MULTIPLE ACTIVE SPREADING CENTERS IN THE HOT NORTH FIJI BASIN (SOUTHWEST PACIFIC) - A POSSIBLE MODEL FOR ARCHEAN SEA-FLOOR DYNAMICS

The North Fiji Basin (NFB), in the Southwest Pacific, is one of the mo st evolved back-are basins. Within the basin, active spreading is taki ng place along several ridges, some of which display the characteristi cs of spreading ridges observed in mature large oceans characterized b y a range of spreading rates (segmented accretion centres, organized m agnetic lineations and propagators, for example). However, the great c umulate length of the spreading centres of the NFB relative to the Bas in's surface makes it an unique example among the world's oceanic basi ns. We have reviewed a comprehensive body of geological, geophysical a nd geochemical evidence (multiple spreading centres of various trends, high heat flow, high geoid and slow upper mantle seismic velocities) which collectively demonstrate that a hot upper mantle underlies the N orth Fiji Basin. Active upper mantle convection is the key process con trolling the large crustal and lithospheric production in the NFB, whi ch would be thus largely independent from the evolution of even the la rgest structural features, such as the Vitiaz and New Hebrides subduct ion zones. Petrological studies of the Archaean greenstone belts have led to a similar picture for the entire Earth during the Archaean: num erous short active ridges connected by unstable triple junctions would have, at this time, provided an efficient convective mechanism allowi ng for a large heat dissipation. We propose that the presently active processes in the North Fiji basin are modern analogues to Archaean sea floor dynamics.