Initiation of subduction at Atlantic-type margins: Insights from laboratory experiments

We have performed scaled lithospheric experiments to simulate the behavior of a ocean-continent plate system subjected to compressional strain over a geological timescale. Experiments have been constructed using sand and silicone putty, representing the brittle upper crust and the ductile lower crust/upper mantle, respectively; the layers floated on glucose syrup simulating the asthenosphere. Compressional stress is achieved by displacing a piston at constant velocity perpendicular to the plate margin. We investigate the influence of four parameters: (1) the negative buoyancy of oceanic lithosphere, (2) the horizontal body forces between continent and ocean, and (3) the brittle and (4) the ductile strength of the passive margin. Two numbers express the importance of these parameters: the Argand number (Ar), representing the ratio between the body force of continent and its integrated strength, and the buoyancy number (F), representing the ratio between the buoyancy force of ocean and its ductile resistance. We obtain three scenarios. In experiments with Ar 3 and F < 1 the ocean deforms by distributed folds, resembling the undulations observed in the Indian Ocean. In experiments with Ar 7 and F > 1 the continent collapses toward the ocean, producing back-are extension and subduction, simulating the post-Alpine Neogene evolution of the Mediterranean area. In experiments with Ar 3 and F > 1 the passive margin slowly evolves toward trench nucleation with the formation of a viscous mantle instability. We conclude that the latter model can be applied to the evolution of Atlantic-type margins, where there is evidence of this ongoing process.