OVER-RIDING PLATE CONTROLS ON MANTLE WEDGE MELTING

Arc magmatism is controlled by interactions between the subducting slab, the over-riding plate, and the mantle wedge that generate and guide mantle melts towards the surface. The distribution of melting is sensitive to temperature, pressure, and composition, ultimately determining the position of the arc front, a narrow chain of active volcanoes. The arc front is often observed to migrate in time, away from the trench. The conventional explanation for this behavior is that slab flattening leads to landward arc front migration. We explore an alternative hypothesis: evolution of the overlying plate, specifically changes to the thickness of the arc root, causes arc front migration. We investigate the effects of varying over-riding plate morphology and viscous decoupling of the shallow slab-mantle interface on melt production using 2D numerical models involving a stationary over-riding plate and a subducting plate with prescribed motion. Initial steady-state simulations suggest that: 1) Localized lithospheric thickening shifts the locus of melt production trenchward while thinning shifts melting landward. 2) Inclined LAB topography modulates the asthenospheric flow field, focusing mantle melt generation beneath the arc. 3) Thickening of the over-riding plate exerts increased torque on the slab, favoring shallowing of the dip angle. 4) Viscous decoupling produces a cold, stagnant forearc mantle but promotes arc front melting due to reduction in the radius of corner flow. We conclude that crustal and lithospheric thickening can significantly affect the spatial pattern of mantle wedge melting and potential pathways of melt migration, in the absence of slab dip changes. Changes in lithospheric thickness may thus be expected to drive arc front migration, facilitate focusing of mantle melt generation, and may contribute to slab rotations through added torque.