A system of subvertical, northeast-striking shear zones collectively called the Cheyenne belt comprises the fundamental boundary between the Archean Wyoming province and the Paleoproterozoic Colorado province in the Medicine Bow Mountains of southeastern Wyoming. These shear zones are generally interpreted as a mid-crustal thrust system rotated to its present-day subvertical orientation during late-stage, orogen-scale folding. However, the map geometry, presence of large domains of S tectonites, and vertical mineral lineations closely match numerical simulations of transpressional shear zones. This transpression hypothesis is tested using two detailed case studies that integrate detailed geologic mapping, kinematic analyses of S and L-S tectonites, quartz crystallographic fabric analyses, and deformation mechanism analyses of shear zones in the western part of the Medicine Bow Mountains. These data point toward general shear deformation that accommodated two coeval deformation components: (1) southeast-side-up, dip-slip or southeast-side-up/dextral, oblique-slip motion and (2) foliation-normal shortening. According to the broadest definition, these are transpressional shear zones. However, the strike-slip component is small, and these data do not require any significant oblique plate motion during the formation of the Cheyenne belt shear zones.
The lack of evidence for overprinting of initial southeast-side-up fabrics by horizontal shortening, the near parallelism of the subvertical shear zones throughout the upper and middle crust, and the presence of subvertical mafic dike swarms in Archean basement rocks immediately adjacent to the Cheyenne belt all indicate that the Cheyenne belt shear zones were not rotated into their present-day subvertical orientations during late-stage deformation. Instead, we interpret the shear zones as a ∼1,750-Ma steeply dipping stretching fault system between the penetratively deformed young, hot, rheologically weak rocks of the Colorado province and the old, cold, rheologically strong Archean rocks north of the belt.