The Lau Basin lies between the Lau and Tonga ridges and extends over more than 1000 km. Its width decreases from 300 km near 16 degree S to less than 100 km near 24S. It is an actively spreading oceanic back-arc basin created in the Plio-Pleistocene the Tonga Trench. The part of the Valu Fa ridge North of 22 degree S was investigated during the Seapso 4 cruise of R/V Jean Charcot in Jan 1986 and complementary data were obtained during the Papnoum transit (Apr 1987). The techniques used included multibeam bathymetry, single-channel seismic reflection, magnetic and gravity field measurements.