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 sili cone putty, representing the brittle upper crust and the ductile lower crus t/upper mantle, respectively; the layers floated on glucose syrup simulatin g the asthenosphere. Compressional stress is achieved by displacing a pisto n at constant velocity perpendicular to the plate margin. We investigate th e influence of four parameters: (1) the negative buoyancy of oceanic lithos phere, (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), repres enting the ratio between the body force of continent and its integrated str ength, and the buoyancy number (F), representing the ratio between the buoy ancy 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 wit h Ar 7 and F > 1 the continent collapses toward the ocean, producing back-a re extension and subduction, simulating the post-Alpine Neogene evolution o f the Mediterranean area. In experiments with Ar 3 and F > 1 the passive ma rgin slowly evolves toward trench nucleation with the formation of a viscou s mantle instability. We conclude that the latter model can be applied to t he evolution of Atlantic-type margins, where there is evidence of this ongo ing process.