Subduction initiation by thermal–chemical plumes: Numerical studies

Prior suggestions for the initiation of subduction have included sediment loading, compression, and plate reconfiguration as potential triggers. Here, we investigate the possibility of subduction initiation by the interaction of the lithosphere with a buoyant mantle plume. Numerical testing of this hypothesis has been conducted in 2D with the I2VIS thermo-mechanical code accounting for phase transitions and a viscoplastic model of a thin oceanic lithosphere hit by a partially molten thermal–chemical or purely thermal plume. We demonstrate that a mantle plume can break the lithosphere and initiate self-sustaining subduction, provided the plume causes a critical local weakening of the lithospheric material above it. The intensity of the required weakening depends on the plume volume, plume buoyancy, and the thickness of the lithosphere and is the highest for the least buoyant purely thermal plumes. Another necessary condition is the presence of high-pressure fluids at the slab upper interface, reducing the effective friction coefficient there to very low values. Based on our results, we suggest that sheet-like instabilities of the Archean mantle convection could have initiated subduction on Earth where ocean was already present in less stable tectonic settings, provided that mantle plumes (sheets) at that time were rich in water and melt, which could drastically reduce the effective friction coefficient in the lithosphere above the plume. Our numerical models are also in good agreement with suggested concepts for corona formation on Venus.