Controls on back-arc crustal accretion: insights from the Lau, Manus and Mariana basins

Abstract

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 backarc 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.

The Effect of Arc Proximity on Hydrothermal Activity Along Spreading Centers: New Evidence From the Mariana Back Arc (12.7°N–18.3°N)

“…Moreover, whereas water is a relatively passive participant in adiabatic melt generation beneath mid‐ocean ridges, in the back arc and arc environment it becomes an active fluxing agent that helps drive melt generation. This enhancement causes the magmatic budget of some BASC sections to increase beyond that expected from spreading rate alone (e.g., Martinez & Taylor, ; Pearce et al, ; Sleeper et al, ). Further, magma supply may be augmented where BASC‐arc separation is small and extensional stress in the thin separating lithosphere broadly distributes rising magma, permitting capture of some magma by the BASC (Brounce et al, ; Stern et al, ).…”

Seismic anisotropy indicates ridge‐parallel asthenospheric flow beneath the Eastern Lau Spreading Center

“…In this view, the flow pattern of the mantle wedge of the subduction zone and the spreading centers are linked, and material is being transported between the two. Martinez and Taylor [, ] further develop this hypothesis in their “piracy model” of along‐strike variation in axial characteristics. They propose that the spatial proximity of the Valu Fa Ridge to the arc causes the ridge to tap arc melts (slab hydrated) and receive (“pirate”) an enhanced melt supply, that mantle wedge return flow of depleted material is responsible for the decreased melt supply to the Northern ELSC where the ridge is farther from the arc, and farther to the north along Central Lau Spreading Center (CLSC), the ridge is sufficiently far away from the slab, such that it taps “normal" mantle and shows typical mid‐ocean ridge characteristics.…”

Basaltic volcaniclastics from the Challenger Deep forearc segment, Mariana convergent margin: Implications for tectonics and magmatism of the southernmost Izu–Bonin–Mariana arc

“…) Swath‐mapped bathymetry is recompiled and matched to predicted bathymetry from S andwell and S mith (). Red line is back‐arc basin spreading axis from M artinez and T aylor (). Islands are black, the largest and southernmost is G uam, USA .…”