Contrasting crustal production and rapid mantle transitions beneath back-arc ridges

The Eastern Lau spreading centre, located on the Lau back-arc basin hydrothermal vent system between Fiji and Tonga, is a focus site of the Ridge 2000 research programme of the US National Science Foundation. The first large-scale seismic study of the area reveals large and abrupt across- and along-strike changes in crustal properties, implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes. The authors conclude that stable, broad triangular upwelling regions, as inferred for mid-ocean ridges, cannot form near the mantle wedge corner. Instead, the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water-rich low-viscosity mantle when it is near the arc. As the ridge moves away from the arc, a tipping point is reached at which the material is rapidly released, resulting in rapid changes in the character of the crust formed at the ridge. This study shows that across-strike and along-strike changes in crustal properties at the Eastern Lau spreading centre are large and abrupt, implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes. It is concluded that stable, broad triangular upwelling regions, as inferred for mid-ocean ridges, cannot form near the mantle wedge corner. Instead, the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water-rich low-viscosity mantle when it is near the arc. As the ridge moves away from the arc, a tipping point is reached at which that material is rapidly released, resulting in rapid changes in the character of the crust formed at the ridge. The opening of back-arc basins behind subduction zones progresses from initial rifting near the volcanic arc to seafloor spreading1. During this process, the spreading ridge and the volcanic arc separate and lavas erupted at the ridge are predicted to evolve away from being heavily subduction influenced (with high volatile contents derived from the subducting plate)2,3,4,5. Current models4,6,7,8 predict gradational, rather than abrupt, changes in the crust formed along the ridge as the inferred broad melting region beneath it migrates away from heavily subduction-influenced mantle. In contrast, here we show that across-strike and along-strike changes in crustal properties at the Eastern Lau spreading centre are large and abrupt, implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes. With incremental separation of the ridge axis from the volcanic front of as little as 5 km, seafloor morphology changes from shallower complex volcanic landforms to deeper flat sea floor dominated by linear abyssal hills, upper crustal seismic velocities abruptly increase by over 20%, and gravity anomalies and isostasy indicate crustal thinning of more than 1.9 km. We infer that the abrupt changes in crustal properties reflect rapid evolution of the mantle entrained by the ridge, such that stable, broad triangular upwelling regions, as inferred for mid-ocean ridges9,10, cannot form near the mantle wedge corner. Instead, the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water-rich low-viscosity mantle when it is near the arc. As the ridge moves away from the arc, a tipping point is reached at which that material is rapidly released from the upwelling zone, resulting in rapid changes in the character of the crust formed at the ridge.