Abstract

The Proterozoic Cheyenne belt marks the southern boundary of the Archean Wyoming province. Within the Medicine Bow Mountains, it consists of strongly deformed, lithologically distinct blocks bounded by mylonite zones. Detailed field and petrographic investigations of the boundary, augmented by study of microscopic kinematic indicators, allow us to refine existing models for tectonic development of the belt. Northward thrusting under amphibolite facies conditions along low-angle mylonite zones emplaced successively deeper crustal blocks over supracrustal rocks of the Wyoming province. Mylonite zones were subsequently steepened and reactivated under greenschist-facies conditions during a period of distributive dextral strike slip. We suggest oblique convergence between an island arc and the Wyoming craton as a possible mechanism for both deformational events. Accretionary events in eastern and western North America, which involved initial convergence followed by strike slip, provide a Phanerozoic analog for our model.

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