TERRANE ASSEMBLAGE MAP OF THE NORTHERN APPALACHIAN AND WESTERN CALEDONIDE OROGENS

Maps of the Appalachian orogen have traditionally been divided into zones, terranes, and domains such as Ganderia, Avalonia and Megumia, reflecting the nature of the basement and overlying pre-accretion cover successions. Most interpreters of regional tectonics seek to use these characteristics to place these entities in their original positions on the margins of Gondwana or Baltica, and imply that the same entities travelled on single plates in their journey across the Iapetus Ocean to North America. The timing of accretion can be examined using detrital zircon distributions and stratigraphic relationships. The approach of a 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 terrane assemblages recording multiple episodes of accretion at the Laurentian margin from Early Ordovician to Devonian. Although these assemblages typically do not coincide with the traditional divisions Ganderia, Avalonia, and Megumia, those divisions still remain useful because they identify distinctive regions on the margins of Gondwana or Baltica from which terranes were derived.

Incorporated into our map are new results from the Munsungun Inlier in Maine, which indicate that it includes rocks belonging to the peri-Laurentian Dashwoods–Tyrone assemblage, and from the Anglo-Welsh basin, which indicate that southern Britain was not accreted until the Devonian. The boundaries separating Laurentia-derived terranes from those that originated near Gondwana or Baltica are of different ages in different parts of the orogen, and are arranged en-echelon. It is therefore not meaningful to identify a single Iapetus suture throughout the orogen.