TESTING TECTONIC MODELS OF THE WYOMING LARAMIDE FORELAND: INTEGRATED STRUCTURAL, AMS, AND PALEOMAGNETIC ANALYSIS OF THE TRIASSIC CHUGWATER GROUP

Detailed structural, anisotropy of magnetic susceptibiliy (AMS), and paleomagnetic data collected from Triassic redbeds around Laramide fold structures (Thermopolis anticline with a 60 degree bend in structural trend, en echelon Derby-Dallas-Sheep Mountain domes, Casper Mountain area with multiple trends) constrain models for origins of diversely trending Laramide foreland structures. The redbeds carry a near primary remnant magnetization for quantifying vertical-axis rotations, have interpretable AMS fabrics, and contain a thin limestone interval with minor faults and systematic fractures that record early layer parallel shortening (LPS). Minor fault systems in structurally simple settings are comprised mostly of conjugate contraction faults with dip slip, which maintain consistent geometries with respect to bedding around folds, indicating development early in the deformation history. Kinematic analysis of early minor faults and AMS fabrics yield estimated LPS directions ranging from 030 to 090 degrees, recording minor spatial dispersion about a regional ENE shortening direction; this dispersion appears partly related to underlying basement anisotropy. Gentle backllimbs typically display simple fracture patterns, including strike- and cross-strike sets. Steep forelimbs of folds display more complex patterns, including later oblique thrust and tear faults that overprint early LPS structures, as well as multiple sets of fractures. Early LPS directions and paleomagnetic declinations are rotated clockwise/ counter-clockwise respectively along more northerly/ easterly trending forelimbs, recording localized wrench shear. Later structures also record minor temporal changes in shortening directions. A model for Laramide foreland deformation combining both spatial and temporal variations in stress directions, and localized wrench shear along variably oriented forelimbs best explains observations.