Abstract: Heterogeneous Laramide Deformation in Rocky Mountain Foreland Arches

Joe D. Gregson, Eric A. Ersley

Relative to thin-skinned thrust belts, structural trends in basement-involved foreland thrust belts are unusually diverse. Previous explanations for the variety of Laramide structural trends in the Rocky Mountain foreland include reactivation of pre-existing structures, multiple phases of deformation, and complex responses to regional rotations or wrench faulting.

Recent studies of minor faulting in the east-trending Uinta arch of the central Rocky Mountains indicate that stresses were controlled by regional structural geometries. Both the northern and southern boundaries of the Uinta arch contain east-west faults and folds, suggesting north-south shortening. In the southern Uintas at Dinosaur National Monument, a complex array of minor faults indicate N-S to NW-SE ^sgr₁ orientations, perpendicular to local faults and fold trends. In the northern Uintas near Farning Gorge Reservoir, however, minor faults indicate a NE-SW ^sgr₁ orientation consistent with regional Laramide trends.

These variable stress directions demonstrate heterogeneous stress within the Uinta arch during the Laramide orogeny. Our data indicates a single stage model of Laramide deformation involving northeast-directed oblique thrusting on a south-dipping master fault system underlying the arch. These faults may have originated during Precambrian extension and Uinta Group deposition. Bending of the basement slab above the listric master fault system is probably responsible for north-south shortening in the southern Uintas, which is not representative of the regional Laramide stress field.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994