WHAT IS THE ACADIAN OROGENY?

The Appalachian orogen involves a number of orogenic events that reflect (i) the Cambrian-Early Ordovician foundering of the Laurentian and Gondwanan passive margins (Taconic and Penobscot orogenies, respectively), (ii) Late Ordovician-Early Silurian collision of Baltica and peri-Gondwanan terranes (e.g. Ganderia, Avalonia, Carolinia) with Laurentia (Caledonian and Salinian orogenies, respectively), events which accompanied the closure of the Iapetus Ocean and the formation of Laurussia, (iii) Siluro-Devonian Acadian Orogeny, and (iv) Permo-Carboniferous terminal collision between Laurussia and Gondwana (Alleghanian and Variscan orogenies) which gave rise to Pangea. Interpretations for the origin of the Siluro-Devonian Acadian orogeny vary from: (a) collision of Avalonia to Laurentia; (b) northward subduction of the Rheic Ocean and overriding of an oceanic plume following the collision between Laurentia and Avalonia; and (c) oblique collision of the Meguma terrane to Avalonia, which resulted in dextral transpression along the boundary. These three events had an important role in the development of the Appalachian orogen, but assignment of each event to the Acadian orogeny results in unnecessary confusion The current nomenclature is profoundly influenced by the varying views on the relationships between the peri-Gondwanan terranes, whether they traveled as separate terranes and collided with Laurentia independently, or whether some (or all) were part of a larger superterrane that became dispersed after collision. We propose a terminology that can accommodate these different viewpoints and respond to changes in interpretation as new data become available. We suggest that collision of all peri-Gondwanan terranes be referred to as the Salinic orogeny, with Ganderian, Avalonian and Carolinian stages for those who interpret these terranes to have collided independently. We suggest that the term Acadian orogeny be reserved for events related to subduction of the Rheic Ocean, with Meguma and Carolinian stages for those who believe that these terranes collided independently from within that ocean. In this scheme a separate Neo-Acadian orogeny would be unnecessary.