The Colorado Proterozoic province is separated from Archean rocks of the Wyoming province by a major structural boundary, the Cheyenne belt. Proterozoic rocks south of the Cheyenne belt are exposed in the Sierra Madre, Medicine Bow Mountains, and Laramie Range of southern Wyoming. They consist of metavolcanic units, metagraywacke, pelitic schist and gneiss, amphibolite, and felsic to mafic intrusive rocks that locally resemble rocks of central Colorado. North of the Cheyenne belt, Archean granite and gneiss of the Wyoming craton are overlain by a Late Archean and Early Proterozoic supracrustal sequence that contains quartzite, metadolomite, phyllite, and subordinate metavolcanic rocks. The eugeoclinal character of the metamorphic rocks south of the Cheyenne belt contrasts sharply with the dominantly siliciclastic supracrustal rocks north of the Cheyenne belt. Although specific sequences south of the belt have not yet been correlated between the Sierra Madre, Medicine Bow Mountains, and Laramie Range, similarities in age, lithology, and major element chemistry suggest that they are part of a single geologic terrane. Macroscopic structure and microscopic kinematic indicators within the Cheyenne belt suggest that accretion of the Proterozoic rocks of northern Colorado to the Archean Wyoming craton was accomplished primarily by large-scale thrustiig. Following accretion of individual thrust blocks, the boundary zone was steepened by folding and reactivated locally during a period of strike-slip movement. Presence of similar lithologies and shear zones south of the Cheyenne belt suggests that the southern margin of the Wyoming craton may have been a long-lived zone of crustal accretion. ~STRODUCTION ary has since been recognized in the Sierra Madre to the west and may be present in the Richeau Hills to the east, across a total. A major structural boundary between the Archean Wyo-distance of over 200 km (Fig. 1). In the Laramie Range, the ml nL! craton to the north and the Proterozoic rocks of northern Middle Proterozoic Laramie Anorthosite and the Sherman Gran-Colorado t~ the south was first recognized in the Medicine Bow ite occupy the projected position of the boundary (Fig. I), but its ''ounbins in southeastern Wyoming by Houston and McCallum location is reasonably well constrained by Nd isotopic data (Geist 1. Originally named the Mullen Creek-Nash Fork shear and others, 1987). In recognition of its regional extent, structural a for exposures in the Medicine Bow Mountains, the bound-complexity, diverse character, and tectonic importance, the
