Tracking slabs beneath northwestern Pacific subduction zones

Abstract

This study uses the amplitudes of bottom-side reflected shear waves to constrain the morphology and dynamics of subducted oceanic lithosphere beneath northwestern Pacific subduction zones. Across Honshu arc, the 410- and 660-km seismic discontinuities are detected at the respective depths of 395±5 and 685±5km within the Wadati–Benioff zone. Their topographies are negatively correlated along slab dip, showing the dominant effect of temperature on the olivine phase changes within the upper mantle transition zone. The base of the upper mantle shows broad depressions as well as localized zones of shallow/average depths beneath Korea and northeast China. The 15+km peak-to-peak topography west of the Wadati–Benioff zones suggests that the stagnant part of the subducted Pacific plate is not as flat as previously suggested. Eastward slab ‘pile-up’ is also possible at the base of the upper mantle. Across southern Kuril arc, the shear wave reflection coefficients of major olivine phase boundaries fall below 5% within the Wadati–Benioff zone. The apparent reflection gaps and the spatial connection between a strong reflector at ~900km depth may imply 1) possible compositional variations at the top and bottom of the transition zone and 2) substantial mass/heat flux across the 660-km seismic discontinuity. We also identify strong reflectors within the subducted oceanic lithosphere at mid transition zone depths. The depths and strengths of these reflectors are highly variable between Honshu and southern Kuril islands.