Gravity and bathymetry data from a detailed ship-based survey of a 40-km length of the Valu Fa ridge allow an analysis of the fine-scale segmentation of this young, back arc spreading center, where the primary morphological segmentation takes the form of small, overlapping spreading centers (OSC) separating ridge segments typically 30 km long. The gravity survey covers one complete segment and the overlap at its northern end and part of the overlap at its southern end. The mantle Bouguer anomaly shows a small (10 mGal), subcircular negative anomaly. The pattern of anomalies implies that the northern OSC is the site of particularly vigorous magmatism, with either increased crustal thickness or density reduction in the mantle (or both) accounting for the observed gravity low. This interpretation confirms the results of seismic analyses of this spreading center, which also imply that the OSC is the site of enhanced magmatism. The observations appear to contrast with widely accepted models of segmentation at mature, ocean basin spreading centers, where morphological offsets are believed to coincide with a relatively weak supply of magma. On a longer (130 km) gravity profile crossing the ridge, the mantle Bouguer anomaly shows a large (~50 mGal), longer-wavelength (~100 km) positive anomaly centered on the spreading axis. This can be modeled by an increase in crustal density and/or a decrease in crustal thickness over a 40- to 50-km-wide area centered on the axis. This is consistent with recent models for the evolution of the basin, involving recent southward propagation of seafloor spreading through preexisting, but stretched, thinned, and intruded island arc lithosphere. ¿ American Geophysical Union 1995