The effect of asthenosphere’s rheology on mantle and surface tectonics : the role of composite rheology

Earth’s upper mantle rheology controls lithosphere-asthenosphere coupling and thus its surface tectonics. Although rock deformation experiments and seismic anisotropy measurements indicate that dislocation creep can occur in the Earth's uppermost mantle, the role of composite rheology (including both diffusion and dislocation creep) on global-scale mantle dynamics and surface tectonics remains largely unexplored.

Here, we investigate the influence of composite rheology on the planform of convection and on the planetary tectonic regime as a function of the lithospheric yield strength in numerical models of mantle convection with plate-like tectonics. We show that the consideration of composite rheology in the upper mantle leads to the self-generation of a discontinuous asthenosphere evolving fast, with a low-viscosity and a maximal thickness that depend on the rheological parameters for diffusion and dislocation creep. In mobile-lid models, the spatio-temporal evolution of the asthenosphere is mainly controlled by the location of slabs and plumes that generate regions of mantle deforming dominantly through dislocation creep. Moreover, the low upper-mantle viscosities caused by composite rheology produce substantial and contrasting effects on surface dynamics. For a strong lithosphere (high yield stress), the large lithosphere-asthenosphere viscosity contrasts promote stagnant-lid convection, while the increase of upper-mantle convective vigor enhances plate mobility for low lithospheric strength (small yield stress). We further show that composite rheology does not facilitate the onset of plate-like behavior at large lithospheric strength due to decoupling between the asthenosphere and the lithosphere.