Magmatic processes and segmentation at a fast spreading mid-ocean ridge: Detailed investigation of an axial discontinuity on the East Pacific Rise crest at 9 degrees 37 ' N

[1] Geophysical and petrological boundaries on mid-ocean ridges provide ide al locations to study the relationships between magmatic, tectonic, and hyd rothermal processes. Alvin-based observational data and geochemical data fo r basalts and hydrothermal fluids are used to investigate these relationshi ps at an axial discontinuity on the East Pacific Rise (EPR) crest between s imilar to9 degrees 36'N and 9 degrees 38'N. This ridge-crest discontinuity is morphologically expressed by the overlap of an eastern and western axial summit collapse trough (ASCT) that delimits the primary volcanic and hydro thermal loci along the ridge crest in this area. The ASCTs overlap by simil ar to3 km and are offset in a right-lateral sense by 0.45 km. Near-bottom i maging of this area in 1989 and 1991 shows changes in volcanic morphology a nd increases in hydrothermal and biological activity consistent with the oc currence of a magmatic event during that time interval. When combined with the inferred age and structure of the seafloor, basalt geochemistry, and hy drothermal fluid chemistry, these temporal changes suggest active southward propagation of the eastern ASCT and show that the western ASCT was unaffec ted by the recent magmatic event. Numerous extinct hydrothermal vents and o lder-looking lava flow surfaces suggest waning of magmatic activity in the western ASCT. Young-looking lava flows within or proximal to the eastern AS CT have anomalously high Mg numbers relative to the regional trend and are chemically similar to lava erupted in 1991 along the 9 degrees 46' - 52'N E PR region. We propose that the young-looking lava flows in the eastern ASCT are related to the 1991 eruption. Data show that the 9 degrees 37'N axial discontinuity marks a magmatic and hydrothermal boundary along the EPR ridg e-crest, and we argue that it be classified as a third-order discontinuity. This result is consistent with geophysical evidence suggesting fundamental differences in the crust and upper mantle north and south of the 9 degrees 37'N discontinuity.