Petroleum exploration beneath Precambrian on the flanks of Rocky Mountain foreland structures has revealed substantial throw on east-west-trending thrusts which has not been predicted by underthrust models of west-directed tangential compression. Recognizing this north-south compressional component in the foreland necessitates a new look at the forces that formed these structures.

Initial compression that developed foreland structures was dominantly from east to west and was caused by westward movement of the North American plate during the opening of the Atlantic Ocean in Late Cretaceous. Atlantic spreading progressed to the North Atlantic and Arctic Oceans in Late Cretaceous and early Tertiary. It is proposed that movement of the North American plate evolved from west to southwest to south, causing not only significant southwest and south movement on several foreland basement-involved thrusts, but also termination of movement in the detached Idaho-Wyoming-Utah thrust belt.

Major east-west-trending foreland structures include the Owl Creek Range, the south flank of the Wind River Range and the south flank of the Granite Mountains in Wyoming, the Uinta Mountains in Utah, and the north flank of North Park basin in Colorado. North-west-trending foreland thrusts, such as the southwest flanks of the Casper arch, and Gros Ventre and Wind rivers in Wyoming developed during the transition from east-west to north-south compression.

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