The Appalachian Orogen is divided into five broad zones based on stratigraphic and structural contrasts between Cambrian–Ordovician and older rocks. From west to east, these are the Humber, Dunnage, Gander, Avalon, and Meguma Zones.The westerly three zones fit present models for the development of the orogen through the generation and destruction of a late Precambrian – Early Paleozoic Iapetus Ocean. Thus, the Humber Zone records the development and destruction on an Atlantic-type continental margin, i.e., the ancient continental margin of Eastern North America that lay to the west of Iapetus; the Dunnage Zone represents vestiges of Iapetus with island arc sequences and mélanges built upon oceanic crust; and the Gander Zone records the development and destruction of a continental margin, at least in places of Andean type, that lay to the east of Iapetus.The Precambrian development of the Avalon Zone relates either to rifting and the initiation of Iapetus or to subduction and a cycle that preceded the opening of Iapetus. During the Cambrian Period, the Avalon Zone was a stable platform or marine shelf.Cambrian–Ordovician rocks of the Meguma Zone represent either a remnant of the continental embankment of ancient Northwest Africa or the marine fill of a graben developed within the Avalon Zone.Silurian and younger rocks of the Appalachian Orogen are mixed marine and terrestrial deposits that are unrelated to the earlier Paleozoic zonation of the system. Silurian and later development of the orogen is viewed as the history of deposition and deformation in successor basins that formed across the already destroyed margins and oceanic tract of Iapetus.

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Since the advent of plate tectonics, the widely accepted model for the development of the Appalachian orogen has involved the opening and closing of a late Precambrian-Paleozoic Iapetus Ocean. Only a few of a growing number of geologically distinctive terranes are easily explained by this model. Vestiges of Iapetus are nowhere coupled to the ancient North American margin. Furthermore, it cannot be demonstrated that any of the extensive Appalachian terranes, now east of the Iapetus tract or its suture, were once connected to the North American miogeocline. All are therefore suspect.The major suspect terranes of the Appalachian orogen are in most respects analogous to previously recognized zones or tectonic lithofacies belts. In the north, these are the Dunnage, Gander, Avalon, and Meguma terranes. In the south, they include easterly parts of the Blue Ridge, the Piedmont, Slate Beit, and the geophysically distinctive Brunswick and Tallahassee-Suwannee terranes beneath the Atlantic Coastal Plain. Most of these are composite and include smaller terranes of uncertain paleogeography. Taconic allochthons are included because they fit the definition of suspect terranes.Stratigraphic and sedimentologic analyses indicate that the Appalachian orogen built up during three major Paleozoic accretionary events. Their timing coincides with the times of structural, metamorphic, and plutonic effects assigned to the Taconian, Acadian, and Alleghanian orogenies.Accretion of the Appalachian orogen progressed from the North American miogeocline outward. The boundaries of the earliest accreted western terranes are marked by melange zones and ophiolite complexes, implying head-on collisions. Later boundaries between eastern terranes are steep mylonite zones and brittle faults, implying oblique movements.The suspect terrane concept, first developed for the North American Cordilleran, provides new insights into the evolution of the Appalachian orogen and solves several enigmas. It is a surgically clean analytical approach and a superior framework in which to view the anatomy of any complex orogen. 33 on May 15, 2015 memoirs.gsapubs.org Downloaded from

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In 1986, 1181 km of marine seismic reflection data was collected to 18–20 s of two-way traveltime in the Gulf of St. Lawrence area. The seismic profiles sample all major surface tectono-stratigraphic zones of the Canadian Appalachians. They complement the 1984 deep reflection survey northeast of Newfoundland. Together, the seismic profiles reveal the regional three-dimensional geometry of the orogen.Three lower crustal blocks are distinguished on the seismic data. They are referred to as the Grenville, Central, and Avalon blocks, from west to east. The Grenville block is wedge shaped in section, and its subsurface edge follows the form of the Appalachian structural front. The Grenville block abuts the Central block at mid-crustal to mantle depths. The Avalon block meets the Central block at a steep junction that penetrates the entire crust.Consistent differences in the seismic character of the Moho help identify boundaries of the deep crustal blocks. The Moho signature varies from uniform over extended distances to irregular with abrupt depth changes. In places the Moho is offset by steep reflections that cut the lower crust and upper mantle. In other places, the change in Moho elevation is gradual, with lower crustal reflections following its form. In all three blocks the crust is generally highly reflective, with no distinction between a transparent upper crust and reflective lower crust.In general, Carboniferous and Mesozoic basins crossed by the seismic profiles overlie thinner crust. However, a deep Moho is found at some places beneath the Carboniferous Magdalen Basin.The Grenville block belongs to the Grenville Craton; the Humber Zone is thrust over its dipping southwestern edge. The Dunnage Zone is allochthonous above the opposing Grenville and Central blocks. The Gander Zone may be the surface expression of the Central block or may be allochthonous itself. There is a spatial analogy between the Avalon block and the Avalon Zone. Our profile across the Meguma Zone is too short to seismically distinguish this zone from the Avalon Zone.