VOLCANIC AND HYDROTHERMAL PROCESSES ASSOCIATED WITH A RECENT PHASE OFSEA-FLOOR SPREADING AT THE NORTHERN CLEFT SEGMENT - JUAN-DE-FUCA RIDGE

The northern portion of the Cleft segment, which is the southernmost s egment of the Juan de Fuca Ridge, is the site of a seafloor spreading episode during the mid-1980s that was originally discovered by the occ urrence of anomalous hydrothermal bursts (megaplumes) and later docume nted by seafloor mapping of new pillow mounds (NPM) that were erupted. Several field seasons of investigations using sidescan sonar, a deep- tow camera system, and the submersible Alvin reveal that about 30 km o f the ridge crest is hydrothermally active and/or has experienced rece nt volcanic and tectonic activity associated with this episode. The mo st intense hydrothermal activity within this area and all the known hi gh-temperature vents lie along a fissure from which a young sheet flow (YSF) erupted. Extinct chimneys located within 100-200 m on either si de of the fissure system represent an older (>100 years) and probably less intense, hydrothermal regime. The bathymetry and the morphology o f the YSF suggest that this eruption occurred over a 1-2 km section of the fissure system that forms its eastern boundary and that it flowed to the south. Fields of lava pillars concentrated at the margins of t he YSF where lava probably formed when the lava stagnated near the edg es of the flow. A comparison of sidescan data sets collected in 1982 a nd 1987 implies that the YSF was erupted at least 7 months prior to th e NPM, consistent with analysis of bottom photographs that suggests th at the eruptions of the YSF and NPM were only separated by a few years . The low hydrothermal flux over the NPM relative to the YSF suggests a rapidly cooled underlying heat source beneath the former. We propose that the NPM were erupted from a dike or dikes injected laterally to the north from a magma body lying beneath the YSF. Recent evidence of a decrease in the intensity of the overlying hydrothermal plumes sugge sts that the system is continuing to cool down.