Interaction of the Rocky Mountain Foreland and the Cordilleran Thrust Belt
Deformational style of Laramide uplifts in the Wyoming foreland
Published:January 01, 1988
William G. Brown, 1988. "Deformational style of Laramide uplifts in the Wyoming foreland", Interaction of the Rocky Mountain Foreland and the Cordilleran Thrust Belt, Christopher J. Schmidt, William J. Perry, Jr.
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The evidence presented in this chapter supports the conclusion that there is no one, single structural style in the Wyoming foreland, and that the orientation of various structures is extremely important when deciding which structural model should apply. For instance, foreland features displaying the basic characteristics of compressional uplifts are those that trend northwest, whereas those features displaying the characteristics of vertical uplifts are those that trend northeast, or east-west.
The development of drape folds and upthrust structures in a compressional system requires a local concentration of the vertical uplift component, satisfying the theoretical and experimental models of Hafner and Sanford. Concentration of these vertical stresses (σ1) takes place along east- or northeast-trending faults that segment or compartmentalize the northwest-trending thrust, or fold-thrust structures. These compartmental faults may have inherited their location and trend from Precambrian zones of weakness reactivated during the Laramide orogeny. Experimental studies have shown that small amounts of lateral movement may occur along these faults without exposure of a strike-slip fault at the surface.
Folding of the sedimentary section is commonly a response to the tectonic loading by the Precambrian basement forcing block. If the forcing block moves along a reverse fault, the resulting fold in the sedimentary section is a compressional fold, which displays the volumetric problems and bed-length adjustments typical of flexural folding. If the forcing block is uplifted along a more nearly vertical fault, the resulting structure is a drape fold in the overlying sedimentary section, which requires certain bed-lengthening mechanisms to develop. Continued uplift will result in the development of an upthrust structure.