Some thoughts on the stability of cratonic lithosphere: Effects of buoyancy and viscosity

Continental cratons have not experienced major tectonic disruptions over a timescale of 10(9) years. The thickness of cratonic lithosphere also appear s to have changed little over this timescale. These observations are often attributed to the presence of chemically buoyant and/or highly viscous subc ratonic roots. Simple physical scaling relationships are developed to explo re the buoyancy and/or viscosity conditions required to stabilize such root s against large-scale deformation and rapid remixing into the mantle. The s calings are tested using idealized numerical simulations with good general agreement. Applied to Earth, the scalings suggest that (1) buoyancy alone i s unlikely to stabilize cratonic roots and (2) if root viscosity is to prov ide stability into the Archean, then roots must be 10(3) times as viscous a s the mantle. Based on:available experimental data, root dehydration cannot account for the required viscosity increase. Temperature-dependent viscosi ty can stabilize roots, but it does so at the expense of stagnating the ent ire mantle lithosphere, i.e., at the expense of sacrificing plate tectonics . This suggests that the plastic yielding properties of rocks at low temper atures will need to be more directly accounted for in future experiments ex ploring root stability.