Along‐arc variation in the 3‐D thermal structure around the junction between the Japan and Kurile arcs

The thermal structure in subduction zones has a strong influence on seismogenesis and arc volcanism. Traditional 2‐D models have been used to provide reasonable agreement between models and observations, but in a number of cases clear 3‐D effects are present. One such case is in the Northern Japan subduction system. At the junction between Japan and Kurile arcs, surface heat flow and the occurrence of intermediate‐depth seismicity are different than in the Tohoku and Hokkaido regions. We investigate the effects of 3‐D slab geometry and a local deepening of slab‐mantle decoupling depth on the thermal structure in this region based on 3‐D finite element approach. We find that both effects produce the along‐arc variation of slab surface temperature, which could reach ∼100°C. The warmer region arises through 3‐D effects of thermal conduction and the colder region arises through localized slow incoming flow in the case where 3‐D slab geometry is taken into account. 3‐D flow arises where a local deepening of slab‐mantle decoupling depth is assumed, which leads to both warmer and colder regions. The effects on surface heat flow are small. While intermediate‐depth seismicity in the subducted crust is suggested to be controlled by temperature‐dependent phase transitions, the predicted changes in thermal structure are not sufficient to cause the observed deepening of seismicity. This suggests that the thermal structure of this subduction zone may be more strongly influenced by time‐dependent deformation of the overriding crust and slab.