Modes of crustal accretion in back-arc basins: Inferences from the Lau Basin

Abstract

As island arc rifting evolves to mature back-arc spreading, the nature of melt generation and mode ofcrustal accretion may vary in response to the interplay of different subduction-related processes and conditions, including (1) changes in mantle dynamics from flux-melting and buoyancy-driven upwelling at the arc volcanic front to decompression melting driven by plate separation at back-arc spreading centers; (2) re-circulation of refractory material through arc and back-arc melting regimes by mantle wedge comer flow, (3) changes in the locus ofmagmatic centers relative to the arc volcanic front; (4) variable locus of initial rifting and breakup; (5) spatially varying rheology attributable to mantle wedge hydration gradients with distance from the slab; (6) slab subduction rate, dip, and length. We discuss the possible influence of these factors on crustal accretion processes in light of observations from intraoceanic back-arc basins, with particular focus on new compilations of swath bathymetry and sidescan imagery from the Lau Basin. In the Lau Basin south of 18°S, the active spreading centers undergo large changes in morphology and crustal characteristics as they separate from the arc volcanic front. Ahead of the southern limit of organized seafloor spreading, a broad area of high acoustic backscatter indicates a wide, distributed form of crustal accretion. Parts of the western Lau Basin have been previously interpreted as remnants of a tectonically rifted preexisting arc. The swath mapping data show, however, that western basin morphology is similar to that formed by the sites of currently active magmatic crustal accretion to the east. These observations support a revised model of Lau Basin evolution in which essentially the entire back-arc basin is formed by magmatic crustal accretion, but crustal thickness and morphology reflect the changing locus of the magmatic centers with respect to a mantle wedge of varying chemical fertility and rheology. Compared to mid-ocean settings, the observations imply an expanded range of crustal accretion variables in arc-proximal magmatic centers in which seafloor morphology is more indicative of mantle wedge chemistry than spreading rate.