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Paleomagnetic and tectonostratigraphic data for the northern Appalachians record Silurian closure of a major ocean, the Iapetus Ocean, that was bordered by the Laurentian craton and the Avalonian microcontinent. In Ordovician times this ocean consisted of at least two basins (Iapetus I and II) and extended from a paleolatitude of 10 to 20°S (Laurentian margin) to ca. 50°S (Avalonian margin); Gondwana was located yet farther south. Paleomagnetic data from the Middle Ordovician Robert’s Arm, Chanceport, and Summerford groups in north-central Newfoundland, which represent intraoceanic arcs and ocean islands, yield paleolatitudes of 30 to 33°S. In contrast, pillow lavas of the upper part of the Late Cambrian to Lower Ordovician Moreton’s Harbour Group, which are currently juxtaposed to the Chanceport Group along the Lobster Cove-Chanceport Fault, acquired their remanence at 11 °S in a marginal island arc setting. Subaerial deposits of the mid-Silurian Botwood Group that unconformably overlie marine sequences in northeastern Newfoundland yield a primary magnetization with a paleolatitude of 24°S, which is indistinguishable from the Early Silurian position of the southeast-facing Laurentian margin. Silurian closure of Iapetus is supported by the timing of thrusting and folding and by the age of angular unconformities in the Central Mobile Belt.

The combined paleomagnetic and tectonostratigraphic data present a working hypothesis for the geometry and tectonic evolution of the northern Appalachians. In Early Ordovician times, a volcanic arc and a back-arc basin (Iapetus I) were located near the Laurentian margin. Following Middle Ordovician obduction of ophiolites onto the Laurentian margin when Iapetus I closed, convergence of Avalon and Laurentia by northward subduction continued until closure of Iapetus II was complete by the Late Silurian.

In view of these data, the traditional subdivision of Ordovician “Taconic” and Devonian “Acadian” orogenies needs to be revised. Maintaining the terminology of orogenic phases, one either has to expand the time interval of the Acadian orogeny to include the Silurian or add an orogenic phase (Caledonian?) in between Taconic and Acadian. In either case, Early to Middle Paleozoic closure of Iapetus should be viewed in terms of a progressive deformation history with peak deformation pulses rather than temporally discrete orogenies.

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