Slab Rollback Instability and Super-Continent Dispersal

Abstract:

Super-continents coalesce over subduction zone complexes and their subsequent dispersal is usually attributed to heating and upwelling of continent-insulated mantle. This dispersal mechanism, however, requires considerable mantle internal heating. Alternatively, the super-continent configuration may be mechanically unstable and disperse regardless of heating mode. In particular, increased drag on plates or subducting slabs (e.g., by accumulating continents) causes them to slow down and trenches to rollback. Once subcontinental slabs are slightly separated, resistance to their descent increases, inducing further trench migration. Slabs thus undergo a rollback instability, which disperses super-continents. A simple theoretical model illustrates this instability and shows there are two equilibrium states, one unstable super-continent state where slabs are conjoined, and one stable state where slabs are widely separated. Slab rollback from the unstable to stable states occurs at typical slow tectonic speeds, and over a period commensurate with the age of ocean basins and the Wilson cycle.