LAURENTIAN-GONDWANAN INTERACTIONS ALONG THE APPALACHIAN MARGIN

The southeast flank of the Appalachian orogen is composed of the peri-Gondwanan crustal blocks of Suwanee, Carolinia, Ganderia, Avalonia, and Meguma. The middle to late Paleozoic accretion of these blocks produced a heterogeneous pattern of tectonism wherein the northern segment of the orogen records a tectonic history distinct from that of the southern segment. Evolutionary divergence between northern and southern segments started in the Late Ordovician to Silurian with the staggered accretion of the first peri-Gondwanan blocks to reach the Laurentian margin, involving the accretion of Carolinia during the Cherokee event in the south followed by the Salinic accretion of Ganderia in the north. Divergence was amplified during the Silurian, specifically with respect to the nature of the Laurentian margin and the history of accretion. During this time frame, the northern margin was convergent whereas the amagmatic southern margin may well have been a transform boundary. In terms of accretion, the Late Silurian-Early Devonian docking of Avalonia imprinted Acadian tectonism on the northern segment, whereas the southern Appalachians appear to have been largely quiescent during this interval. The evolutionary paths of the two segments of the margin converge on a common history in the Late Devonian, with the Fammenian event; this event is characterized by a significant component of dextral shear. We suggest that the Fammenian event is related to the initial margin-wide interaction of Laurentia with the peri-Gondwanan blocks of Meguma and Suwanee, providing a uniform tectonic template for margin evolution. It is likely that an ocean basin intervened between Meguma and Gondwana; however, it is unclear whether Suwanee was attached or was separated from Gondwana. The culminating Laurentian-Gondwanan collision in the late Paleozoic is marked by second order divergences in history; specifically during the Carboniferous, the southern segment records a larger component of shortening than the northern Appalachians.