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



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.

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Figure 1: Layout of the seismic experiment and map view of compressional wave velocity variations in the upper crust.
Figure 2: Seafloor bathymetry and Bouguer gravity anomalies in the study area.
Figure 3: Map of the Lau basin and cartoon interpretations of the formation of the crustal domains via changes in mantle wedge composition and flow patterns.


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We thank the captain, crew, and science parties of the R/V Langseth leg MGL0903 and R/V Kilo Moana leg KM0804. This work was funded as part of the NSF Ridge 2000 Program by grants OCE0426428 (R.A.D.) and OCE0732536 & OCE0727138 (F.M.). We thank J. Hammer, G. Ito, J. Sinton, B. Taylor and C. Wolfe for comments on an early version of the manuscript.

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R.A.D. carried out the seismic data collection, analysis and modelling; F.M. carried out bathymetry data collection and processing and gravity analysis. Both authors contributed to writing the paper.

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Correspondence to Robert A. Dunn.

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The authors declare no competing financial interests.

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Dunn, R., Martinez, F. Contrasting crustal production and rapid mantle transitions beneath back-arc ridges. Nature 469, 198–202 (2011).

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