Abstract

One clear distinction between thin-skinned thrust belts and the basement-cored structures of the Rocky Mountain uplifts is the greater variability of thrust vergence in Rocky Mountain uplifts. In thin-skinned thrusting, nearly all the major thrusts verge toward the stable craton. This uniform polarity of thrusting is probably the result of a combination of preferential sliding on hinterland-dipping beds and topography-induced gravity spreading, factors of more limited importance in foreland uplifts.

Important back-thrusting occurs in triangle zones at multiple scales in basement-cored foreland uplifts. Small-scale back thrusts occur in the sedimentary rocks in front of basement overhangs to facilitate the transition from basement block motion to flexural slip in the sedimentary cover. These can be seen on both sides of the west-vergent Rattlesnake Mountain and Forellen structures, which are actually larger-scale back-thrusts of opposite polarity. Increasing amounts of back-thrusting, commonly on several spaced structures (e.g., northwestern Beartooth and northeastern Front Range), reduces the slip on the master thrust, resulting in the termination of the master thrust in the basement. The southwestern vergence of the Wind River and northern Front Range suggests that their master thrusts are actually backthrusts off a mid-crustal detachment. Crustal wedging and basement involvement provide additional complexities to basement-cored structures that must be addressed in analytical and regional models of foreland uplifts.