The American‐Antarctic Ridge

Abstract

The America‐Antarctic Ridge forms the boundary between the South America and Antarctic plates in the South Atlantic. A recent cruise on R/VMelville surveyed and dredged the America‐Antarctic Ridge between the Conrad Fracture Zone at 56°S, 4°W and a spreading center at 60°S, 18.5°W. This section of the plate boundary consists of five transform faults and six spreading center segments. Prior to our work the plate boundary was generalized on the basis of seismicity and was thought to consist of only two or three spreading centers offset by as many transform faults. Magnetic anomaly correlations indicate that the spreading rate on the America‐Antarctic Ridge near the Bouvet triple junction is about 9 mm/yr (half rate). The ridge has the rough topography and great relief characteristic of slowspreading ridges. The average relief of the median valleys (2.55±0.33 km) is even greater than that of the Mid‐Atlantic Ridge (1.36±0.47 km (Shih, 1980)), consistent with the slower spreading rate of the America‐Antarctic Ridge. Dredging of the fracture zones recovered diabase, greenstone, gabbro, and abundant peridotites (in addition to pillow basalt): rocks which are rare from fast‐spreading ridges. On the basis of the new bathymetrie and magnetic data we have revised the pole of rotation for the Antarctic and South American plates. Depending on the actual instantaneous plate velocities for the American‐Antarctic Ridge at the Bouvet triple junction, the newly determined Antarctic‐South America pole of rotation is either 85°S, 22°E, if the half spreading rate on the American‐Antarctic Ridge is 9.2 mm/yr, or between 84.5°S, 14°E and 85.50S, 19°E, if the rate is 9.0 mm/yr.