Together, the Lau, Manus and Mariana basins encompass a broad range of conditions of back-arc basin development. Marine surveys have determined the tectonic setting and reconnaissance-scale geophysical and geochemical properties of the extension axes in these basins. We review these data to examine crustal accretion characteristics in the back-arc setting. In each basin magmatism is enhanced in spreading centres near the arc volcanic front, but decreases becoming "deficient" in axes further from the arc. In the Lau and Manus basins the axes extend far behind the arc and develop typical mid-ocean ridge (MOR)-like characteristics. We propose that these variations are controlled by the subducting slab, which shapes composition and flow within the mantle wedge: water released from the slab lowers the mantle solidus and increases in concentration toward the volcanic arc. Spreading centres near the arc advect the hydrated mantle material, enhancing melt production. Slab-induced flow carries melt-depleted mantle material back beneath the basin where it mixes with ambient mantle. Spreading centres further from the arc advect this mantle mixture and produce thinner than normal crust. Far from the arc, spreading centres advect essentially mid-ocean ridge basalt (MORB)-source mantle and their characteristics are MOR-like. Thus, in addition to spreading-rate effects on mantle melting that predominate at MORs, in back-arc basins the subducting slab introduces additional systematic effects.