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Southwest Indian Ridge
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Southwest Indian Ridge
The Southwest Indian Ridge (SWIR) is a mid-ocean ridge located along the floors of the south-west Indian Ocean and south-east Atlantic Ocean. A divergent tectonic plate boundary separating the Somali plate to the north from the Antarctic plate to the south, the SWIR is characterised by ultra-slow spreading rates (only exceeding those of the Gakkel Ridge in the Arctic) combined with a fast lengthening of its axis between the two flanking triple junctions, Rodrigues (20°30′S 70°00′E / 20.500°S 70.000°E) in the Indian Ocean and Bouvet (54°17′S 1°5′W / 54.283°S 1.083°W) in the Atlantic Ocean.
The spreading rate along the SWIR varies: the transition between slow (30 mm/yr) and ultra-slow (15 mm/yr) spreading occur at magnetic anomaly C6C (c. 24 Ma). This occurs between 54° and 67°E, the deepest, and perhaps coldest and most melt-poor, part of Earth's mid-ocean ridge system. Crustal thickness decreases quickly as spreading rates drop below c. 20 mm/yr and in the SWIR there is an absence of volcanic activity along 100 km (62 mi) stretches of ridge axis.
Along large sections, the SWIR runs obliquely relative to the spreading direction, typically about 60°. Because obliquity increases ridge length while decreasing mantle upwelling rates, the SWIR is transitional between slow and ultra-slow ridges. The slow-spreading sections of the SWIR have magmatic segments linked by transform faults, while the ultra-slow sections lack such transforms and have magmatic segments linked by amagmatic troughs.
Spreading in the SWIR is slow, but the plate boundary is intersected by the much slower but more diffuse Nubian–Somalian boundary. The variation in spreading rates indicate the SWIR is not a spreading centre between two rigid plates, but that the previously assumed single African plate north of the SWIR is in fact divided into three plates: the Nubian, Lwandle, and Somalian plates.
The location on the SWIR of this "diffuse" triple junction between the Nubian, Somali, and Antarctic plates has been estimated to between 26°E and 32°E or just west of the Andrew Bain transform fault. This diffuse triple junction forms the southern end of the East African Rift system.
180 Ma-old rocks, dated from zircons in diorite and gabbro, were dredged from a location 60 km (37 mi) south of the SWIR in 2010. This age is comparable to that of the break-up of Gondwana, the opening of the Indian Ocean, and emplacement of the Karoo Large Igneous Province (179–183 Ma) – in sharp contrast the Neogene age of the ocean floor near the SWIR. It can be assumed the rocks were deposited near the SWIR by an external force, such as an ice-rafting or a tsunami, but the SWIR is located far away from any continental margin and rocks of similar age have been reported from the Mid-Atlantic Ridge. If the rocks came directly out of the mantle it would have lost most of its isotopic lead. Ice-rafted dropstones commonly show signs of rounding.
Hydrothermal circulation at mid-ocean ridges can, however, bring intrusive rocks into the shallow mantle, and it is possibly a good candidate in this case. Most rocks in Africa facing the SWIR are Archean cratons. The Neoproterozoic Pan-African Orogenic Belt, however, was accreted during the closure of the Mozambique Ocean and some rocks from eastern Africa, Madagascar, and Antarctica are associated with this event. During the break-up of Gondwana the Karoo volcanics intruded the Pan-African rocks and it is possible, rather than evident, that these rock found their way to the SWIR this way. Because spreading in the SWIR is ultra-slow, the mantle beneath should be abnormally cool, which could prevent melting of the rocks.
The western end of the SWIR, known as the Bouvet Ridge, is bounded by the Bouvet and Moshesh transforms north and south of it respectively. The Bouvet Ridge is 110 km (68 mi) long with a full spreading rate of 14.5 mm/a (0.57 in/year) during the last 3 Ma. The axial valley is a kilometre deep, typical of slow-spreading ridges, and 16 km wide, which is unusually wide. The zero-age axis lies 2,000 m (6,600 ft) below sea level in the central segment, but deeper closer to the two transforms: This is roughly a kilometre shallower than similar slow-spreading ridges, probably because of the vicinity to the BTJ.
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Southwest Indian Ridge
The Southwest Indian Ridge (SWIR) is a mid-ocean ridge located along the floors of the south-west Indian Ocean and south-east Atlantic Ocean. A divergent tectonic plate boundary separating the Somali plate to the north from the Antarctic plate to the south, the SWIR is characterised by ultra-slow spreading rates (only exceeding those of the Gakkel Ridge in the Arctic) combined with a fast lengthening of its axis between the two flanking triple junctions, Rodrigues (20°30′S 70°00′E / 20.500°S 70.000°E) in the Indian Ocean and Bouvet (54°17′S 1°5′W / 54.283°S 1.083°W) in the Atlantic Ocean.
The spreading rate along the SWIR varies: the transition between slow (30 mm/yr) and ultra-slow (15 mm/yr) spreading occur at magnetic anomaly C6C (c. 24 Ma). This occurs between 54° and 67°E, the deepest, and perhaps coldest and most melt-poor, part of Earth's mid-ocean ridge system. Crustal thickness decreases quickly as spreading rates drop below c. 20 mm/yr and in the SWIR there is an absence of volcanic activity along 100 km (62 mi) stretches of ridge axis.
Along large sections, the SWIR runs obliquely relative to the spreading direction, typically about 60°. Because obliquity increases ridge length while decreasing mantle upwelling rates, the SWIR is transitional between slow and ultra-slow ridges. The slow-spreading sections of the SWIR have magmatic segments linked by transform faults, while the ultra-slow sections lack such transforms and have magmatic segments linked by amagmatic troughs.
Spreading in the SWIR is slow, but the plate boundary is intersected by the much slower but more diffuse Nubian–Somalian boundary. The variation in spreading rates indicate the SWIR is not a spreading centre between two rigid plates, but that the previously assumed single African plate north of the SWIR is in fact divided into three plates: the Nubian, Lwandle, and Somalian plates.
The location on the SWIR of this "diffuse" triple junction between the Nubian, Somali, and Antarctic plates has been estimated to between 26°E and 32°E or just west of the Andrew Bain transform fault. This diffuse triple junction forms the southern end of the East African Rift system.
180 Ma-old rocks, dated from zircons in diorite and gabbro, were dredged from a location 60 km (37 mi) south of the SWIR in 2010. This age is comparable to that of the break-up of Gondwana, the opening of the Indian Ocean, and emplacement of the Karoo Large Igneous Province (179–183 Ma) – in sharp contrast the Neogene age of the ocean floor near the SWIR. It can be assumed the rocks were deposited near the SWIR by an external force, such as an ice-rafting or a tsunami, but the SWIR is located far away from any continental margin and rocks of similar age have been reported from the Mid-Atlantic Ridge. If the rocks came directly out of the mantle it would have lost most of its isotopic lead. Ice-rafted dropstones commonly show signs of rounding.
Hydrothermal circulation at mid-ocean ridges can, however, bring intrusive rocks into the shallow mantle, and it is possibly a good candidate in this case. Most rocks in Africa facing the SWIR are Archean cratons. The Neoproterozoic Pan-African Orogenic Belt, however, was accreted during the closure of the Mozambique Ocean and some rocks from eastern Africa, Madagascar, and Antarctica are associated with this event. During the break-up of Gondwana the Karoo volcanics intruded the Pan-African rocks and it is possible, rather than evident, that these rock found their way to the SWIR this way. Because spreading in the SWIR is ultra-slow, the mantle beneath should be abnormally cool, which could prevent melting of the rocks.
The western end of the SWIR, known as the Bouvet Ridge, is bounded by the Bouvet and Moshesh transforms north and south of it respectively. The Bouvet Ridge is 110 km (68 mi) long with a full spreading rate of 14.5 mm/a (0.57 in/year) during the last 3 Ma. The axial valley is a kilometre deep, typical of slow-spreading ridges, and 16 km wide, which is unusually wide. The zero-age axis lies 2,000 m (6,600 ft) below sea level in the central segment, but deeper closer to the two transforms: This is roughly a kilometre shallower than similar slow-spreading ridges, probably because of the vicinity to the BTJ.
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