POST-ORDOVICIAN CONTRASTS IN THE GEOLOGICAL EVOLUTION OF THE NORTHERN AND SOUTHERN APPALACHIAN OROGEN

The New York promontory serves as the divide between the northern and southern segments of the Appalachians. The first order character of the pre-Silurian crustal building blocks of the orogen is essentially uniform along the length of the orogen. Following Late Ordovician-Silurian sinistral oblique accretion of Carolinia and Ganderia along the Appalachian margin, the northern and southern segments of the orogen appear to record distinctly different histories. The northern Appalachians contain a robust middle to late Paleozoic lithotectonic record of Silurian to Early Devonian tectonism with an extensional component followed by Acadian, Neoacadian, and Alleghanian events across most of the orogen. Although the tectonic details of this record are controversial, e.g. subduction polarity and bulk kinematics, it is generally agreed that this record relates to the accretion of the peri-Gondwanan tracts of Avalonia and Meguma and the culminating interaction of Gondwana with Laurentia. The southern Appalachian middle to late Paleozoic lithotectonic record is sparse, with relevant strata preserved as clastic basins and wedges along the western margin of the orogen and magmatic rocks of ambiguous origin in the hinterland; this meager record reflects Neoacadian and Alleghanian events. The Alleghanian event is explicitly tied to the collision of Gondwana with Laurentia, but the nature of the Neoacadian event is unknown, as no counterpart to either Avalonia or Meguma is recognized in the southern orogen. The contrast in lithotectonic evolution between the northern and southern segments of the orogen during the middle to late Paleozoic appears to be related mainly to the limited distribution of Avalonia and Meguma. This difference implies that the oceanic tract trailing Carolinia and Ganderia, i.e. the Rheic Ocean, was more complex than the older Iapetus Ocean in that it harbored first-order lateral variations.