Consequences of 3-D flow on crustal production along the Lau back-arc spreading center

Observed axial morphology of Valu Fa (VF) and Eastern Lau (EL) (southern and central regions along the Lau back-arc spreading center) suggest that VF is more magmatically robust than EL despite EL spreading nearly twice as fast as VF. Early geochemical and geophysical studies showed a gradational decrease in subduction enhanced melting moving north from VF to EL. More recent geochemical and seismic observations detail a rapid stepwise decrease in subduction influence and melt production as the spreading center axis sweeps away from the Tofua Volcanic-Arc. As the ridge sweeps away from the volcanic-arc, the influence of the slab hydrated mantle in the melting structure of the ridge decreases. Furthermore, EL produces an anomalously thin crust for a robust spreading center. While 2-D numerical studies show a decrease in subduction influence and melting going from VF to EL, they have difficulty explaining the thinning of EL’s crust.

To understand the geophysical observations, we implement the effects of 3-D flow and slab hydration in three numerical experiments. The first model with no slab hydration shows that observed geometric and surface kinematic boundary conditions cause a steep northward gradational increase in relative melting (anhydrous) with increasing spreading rate. A peak in the relative melting appears particularly close to EL where crust is shown to be thinnest resulting from a northwestern along axis mantle flow from slower spreading VF to the faster spreading ELSC. These predictions run in opposition to the observations. The second model including a viscosity reduction in the mantle wedge due to slab hydration causes a subdued relative melting increase with spreading rate and a “saddle” shaped decrease in relative melting north of 20.9°S. This saddle shaped melting structure is caused by a reversal in along axis flow towards the southeast. Mantle is driven from below EL towards the lower viscosity region beneath VF, accounting for the thin crust observed at EL. Finally, introducing a direct hydration of the wedge in the third model increases the melt production under VF and causes a stepwise decrease in melt production at EL due to its decreased proximity to the slab-hydrated region and its position above the saddle point in melt production, consistent with geophysical observations.