A buckling stagnant slab observed inside the mantle transition zone

Subducting lithosphere is a primary driver of plate tectonics. Seismic tomography shows that some slabs are stagnant in the mantle transition zone (MTZ), and it is debated why they do not continue directly across the MTZ base ( d 660). Here we demonstrate by seismic Receiver Function images of the Japan-Kurile subduction system that stagnant slabs may be flat or buckling inside the MTZ, depending on the dip of the incident slab. Shallow dip may result in a flat stagnant slab following a broad, gentle d 660 depression as predicted by thermal effects. Moderate dip (20–35º) causes a buckling stagnant slab and a localized ∼40 km deep d 660 depression, which is deeper than predicted from pure thermal effects. In-phase deformation of a positive converter around 520 km depth (labelled x 520 and unlikely to be caused by exothermic phase transition) and the endothermic d 660 inside the MTZ cannot have a pure thermal origin and indicates that these interfaces correspond to the top and bottom of the buckling stagnant slab. We speculate that the observed dip-dependent in-phase deformation of the d 660 and the x 520 boundaries is affected by both cooling and mechanical impact by the slab, which control their temperature and depth (pressure), respectively. Our findings suggest that slab impact on MTZ may also affect localized d 660 depressions in steeper subduction systems and that thermal effects of slabs on MTZ deformation may often be overestimated.