GEOLOGICAL STRUCTURE OF THE CENTRAL SPREADING SYSTEM, NORTH FIJI BASIN

SeaBeam mapping, single-channel and multichannel seismic profiling, sonobuoy-OBSH refraction measurements and geomagnetic anomaly observations obtained during the cruises KAIYO87 and 88 in the North Fiji Basin reveal the detailed structure and history of jumps of young spreading systems. Currently active spreading systems north-northwest and south-southwest of a triple junction at about 17-degrees-S, 174-degrees-E, where active hydrothermal activity was discovered during KAIYO88, jumped to their present positions possibly in the late Quaternary. The spreading ridge south-southwest of the triple junction, which is oriented N15-degrees-20-degrees-E, has a 0.7-3.5 km wide axial graben and a 15-35 km wide trapezoidal morphological profile. It cuts into a wide N40-degrees-60-degrees-E oriented graben whose side walls themselves sharply cut previously formed structures, including a high seamount which is now separated into three highs.