Quantitative analysis of the inclined low-velocity zone in the mantle wedge of northeastern Japan: A systematic change of melt-filled pore shapes with depth and its implications for melt migration

Travel-time tomography beneath the northeastern Japan arc reveals that an inclined low-velocity zone exists in the mantle wedge, sub-parallel to the down-dip direction of the slab. This zone, distributed continuously along the arc as a single inclined sheet, has been considered as the main source of arc magma. A quantitative interpretation of both P- and S-wave velocity structures precisely determined from the recent travel-time tomography is done in terms of thermal heterogeneity and fluid content. The combined analysis of both P- and S-wave velocity structures makes it possible to independently derive the information on the shape and volume fraction of the fluid-filled pores. The reference P- and S-wave velocities representing the host rock velocities at reference temperature T₀ are estimated from the tomographic data, while an alternative method based on high-temperature and high-pressure elasticity of upper mantle minerals is used to confirm the validity of the present method. The calculated velocity anomalies are corrected for the thermal effect using the three-dimensional thermal structure estimated from the P-wave attenuation data, showing that the observed low-velocity anomalies cannot be explained by the thermal effect alone. The remaining velocity anomalies are explained by the existence of melt-filled pores and effective aspect ratio and volume fraction of the pores are determined. The results show a systematic change in melt-filled pore shapes with depth, suggesting the existence of 3-6 vol.% melts as grain boundary tubules at a depth of 90 km, 0.04-0.05 vol.% melts as thin cracks or dikes with aspect ratio of ∼0.001 at a depth of 65 km, and 1-2 vol.% melts as cracks or dikes with aspect ratio of 0.02-0.04 at a depth of 40 km.