Seafloor spreading in the North Fiji Basin (Southwest Pacific)

Abstract

The Seapso Leg 3 cruise (December, 1985) was carried out in the North Fiji Basin aboard the IFREMER R.V. “Jean Charcot”. The main purpose of the cruise was to carry out a geophysical and geological study of the central part of the basin between 16° S and 22°S. To this end. bathymetric profiles (with the help of the Seabeam multichannel echosounder), single-channel seismic reflection profiles and magnetic and gravimetric profiles were made. Moreover, geological samples were taken (by dredging and piston coring) as well as water samples (with the help of a multiprobe). One of the aims of the cruise was to explore the present-day spreading axis of the North Fiji Basin. We were able to show that accretion (the rate of which is about 7 cm/yr) in the North Fiji Basin has been constrained in a N-S direction for 3 Ma. Very recently, between 0.6 and 0.7 Ma ago, the accretion process changed, resulting in the creation of a ridge-ridge-trench triple junction centered at 173°30′E and 16°40′S. Water samples taken in the vicinity of this triple junction show significant anomalies of methane and manganese, which are probably linked to hydrothermal events. In its morphological characteristics (transversal as well as longitudinal) the ridge of the North Fiji Basin shows striking similarities to the East Pacific Rise. It presents the same differences in level, the same sizes of structures and the same particular features (overlapping spreading centers, offsets, etc.). Another aim of the SEAPSO cruise was to study the complex deformation area located immediately to the west of the Fiji Islands, which we interpret as a distensive type deformation in a strike-slip system.