Convection Dynamics of Stagnant Slabs and Their Return Flows from Seismic Imaging

Stagnant slabs in the mantle transition zone have been widely imaged in seismic tomographic studies. The dynamics of stagnant slabs, in particular, the sinking of the slabs into the lower mantle and their associated return flows are important for the understanding of material exchange between the upper mantle and the lower mantle. The horizontal extent of stagnant slabs is often well resolved in seismic tomography but the interpretation of wavespeed anomalies in the mantle transition zone remains challenging due to limited vertical resolutions. Structures with a vertical length scale smaller than the 250 km extent of the mantle transition zone are often not well resolved and global wavespeed models can differ significantly depending on their vertical parametrization. In finite-frequency imaging of the mantle transition zone discontinuities using SS waves and their precursors, we show that both the 410-km and the 660-km discontinuities occur at greater depths beneath major subduction zones. This observation suggests possible warm and dry return flows in the upper mantle transition zone above the cold stagnant slabs. We simulate the sinking of continental-sized stagnant slabs in a super adiabatic mantle using CitcomS. In the absence of slab stagnation, the return flows produced by a sinking slab are overall weak and diffused. As the length of the horizontal (stagnation) extent of the slab increases, return flows become stronger and more concentrated in the vicinity of the sinking slab. The return flows can be further facilitated by a local low viscosity layer above the mantle transition zone if partial melts are present in the continental asthenosphere. The far-field return flows are in general much weaker, consistent with a weak anti-correlation between the 410-km and 660-km discontinuities in global oceanic regions.