Abstract

A mechanical model of the continental crust of the western U.S. during the Laramide is proposed. It is given the attribute of being an elastic continuum and is endowed with boundary conditions placing it under uniform horizontal compression. The stress-system derived from the model, used in conjunction with data supplied by experimental rock mechanics, yields a model scenario of structural development that closely parallels that inferred from the observed structures of the Rocky Mountain Foreland. Those features predicted by the model are small basement-cored folds (less than 1500 meters - 5,000 feet amplitude) whose location and trend is determined by the loci of basement faults. The small folds are found in basins formed by later basin-bounding fault sytems with as much as 13,000 meters (43,000 feet) of vertical relief.

The model predicts that basement faults should form at 30 to 35 degrees to the horizontal. The rock mechanics data suggest that, under uniform horizontal compression (response of the elastic model), faults will initiate at the upper basement surface and propagate downward (at 30 to 35 degrees dip) with increased shortening.

The model, combined with the rock mechanics data, accurately predicts the regional basement features of the Rocky Mountain Foreland. The model also yields insight into the chronological development of foreland deformation.