Carbonates and intermediate-depth seismicity: Stable and unstable shear in altered subducting plates and overlying mantle.

A model for intermediate-depth earthquakes of subduction zones is evaluated based on shear localization, shear heating, and runaway creep within thin carbonate layers in an altered downgoing oceanic plate and the overlying mantle wedge. Thermal shear instabilities in carbonate lenses add to potential mechanisms for intermediate-depth seismicity, which are based on serpentine dehydration and embrittlement of altered slabs or viscous shear instabilities in narrow fine-grained olivine shear zones. Peridotites in subducting plates and the overlying mantle wedge may be altered by reactions with CO-bearing fluids sourced from seawater or the deep mantle, to form carbonate minerals, in addition to hydrous silicates.