Lithosphere–asthenosphere viscosity contrast and decoupling

The coupling/decoupling between the lithosphere and asthenosphere has significant implications for understanding many important aspects of plate tectonics and geodynamics. To drive plate motion, mantle convection requires coupling at the lithosphere–asthenosphere (LA) interface. Meanwhile a low viscosity layer in the asthenosphere is indicative of possible LA decoupling. Here we present an analytical model of a stratified uppermost mantle structure disturbed by a long-wavelength perturbation (such as the body tide) to analyse the influence of LA viscosity contrast on the growth (or decay) rates of the perturbation. We show that the viscosity contrast of 8–10 orders of magnitude would allow a relative motion of the lithosphere over the asthenosphere due to the long-wavelength perturbations at the rate of about 10 cm yr−1. These constrains on the viscosity contrast can allow to discriminate between the LA coupling and decoupling. The growing seismic and mineralogical evidences of a possible ultra low viscosity asthenospheric layer may be indicative of the LA decoupling and their relative motions due to long-wavelength perturbations, and a contribution of the tidal drag on the plate motion should not be neglected in the regions of high viscosity contrasts.