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 CO2-bearing fluids sourced from seawater or the deep mantle, to form
carbonate minerals, in addition to hydrous
silicates. Effective viscosities of magnesian
carbonates are higher than those for
antigorite serpentine and they are markedly lower than those for H2O-saturated
olivine. However, magnesian
carbonates may extend to greater mantle depths than hydrous
silicates at temperatures and pressures of subduction zones. Strain rates within altered downgoing mantle peridotites may be localized within carbonated layers following slab
dehydration. A simple model of shear heating and temperature-sensitive creep of
carbonate horizons, based on experimentally determined creep laws, predicts conditions of stable and unstable shear with strain rates up to 10/s, comparable to seismic velocities of frictional fault surfaces. Applied to intermediate-depth earthquakes of the Tonga subduction zone and the double Wadati-Benioff zone of NE Japan, this mechanism provides an alternative to the generation of earthquakes by
dehydration embrittlement, beyond the stability of
antigorite serpentine in subduction zones.