PALEOZOIC ACCRETION AND TRANSLATION OF TERRANES IN THE NORTHERN APPALACHIANS, ARCTIC AND CORDILLERA

The Laurentian margin shows evidence for long-lived extension culminating in hyperextended margins in several segments during its break-out from Rodinia and isolation as a large continent during the late Neoproterozoic to Early Cambrian. Early Cambrian amalgamation of Gondwana probably led to a major plate reorganization and initiated subduction in the oceans surrounding Laurentia. Some of the convergence zones were inherited from the Mirovoi Ocean zone (Timanide-Avalonian arcs around Baltica and West Gondwana). As a result, Laurentia was subjected to episodic Late Cambrian to Devonian accretion of peri-Laurentian, peri-Gondwanan and/or Baltican (composite) terranes along its Appalachian and Caledonian margins. The locus of associated deformation, metamorphism and magmatism progressively migrated outboard following addition of new terranes. Detailed analysis of terranes and their bounding sutures reveals a significant diversity of tectonic processes, including soft and hard collisions, which affected preservation of these terranes. Many of the accretionary episodes are characterized by soft collisions that mostly concentrated contractional deformation and associated metamorphism into the lower, subducting plate, followed by subduction step-back or polarity reversal. Soft collisions were locally followed by hard collisions, which are characterized by thickening and metamorphism of the upper plate (e.g., Baie Verte margin – Dashwoods block collision). Lateral translation of terranes has been difficult to constrain, except for younger motions.

Some Caledonian terranes were laterally displaced along the Laurentia’s Arctic margin and accreted to the Franklinian margin during the M’Clintock and/or Elsmerian orogenies (e.g. Pearya terrane). The Cordilleran Alexander terrane, which originated in the Timanides of northern Baltica was subjected to related Late Silurian to Early Devonian Appalachian-Caledonian orogenesis and progressively migrated west until its Late Devonian arrival on the northwestern Panthalassan margin of Laurentia. Its Caribbean-style arrival may have ‘infected’ Panthalassa with subduction. The start of subduction in the Laurentian realm of Panthalassa overlapped with intense Neo-Acadian and Alleghenian orogenesis in the Appalachians.