MECHANICS OF LARAMIDE DEFORMATION ALONG THE EAST FLANK OF THE LARAMIE RANGE, WYOMING AND FRONT RANGE, COLORADO

The area studied lies along the east flank of the Laramie Range, Wyoming and the Front Range, Colorado. These ranges were uplifted during the Laramide orogeny which occurred during Late Cretaceous-Early Tertiary time. The Precambrian basement rocks consist of both igneous and metamorphic rocks that range from massive to well-foliated in texture. The lower 280 meters of the sedimentary section is dominated by carbonates and arkoses of the Casper Formation. The overlying strata include Permo-Triassic redbeds and Jurassic and Cretaceous shales and sandstones. Tertiary strata post-date the Laramide orogeny and in places cover the deformation along the flanks of the ranges.;Dynamic analysis of twinned calcite in nineteen samples collected from the Casper and Ingleside Formation gives an indication of the stress history of these strata during Laramide deformation. Samples from unfaulted drape folds generally show C axes oriented parallel to bedding and normal to the fold axes. These data may be due to bending stresses or to layer parallel shortening during early stages of drape folding over reverse basement faults. Samples collected from more intensely deformed rocks show scattered C axes suggesting complex stress histories. Microfractures were measured in eight samples collected from the Cretaceous Cloverly and Muddy Sandstone Formations and the Casper Formation. These data showed no obvious shear sets that could be related to a Laramide stress system.;Generally the basement deformation can be modeled as the movement of large, intact blocks separated by narrow fault zones. No ductile deformation was observed in any of these zones. These blocks were displaced vertically and tilted with respect to each other. At several places the edges of these blocks were more intensely deformed, which allowed the basement block surfaces to fold. The overlying sedimentary rocks were drape folded as vertical displacement occurred along the basement faults. With increased displacement, faults cut through the strata, thinning the section. Where displacements are greatest, the entire exposed section is faulted through, with rocks as young as Cretaceous in fault contact with the basement.