Appalachian suspect terranes

Abstract

Since the advent of plate tectonics, the widely accepted model for the development of the Appalachian orogen has involved the opening and closing of a late Precambrian-Paleozoic Iapetus Ocean. Only a few of a growing number of geologically distinctive terranes are easily explained by this model. Vestiges of Iapetus are nowhere coupled to the ancient North American margin. Furthermore, it cannot be demonstrated that any of the extensive Appalachian terranes, now east of the Iapetus tract or its suture, were once connected to the North American miogeocline. All are therefore suspect. The major suspect terranes of the Appalachian orogen are in most respects analogous to previously recognized zones or tectonic lithofacies belts. In the north, these are the Dunnage, Gander, Avalon, and Meguma terranes. In the south, they include easterly parts of the Blue Ridge, the Piedmont, Slate Beit, and the geophysically distinctive Brunswick and Tallahassee-Suwannee terranes beneath the Atlantic Coastal Plain. Most of these are composite and include smaller terranes of uncertain paleogeography. Taconic allochthons are included because they fit the definition of suspect terranes. Stratigraphic and sedimentologic analyses indicate that the Appalachian orogen built up during three major Paleozoic accretionary events. Their timing coincides with the times of structural, metamorphic, and plutonic effects assigned to the Taconian, Acadian, and Alleghanian orogenies. Accretion of the Appalachian orogen progressed from the North American miogeocline outward. The boundaries of the earliest accreted western terranes are marked by melange zones and ophiolite complexes, implying head-on collisions. Later boundaries between eastern terranes are steep mylonite zones and brittle faults, implying oblique movements. The suspect terrane concept, first developed for the North American Cordilleran, provides new insights into the evolution of the Appalachian orogen and solves several enigmas. It is a surgically clean analytical approach and a superior framework in which to view the anatomy of any complex orogen.