From subduction to collision: Control of deep processes on the evolution of convergent plate boundary

Abstract

Using laboratory experiments, we investigate the dynamics of the collisional process that follows the closure of an oceanic basin. The evolution of these experiments systematically shows four successive episodes of deformation, which correspond to (1) the initiation of oceanic subduction, (2) a mature period of oceanic subduction, (3) an episode of continental subduction, during which the trench absorbs all the convergence and the superficial tectonic regime does not change significantly within the continental plates, and (4) continental collision that starts when the trench locks and convergence is absorbed by a system of thrust faults and folds. We observe that the amount of continental material that subducts before the onset of collision depends on the slab pull exerted by the subducted oceanic lithosphere. The slab‐pull force, in turn, depends on the amount of subducted oceanic material, on the thickness of the convective mantle, and also on the rheology of the slab. Our experiments, indeed, suggest that parts of the oceanic slab may separate from the superficial slab to sink rapidly in the mantle, decreasing the slab‐pull level and triggering the rapid onset of collision. We observe two possible modes of slab deformation: slab break‐off and development of viscous instabilities. We define two dimensionless rheological numbers to characterize the possible occurrence of these modes of deformation. In all cases, oceanic closure is followed by episode of continental subduction during which the continental lithosphere may reach depths varying between 50 and 450 km, prior to the onset of continental collision.