South American-Antarctic plate motion over the past 50 Myr, and the evolution of the South American-Antarctic ridge
Magnetic and bathymetric data from the South American-Antarctic plate boundary east of the South Sandwich trench have been interpreted to produce ocean floor ages, spreading rates and directions. SAM-ANT motion over the past 50 Myr has been slow (10–15 mm yr‒1 half rates). About 20 Ma the spreading direction changed from 120°-300° to the present E-W. These results have been combined with similar data from the South Atlantic and Southwest Indian Oceans to calculate six poles and rates of SAM-ANT motion covering the past 50 Myr. The change at 20 Ma appears to have originated the long-offset Bullard and South Sandwich fracture zones. The change may have been facilitated, or even triggered, by ridge crest-trench collision along the South Scotia Ridge, east of the South Orkney Islands. This event however, could not alone have caused a stable change. It is concluded that the global balance of forces may for long periods be imperfectly reflected in plate motions, because of the constraining effects of long fracture zone offsets on the directions of plate motion, and that the ‘causes’ of abrupt changes in direction may precede them by several million years.