Southwest Indian Ridge

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Southwest Indian Ridge on the left

The Southwest Indian Ridge (SWIR) is a mid-ocean ridge located along the floors of the southwest Indian Ocean and southeast Atlantic Ocean. A divergent tectonic plate boundary separating the African Plate to the north from the Antarctic Plate to the south, the SWIR is characterised by ultra-slow spreading rates combined with a fast lengthening of its axis between the two flanking triple junctions, Rodrigues in the Indian Ocean and Bouvet in the Atlantic.[1]

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 (ca. 24 Ma). This occurs between 54°–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.[2]

Geological setting[edit]

The SWIR extends for nearly 8,000 km (5,000 mi) between the Rodrigues Triple Point in the southern Indian Ocean to the Bouvet Triple Junction in the south Atlantic. It formed shortly after the breakup of Gondwana during the Mesozoic Era.[3][4]

The SWIR is one of the slowest spreading ridges with spreading rate of 30–36 millimetres per year (0.037–0.045 in/Ms). Marion Island, where the Marion hotspot is located, lies 250 km (160 mi) from the SWIR on 28 Ma crust. Bouvet Island, located 300 km (190 mi) from the Bouvet triple junction and 55 km (34 mi) from the SWIR, is located on 7 Ma crust, though the exact location of the Bouvet hotspot has not been determined.[4]

East of Bouvet at 25°E, SWIR is off-setted 1,800 km (1,100 mi) to the north-east by the Du Toit, Andrew Bain, Marion, and Prince Edward fracture zones (FZ). South of this offset is the geoid high of the Southern Ocean, the flank of which the SWIR crosses before reaching the Marion hotspot at 36°E. 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[5]

Major changes at Discovery FZ (42°E), Galliene FZ (52°E), and Melville FZ (60°E) define large-scale segmentation of the SWIR. Mean axial depth varies between 4,730 m (15,520 ft) between Melville FZ and Rodrigues TJ, a section underlain by either thin crust or cold mantle, to 3,050 m (10,010 ft) between Andrew Bain FZ and Discovery FZ, a section affected by the Marion hotspot.[6]

The SWIR is highly segmented, especially near Marion and Bouvet, and can be divided into seven subsections with distinct morphologies:[4]

  • Bouvet–10°E: short ridge segments closely-spaced transforms.
  • 10°E–15°E: 400 km (250 mi)-long with extremely low spreading rate; forms an oblique angle to the regional spreading direction resulting in a chaotic segmentation pattern.
  • 15°E–25°E: a series of short segments separated by non-transform offsets.
  • Andrew Bain Fracture Zone (700 km (430 mi))–Du Toit Fracture Zone (150 km (93 mi)): south of Africa SWIR makes a sharp northward turn between these two long fracture zones.
  • Marion–Gallieni Fracture Zone: irregular segmentation with relatively shallow axial depth.
  • Gallieni–Atlantis II Fracture Zone:
  • Melville Fracture Zone–Rodrigues: created by the 45 Ma eastward migration of the triple junction, resulting in an obliquity produced by a second-order segmentation. An increase in axial depth east of 49°E probably reflects non-magmatic extension.

The SWIR is classified as an ultra-slow spreading ridge with a current spreading rate of around 8 mm/yr, making it the second slowest in the world after the Gakkel Ridge. As with other slow-spreading ridges, magma supply along the ridge is often low or intermittent, leading to the formation of oceanic core complexes that expose lower crustal plutonic rocks through tectonic unroofing along low-angle detachment faults.

The SWIR has been studied for a range of issues.[7]

In situ Jurassic rocks[edit]

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.[8] 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 sign of rounding.[9]

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.[9]

References[edit]

Notes[edit]

  1. ^ Patriat et al. 1997, Abstract
  2. ^ Sauter et al. 2011, Introduction, p. 911
  3. ^ Norton & Sclater 1979
  4. ^ a b c Georgen, Lin & Dick 2001, Geological setting, pp. 11–12
  5. ^ Zhou & Dick 2013, Tectonic setting, p. 196
  6. ^ Mendel et al. 2003, Regional setting, pp. 3–4
  7. ^ Sclater & Harrison 1971; Ansorge et al. 2010; Hamelin & Allègre 1985; Leg 118 Shipboard Scientific Party 1988
  8. ^ Cheng et al. 2016, Samples and results, p. 1
  9. ^ a b Cheng et al. 2016, Discussion, pp. 4–7

Sources[edit]

Coordinates: 25°30′00″S 70°00′00″E / 25.5000°S 70.0000°E / -25.5000; 70.0000