Flat 410 and 660 discontinuities beneath northeastern Japan: Implication for a sub-slab wet plume hypothesis

Abstract

Recent seismic tomography studies have shown that distinct low-velocity anomalies exist below subducting slabs in many subduction zones and these anomalies are interpreted as a hot plume from the lower mantle. However, it is still unclear how high are the temperatures in the sub-slab low-velocity anomaly regions. Here, we conduct receiver function analysis and estimate the horizontal temperature variation in the mantle transition zone by determining the depth variation of 410 and 660 discontinuities beneath northeastern Japan. The obtained results show that the depth of the two discontinuities changes little, which suggests no distinct thermal heterogeneities over the study area. Therefore, we infer that the major cause of the sub-slab low-velocity anomaly is attributable not to high-temperature anomaly but to the presence of a small amount (∼0.2 wt%) of hydrous minerals, which can explain the sub-slab low-velocity anomalies and the flat 410 and 660 discontinuities.