Robert J. Lillie, Robert A. Enos
Kinematic models at various stages of continental collision illustrate the evolution of crustal structure, topography, and resulting gravity anomalies. The models are structural cross sections that are balanced in terms of continental crustal volume, and retain a state of local isostatic equilibrium. Widths of Bouguer anomalies, computed from the models, are indicative of the amount of continental crustal shortening; the decay in anomaly amplitude signifies the amount of erosion accompanying isostatic rebound.
The Ouachita Mountains of Arkansas and the western Carpathians of Slovakia have narrow Bouguer anomaly lows characteristic of the early stages of collision, with less than 100 km of continental crustal shortening. Since collision, these areas have undergone less than 10 km of erosion during the process of rebounding the synorogenic crustal root. Thick flysch deposits remain in the foreland of the mountains because the deposits are isostatically compensated by shallow mantle of the former continental margin.
More advanced stages of collision are indicated by Bouguer anomaly lows that broaden as crustal shortening progresses. Anomaly widths suggest 175 km and 250 km of shortening for the eastern Alps and southern Appalachians, respectively. High elevations and a Bouguer anomaly amplitude of -140 mGal reflect a thick crustal root that remains because the area has rebounded only 10 km since convergence ceased. The southern Appalachians, on the other hand, retain a very small Bouguer amplitude after undergoing more than 20 km of erosion and isostatic rebound. In both of these areas, the rebound and erosion have been sufficient to remove most of the synorogenic deposits in the foreland.
AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994