GEOMORPHOLOGY AND STRUCTURAL SEGMENTATION OF THE CREST OF THE SOUTHERN (PACIFIC-ANTARCTIC) EAST PACIFIC RISE

Geomorphology of the boundary between Pacific and Antarctic plates was mapped with a Sea Beam multibeam echosounder and a SeaMARC II bathymetric side scan sonar, from the southern end of Juan Fernandez microplate at 35-degrees-S to Heezen transform at 56-degrees-S. There are six spreading center systems separated by two large, left-stepping nontransform offsets (at 36.5-degrees-S and 41.5-degrees-S), two right-stepping transform-fault systems, and a left-stepping hybrid structure with equally long strike-slip and nontransform offsets. Axial rift zones (spreading axes) are generally within 1-degrees of normal to the relative motion predicted by current plate rotation models. Most axial rift zones crop out along the crests of typical East Pacific Rise (EPR) axial ridges which commonly have flat rather than humped long profiles. About 2% of this fast-spreading (84-100 mm/yr) rise crest has an axial rift valley instead of an axial ridge. The longest rift-valley segment is midway between Menard and Vacquier transforms near 51-degrees-S, where an anomalously deep rise crest may mark a zone of below-average mantle upwelling. Nontransform offsets structurally similar to those on the tropical EPR, but mostly formed by differential asymmetric spreading, subdivide the four longest spreading center systems. On the northern, transform-free two thirds of the rise, net rift propagation at migrating offsets has been into the faster Pacific plate. This causes fight steps to migrate north, left steps to migrate south, and crust to be transferred to the east flank. This pattern has prevailed for several million years, judging from the oblique fracture zones (pseudofaults) mapped on the rise flanks with older marine data and Geosat altimetry. On this rise crest, the difference in stress on two plates with very different velocities may control the direction of offset migration.