Fluid flow during unbending: Implications for slab hydration, intermediate-depth earthquakes and deep fluid subduction

We calculate the tectonic stress profile and associated direction of fluid flow during unbending and dehydration of oceanic plates, for a range of critical parameters that affect their combined elasto-plastic and viscous behaviour, such as bending curvature, age, pore fluid pressure and viscous flow laws. In all models, negative pressure gradients are established at Moho depths, down to the base of the slab elastic core. Fluids released at these depths flow downward across the plate, “wetting” or further hydrating the underlying dry levels, and ultimately accumulate at the base of the elastic core, increasing the pore fluid pressure and triggering deep seismicity. The thickness of the “wet” layer increases for low bending curvatures, low pore fluid pressure, old slabs and dry viscous rheologies. “Wetting” of the upper 10–30 km of the slab has important implications for its rheological and anisotropic structure. Unbending favours the redistribution and trapping of significant amounts of fluids in subducting oceanic plates that will subsequently be released at the base of the upper mantle.