DECONSTRUCTING THE "IAPETUS SUTURE": DIACHRONOUS ACCRETION OF TERRANE ASSEMBLAGES IN THE APPALACHIAN COLLAGE

Maps of the Appalachian–Caledonide orogen have traditionally sought to identify a unique Iapetus suture marking either the collision between Laurentian and Gondwanan continental crust, or the closure of the last remnant of the Iapetus Ocean. This seemingly straightforward task is fraught with perils in an orogen that was built by the accretion of multiple fragments over substantial time. Orogen syntheses based in Britain and Ireland typically show the Iapetus suture as a Silurian feature; those based in Newfoundland show a Late Ordovician suture; those in Cape Breton Island have no record of an Iapetus suture, whereas those in southern New England show the Iapetus closing in the Early Ordovician.

Both the provenance and the timing of accretion can be examined using detrital zircon distributions and stratigraphic relationships in terranes within the orogen. For example, the approach of a Ganderian terrane to the Laurentian margin is typically marked by an influx of ~1 Ga zircon derived from the Grenville orogen, in sedimentary rocks conformable with underlying Gondwana- or Baltica-derived strata. The end of accretion is typically bracketed by an angular unconformity, above which forearc basin sedimentary and volcanic rocks contain a mixture of Laurentian and non-Laurentian zircon. This approach allows identification of multiple sutures arranged en echelon within the orogen, ranging in age from Early Ordovician to mid-Devonian. Terranes derived from the peri-Gondwanan domain Ganderia arrived diachronously, at increasingly later times from south to north, such that the Laurentia–Gondwana boundary (Mekwe'jit line) is marked by sutures of different age along the orogen. Traced along orogen strike from southwest to northeast, a suture of a given age may separate: (1) two Gondwana-derived terranes; (2) a Laurentia-derived terrane and a Gondwana-derived terrane; or (3) two Laurentia-derived terranes.

Naming a single Iapetus suture is further complicated by recent detrital-zircon provenance results that show a potential second suture, of Devonian age, within Britain, and other results that show the Munsungun inlier of northern Maine as Laurentian, not Gondwanan as previously thought.

We therefore argue that efforts to portray and discuss a single Appalachian–Caledonide "Iapetus suture" should be abandoned.