TheSouth American–Antarctic Ridge or simplyAmerican-Antarctic Ridge (SAAR orAAR) (in Spanish:Dorsal Antártico-Americana) is thetectonicspreading center between theSouth American plate and theAntarctic plate. It runs along the sea-floor from theBouvet triple junction in the SouthAtlantic Ocean south-westward to a majortransform fault boundary east of theSouth Sandwich Islands. Near the Bouvet triple junction the spreading half rate is 9 mm/a (0.35 in/year), which is slow, and the SAAR has the rough topography characteristic of slow-spreading ridges.[1]
The boundary between the South American and Antarctic plates can be divided into three parts of which the SAAR forms the eastern third:[2]
The first stretches from theChile triple junction in theChile Trench at 46°S to the westernStraits of Magellan at 52°S. Since 15 Ma, theoceanic crust of the Antarctic plate is being slowly subducted (20–24 mm/a (0.79–0.94 in/year)) under South America along this trench which is currently extending northward. In the central part, between the Straits of Magellan and theSouth Sandwich Trench, the two large continental plates are separated by theScotia plate and a number of smaller plates east of it. During the past 40 Ma (or since theopening of the Drake Passage) the South Sandwich Trench has been migrating eastward due to the evolution of aback-arc basin, effectively consuming the SAAR.[2]
The third eastern part, i.e. the 'SAAR proper', has two long and several shorter transform faults separating short north-to-south-directed ridge crests. The motion in the SAAR is currently c. 20 mm/a (0.79 in/year) westward but it was originally closer to north–south. It can be inferred, based on fracture zone topography andmagnetic anomalies in theWeddell Sea, that this change in direction occurred during the Cretaceous and Cenozoic.[2]The western part of the SAAR is dominated by the earthquake-intensiveSouth Sandwichisland arc,fore-arc, andtrench. East of these structures the SAAR is composed of a series of north–south-oriented ridge crests, median valleys, and east–west-oriented transform faults. The major transform faults (from the east to the west) are the Konrad (displacement about 190 km),the Bullard (displacement more than 700 km), the Volcano (displacement about 100 km), and the southernmost South Sandwich (displacement up to 320 km) transform faults.[3] The topography of the SAAR is extreme, with valleys reaching 1.5–2 km (0.93–1.24 mi) deeper than adjacent ridges in average and maximum depth exceeding 3 km. The SAAR is more shallow near the Bouvet triple junction.[4] The Bouvet triple junction is between the American, African, and Antarctic lithospheric plates which has geographical coordinates of 55°S, 0°E.[3]
North of the SAAR theSouth Sandwich plate consumes the South American plate at a rate of 65.8 mm/yr driven by back-arc extension.This fast subduction has broken off the southern part of the South American plate between the north-eastern end of the South Sandwich Arc and the Mid-Atlantic Ridge, leaving a separate microplate called 'Sur' (Spanish for 'South') north of the SAAR. The southern part of this Sur microplate has probably also been broken off and is subducting independently under the South Sandwich Arc.[5]
The break-up ofGondwana began in the Mid- to Late Jurassic in what is today theMozambique Basin east of Africa, whereas the South American and African plates started to break apart during the Early Cretaceous. Between these events neither theMid-Atlantic Ridge nor the Bouvet triple junction existed and the SAAR formed a continuous ridge together with theSouthwest Indian Ridge.[6]
Around 106 Ma the eastern end of theFalkland Plateau separated from theAgulhas Bank, opening the South Atlantic which, however, remained an enclosed basin north of the Falkland Plateau until 85–83 Ma. Around 97 Ma theNortheast Georgia Rise (today north of the Scotia plate) and theMaud Rise (off Antarctica) were located next to theAgulhas Plateau (south of South Africa) where the Bouvet hotspot formed the Southern Ocean Large Igneous Province (112–93 Ma).[7]
The Bouvet triple junction, today considered an R-F-F (ridge-fault-fault) typetriple junction, was an R-R-R type beforeanomaly 28 (c. 64 Ma), which means that before the Scotia plate started to develop in the Mid-Tertiary, only ridges and transform faults separated Africa, Antarctica, and South America. The north–south motion of Antarctica relative to Africa and South America before anomaly 28 changed to a slow east–west clockwise motion around 60 Ma, an abrupt change coincident with change in triple junction configuration.[6]
During the Late Paleocene–Early Eocene Antarctica and South America separated at a rate of only 0.3 cm/yr. The brief opening of small extensional basins south ofTierra del Fuego initiated the opening of theDrake Passage around 49 Ma after which the spreading rate increased eight-fold to 2.4 cm/yr.[8]As the Antarctic—South American plate motion changed from north to north-west during this period, oceanic crust in the north-westWeddell Sea started to subduct on the eastern side of the Drake land bridge—the eastward migration of the South Sandwich Trench had begun.[9]
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