Deconstructing the Iapetus Suture: Terrane assemblage map of the northern Appalachians and western Caledonides

Maps of the Appalachian–Caledonide Orogen have sought to identify a unique Iapetus suture marking either a collision between Laurentian and Gondwanan crust, or final closure of the Iapetus Ocean. However, orogen syntheses based in Britain and Ireland show the Iapetus suture as Silurian; those in Newfoundland show a Late Ordovician suture; those in Cape Breton Island show no Iapetus suture, and those in southern New England show closure in the Early Ordovician. The provenance and the timing of accretion can be examined using detrital zircon distributions and stratigraphic relationships. For example, the approach of a Ganderian terrane to the Laurentian margin is typically marked by an influx of ~1 Ga zircon from the Grenville Orogen. The end of accretion is typically bracketed by an angular unconformity, above which forearc basin sedimentary and volcanic rocks contain both Laurentian and non-Laurentian zircon. This approach allows identification of terrane assembages separated by multiple anastomosing sutures, ranging in age from Early Ordovician to Devonian. Terranes derived from peri-Gondwanan Ganderia arrived diachronously, such that the Laurentia–Gondwana boundary is marked by sutures of different age along the orogen. We therefore argue that efforts to identify a single Appalachian–Caledonide "Iapetus suture" are not worthwhile.