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fz rodrigues tj 3 tectonic history 4 see also 5 references toggle references subsection 5 1 notes 5 2 sources toggle the table of contents southwest indian ridge 26 languages العربية azərbaycanca беларуская català deutsch español euskara français galego hrvatski italiano 日本語 қазақша 한국어 ripoarisch latina lietuvių polski русский српски srpski kiswahili українська oʻzbekcha ўзбекча 吴语 粵語 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikimedia commons wikidata item appearance move to sidebar hide coordinates 42 s 41 e 42 s 41 e 42 41 from wikipedia the free encyclopedia mid ocean ridge in the indian and atlantic oceans approximate surface projection on southern oceans of southwest indian ridge approximate surface projection on oceans of fracture zones is shown key fracture zones probable extension of fracture zones fracture zone of relevance to article probable extension relevant to article mid oceanic ridges sea land clicking on the rectangle icon enables full window and mouse over with text detail 1 separating the african or nubian somali plates and antarctic plates the southwest indian ridge swir stretches 7 700 km 4 800 mi from the atlantic ocean to the indian ocean with an average spreading rate of 14 15 millimetres per year 0 55 0 59 in year the swir is one of the slowest spreading mid ocean ridges on earth characterised by numerous large transform offsets most of the swir is highly segmented and oblique relative to the spreading direction 2 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 20 500 70 000 in the indian ocean and bouvet 54 17 s 1 5 w 54 283 s 1 083 w 54 283 1 083 in the atlantic ocean 3 geological setting edit topography of the swir white dots are hotspots dashed lines are fracture zones spreading rates edit 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 4 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 5 diffuse plate boundaries edit spreading in the swir is slow but the plate boundary is intersected by the much slower but more diffuse nubian somalian boundary 6 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 7 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 8 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 9 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 10 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 10 subsections edit bouvet tj andrew bain tf edit 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 11 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 12 between 9 and 25 e the swir trends e w and lacks transforms this section is composed of orthogonal magmatic accretionary segments linked by oblique amagmatic accretionary segments 2 the oblique portion of this area 9 to 16 e the oblique supersegment is highly variable in axial orientation ranging from orthogonal to 56 and its series of magmatic and amagmatic segments results in abruptly fluctuating magmatism and ultra slow spreading 13 west of a discontinuity at 16 e axial depth drops 500 m and there is an abrupt change in morphology and magnetism in the western end of this area 9 30 11 45 a short magmatic ridge segment intersects the shaka fz 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 east of the seamount 11 30 10 24 e there is a 180 km long and 4 200 m deep amagmatic segment 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 5 the orthogonal supersegment 16 to 25 e in contrast is almost perfectly orthogonal relative to the spreading direction and is composed of magmatic accretionary segments linked by short non transform offsets where the obliquity of the swir increases so does its length this lengthening results in a decrease in mantle upwelling and a ridge geometry characteristic of ultra slow spreading ridges 12 mm yr 13 the orthogonal supersegment is similar to larger ridge segments of the mid atlantic ridge 5 andrew bain tf edit 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 and 53 s 27 e 14 15 the largest of these the 750 km long long andrew bain fz is where the nubia somalia boundary intersects the swir 15 the active section of the andrew bain tf represents the largest age offset 65 ma of any oceanic transform fault and it is also the widest 120 km its extension extends south from the mozambique escarpment between the mozambique ridge and basin to the astrid ridge off antarctica 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 16 the swir crosses the flank of the swell before reaching the marion hotspot at 36 e 17 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 18 andrew bain tf melville fz edit between the marion hotspot and gallieni fz there is an irregular segmentation with relatively shallow axial depth 18 between prince edward fz and atlantis ii fz 35 57 e all major transform faults and their 35 ma associated magnetic anomalies are increasingly trending more directly north south magnetic anomalies in the mozambique basin indicate this is the dominant spreading direction for the past 80 ma 19 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 20 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 this is probably due to the interaction with the crozet hotspot the increased magmatism of which resulted in the large crozet volcanic plateau at c 10 ma the hotspot also triggers thermal plumes and incorporates small amounts of lower mantle material resulting in a mixed ocean island basalt oib mid ocean ridge basalt morb signature the crozet hotspot bank is however located more than 1000 km from the swir and ridge hotspot interaction at distances beyond 500 km is theoretically supposed to be insignificant the kerguelen and réunion hotspots are however probably interacting with the southeast indian ridge and central indian ridge over similar distances as suggested by volcanic chains and lineaments connecting those ridges and hotspots the absence of such lineaments between the swir and crozet can be explained by plate age and thickness plates older than 25 ma are thought to be too thick for plumes to penetrate 21 between the gallieni and melville fzs the swir was originally roughly perpendicular to the spreading direction with few and small offsets about 40 ma a clockwise change in spreading direction quickly resulted in evenly spaced offsets and a more rugged terrain since then the atlantis ii transform fault has grown while the offsets west and east of it have begun to disappear 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 22 melville fz rodrigues tj edit 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 17 an increase in axial depth east of 49 e reflects non magmatic extension 18 the segmentation and morphology in the axial valley of the easternmost swir is unique to ultra slow spreading ridges 3000 m high ridge segments are linked by more than 100 km long axial segments there is no volcanism along this section the flanks of the ridge axis are wide and lack a volcanic crustal layer these flanks are rounded and smooth and lack the corrugated pattern associated with oceanic core complexes this non volcanic sea floor is made of seawater altered mantle derived rocks brought to the surface by large scale detachment faults during the last 10 ma these detachment faults have flipped back and forth across the ridge axis and produced almost all the divergence along this section of the swir 23 in the easternmost swir east of melville fz 60 45 e the mantle is unusually cold and the crust thin 3 7 km in average resulting in only partial melting in the mantle and a decrease in melt supply to the swir in this region 23 this shortage in magma supply has resulted in fewer but taller seamounts east of melville there are more than 100 seamounts per 10 3 km 2 about 50 m tall west of melville whereas east of melville there are fewer than 10 seamounts per 10 3 km 2 more than 100 m tall 24 tectonic history edit the swir is characterised by deep sub parallel and well delineated fracture zones sometimes deeper than 6 000 m 20 000 ft delineated by elevated rims sometimes reaching up to 2 000 m 6 600 ft below sea level these fracture zones are very long and often align with older structures near the continental shelves 14 these fracture zones and their extensions into the agulhas basin are flow lines describing the motion of africa and antarctica since break up of gondwana in the late cretaceous 14 25 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 26 the spreading direction between the continents started to change around 74 ma and 69 64 ma spreading slowed c 1 cm yr then changed orientation to ne sw fracture zones near prince edward fz are from the eocene much younger than could be assumed from their length 27 see also edit central indian ridge southeast indian ridge rodrigues triple junction references edit notes edit general citations for named fracture zones are at page wikipedia map data fracture zone and specific citations are in interactive detail 1 2 standish et al 2008 regional setting p 3 5 patriat et al 1997 abstract sauter et al 2011 introduction p 911 1 2 3 dick lin schouten 2003 the swir from 9 to 25 e pp 406 409 chu gordon 1999 pp 64 67 demets gordon argus 2010 southwest indian ridge plate motions p 38 fig 29 p 37 horner johnson et al 2005 abstract cheng et al 2016 samples and results p 1 1 2 cheng et al 2016 discussion pp 4 7 trukhin et al 1999 introduction pp 1 2 ligi et al 1999 westernmost southwest indian ridge pp 29372 29375 1 2 standish et al 2008 regional setting p 6 6 7 1 2 3 ...