DETERMINING THE 3-D KINEMATIC HISTORY OF THE WYOMING LARAMIDE FORELAND: PRELIMINARY RESULTS FROM A PALEOMAGNETIC INVESTIGATION OF THE TRIASSIC CHUGWATER GROUP

Paleomagnetic analysis, along with complimentary structural, strain, and anisotropy of magnetic susceptibility (AMS) studies, are underway to determine the 3-D kinematic evolution and mechanical processes that generated Cordilleran foreland deformation in Wyoming. The Wyoming Laramide is an ideal case study for understanding foreland deformation because there are numerous basement arches and cover folds with a wide range of trends; abundant seismic and drill hole data that provide key constraints for constructing cross sections and 3-D fold-fault surfaces; previous studies and our preliminary work that indicate systematic patterns of minor layer-parallel shortening and local vertical axis rotations; and excellent exposures of basement and cover rocks that allow analysis of deformation styles and mechanisms over a wide range of structural levels and scales. Results from this study will help constrain previously proposed models that predict disparate patterns of rotation and internal strain for Laramide uplifts, including: temporal changes in shortening directions, spatial changes in stress/fault direction, and bulk shortening and transpression.

Paleomagnetic sampling is concentrated in the Triassic Chugwater Group. Preliminary paleomagnetic analysis reveals the presence of reversals at the sample and site level, and positive fold tests for restored site means, indicating a near primary magnetic acquisition for most Chugwater sites. Analysis of individual fold structures (Thermopolis anticline and the en echelon Derby and Dallas domes) indicates local rotation during fold formation. In general declinations from the gently dipping backlimbs of uplifts are consistent with expected Triassic reference directions for Wyoming, whereas declinations along steep frontal limbs show systematic rotations where fold trends curve. Observed paleomagnetic patterns, combined with structural relations and AMS fabrics, are consistent with a hybrid model of spatial variations in shortening directions likely associated with basement anisotropy, temporal changes in shortening directions, and varying amounts of wrench shear related to variations in structural trend.