GLORI-B / Sea Beam 2000 Survey of the Fastest Seafloor Spreading Center

R N Hey, F Martinez, P D Johnson

M Somers, Q Huggett, J Campbell, T Le Bas, R Beale,R Rusby

J Korenaga

During February to March 1993 we surveyed the fastest-spreading segments of the global seafloor spreading system, the East Pacific Rise between the Easter and Juan Fernandez microplates. We used the new GLORIA system (GLORI-B) and Sea Beam 2000, as well as gravity, magnetics, and the new Japanese shipboard 3-component magnetometer system (STCM). We fully imaged the spreading center between the microplates, although a propulsion problem on the R/V Melville forced us to greatly reduce the flank coverage in the south. The stunning GLORIA sidescan sonar mosaic reveals a giant duelling propagator system near 29S which has recently tried or is presently trying to become a new microplate, although a minimum of 3 huge failed rifts on the Nazca plate show a past history of massive and continuous spreading center reorganization by duelling propagation, and lithospheric rotation and transfer from the Pacific to Nazca plate with no obvious previous attempts at microplate formation. The young volcanism previously proposed as the Ahu volcanic field is concentrated along a linear ridge with a western terminus near 26.5S, 111W. Easter Island lies at the intersection of this Ahu ridge and the initial microplate rift pseudofault, confirming its formation at the initial propagator tip early in the microplate history. Other discoveries include a fracture zone that may have been the initial southern boundary of the Easter microplate, evidence for probable subsequent southward growth of the microplate, and a new seamount chain on the Pacific plate extending away from a shallow (2,000m) area of propagating and overlapping ridges near 27.7S, 113.1W.