Formation and Prediction of Structural Traps in Northwest Beartooth Mountains Near Livingston, Montana: ABSTRACT

Abstract

The bounding faults on the northeastern flank of the Beartooth Mountains display a reversal of thrust vergence, with southwest-dipping thrust faults in the southeast, and northeast-dipping thrust faults in the northwest. Previous structural models of the Beartooth uplift do not explain this reversal in vergence because they do not balance by sedimentary bed length or basement block area and are not restorable to conceivable initial geometries. In addition, the models do not account for the thin-skinned deformation exposed along the northwestern corner. Restorable models for the northwest Beartooth uplift predict a blind master thrust, dipping to the southwest with wedges of basement back thrust over the main block. The basement-wedge formation is probably responsible for complex thin-skinned thrusts, duplexes, and ramp structures that fill apparent strain incompatibilities (space problems) for the southeast and northwest corners of the uplift. Estimates of 8 to 14 km (5 to 7 mi) of upper crustal shortening are consistent with published estimates from gravity modeling to the southeast. Geometric balancing techniques that conserve basement area and sediment bed length provide strong constraints on the structural geometry of foreland uplifts in the Rocky Mountain region. In the Beartooth Mountains, the complex interplay between the postulated southwest-dippingmore » master fault, northeast-dipping back thrusts, and thin-skinned deformation allow for multiple structural traps for petroleum accumulation. The surface exposures in the Beartooth Mountain uplift do not suggest the subsurface complexity predicted by the balanced cross sections.« less