Variations of slab dip and overriding plate tectonics during subduction: Insights from analogue modelling
Résumé
We present small-scale laboratory models of oceanic subduction in which plates motion is imposed by lateral boundary conditions. The oceanic plate moves trenchward at constant speed and subducts below a fixed overriding plate. In this configuration, the long-term process of subduction is not steady-state. Slab interaction with the upper mantle-lower mantle boundary results in periods of slab flattening during which the dip of the slab diminishes, followed by periods of slab steepening. The overriding plate tectonic regime is influenced by the dynamics of subduction, slab anchoring favouring trench perpendicular shortening. When the slab is anchored, slab flattening further favours shortening, while slab steepening favours extension or smaller shortening rates. Non-steady-state long-term subduction may explain part of the variability of slab geometries evidenced by statistical analyses of present-day subduction zones. Experiments suggest that, despite boundary conditions applied on the converging plates do not change, tectonics pulses within the overriding plate may be caused by this non-steady-state behaviour.