Four-dimensional structural evolution of a complex fold-fault system, Laramide foreland, Wyoming
Arlo Weil, Bryn Mawr College
Adolph Yonkee, Weber State University
Lauren Reeher, Utah Geological Survey
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Abstract
Characteristics of fault networks, anisotropy of magnetic susceptibility (AMS) fabrics, and remanent paleomagnetism within Triassic to Jurassic red beds and limestone record patterns of early layer-parallel shortening (LPS) to large-scale fault propagation and fold growth during evolution of the Thermopolis fold-fault system in the Wyoming sector of the Laramide foreland. This system, located on the north flank of the overall W-E trending, basement-cored Owl Creek arch in central Wyoming, includes a main NW- to W-trending anticline and additional NNW- to NW-trending folds. The main anticline is segmented into domes with steep, SE- to S-vergent forelimbs connected across oblique transfer zones. Early LPS within limestone was accommodated mostly by minor reverse-slip (wedge) faults with consistent relationships relative to bedding around fold limbs. Within red beds, LPS formed subtle fabrics with micro-kinked phyllosilicate grains that give rise to AMS Kmax lineations. Minor faults and AMS fabrics record WSW-ENE oriented LPS along fold backlimbs and NW-trending forelimbs, subparallel to the regional Laramide shortening direction. Along W-trending forelimbs, LPS directions are oriented SSW-NNE, reflecting local stress refraction plus vertical-axis rotations near transfer zones. Secondary reverse and oblique-slip faults with complex linkages formed during steepening and shear of fold forelimbs. Thinning of stratigraphic units cut by reverse faults in steep forelimbs is consistent with a trishear model in which basement fault segments propagated upward during progressive folding. Trishear modeling indicates basement fault dips of ~40°, initial fault tip depths of ~2 km below the basement-cover contact, and propagation-to-slip ratios of ~2. Boundary-element modeling indicates an evolving stress field with increased differential stress in forelimbs and refraction of stress directions along oblique fault segments. Evolution of the system is interpreted to reflect fault nucleation along basement weaknesses, upward propagation and lateral linkage of faults, and an increasingly heterogeneous stress/ strain field during Laramide shortening, which was associated with stress transfer from a flat-slab segment of the subducting Farallon plate.
Session
Deformation in the upper crust