The postorogenic collapse of the early Paleozoic Caledonian orogeny is well documented; however, several different plate tectonic models exist for the convergent phase involving closure of the Iapetus Ocean and the collision of Laurentia and Baltica. Receiver function analysis of 11 broadband seismometers along a 270 km transect in the East Greenland Caledonides reveals the existence of an east-dipping high velocity slab. Numerical modeling demonstrates that relict subducted and eclogitized crust is a plausible explanation. Thus, eastward subduction preceded subsequent west-dipping subduction during the formation of the East Greenland and Scandinavian Caledonides. This is a key constraint for understanding the Caledonian and continental margin evolution in the North Atlantic realm.