Low-temperature cooling histories of the Cheyenne belt and Laramie Peak shear zone, Wyoming, and the Soda Creek-Fish Creek shear zone, Colorado

Abstract

The base of a fossil apatite fission-track (AFT) partial annealing zone (PAZ), which formed when the area now occupied by the central and southern Rocky Mountains was at sea level in Late Cretaceous time, has since been disrupted by Laramide and post-Laramide tectonism and denudation. New AFT data are used to identify this marker and to examine its disruption across Proterozoic boundaries in north-central Colorado and south-central Wyoming. The cooling history recorded by the AFT data in the Laramie Range is not strongly controlled by basement structures, but instead reflects either long-wavelength warping of the base of the PAZ during Laramide deformation or N-S variations in Paleozoic to Mesozoic sediment thickness across this range. In contrast, at least one structure associated with the Cheyenne belt in the Medicine Bow Mountains, the Rambler shear zone, influenced the Laramide cooling history of this range. The Rambler shear zone separates Laramide AFT cooling ages (60 to 79 Ma) to the northwest from >100 Ma AFT ages to the southeast. In the Sierra Madre, Wyoming, AFT ages from Archean rocks north of the Cheyenne belt and from Proterozoic rocks to the south are nearly equivalent (49-79 Ma); the Late Cretaceous PAZ is not preserved in this mountain range. Similarly, AFT ages north and south of the Proterozoic Soda Creek-Fish Creek shear zone the Park Range, Colorado are about the same (45-75 Ma). Thus, these shear zones apparently were not strongly reactivated during Laramide deformation.