DETERMINING THE 3-D KINEMATIC HISTORY OF THE WYOMING LARAMIDE FORELAND: PRELIMINARY RESULTS FROM DETAILED STRUCTURAL ANALYSIS OF THE TRIASSIC CHUGWATER GROUP

Detailed structural, anisotropy of magnetic susceptibility (AMS), and paleomagnetic data collected from the Triassic Chugwater Group around two Laramide fold systems (the Thermopolis anticline that varies in structural trend from NW to ENE, and the left-stepping en echelon Derby and Dallas domes) constrain models for the origin of diversely trending Laramide foreland structures. Chugwater redbeds carry a near primary remnant magnetization for quantifying vertical-axis rotations, have interpretable AMS fabrics related to layer-parallel shortening (LPS), and contain a thin limestone interval with minor fault systems that display fibrous slip lineations. Minor fault systems include conjugate contraction faults with dip slip, and conjugate subvertical faults with strike slip. Typically one conjugate set dominates at a given site, recording either LPS with vertical thickening, or wrench shear with strike-parallel extension. Octahedral fault patterns at some sites reflect 3-D strain and varying stress ratios. Minor fault sets typically maintain consistent geometry with respect to bedding around folds, indicating development during early deformation. Kinematic analysis of minor fault data in structurally simple settings yields estimated shortening and sigma1 directions from 050 to 070 degrees that are subperpendicular to regional NW structural trends and consistent with AMS fabric orientations. In detail, variations in slip lineations at individual sites may record minor changes in shortening directions from more E-W to more N-S during progressive deformation. Sites in the steep forelimb of the Thermopolis anticline have shortening directions that vary around a distinct structural bend that records components of vertical-axis rotation and sinistral wrench shear along the ENE-trending part of the fold. Most sites in the Derby and Dallas domes have ENE trending sigma1 directions, consistent with regional shortening directions, with additional reverse and tear faulting in stepovers between domes. Observed structural patterns 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 depending on structural trend.