RECENT TECTONIC, MAGMATIC, AND HYDROTHERMAL ACTIVITY ON THE EAST PACIFIC RISE BETWEEN 17-DEGREES-S AND 19-DEGREES-S - SUBMERSIBLE OBSERVATIONS

The objective of the Naudur cruise (December 1993) of the submersible Nautile was to study the interaction among magmatic, tectonic, and hyd rothermal processes at a very fast spreading mid-ocean ridge axis. Twe nty-three dives were completed, both along and across the axis, in fou r areas located between 17 degrees 10' and 18 degrees 45'S on the East Pacific Rise. Rock, sulfides, water, and biological samples have been collected along each of the segments. Two main types of segments have been distinguished, characterized either by the predominance of prese nt-day volcanic activity or by predominant tectonic activity. Linked t o both types of activity, 69 hydrothermal sites have been discovered a nd sampled. They comprise four types, interpreted as successive evolut ionary stages. The first are shimmering water sites which occur immedi ately after the formation of lava lakes and are characterized by large surface area and poorly developed associated fauna. The second, in ar eas dominated by recent volcanic activity, have waters venting directl y from lava fissures and more focused discharge areas through black sm oker chimneys. The third stage is represented by more mature hydrother mal vents and deposits, along the faults bounding the eastern side of the axial graben in tectonic-dominated areas. The associated fauna is well developed. The fourth stage corresponds to the reactivation of vo lcanic activity with lava flows, young black smokers, and diffuse vent ing associated with the faults bounding the axial graben. Fluids colle cted range from 200 degrees to 340 degrees C and show a wide variabili ty in chemical and gas composition. Within each of the explored areas, evidence of recent volcanic activity has been observed.