The inception and termination of the ca. 670-550 Ma Avalonian-Cadomian orogenic cycle are interpreted in terms of the amalgamation and breakup of a late Precambrian supercontinent. Synorogenic, arc-related volcano-sedimentary and granitoid rocks are attributed to oblique, southward-directed subduction along the margin of the supercontinent after its amalgamation. Breakup of the supercontinent and inception of the Iapetus cycle led to termination of subduction and its replacement by transform activity. The resulting closure of late Precambrian arc-related basins and the development of Early Cambrian platform successions with Acado-Baltic fauna accompanied the formation of Iapetus, but took place on the active margin of a preexisting ocean and do not record the evolution of its rifted European flank.