Thrusts and nappes in the North American Appalachian Orogen

Abstract

Summary The Appalachian Orogen in North America was subjected to three major deformational-thermal events: the Taconic (Ordovician-Silurian), Acadian (late Devonian), and Alleghanian (Permian). Each event involved large-scale horizontal transport of thrust-nappes ranging from tens to hundreds of kilometres in different parts of the orogen. Transport was dominantly westward although major Acadian-generated eastward transport occurred in southern New England. There is a direct relationship between chronological proximity to a thermal peak and numbers of thrusts. Thrusts were produced during the Taconic and Acadian events which pre- and post-dated the thermal peak, as well as being synchronous with it. Transport of the Bay of Islands ophiolites and other large masses along the western margin of the orogen occurred before Taconic metamorphism, but probably only immediately before. Many large thrusts of the Appalachians were active during two or even three of the deformational-thermal events, and more than once within a single event. This is particularly true for the Blue-Ridge-Inner Piedmont mega-nappe, which involves at least 225 km of horizontal transport. Compressional tectonics were probably the dominant process responsible for all thrusts in the Appalachians, except the Taconic klippes. The Alleghanian décollement Valley and Ridge thrusts are overridden by crystalline Alleghanian and older thrusts of the Blue Ridge in the southern Appalachians. The same mechanism must apply to the central Appalachians. Thrusting and formation of crystalline thrust-nappes in the Appalachians and other mountain chains may be an adiabatic process which functions to dissipate much of the thermal energy produced during subduction (both A and B types) and collision events.