MULTISCALE MORPHOLOGIC VARIABILITY OF THE NORTH FIJI BASIN RIDGE (SOUTHWEST PACIFIC)

Two cruises of the French-Japanese STARMFR joint project carried out with the submersibies Nautile (1989) and Shinkai 6500 (1991), have been devoted to the in situ exploration of the North Fiji Basin ridge. Four areas, located near 16-degrees-20', 17-degrees, 18-degrees-50' and 19-degrees-S have been explored during 42 dives. At 16-degrees-20'S, the axial zone trending N 160-degrees corresponds to a prominent graben with walls 1000 m high. Tectonic features and recent screes are present, and the hydrothermal activity is characterized by sporadic fluid discharge. At 17-degrees-S, present accretion is confined to a high dome less than 1900 m deep and cut in its axial part by a wide graben trending N15-degrees. At this site, hydrothermal features of two types are observed: fossil chimneys of sulfides, and active vents, made of anhydrite and expelling water at 285-degrees-C. The site at 18-degrees-50'S is located on top of an axial dome, very similar to those of the East Pacific Rise. Small fissures and very fresh volcanic flows argue for a present-day activity, with weak evidence of tectonics. Hydrothermal emanations with animal colonies are found scattered over the whole area. The fourth site, at 19-degrees-S, shows old and viscous lavas and a very mature tectonic activity, compared to the preceding station which is located only 20 km northward. Large-scale variations have been detected between three segments. Different thermal regimes can explain the distinct morphologies observed. Decakilometric scale variations do occur between stations 14, with volcanic predominance, and ''19-degrees-S'', where tectonic activity is dominating. Finally, small-scale variations are observed on sites 4 and 14, where volcanic features gradually give way to tectonic structures. The North Fiji Basin ridge-with an intermediate average spreading rate-shows tectonic, volcanic and hydrothermal variability, ranging from 1 km to tens of kilometres and related to a temporary fine scale organisation of the accretionary processes.