Montana transform: A tectonic cam surface linking thin- and thick-skinned Laramide shortening across the Rocky Mountain foreland

Abstract

The Montana transform is here defined as the northern boundary of the Wyoming Laramide foreland. This sinistral-transpressional shear zone followed a crustal-scale structure that defined the southern margin of the Mesoproterozoic Belt basin of western Montana. The structural zone transferred clockwise tectonic rotation of the thick-skinned Wyoming foreland to clockwise rotation of thin-skinned thrust plates of the Montana thrust belt. Traction above the Farallon plate, which rotated northeastward beneath the Wyoming foreland, may have driven the rotation of the basement-involved Laramide ranges. The ancestral crustal structure that controlled the transform did not coincide exactly with a small circle of rotation about the Euler pole for the Wyoming foreland. Instead, the transform rotated eccentrically, like a cam surface, and generated sufficient sinistral transpression against the Belt Supergroup to thrust it out of its basin.