ANISOTROPY OF MAGNETIC SUSCEPTIBILITY, PALEOMAGNETIC, AND STRUCTURAL STUDIES OF TRIASSIC RED BEDS FROM THE SOUTHEAST FLANK OF THE LARAMIDE BIG HORN ARCH, WYOMING

The southeast flank of the Big Horn Arch is characterized by a series of right-stepping en echelon folds that are used as a case study for testing different kinematic models for the deformation history of diversely oriented basement-cored uplifts in the Laramide foreland. Thirty-one sites were sampled in the Triassic Chugwater Group, which consists of red beds and a thin interval of limestone, display well developed minor faults and outcrops continuously around the fold train. The hematite-cemented red beds carry a near-primary remanent magnetization that is used to test models of local and regional rotation. Anisotropy of magnetic susceptibility (AMS) and paleomagnetic samples were collected from each site, and detailed structural features were measured – including minor wedge and strike-slip fault, tectonic foliation and fracture set orientations. AMS fabrics and structural data record evidence of early layer parallel shortening (LPS) that d a systematic change along structural trend - from NE along the southern flank to E along the eastern flank. The change in LPS directions can be explained by either temporal changes in regional shortening, spatial changes in stress trajectories, or wrench shear and vertical-axis rotations. Distributions and magnitudes of potential vertical-axis rotation will be constrained from paleomagnetic analysis of the red beds. Preliminary results indicate a composite deformation model that combines both spatial and temporal variations in stress directions, and localized wrench shear along variably oriented forelimbs of Laramide uplifts. Results will be further compared to the results of similarly analyzed left-stepping en echelon folds from the southeastern flank of the Laramide Wind River Arch where minor counter-clockwise rotation of shortening directions suggest localized dextral wrench shear.