Multiple active spreading centres in the hot North Fiji Basin (Southwest Pacific): a possible model for Archaean seafloor dynamics?

The North Fiji Basin (NFB), in the Southwest Pacific, is one of the most evolved back-arc basins. Within the basin, active spreading is taking place along several ridges, some of which display the characteristics of spreading ridges observed in mature large oceans characterized by a range of spreading rates (segmented accretion centres, organized magnetic lineations and propagators, for example). However, the great cumulate length of the spreading centres of the NFB relative to the Basinʹs surface makes it an unique example among the worldʹs oceanic basins. We have reviewed a comprehensive body of geological, geophysical and 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 North Fiji Basin. Active upper mantle convection is the key process controlling the large crustal and lithospheric production in the NFB, which would be thus largely independent from the evolution of even the largest structural features, such as the Vitiaz and New Hebrides subduction zones. Petrological studies of the Archaean greenstone belts have led to a similar picture for the entire Earth during the Archaean: numerous short active ridges connected by unstable triple junctions would have, at this time, provided an efficient convective mechanism allowing for a large heat dissipation. We propose that the presently active processes in the North Fiji basin are modern analogues to Archaean seafloor dynamics.