Lawver and Dalziel OPP 9526687 Bevis OPP 9530383 Smalley OPP 9527529 Abstract Antarctica is the Earth's most isolated continent. It is surrounded by actively spreading ridges except in the South American sector. The motion of South America with respect to Antarctica is latitudinal and left-lateral at approximately 22 mm per year and is distributed along the boundaries of the intervening Scotia plate. A prominent but discontinuous bathymetric high known as the Scotia Ridge surrounds the Scotia plate on three sides. This feature includes some continental material detached from the South American and Antarctic continents, but its eastern closure is a volcanically and seismically active group of islands, the South Sandwich arc, that is separated from the Scotia plate by a vigorously spreading back- arc ridge. The entire east-closing, locally emergent bathymetric feature joining the two continents, is known as the Scotia arc. The D-shaped Sandwich plate and arc appear to be moving rapidly east with respect to both South America and Antarctica, thereby for the first time introducing a subduction system into the otherwise rift-bounded South Atlantic Ocean basin. This motion may constitute the best evidence for mantle return flow from the closing Pacific Ocean basin to the expanding Atlantic Ocean basin. The Scotia arc is nonetheless one of the most poorly constrained of the major tectonic systems on Earth, yet it is a critical and enigmatic link in global plate-motion circuits. This SCotia ARc GPS Project (SCARP) will use the Global Positioning System (GPS) to measure the plate motions between South America, Antarctica, and Africa, and around the Scotia arc using a newly developed geodetic strategy known as a multimodal occupation strategy (MOST). This involves setting up permanent GPS receivers at a small number of sites in South America and Antarctica, and using additional receivers to position numerous stations relative to this continuously operat ing network. Two seasonally occupied stations in the South Sandwich islands will be tied to permanent GPS sites in South America, Antarctica, and Africa, and to intervening stations in the Falkland, South Georgia, and South Orkney islands that will be occupied on an occasional basis by British collaborators. During the initial three years the South Sandwich arc motion will be easily resolved, and using roving stations in the Antarctic Peninsula- South Shetland Islands area, it should be possible to determine if extension is occurring across Bransfield Strait. This project will also construct a relatively dense subnetwork in Patagonia/Tierra del Fuego, and a moderately dense subnetwork in the Antarctic Peninsula. While achieving sub-millimeter/year velocity resolution in the initial three year project with these subnetworks is not expected, this project will establish the baseline necessary for a follow-on suite of measurements in perhaps six to eight years. The follow-on project will allow characterization of the slow motions and deformations that occur across and within the boundaries of the Scotia plate. The objectives of SCARP are to determine: 1. the relative motions of the Antarctic, South American, Scotia and South Sandwich plates; 2. strain partitioning within the South America-Scotia plate boundary zone, Tierra del Fuego; 3. the rate of extension across the volcanically active Bransfield trough and the present rate of uplift or subsidence of the extinct South Shetland Islands volcanic arc; 4. and with our British colleagues, the rate of rollback of the South Sandwich Trench in a South American-African framework; These objectives in turn will allow: 1. An important contribution to a geodetic assessment of the elastic displacement field associated with extension in Bransfield Strait and the accumulation or loss of ice on the Antarctic continent; 2. Determination of why there is transpression along the northern boundary of the Scotia plate and tra nstension along its southern boundary; 3. Testing for motion between East and West Antarctica postulated as a source of error in global plate circuits; and 4. Evaluatation of what the South Sandwich trench rollback means in terms of mantle flow and the potential for transforming a passive rifted ocean basin into a subducting or disappearing ocean basin.
