Role of a low‐viscosity zone in stabilizing plate tectonics: Implications for comparative terrestrial planetology

Abstract

Earth's near‐surface layer, its lithosphere, is broken into quasi‐rigid plates that form the upper thermal boundary layer for mantle convection. Since the discovery of plate tectonics, it has been widely conjectured but only recently demonstrated that this peculiar style of convection may be facilitated by an upper mantle low viscosity zone (LVZ) over which the plates glide easily. The LVZ, or “asthenosphere,” concept dates from 19th century investigations of isostatic support of mountain belts and is supported by modern evidence for a seismic low velocity zone and by studies of postglacial rebound and dynamic compensation of the Earth's gravity field. Here we show in both two‐dimensional (2‐D) Cartesian and 3‐D spherical Earth models that combining a pronounced LVZ and a plastic yield stress to allow localized weakening of the cold thermal boundary layer results in a distinctly plate tectonic style of convection, with ∼30% toroidal surface motion for the 3‐D case. Recycling of water into the upper mantle at subduction zones is a plausible cause of Earth's LVZ, whereas Venus is dry and lacks both an LVZ and plate tectonics.