Recent geological mapping in the northern Appalachians has provided an improved data base for delineating terranes and providing better constraints on their accretionary history. A revised terrane map of the northern Appalachians is presented along with a series of diagrams outlining their accretionary history. The terranes may be grouped into the following categories from northwest to southeast: (1) North American terrane (continent), characterized on its eastern side by a Grenvillian basement overlain successively by a late Precambrian–Ordovician miogeocline and an Ordovician foredeep; (2) terranes composed of continental rise and oceanic lithosphere thrust over the North American terrane; (3) diverse terranes made up of volcanic arc complexes, oceanic lithosphere, and microcontinents, which represent the vestiges of Iapetus; (4) diverse Precambrian terranes (Avalon Composite terrane) characterized by metamorphosed mid–Late Proterozoic rocks, Late Proterozoic rifted cratonic volcanic arc complexes, overlain by outliers of platformal Cambro-Ordovician and Silurian–Early Devonian rocks containing an Atlantic and Rhenish fauna, respectively; (5) Meguma terrane composed of Cambro-Ordovician turbidites overlain by Upper Ordovician–Lower Devonian rift volcanic rocks and shelf sediments containing a Rhenish fauna.
The accretionary history of these terranes may be broadly reconstructed in the following sequence: (a) Late Precambrian amalgamation of the Avalon Composite terrane during the Cadomian Orogeny is constrained by the similarity of the Cambro-Ordovician outliers, which are inferred to represent a formerly continuous overstep sequence; (b) progressive oblique dextral convergence in the northwestern part of Iapetus between North America and a volcanic arc (Ascot–Weedon–Notre Dame) throughout the Ordovician Period culminated in continent-arc collision and accretion of intervening terranes; (c) progressive oblique sinistral convergence in the southeastern part of Iapetus beneath a volcanic arc complex (Bronson Hill–Mirimichi–La Poile) throughout Ordovician, Silurian, and early Devonian time led to closure of Iapetus and amalgamation of intervening terranes and sinistral emplacement of the Avalon Composite terrane along a deep vertical fault (Dover–Turtle Head Fault); (d) progressive oblique dextral closure of the Theic Ocean between Avalonia and Gondwanaland led to accretion of Meguma terrane by earliest Devonian time and culminated in progressive continent-continent collision throughout Devonian, Carboniferous, and Early Permian time.
Reactivation of primary terrane boundaries during later deformational events has obscured original accretionary relationships. Thus, overstep sequences, stitching plutons, accretionary fabrics, and metamorphic overprinting may only provide upper limits on the reactivation rather than initial accretion.
