Structural style of the Laramide orogeny, Wyoming foreland. (Volumes I and II)

Abstract

This study documents the styles of Laramide structures of the Wyoming foreland, and presents an integrated synthesis of foreland development. Theoretical data and experimental results are compared to field observations, subsurface well control, and geophysical data to judge their application to the foreland. The direction of collision (N40-50E) between the North American and Farallon plates represents the direction of maximum compressive stresses during the Laramide orogeny. A "tectonic front" progressed from west to east across the foreland during Campanian to middle Eocene time. No single model of uplift applies throughout the foreland. Dips of basement faults vary with changes in strike (i.e. northwest strike: 20-45 degrees; north to north-northwest strike: 45-60 degrees; east to northeast strike: 60 degrees to vertical). Northwest-trending uplifts are fold-thrust structures exhibiting dual fault systems and rotated flanks. North-northwest trending uplifts are thrust uplift structures bounded by single reverse faults. East- and northeast-trending uplifts are drape folds and upthrust structures along which displacements were dominantly vertical. The forms of Laramide structures were controlled by upward movement of basement forcing blocks, arrangement of lithology into mechanical packages, and deformational mechanisms which were determined by dominant lithology, applied stresses, and depth of burial. "Crowd" structures (i.e. back-limb, cross-crestal, and rabbit-ear folds) developed as volumetric adjustments in upward-tightening, parallel-folded synclines. There is no consistent relationship between basement fabric and northwest-trending Laramide structures; however, reactivated Precambrian-age zones of weakness controlled east- and northeast-trending compartmental faults, which intersect and segment other trends. As a result, northwest-trending thrust and fold-thrust structures may change directions of asymmetry, be offset, or even replaced by other structural styles, while maintaining overall structural balance. This study has documented approximately 30 miles of crustal shortening (representing 15% strain), which was pervasive and uniform across the foreland.