Plate bending at subduction zones: Consequences for the direction of plate motions

Bending of lithospheric plates at subduction zones is thought to be an important source of dissipation for convection in the Earth's mantle. However, the influence of bending on plate motion is uncertain. Here we use a variational description of mantle convection to show that bending strongly affects the direction of plate motion. Subduction of slabs and subsidence of oceanic lithosphere with age provide the primary driving forces. Dissipation is partitioned between plate bending and various sources of friction at plate boundaries and in the interior of the mantle due to viscous flow. We determine the poles of rotation for the Pacific and Nazca plates by requiring the net work to be stationary with respect to small changes in the direction of motion. The best fit to the observed rotation poles is obtained with an effective lithospheric viscosity of 6 × 1022 Pa s. Bending of the Pacific plate dissipates roughly 40% of the energy released by subduction through the upper mantle.