PREEXISTING FABRIC CONTROLS MID-CRUSTAL RESPONSE DURING PROTEROZOIC ARC-CONTINENT COLLISION, CENTRAL LARAMIE MOUNTAINS, SOUTHEAST WYOMING

Precambrian rocks exposed in the central Laramie Mountains (CLM) between the Cheyenne belt and the Laramie Peak Shear Zone (LPSZ) 50 km north were ~27 km deep while being deformed in the absence of magmatism during two Proterozoic orogenic events; the Medicine Bow orogeny and the Trans-Hudson orogeny (THO). Both styles and intensities of deformation vary within this block. Early west-vergent folds, seen only in the eastern margin of the range, are attributed to east-west shortening during the THO. This study addresses whether one strain field can resolve the remaining variability, and if so, what factors control deformational style.

Rocks within and east of the Owen Creek greenstone belt (OCGB) were chosen for closer study because both folds and shear fabrics are present, and the relationship between the varying structural styles can be examined. Rocks in the central and eastern portion of the OCGB are deformed into SW-plunging, upright, S-symmetry folds. Fold intensity decreases to the west as strain is increasingly taken up by shearing. Both folding and shearing involve left-lateral transpression during NW-SE shortening. No clear crosscutting relationships exist, therefore they are interpreted to be coeval.

Regionally, folds to the east and south of the OCGB as well as shearing in the LPSZ to the northwest can be resolved by the same strain regime as in the study area. Consistent shortening and transpression across the CLM, combined with a lack of crosscutting relationships, is interpreted to suggest only one event is required to produce the varying structural styles. This single-event interpretation requires a controlling mechanism to explain the variety of structures. Shearing is concentrated at major crustal structures including a 100 m-wide ultramafic dike within the OCGB, the western greenstone-gneiss contact, and the Laramie Peak granite-gneiss contact. Additionally, upright folds correspond to areas where preexisting foliation is near vertical and inclined folds correspond to shallow-dipping preexisting foliation. We propose that in the absence of thermal softening by magmatism preexisting fabrics control local deformational style during a single regional event.