NEW EVIDENCE FOR SINISTRAL TRANSPRESSION IN THE CHEYENNE BELT

The movement history of the Cheyenne belt shear zones in SE Wyoming is key to developing plate tectonic models for the Paleoproterozoic reactivation of the southern margin of the Archean Wyoming province. Historically, these shear zones have been interpreted as strike-slip, reverse, and thrust fault systems leading to a variety of plate tectonic models. To test these interpretations and the plate tectonic models based on them, we combined detailed geologic mapping, kinematic analyses of S and L-S tectonites, quartz crystallographic fabric analyses, and deformation mechanism analyses in the northern mylonite zone of the eastern Medicine Bow Mountains. From SE to NW, mylonites of this subvertical shear zone record: sinistral strike-slip; northwest-side-up, dip-slip; southeast-side-up, dip-slip; northwest-side-up/sinistral oblique-slip; and pure-shear-dominated, foliation-normal shortening coupled with local sinistral strike-slip motion. Throughout the zone, feldspars record grain-boundary bulging and minor subgrain rotation recrystallization. Thus, this zone exhibits complex strain path partitioning and significant evidence for high-temperature deformation that accommodated sinistral transpression. Evidence for southeast-side-up, dip-slip motion is confined to ultramylonites immediately adjacent to the terrane boundary.

The southern mylonite zone also contains high-temperature protomylonites that record sinistral strike-slip motion and narrow bands of mylonite that record southeast-side-up, dip-slip motion. These field relationships indicate that sinistral transpression predates southeast-side-up motion. Sinistral transpression and southeast-side-up motion probably took place during a single progressive deformation event because quartz and feldspar recrystallization textures in tectonites that record both kinematic styles are remarkably similar. Synmetamorphic fabrics in the French slate record sinistral strike-slip and northwest-side-up/sinistral oblique-slip motion. Hence, this deformation style was coeval with the formation of the pronounced metamorphic gradient in this unit. These results indicate that sinistral strike-slip motion probably played a major role in the accretion of juvenile terranes along the margin of the Wyoming province.