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Montana transform

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

Department of Geosciences, University of Montana, Missoula, MT 59812, U.S.A.

e-mail: james.sears{at}umontana.edu

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.

Key Words: Belt basin • Laramide • Rocky Mountains • tectonic rotation • Wyoming foreland

This article has been cited by other articles:

				J.W. Sears Belt-Purcell Basin: Keystone of the Rocky Mountain fold-and-thrust belt, United States and Canada Geological Society of America Special Papers, January 1, 2007; 433(0): 147 - 166. [Abstract] [Full Text] [PDF]