More free publications from Archimer Article No 529 Contribution C.O.B. No 718 FIRST MANNED SUBMERSIBLE DIVES ON THE EAST PACIFIC RISE ÀT 21°N (PROJECT RITA): GENERAL RESULTS CYAMEX SCIENTIFIC TEAM

the axis of the East Pacfic Rise near 21°N. The expedition CYAMEX, the first submersible program to be conducted on the East Pacific Rise, is part of the French-American-Mexican project RiTA (Rivera-Tamayo), a 3-year study devoted to detailed geological and geophysical investigations of the East Pacific Rise Crest. On the basis of the 15 dives made by CYANA in the axial area of the Rise, a morphological and tectonic zonation can be established for this moderately-fast spreading center. A narrow, 0.6 to 1.2 km wide zone of extrusion (zone l), dominated by young lava flows, is flanked by a highiy fissured and faulted zone of extension (zone 2) with a width of 1 to 2 km. Further out, zone 3 is dominated by outward tilted blocks bounded by inward-facing fault scarps. Active or recent faults extend up to 12 km from the axis of extrusion of the East Pacfic Rise. This represents the iüst determination from direct field evidence of the width of active tectonism associated with an accreting plate boundary. Massive suffide deposits, made principally of zinc, copper and uon, were found close to the axis of the Rise. Other signs of the intense hydrothermal activity included the discovery of benthic fauna of giant size similar to that found at the axis of the Galapagos Rift. We emphasize the cyclic character of the volcanicity. The main characteristics of the geology of thi