THREE-DIMENSIONAL STUDIES OF OBLIQUE DEFORMATION WITHIN LARAMIDE FOLDS

Fold-axis parallel shear within fault-related folds is increasingly recognized in many traditional Laramide folds as a mechanism to absorb transpressional stress. Structural and geophysical evidence from Laramide folds in the Rocky Mountain Foreland and the Colorado Plateau supports oblique shear within folds. Vertical-axis rotations are one means by which such shear can be accommodated and so identified by paleomagnetic measurements. Clockwise and counterclockwise rotations on the Grand Hogback and Grayback monoclines of Colorado correlate with oblique deformation. 9 to 24° of clockwise rotation are found in the north-south trending segment of the Grand Hogback Monocline, and 15° ± 7.8° of counterclockwise rotation are found in the east-west trending segment (Tetreault et al., 2003). Clockwise paleomagnetic rotations of 39° ± 14° are found in the steeply dipping forelimb of the Grayback Monocline (Holdaway, 1998). These paleomagnetic rotations in both folds are probably produced as northeast shortening during the Laramide Orogeny acted upon monoclines trending obliquely to this shortening. However, the mechanisms of oblique folding are not well understood; similar folds on the Colorado Plateau inferred to have absorbed oblique shortening have not yielded paleomagnetic rotations. For example, right-lateral shear across the trends of the East Kaibab and Nacimiento monoclines are widely reported, yet preliminary paleomagnetic studies have not found vertical-axis rotations to support large right-lateral slip. In order to understand how oblique deformation is being absorbed in these folds and why they are perhaps being absorbed with very different mechanisms, we are thoroughly investigating two folds to understand the three-dimensional deformation. Research is focused on constraining mechanisms of oblique deformation by making paleomagnetic and structural measurements on the Grayback and Grand Hogback monoclines over varying structural depth and along transects across and along the monoclines. Our goals are to understand how oblique shear is manifested in Laramide folds, how the magnitude and mechanism of shear varies in three-dimensions, and to determine which kinematic or mechanical model of folding best fits these Laramide folds.