Controls on melt focusing beneath old subduction zones: A case study of northeast Japan

Arc volcanoes in global subduction zones are geographically focused regardless of subduction parameters, and the Japan subduction zone is an excellent natural laboratory to examine arc volcanoes because young and old oceanic plates subduct along southwest and northeast Japan, respectively. Compared with the arc volcanoes in southwest Japan, which are formed by restricted sub-arc melting in the mantle wedge, those in northeast Japan should result from melt transport from the sub-backarc to the sub-arc mantle (i.e., melt focusing) because the liberated water from the sub-arc to the sub-backarc slab generates flux melting in the sub-arc to sub-backarc mantle. In this study, we quantitatively evaluated the partial melting and melt transport in the mantle wedge beneath northeast Japan using a series of two-dimensional numerical models with controlling factors, such as melt viscosity, melt density, mechanical decoupling depth at the slab interface, and melt freezing. Model calculations showed that a large melt viscosity and/or density allow corner-flow dominant melt behavior, which focuses the melt from the sub-backarc to the sub-arc mantle. A greater mechanical decoupling depth prevents the melt from being dragged toward the forearc mantle, enhancing the sub-arc melt focusing. Melt freezing prevents the melt from being dragged toward the back-arc mantle, further enhancing the sub-arc melt focusing. Calculated melt distribution was consistent with geographically focused arc volcanoes, seismic tomography, and geochemically estimated melt production in the mantle beneath northeast Japan; that is, the melt focusing in the old subduction zones is attributed to the corner flow and melt freezing in the mantle wedge.