Melt Focusing Along Permeability Barriers at Subduction Zones and the Location of Volcanic Arcs

Abstract

Abstract Fluids released from dehydration reactions occurring in subducting slabs trigger partial melting in the mantle wedge. The resulting magma rises through the overlying mantle wedge and lithosphere and forms arc volcanoes at subduction zones. In general, the location of the volcanic arc is narrowly defined even though the melting region in the mantle wedge can be broad. We propose here that a thermally controlled low permeability barrier at the base of lithosphere is able to focus melts to the place where the volcanic arc is actually observed. As the melt ascends, the permeability of the mantle rock decreases as a result of melt crystallization. A low permeability barrier may form in the cooler lithosphere and can trap ascending melt and redirect it laterally according to the slope of the permeability barrier, so that the ascending melt is focused at the apex of the permeability barrier. We model the location and shape of isotherms that approximate the permeability barriers in the mantle wedge based in two‐dimensional numerical subduction models that follow the specific geometry of various subduction zones. In 28 of 31 globally distributed test regions, the arc locations estimated from our model show good agreement with the actual arc locations. The modeling results indicate that volcanic arcs can be explained as the surface projection of the apex of the permeability barrier, regardless of the distribution of melt deeper in the mantle wedge.

Keywords

• mantle wedge
• melt focusing
• numerical modeling
• permeability barrier
• subduction zone
• volcanic arc