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The Scotia plate () is a minor plate tectonics on the edge of the Atlantic Ocean and Southern Ocean oceans. Thought to have formed during the late Eocene with the opening of the Drake Passage that separates Antarctica and South America, it is a minor plate whose movement is largely controlled by the two major plates that surround it: the Antarctic plate and the South American plate. The Scotia plate takes its name from the steam yacht Scotia of the Scottish National Antarctic Expedition (1902–04), the expedition that made the first bathymetry of the region.
Roughly rhomboid, extending between and , the plate is wide and long. It is moving WSW at /year and the South Sandwich plate is moving east at /year in an absolute reference frame. Its boundaries are defined by the East Scotia Ridge, the North Scotia Ridge, the South Scotia Ridge, and the Shackleton fracture zone.
The Scotia plate is made of oceanic crust and continental fragments now distributed around the Scotia Sea. Before the formation of the plate began (40Ma), these fragments were at least partly exposed above the water, and formed a continuous landmass from Patagonia to the Antarctic Peninsula along an active subduction margin. It served as one end of the Antarctic land bridge, a wider land bridge that connected South America, Antarctica, and Australia. At present, the plate is almost completely submerged, with only the small exceptions of South Georgia on its northeastern edge and the southern tip of South America.
The northern ridge stretches from Isla de los Estados off Tierra del Fuego in the west to the microcontinent South Georgia in the east, with a series of shallow banks in between: Burdwood Bank, Davis, Barker, and Shag Rocks. North of the ridge is the deep Falkland Trough.
The South Georgia microcontinent was originally connected to the Roca Verdes back-arc basin (southernmost Tierra del Fuego) until the Eocene. Before that, this basin went through a series of geological transformations during the Cretaceous, through which South Georgia was first buried, then made a topographic feature again by the Late Cretaceous. At about 45 Ma, South Georgia, still part of the South American plate, got buried again and something, possibly rotation of the Fuegian Andes, completed the break-up and allowed South Georgia a second exhumation. During the Oligocene (34–23 Ma) South Georgia was reburied again as seafloor spreading took place in the West Scotia Sea. 10 Ma, finally, the South Georgia microcontinent was uplifted as a result of the collision with the Northeast Georgia Rise.
At the eastern tip of the Antarctic Peninsula, the beginning of South Scotia Ridge, a small and heavily dissected bank degenerates into several outcrops dominated by Paleozoic and Cretaceous rocks. A small basin, Powel Basin, separates this cluster from the South Orkney microcontinent composed of Triassic and younger rocks.
The eastern continuation of the ridge, the Scotia Arc east of the South Sandwich plate, are the South Sandwich island arc and trench. This volcanic active island arc has submerged ancestors in Jane and Discovery banks in the southern ridge.
North of South Shetland Islands and along the southern half of the Shackleton fracture zone is the remnant of the Phoenix plate (also known as Drake or Aluk Plate). Around 47 Ma the subduction of the Phoenix plate started as the propagation of the Pacific-Antarctic Ridge continued. The last collision between Phoenix ridge segments and the subduction zone was 6.5 Ma and at 3.3 Ma movements had stopped and the remnants of the Phoenix plate was incorporated into the Antarctic plate. The southern part of the Shackleton fracture zone is the former eastern edge of the Phoenix plate.
The banks of northern Central Scotia Sea are superposed on oceanic basement and the spreading centre of the West Scotia Sea. Analyses of samples of volcaniclastic rocks from these sites indicate they are constructs of a continental arc and in some cases oceanic arc similar to those being formed in the currently active South Sandwich Arc. The oldest volcanic arc activity in the central and eastern regions of the Scotia Sea are 28.5 Ma. The South Sandwich forarc originated in the Central Scotia Sea at that time but has since been translated eastward by the back-arc spreading centre of the East Scotia Ridge.
The earliest marine fossils found on the Maurice Ewing Bank, on the eastern end of the Falkland Plateau, are associated with the Indian and and probably slightly more than 150 Ma. The Weddell Sea opened and spread along the southern margin of the Falkland Plateau and into the Rocas Verdes back-arc basin which extends from South Georgia and along the Patagonian Andes. Fragments of the inverted oceanic basement of this basin are preserved as Ophiolite complexes in this area, including the Larsen Harbour complex on South Georgia. This makes it possible to restore the original position of the South Georgia microcontinent south of the Burdwood Bank on the western North Scotia Ridge south of the Falkland Islands.
The Rocas Verdes basin was filled with derived from the volcanic arc on its Pacific margin and partly from its continental margin. The Weddell Sea continued to expand which led to the extension between the Patagonian Andes and the Antarctic Peninsula. In the Mid-Cretaceous (100 Ma) the spreading rate in South Atlantic increased significantly and the Mid-Atlantic Ridge grew from 1200 km to 7000 km. This led to compressional deformations along the western margin of the South American plate and the obduction of the Rocas Verdes basement onto this margin. Structures associated with this obduction are found from Tierra del Fuego to South Georgia. The acceleration of the westward motion of South America and the inversion of the Rocas Verde Basin finally lead to the initiation of the Scotia Arc. This inversion had a strike-slip component which can be seen in the Cooper Bay dislocation on South Georgia. This geological regime lead to the uplift and elongation of the Andes and the embryonic North Scotia Ridge, which resulted in the initial eastward relocation of the South Georgia microcontinent and formation of the Central Scotia Sea.
During the early Eocene (50 Ma), the Drake Passage between the southern tip of South America at Cape Horn and the South Shetland Islands of Antarctica was a small opening with limited circulation. A change in relative motion between the South American plate and the Antarctic plate would have severe effects, causing seafloor spreading and the formation of the Scotia plate. Marine geophysical data indicates that motion between the South American plate and the Antarctic plate shifted from N–S to WNW–ESE accompanied by an eightfold increase in the separation rate. This shift in spreading initiated crustal thinning and by 30–34 Ma, the West Scotia Ridge formed.
A complex pattern of spreading prior to 26 Ma is probably present in the oldest parts of this spreading regime; i.e. west of Terror Rise (north of Elephant Island) and on the shelf slope of Tierra del Fuego. Until 17 Ma the Central Scotia plate moved quickly eastwards, fuelled by the eastern trench migration, but both plates have moved very slowly since.
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