A free plate surface and weak oceanic crust produce single‐sided subduction on Earth

Earth's lithosphere is characterized by the relative movement of almost rigid plates as part of global mantle convection. Subduction zones on present‐day Earth are strongly asymmetric features composed of an overriding plate above a subducting plate that sinks into the mantle. While global self‐consistent numerical models of mantle convection have reproduced some aspects of plate tectonics, the assumptions behind these models do not allow for realistic single‐sided subduction. Here we demonstrate that the asymmetry of subduction results from two major features of terrestrial plates: (1) the presence of a free deformable upper surface and (2) the presence of weak hydrated crust atop subducting slabs. We show that assuming a free surface, rather than the conventional free‐slip surface, allows the dynamical behavior at convergent plate boundaries to change from double‐sided to single‐sided. A weak crustal layer further improves the behavior towards steady single‐sided subduction by acting as lubricating layer between the sinking and the overriding plate. This is a first order finding of the causes of single‐sided subduction, which by its own produces important features like the arcuate curvature of subduction trenches.