THE VALU FA RIDGE - THE PATTERN OF VOLCANIC ACTIVITY AT A BACK-ARC SPREADING CENTER

The Central Valu Fa Ridge, an active back-arc spreading segment bounded by small overlapping spreading centres, can be divided into three regions, each 8-12 km long, with distinct bathymetric, morphological and magnetic characteristics. These observations suggest that each region has a different volcanic history, driven by variations in melt supply along axis. Two of the three regions are centred on small deviations of the ridge axis, which appear to be the local foci of extrusion. There appears to have been a recent re-organisation of the magmatic supply to the ridge. In the past the highest magmatic budget was at the southern end of the ridge but it has now switched to the northern end, where it coincides with a small overlapping spreading centre. This re-organization may be associated with island arc activity. Beneath the ridge a bright reflector, which is interpreted as being from the roof of a magma chamber, is laterally more continuous than the inferred volcanic segmentation. This implies that mixing of magma within the chamber must be limited along strike. The magmatic budget at various parts of the ridge, inferred from axial morphology, can be correlated with variations in the width and brightness of the reflector. This correlation suggests that the size of the chamber is dynamic and changes in response to variations in supply from below or delivery upwards.