Incipient shortening of a passive margin: the mechanical roles of continental and oceanic lithospheres

By definition, passive margins deform little or no further after continental break-up. Nevertheless, post-break-up deformation and associated uplift have been documented along many margins (for example Norway, Greenland, West Africa, South America, India and Australia). We have conducted a series of analogue experiments in which we subjected scaled multilayered models of the lithosphere (made of sand and viscous layers) to horizontal shortening. A multilayered lithosphere rested on an asthenosphere of low viscosity that allowed isostatic adjustments. Models with various strength profiles represented oceanic lithospheres of different ages. The continental lithosphere reacted to an applied boundary velocity according to the strength and density of oceanic lithosphere. Where oceanic lithosphere was stronger and heavier than continental lithosphere it subducted; where oceanic crust was stronger and lighter than continental crust it obducted. On this basis we suggest that the style of incipient shortening of a passive margin depends on the age of oceanic lithosphere. Where the ocean is young we expect obduction; where it is old we expect incipient subduction. In nature, ophiolites and shortened passive margins appear to conform to this simple rule.