PROPAGATING RIFT WEST OF THE FIJI ARCHIPELAGO (NORTH FIJI BASIN, SW PACIFIC)

A Sea Beam survey of an area located west of the Fiji Islands in the North Fiji Basin, between 17 degrees 10 degrees S and 18 degrees S and 175 degrees 40'E and 176 degrees 40'E, shows a complex structure characterized by two deep grabens bounding a central plateau. In previous papers, we interpreted this area as an intraoceanic strike-slip deformation zone, rejecting the possibility of oceanic spreading in the area. By comparison with the structures observed in the Galapagos 95.5 degrees W area, we now reinterpret the area as a southward propagating spreading system. The analysis of structural and magnetic data and earthquake focal mechanisms confirms this new interpretation. The full spreading rate for the North Fiji Basin ridge, deduced from magnetic lineations, varies from 50 to 80 mm/yr. For the West Fiji spreading ridge, the magnetic anomaly analysis gives a spreading rate of about 56 mm/yr for the northern segment and slightly slower (40 mm/yr) for the southeastern segment. Using a 50 mm/yr spreading rate, the angles between the pseudofaults allow us to calculate the propagation velocity. For an angle of 75 degrees north of,17 degrees 35'S, the Propagation velocity is 33 mm/yr. The tip of the propagator is characterized by a 20 degrees angle between the pseudofaults, which implies a recent increase of propagation velocity to 141 mm/yr. The initiation of this propagation could be related to the emplacement of the North Fiji fracture zone 1 to 1.5 m.y. ago, resulting in the formation of the 16 degrees 50'S triple junction. The main kinematic implication of the existence of a propagating rift west of the Fiji Islands is that in the North Fiji Basin the present-day accretion is distributed on two parallel active spreading ridges, one in the central part of the basin, the other one in its southeastern quadrant. Such a phenomenon, unusual on mid-oceanic ridges, except for microplates such as Galapagos, Easter and Juan Fernandez, may be more common in marginal basins because of their peculiar geodynamic setting.