South American-Antarctic Ridge

The South American–Antarctic Ridge (SAAR or AAR) is the tectonic spreading center between the South American Plate and the Antarctic Plate. It runs along the sea-floor from the Bouvet Triple Junction in the South Atlantic Ocean south-westward to a major transform fault boundary east of the South Sandwich Islands. Near the Bouvet Triple Junction the spreading half rate is 9 mm/a (0.011 in/Ms), which is slow, and the SAAR has the rough topography characteristic of slow-spreading ridges.

The boundary between the South American and Antarctic plates can be divided into three parts of which the SAAR forms the eastern third:

The first stretches from the Chile Triple Junction in the Chile Trench at 46°S to the western Straits of Magellan at 52°S. Since 15 Ma, the oceanic crust of the Antarctic plate is being slowly subducted (20–24 mm/a (0.025–0.030 in/Ms)) under South America along this trench which is currently extending northward. In the central part, between the Straits of Magellan and the South Sandwich Trench, the two large continental plates are separated by the Scotia Plate and a number of smaller plates east of it. During the past 40 Ma (or since the opening of the Drake Passage) the South Sandwich Trench has been migrating eastward due to the evolution of a back-arc basin, effectively consuming the SAAR.

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 (0.025 in/Ms) westward but it was originally closer to north-south. It can be inferred, based on fracture zone topography and magnetic anomalies in the Weddell Sea, that this change in direction occurred during the Cretaceous and Cenozoic. The western part of the SAAR is dominated by the earthquake-intensive South Sandwich island arc, fore-arc, and trench. 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. 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. The Bouvet Triple junction is between the American, African, and Antarctic lithospheric plates which has geographical coordinates of 55°S, 0°E.

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