of the world's oceans involves the flow of warm surface waters from the ...

(NADW) results in warmer water than other CDW masses.

Antarctic Intermediate Water Antarctic Intermediate Water (AAIW) is a cold, relatively low salinity water mass found mostly at intermediate depths in the Southern Ocean. The AAIW is formed at the ocean surface in the Antarctic Convergence zone or more commonly called the Ant ...

(AAIW) flows above the CDW in the Agulhas Basin in an anti-cyclonic path (in contrast to the cyclonic path followed by CDW.)

References

Notes

Sources

* * * * * * * * * {{Coord, 45, S, 20, E, display=title Oceanic basins of the Atlantic Ocean Oceanic basins of the Indian Ocean