Agulhas Plateau
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The Agulhas Plateau is an oceanic plateau located in the 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 ...
about south of South Africa. It is a remainder of a large igneous province (LIP), the Southeast African LIP, that formed (Ma) at or near the triple junction where Gondwana broke-up into
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest co ...
,
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 souther ...
, 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 ...
. The plateau formed together with Northeast Georgia Rise and Maud Rise (now located near the Falkland Island and Antarctica respectively) when the region passed over the
Bouvet hotspot Bouvet can have the following meanings: Places * Bouvet Island, an uninhabited Norwegian island in the South Atlantic People * Joachim Bouvet (1656–1730), French Jesuit who worked in China, leading member of the Figurist movement * Jean-Baptiste ...
.


Geology


History of research

The Agulhas Plateau is one of the key structures in the reconstruction of the Gondwana break-up. It was first mapped in 1964 (i.e. part of what would become the Heezen- Tharp map of the world's ocean floor finally published in 1977), but its crustal composition, paleoposition, and geological origin remained enigmatic for decades. The boundary between the Earth's crust and the mantle (the Moho) rises from between the Agulhas Bank (south of South Africa) and the Agulhas passage (south of the bank), typical for a continent-ocean transition. The Agulhas Passage consists of 120-160 Ma-old oceanic crust, whereas the 100-80 Ma-old Agulhas Plateau rises above the surrounding ocean floor while the Moho dips to between below it. The morphology of the basement below the northern plateau is irregular, suggestive of an oceanic origin. The basement below the southern plateau, however, is smooth, which has been interpreted as indicative of a possible continental origin. ODP drilling at the Northeast Georgia Rise (north-east of
South Georgia South Georgia ( es, Isla San Pedro) is an island in the South Atlantic Ocean that is part of the British Overseas Territory of South Georgia and the South Sandwich Islands. It lies around east of the Falkland Islands. Stretching in the east ...
) indicated that the Agulhas Plateau and the rise formed together and must have an oceanic origin. Some researchers, nevertheless, remained convinced that the plateau was at least partly of continental origin. Over several decades analyses of geoid, MAGSAT, gravitational, and magnetic anomalies data collected across the plateau were used as arguments for both an oceanic and a continental origin. Uenzelmann-Neben, Gohl and Ehrhardt could finally present seismic evidence that showed that the Agulhas Plateau was a large igneous province (LIP) made entirely of oceanic crust.


Large igneous province

The Agulhas Plateau is the remaining core of a large-scale volcanism that started in the
Lazarev Sea The Lazarev Sea (, ''More Lazareva'') is a proposed name for a marginal sea of the Southern Ocean. It would be bordered by two proposals from a 2002 International Hydrographic Organization (IHO) draft, a King Haakon VII Sea to the west and a R ...
(today off Antarctica) with the emplacement of the Karoo basalts 184 Ma. This process continued with the formation of the Mozambique Ridge (MOZR)-Agulhas Plateau LIP which was active in phases between 140-95 Ma. This formation coincides with the formation of the Kerguelen-Heard Plateau. The MOZR formed 140-122 Ma and must have reached its maximum extent about 120 Ma while the spreading zone between Africa and Antarctica was located under its eastern flank. The South Atlantic Ocean started to open-up 130 Ma when the Falkland Plateau moved westwards along what was becoming the Agulhas-Falkland Fracture Zone (AFFZ). In the wake of the Falkland Plateau, during the Cretaceous quiet interval, first the Natal Valley formed, then the
Transkei Basin Transkei (, meaning ''the area beyond he riverKei''), officially the Republic of Transkei ( xh, iRiphabliki yeTranskei), was an unrecognised state in the southeastern region of South Africa from 1976 to 1994. It was, along with Ciskei, a Ban ...
, a process completed 90 Ma. The process continued with the formation of the Agulhas Plateau— Northeast Georgia RiseMaud Rise LIP (AP-NEGR-MR LIP or Southeast African LIP) at the end of the
Early Cretaceous The Early Cretaceous (geochronological name) or the Lower Cretaceous ( chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145  Ma to 100.5 Ma. Geology Pr ...
(100 Ma). The AP-NEGR-MR LIP formed when the region passed over the Bouvet hotspot. About 94 Ma the main eruption ended and seafloor spreading detached the NEGR and MR from the AP. Before this separation the AP-NEGR-MR LIP consisted of of oceanic plateau. The MOZR and AP are today connected by a crustal corridor, the Transkei Rise, which rises above the surrounding ocean floor. This rise is thought to be the product of continuous but reduced volcanism during the 20 Ma period between the formation of the MOZR-AP LIP and AP-NEGR-MR LIP. Volcanic layers on the southern Agulhas Plateau where later overlaid by sediments in which traces of either sub-areal or shallow marine erosions indicate that the plateau was near sea-level. Southern Africa experienced two periods of
erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is di ...
and denudation during the Early and Mid-late Cretaceous. The driving forces behind these events is poorly understood, but both periods coincide with LIP formation: the first period (130-120 Ma) coincides with the initial stages of the Gondwana break-up and the second period (100-90 Ma) with the formation of the Agulhas LIP. Somehow, these two events led to the Mesozoic uplift of southern Africa.


Oceanography

The Antarctic Bottom Water (AABW) flows north-eastward into the Transkei Basin throw the Agulhas Passage and across the southern margin of the Agulhas Plateau. AABW then flows into the Mozambique Basin. Palaeoceanographic evidences show the presence of proto-AABW during the
Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but ...
(34-23 Ma) and that proto-AABW was strengthened 15 Ma and deflected southward by the increased flow of North Atlantic Deep Water (NADW). NADW flows north of the Agulhas Plateau through the Agulhas Passage into the Transkei Basin where it splits in two and continues into the Natal valley and the Indian Ocean.
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) originates at the water surface around Antarctica and flows northward into the Indian Ocean. At , it then flows westward along the African east-coast and the Agulhas Bank before retroflecting eastward across the Agulhas Plateau into the Indian Ocean. The Agulhas Current, the western boundary current of the Indian Ocean, retroflects abruptly into the Indian Ocean south-west of South Africa and becomes the Agulhas Return Current. Over the Agulhas Plateau the return current forms a major northward loop to bypass it.


See also

* African superswell


References


Notes


Sources

* * * * * * * * * * * * * * {{Large igneous provinces Plateaus of the Indian Ocean Large igneous provinces