Dynamics of Subduction and Plate Motion in Laboratory Experiments.

3-D laboratory experiments have been designed to investigate the way slab-bearing plates move during subduction inside the mantle. The boundary conditions are as simple as possible: a viscous plate rests in the center of a large tank filled up by honey and subducts under its negative buoyancy once a small instability at the plate edge is created. Varying thickness, width, viscosity, density of the plate and mantle, three characteristic modes of subduction are observed: a retreating trench mode (Mode I), a retreating trench mode following a transient period of advancing trench (Mode II), and an advancing trench mode (Mode III). These modes are characterized by different partitioning of the amount of subduction into plate and trench motion. Our experiments show that the velocity of subduction can be roughly modeled by the dynamic interaction between acting and resisting forces and that some parameters such as the slab viscosity or thickness have the opposite influence than the one usually suggested in the literature. This result is interpreted as the consequence of the dependence (measured in the experiments) of the slab radius of curvature on the plate viscosity and thickness. However, it is still far from being simple to predict how the trench and plate move. Our results suggest that the complexity of the style of subduction could also be controlled by simple geometrical rules of a plate bending inside a stratified mantle: our planet system is in the crucial range where the length of the slab pulling down the plate is about the double of its radius of curvature.