Evidence in the geological record shows that continental interiors periodically undergo enigmatic episodes of large-scale subsidence. We propose that mantle flow associated with the descent of cold plumes and slabs, and their interaction with the endothermic phase change at 660 km depth, may provide a plausible mechanism for these epeirogenic events. Simulations of mantle convection that incorporate the thermodynamic effects of the phase change and both depth- and temperature-dependent viscosity are used to model the descent of plumes and slabs through the 660 km boundary. We find that the plume-slab flow scenarios are capable of supporting topographic deflections of amplitudes of ∼1 km and horizontal wavelengths of ∼1000 km that persist over time scales of 100–150 m.y.

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