APPALACHIAN SALIENTS AND REENTRANTS: SECONDARY FEATURES ATTRIBUTABLE TO BENDING OF AN ORIGINALLY LINEAR OROGEN

Thomas' groundbreaking papers of the seventies and eighties established a basic assumption in models of Appalachian orogeny - that being that the map-view geometry of the orogen is a reflection of the primary shape of the pre-collisional continental rifted margin. Hence, the salients and reentrants that characterize the length of the orogen are interpreted as being inherited from primary promonotories and embayments that characterized by original passive margin. The bends, however, form a regular repeating pattern, approximating a Sine wave Y = 55 km SIN (0.0084) x, where 55 km = amplitude, and 0.0083 = 2p/l (average l = 750 km). Furthermore, given that the width of the bending beam in the Appalachians is <50 km, the bends adhere to Biot's relationship: a plot of the wavelength of the bends against the width of the bending beam shows that the bends obey a cube root relationship similar to that for folds of a high viscosity beam in an incompetent low viscosity matrix. Hence the Appalachian salients and promontories are the result of buckling of a rigid, competent beam and the resulting bends can be modeled as rotations about vertical axes of rotation. Additional observations consistent with a “secondary” origin for the salients and reentrants include: (1) paleomagnetic data which, while hotly debated, require that at least parts of the orogen began as more linear features that were subsequently bent; and (2) along strike bends of the correlative Variscan orogen in Europe, including the Cantabrian and Bohemian oroclines, are “isoclinal” and, as such, are far too tight to reflect wrapping of orogenic features around pre-existing continental margins. Modern analogues similarly support a secondary origin for the bends of the Appalachians. The Bolivian orocline, a salient in the modern Andean orogen, has been demonstrated through paleomagnetic and GPS studies, to be the result of ongoing bending of an originally more linear orogen. The bends of the Panamanian Isthmus approximate a Sine wave Y = 140 km SIN (0.083) x, where 140 km = amplitude, and 0.011 = 2p/l (average l = 760 km), and also adhere to Biot's relationship. The Andean and Panamanian bends formed in the absence of any pre-existing continental template that acted as a backstop during collisional orogenesis, and provide us with modern analogues for formation of the Appalachian salients and reentrants.