Southwest Indian Ridge
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.
[6] 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.
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.
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.
The rough topography here obscures the SWIR which runs into the western flank of the Joseph Mayes Seamount, one of few volcanic centres along the oblique supersegment.
The seamount splits an old peridotite block, the remains of which project on either side of the ridge, and fills the rift valley in between, resulting in a double-peaked volcano sitting on the SWIR.
Reaching a maximum depth of 4,700 m, its deepest part has a rough floor void of signs of recent volcanism but filled with irregular horst blocks partially made of serpentinised peridotite.
[4] A series of fracture zones — Du Toit, Andrew Bain, Marion, and Prince Edward — offsets the SWIR 1,230 km (760 mi) between 45°S,35°E—53°S,27°E.
[14] The active section of the Andrew Bain TF represents the largest age-offset (65 Ma) of any oceanic transform fault and it's also the widest (120 km).
East of the Andrew Bain TF is the "Marion Swell", the geoid high of the Southern Ocean, between 35°E and 50.5°E, and the Madagascar Plateau and the Del Cano Rise.
[19] Between Indomed and Gallieni FZs the SWIR is more shallow and has a higher magma supply than neighbouring deeper sections; the crust is also thicker and/or the mantle hotter.
About 40 Ma in the future the Gallieni, Atlantis II, and Melvilles transform faults will continue to grow while the SWIR segments between them will keep most of their present length and shape.
[21] East of the Indomed FZ (south of Madagascar) the SWIR is the product of the 64 million years of eastward propagation of the Rodriguez triple junction.
This section is composed of regularly spaced non-transform discontinuities, short oblique amagmatic segments, and the Atlantis II, Novara, and Melville transforms.
[23] The SWIR is characterised by deep, sub-parallel, and well-delineated fracture zones, sometimes deeper than 6,000 km (3,700 mi), delineated by elevated rims, sometimes reaching up to 2,000 m (6,600 ft) below sea level.
[13][24] The SWIR opened during the break-up of Gondwana when Antarctica broke off from Africa during the Permian-Triassic Karoo large igneous province c. 185–180 Ma in what is now the Mozambique Basin and the Riiser-Larsen Sea.
