A low viscosity wedge in subduction zones

Geochemical, petrologic and seismological observations indicate that there may be high concentrations of water in the region above a subducting slab (the mantle wedge), which could decrease the viscosity of the mantle locally by several orders of magnitude. Using numerical models we demonstrate that a low viscosity wedge has a dramatic influence on the force balance in a subduction zone and leads to an observable signal in the topography, gravity and geoid. A regional dynamic model of the Tonga¯Kermadec subduction zone shows that the viscosity of the wedge is at least a factor of 10 smaller than surrounding mantle lithosphere and asthenosphere, consistent with estimates from seismic dissipation and deformation experiments.