MORPHOLOGY OF THE VALU FA SPREADING RIDGE IN THE SOUTHERN LAU BASIN

Multibeam echosounder surveys and seismic reflection profiling were conducted in the southern Lau back arc basin (SW Pacific). The bathymetric survey covered a 130-km-long section of the active spreading ridge (3 cm/yr half spreading rate), the N-S trending Valu Fa Ridge (VFR), while seismic profiles concentrated on a 35-km-long ridge section. Analysis of these data shows a morphologically segmented ridge which is underlain by a continuous magma chamber. The ridge shows similarities, but also differences, to the segmentation model developed for mid-ocean ridges. Analogous to this model the section of the VFR studied displays third- and fourth-order segmentation. The amplitudes of the along-axis undulations, however, are a factor of 5 to 10 greater than at mid-ocean ridges. Additionally, the VFR has numerous small volcanic cones along the axis, which have previously only been observed at much slower spreading ridges. In cross section the VFR has a very steep, narrow triangular shape which is thought to result from the high viscosity of its andesitic lavas. The axial magma chamber appears to vary in width (1-4 km) and location relative to the axis beneath different morphological segments. Perhaps the most significant deduction from our combined data is that the magma chamber is widest beneath the most prominent axial discontinuity, a small overlapping spreading center (OSC). This is counter to observations from mid-ocean ridges. Off-axis traces of ridge segments allow us to reconstruct the evolution of this OSC for the past 220 kyr. We conclude that this back arc spreading axis shows a segmentation pattern which is similar to mid-ocean ridges, but it is influenced and modified by the geochemistry of its lavas and its proximity to the island arc.