Dynamic Component of the Asthenosphere: Lateral Viscosity Variations Due To Dislocation Creep at the Base of Oceanic Plates

Abstract

The asthenosphere is commonly defined as an upper mantle zone with low velocities and high attenuation of seismic waves, and high electrical conductivity. These observations are usually explained by the presence of partial melt, or by a sharp contrast in the water content of the upper mantle. Low viscosity asthenosphere is an essential ingredient of functioning plate tectonics. We argue that a substantial component of asthenospheric weakening is dynamic, caused by dislocation creep at the base of tectonic plates. Numerical simulations of subduction show that dynamic weakening scales with the surface velocity both below the subducting and the overriding plate, and that the viscosity decrease reaches up to two orders of magnitude. The resulting scaling law is employed in an apriori estimate of the lateral viscosity variations (LVV) below Earth's oceans. The obtained LVV help in explaining some of the long-standing as well as recent problems in mantle viscosity inversions. The motion of lithospheric plates at the Earth's surface is enabled by a weak underlying layer-the asthenosphere. The origin of this low viscosity layer is still subject of discussion. Presence of water or partial melt were proposed as possible reasons of its reduced viscosity. Another mechanism that may lead to weakening is non-linear deformation. Rheological description of asthenospheric material includes dislocation creep, a deformation mechanism that depends on the velocity contrast between the lithospheric plate and underlying mantle-the faster the plates are, the weaker the underlying layer becomes and vice versa. Here we argue that a substantial component of asthenospheric weakening is dynamic, caused by this deformation mechanism. We evaluate numerical models of subduction including dislocation creep and derive a relation between the surface velocity of oceanic plates and the magnitude of the underlying asthenospheric viscosity. This allows us to estimate how the viscosity varies under different oceanic plates on Earth, which is otherwise hard to constrain. Our results indicate that the asthenosphere below the Pacific plate should be particularly weak. Substantial component of asthenospheric weakening is dynamic, caused by dislocation creep at the base of tectonic plates Dynamic weakening scales with the surface velocity both below the subducting and the overriding plate The resulting scaling law is employed in an apriori estimate of the lateral viscosity variations below Earth's oceans