Abstract

Paleomagnetic data from northern Appalachian terranes identify several arcs within the Iapetus ocean in the Early to Middle Ordovician, including a peri-Laurentian arc at ∼ 10°–20°S, a peri-Avalonian arc at ∼ 50°–60°S, and an intra-oceanic arc (called the Exploits arc) at ∼ 30°S. The peri-Avalonian and Exploits arcs are characterized by Arenigian to Llanvirnian Celtic fauna that are distinct from similarly aged Toquima-Table Head fauna of the Laurentian margin, and peri-Laurentian arc. The Precordillera terrane of Argentina is also characterized by an increasing proportion of Celtic fauna from Arenig to Llanvirn time, which implies (1) that it was in reproductive communication with the peri-Avalonian and Exploits arcs, and (2) that it must have been separate from Laurentia and the peri-Laurentian arc well before it collided with Gondwana. Collectively, the paleomagnetic and faunal data require an open Ordovician ocean adjacent to the Appalachian margin and argue against a Taconic-Famatinian collision between North and South America.

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