Dynamics of continental accretion

Subduction zones become congested when they try to con sumebuoyant,exotic crust. The accretionary mountain belts (orogens) that format these convergent plate margins have been the principal sites of lateral continental growth through Earth's history.Modern examples of accretionary margins are the North American Cordilleras and southwest Pacific subduction zones. The geologic record contains abundant accretionary orogens, such as the Tasmanides1, along the easternmargin of the supercontinentGondwana, and the Altaïdes, which formed on the southern margin of Laurasia2. In modern and ancient examples of long-lived accretionary orogens, the overriding plate is subjected to episodes of crustal extension and back-arc basin development, often related to subduction rollback3 and transient episodes of orogenesis andcrustal shortening4-7, coincidentwith accretion of exotic crust. Here we present three-dimensional dynamic models that show how accretionary margins evolve from the initial collision, through a period of plate margin instability, to re-establishment of a stable convergent margin. The models illustrate how significant curvature of the orogenic system develops, as well as the mechanism for tectonic escape of the back-arc region.The complexity of themorphology and the evolution of the systemare caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction.Wefind geological and geophysical evidence for this process in the Tasmanides of eastern Australia, and infer that this is a recurrent and global phenomenon.