The Fonualei Rift and Spreading Center: Effects of ultraslow spreading and arc proximity on back-arc crustal accretion

Back-arc spreading center characteristics reflect interactions between plate-driven mantle advection and melting and slab-driven hydrous melting and buoyant upwelling in the mantle wedge. At the Fonualei Rift and Spreading Center (FRSC) in the Lau Basin, spreading rates decrease from slow to ultraslow, providing an opportunity to examine crustal accretion as the plate-driven component is minimized. A new Lau Basin kinematic analysis predicts FRSC spreading rates of ~32–8 mm/yr southward, much slower than previous estimates of ~95–47 mm/yr. Here we examine FRSC morphology and geophysical characteristics as it approaches the Tofua arc volcanic front southward and spreading rates decrease, minimizing the plate-driven component of mantle advection and maximizing buoyant hydrous flux melting. Axial morphology changes abruptly from a deep, flat, faulted axis ~100 km away from the arc to a volcanic ridge that shoals and increases in relief southward. Within ~50 km of the arc at the south end, the volcanic ridge is abruptly replaced by isolated volcanic cones bisected by volcanic rift zones and surrounded by anomalously deep seafloor. These morphologic changes likely reflect along-axis focusing of mantle upwelling and melting similar to that seen at ultraslow mid-ocean ridges, causing the change in morphology from a segmented ridge to spaced axial cones. We propose that as opening rates slow and the ridge approaches the arc, more of the inherently three-dimensional pattern of hydrous flux melting and buoyant upwelling in the mantle wedge is expressed volcanically. With faster opening, two-dimensional plate-driven mantle advection dominates melt production, favoring ridges over point-source features.