From nanoparticles to plate tectonics : insights for laboratory experiments using colloidal dispersions

We recently discovered a material, aqueous dispersions of colloidal nanoparticles, whose rheology depends strongly on solid particle fraction f_p, being Newtonian at low f_p, and presenting yield stress, shear thinning, elasticity, and brittle properties as f_p increases. Moreover, the rheology is time-dependent, with shear stress causing damage, which can heal due to electrostatic interactions between the colloidal particles. The competition between damage and healing results in long-term weak zones. Such a behaviour is analogue to the rheology of mantle rocks as temperature decreases.

We therefore undertake a systematic laboratory study of convection in such fluids, where the system is continuously cooled and dried from above and heated or not from below. As the dispersion is dried, a skin ("lithosphere") forms at the surface on the convective fluid, and instabilities develop on several scales as shear bands (0.01 mm-scale), folds (mm-scale), fractures (0.1-10 mm-scale), small-scale convection (cm-scale) and plates (2-20 cm-scale). The system always describes several dynamic regimes through time: an initially very soft lithosphere would result in a stagnant lid regime of convection, which can then evolve in episodic and/or partial subduction, sometimes continuous plate tectonics, and finally stagnant lid. Regime diagrams will be presented as a function of the effective rheology of lithosphere. This "effective" rheology depends on the cascade of instabilities at smaller scales, thanks to which it is much weaker than the material properties measured on small samples.