TRANSFORM MOTION ACROSS THE SCOTIA ARC: INFLUENCE ON OCEANIC CIRCULATION

The Scotia arc is the eastward closing loop of locally emergent submarine ridges joining the southern tip of the South American continent to the northern tip of the Antarctic Peninsula and enclosing the Scotia Sea. South America has moved westward relative to Antarctica at 20-30mm/yr over the past 50 Myr. The present day motion is taken up by transform motion along the east-west trending South Sandwich Fracture Zone in the South Atlantic Ocean basin and by subduction along the north-south trending Andean margin. Across the Scotia arc itself, it is taken up by a combination of transform motions on the North and South Scotia ridges and the Shackleton Fracture Zone which closes off the Scotia Sea to the west.

The North Scotia Ridge transform fault (NSRT) separates the South American plate from the Scotia plate to the south. It extends from the Antarctic-South American-Scotia plate triple junction at the Chile Trench in the west to the South Sandwich trench in the east where the South American plate is being subducted westward. Global Positioning System measurements in southernmost South America (the Magallanes-Fagnano fault system) demonstrate 6.6 ±1.3mm/yr of left-lateral motion is occurring there. Hence the transform takes up 20-30% of the westerly motion of South America relative to the Antarctic Peninsula across the Scotia arc. The remainder is taken up along the South Scotia Ridge transform (SSRT) that separates the Scotia and Antarctic plates and joins the Antarctic Peninsula to a triple junction with the South Sandwich trench and South Sandwich Fracture Zone, and the northwest-southeast trending Shackleton Fracture Zone (SFZ) that bounds the Scotia plate between South America and the Antarctic Peninsula. The Euler poles for the motions of South America-Scotia and of Scotia-Antarctica are closely located, so displacement on the NSRT and SSRT are nearly parallel and the Scotia plate is not deforming or rotating

Although the trace of the NSRT is clearly marked across South America, displacements are difficult to determine because the stratigraphic units and main structural fabric are parallel to the fault. There is a reasonably reliable left-lateral offset of ~25 km measured on an Upper Jurassic volcanic unit. Less certain is an observed 80 km left-lateral offset on the margin of the Mesozoic-Cenozoic Patagonian batholith. To the east along the North Scotia Ridge itself its location is unclear, though a few earthquakes with east-west left lateral motion fault-plane solutions have been recorded. Burdwood Bank, the submerged ridge immediately offshore Tierra del Fuego, is a continuation of the Andean Cordillera. The Davis and Aurora banks and the Black and Shag rocks platform to the east appear to be displaced fragments of the Pacific side of the Cordillera. South Georgia Island (~3000m elevation) is the emergent part of a microcontinental block at the eastern end of the ridge. It has been displaced ~ 1700 km eastward from the Pacific margin. Interestingly there are clusters of earthquake epicenters located at the eastern ends of Burdwood, Davis and Aurora banks, and the South Georgia platform. They appear to form restraining bends along the NSRT. South Georgia collided with the Northeast Georgia Rise on the South American plate at ~ 12-10 Ma. This probably accounts, for underthrusting of the Scotia plate beneath the microcontinent as well as the rollback of the South Sandwich arc to form the active backarc spreading center beneath the East Scotia Sea.

Though complex in detail, the SSRT is marked by a fairly continuous narrow zone of epicenters offset in releasing bends. Powell Basin, which separates the South Orkney Islands continental platform from the Antarctic Peninsula, may represent a Neogene pull-apart basin. The South Orkney block is now within the deforming zone of the SSRT. A number of epicenters are located along the SFZ which is transitional southeastward from the Chile Trench subduction zone to a larger component of strike-slip motion south east of the extinct Antarctic-Phoenix Ridge to the west. At the southern end of the ridge the Elephant Island block, northernmost part of the South Shetland Islands platform, appears to be moving with the Scotia Sea block to the north of the SSRT.

The zone of left-lateral transform motion of South America with respect to Antarctica through the Scotia arc region dates back at least until 84 Ma. It may have been initiated in the Early Cretaceous, ~130 Ma, with the opening of the South Atlantic Ocean basin. During the interval between 30 Ma and 9 Ma most of the motion may have been taken up in the NW-SE opening of the West Scotia Sea with little or none in an east-west direction along the North and South Scotia ridges. This would explain the limited displacement mapped on shore in South America. At this time the Shackleton Fracture Zone was the western boundary of the spreading system. The late Mesozoic-Cenozoic development of the transform zone in the Scotia Sea region played a major role in controlling the development of oceanic circulation in the Southern Ocean. It was critical in the onset and development of the globe-encircling Antarctic Circumpolar Current.