DETERMINING THE 3-D KINEMATIC HISTORY OF THE WYOMING LARAMIDE FORELAND: PRELIMINARY ANISOTROPY OF MAGNETIC SUSCEPTIBILITY RESULTS FROM THE TRIASSIC CHUGWATER GROUP

Anisotropy of magnetic susceptibility (AMS) analysis, along with complimentary paleomagnetic, structural and strain studies, are underway to determine the 3-D kinematic evolution and mechanical processes that that generated Cordilleran foreland deformation in Wyoming. AMS sampling is concentrated in the Triassic Chugwater Group from multiple foreland arches from throughout the Laramide of Wyoming. In general, an idealized suite of AMS ellipsoids are developed as primary sedimentary fabrics are progressively overprinted by weak tectonic fabrics. Primary sedimentary fabrics have distinctly oblate AMS ellipsoids with Kmin axes perpendicular to bedding, which grades with progressive deformation to a composite fabric with triaxial AMS ellipsoids and Kmax axes (magnetic lineations) subparallel to the intersection of layer-parallel shortening (LPS) fabrics with bedding. Preliminary data show well developed Kmax directions with alpha95 uncertainties < 10 degrees at most sites, reflecting intersection of weak LPS fabrics and bedding, and in general lying sub-parallel to structural trend. Sites from gently dipping, structurally simple areas have Kmax trends mostly between 320 and 340 degrees (LPS directions ~ 050 to 070 degrees). Sites from steeper fold limbs, however, display changes in Kmax directions that correlate with changes in structural trend. This pattern may record either vertical-axis rotation or localized stress/strain refraction. Such disparate models can be tested by integrating paleomagnetic and structural data. Some variation in AMS directions also appears to be related to local structural noise and lithology, illustrating the importance of adequate sampling density.

Observed AMS fabrics, combined with structural relations and paleomagnetic data, 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. Moreover, significant correlation between AMS fabric geometry and trend with observed structural style and LPS orientations, indicate that AMS is a potentially powerful tool in defining structural styles in weakly deformed foreland rocks.