Evidence for Hot Material Subducted beneath the Kurile Slab

The upper mantle discontinuities, located at average depths of 410 and 660 km, are generally attributed to high-pressure solid-to-solid phase changes in the mineral olivine, the 410 a result of the olivine to wadsleyite phase transition, and the 660 from the decomposition of ringwoodite to Mg-perovskite+magnesiowüstite. The depth and sharpness of these phase changes are sensitive to mantle temperature and composition, resulting in topography on the seismic discontinuities in regions of mantle thermal and chemical heterogeneity. We determine the depth of the discontinuities beneath the Kuriles using P-wave energy reflected off the underside of each discontinuity, which arrive as a precursory seismic phases to the PP arrival. Our dataset consists of 31 Sumatran earthquakes recorded by the USArray and the High Lava Plains Seismic Experiment that sample the discontinuities beneath the Kurile subduction zone. To obtain discontinuity depths, we analyze the data using a migration technique and determine the robustness of each migrated arrival by examination of vespagrams for each event. We detect a relatively flat 410 boundary in this region, with the exception of near the Kurile slab, where the boundary is elevated by 5-10 km. In most migrated events, we do not detect an underside P-wave reflection from the 660 km discontinuity, consistent with past results for this seismic phase. However, beneath the oceanward side of the subducting Kurile slab, we detect reflections from the 660 km boundary and determine that it is deep (>670 km). The presence of a P-wave reflection at this depth implies an increase in the impedance contrast at this boundary relative to the global average mantle. We propose the enhanced impedance contrast in this region represents the subduction of hot material beneath the slab into the mantle. Mineral physical experiments predict that at high transition zone temperatures (>2000 K) the ringwoodite phase becomes unstable and decomposes into majorite garnet. This reverses the sign of the Clapeyron slope (dP/dT) of the phase transition at 660 km depth and enhances the impedance contrast across the 660 km discontinuity. Our interpretation is further supported by seismic tomography that detects a robust low velocity anomaly in the transition zone below the subducting Kurile slab. This feature is consistent with higher mantle temperatures at these depths.