A new view of an old suture zone: Evidence for sinistral transpression in the Cheyenne belt

Abstract

The type locality of the Archean–Paleoproterozoic suture zone in the southern Rocky Mountains is marked by a series of subvertical shear zones collectively called the Cheyenne belt. The Cheyenne belt is a key structure for developing models for 1780–1740 Ma tectonism along the southern margin of the Archean Wyoming Province, which heralded a rapid period of continental amalgamation. This paper tests existing structural and plate-tectonic models for the Cheyenne belt with detailed geologic mapping, kinematic analyses, quartz crystallographic fabric analyses, and deformation mechanism analyses of the northern mylonite zone of the eastern Medicine Bow Mountains. Mylonites of this zone record a complex deformation history, but the main deformation phase was sinistral/northwest-side-up oblique transpression. Evidence for southeast-side-up, dip-slip motion that characterizes many other areas of the belt is confined to ultramylonites immediately adjacent to the terrane boundary. Hence, fabrics related to sinistral transpression were likely overprinted by southeast-side-up motion. Sinistral strike-slip motion is recorded in at least two other localities in the Cheyenne belt. Because synmetamorphic fabrics on both sides of the suture zone record sinistral strike-slip and northwest-side-up motion, this was probably the dominant deformation style in the field area and may have been the dominant deformation style throughout the Cheyenne belt. Based on these data and regional constraints, we interpret the Cheyenne belt as a subvertical transpressional stretching fault system that simultaneously accommodated sinistral strike-slip motion, penetrative horizontal shortening, and dip-slip motion related to differential crustal thickening between the relatively cold Wyoming Province and younger, hotter rocks to the south.