Stagnant Slabs and Their Return Flows from Finite-Frequency Tomography of the 410-km and 660-km Discontinuities

Abstract:

Slab pull is generally considered as the dominant force that drives the global movement of tectonic plates. This convection mode is well constrained in the upper mantle but the convection pattern in the mid mantle is more speculative. In this study, we present high-resolution global models of the 410-km and 660-km discontinuity structure from finite-frequency tomography of SS precursors. The new models reveal a strong positive correlation between the two discontinuities under major subduction zones. In regions where large- scale stagnant slabs have been reported in the mantle transition zone, both discontinuities occur at depths greater than the global average. This structure correlates well with seismic wavespeed anomalies, suggesting possible return flows from the lower mantle warming up the shallow mantle transition zone above the stagnant slabs. Using a simple model for stagnant slabs in geodynamical simulations, we show that this mode of mass exchange between the upper mantle and the lower mantle occurs predominantly in the vicinity of the slabs, and, the return flows become stronger as the extent of slab stagnation increases.