Controls on segmentation and morphology along the back-arc Eastern Lau Spreading Center and Valu Fa Ridge

Back-arc spreading centers increasingly depart from mid-ocean ridge (MOR) characteristics with proximity to the arc volcanic front. The close association of these departures with slab-derived materials in erupted lavas suggests that subduction-related chemical effects are their primary cause. The Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR) are type examples of this process. Together they constitute a first-order spreading center in the Lau back-arc basin that progressively converges on the Tofua arc volcanic front from north to south. Here we use ship multibeam and deep-towed side-scan sonar data to examine variations in axial morphology and volcanism at the second-and third-order segment scale along these ridges and develop a model for the processes that control them. Closest to the arc, VFR, and the southern segment of the ELSC shoal toward second-order segment ends, in contrast to MORs. Northward and beyond similar to 70 km from the arc, the axis becomes abruptly deeper and flatter and no longer shoals toward second-order segment ends. At VFR, along-axis topographic highs correlate with the location of arc volcanoes along slab flow lines. These correlations are weaker along the southernmost ELSC segment and absent along ELSC segments farther north. The observations show a modulation of back-arc segmentation with arc proximity that rapidly diminishes with distance. They support a model of the mantle wedge with a strongly hydrous domain within similar to 70 km of the arc within which the arc and ridge interact and a much less hydrous domain farther from the arc without evident arc-ridge interactions.