THICK-SKINNED DEFORMATION OF THE ARCHEAN WYOMING PROVINCE DURING PROTEROZOIC ARC-CONTINENT COLLISION

Two major mylonite zones are exposed in the southeastern Archean Wyomi ng province: the Cheyenne belt, which marks the boundary between the W yoming and Colorado provinces, and the Laramie Peak shear zone, across which Archean rocks have been uplifted differentially at least 10 km. Between the two mylonite zones is a 60-100-km-wide belt of Archean mi gmatitic gneiss, which has been intruded by diabase dikes and peridoti tes and contains enclaves of high-grade metasedimentary rocks. We inte rpret this belt of high-grade rocks as a tectonically reactivated bloc k of Archean crust, uplifted during the development of the Cheyenne be lt collisional zone at ca. 1.8 Ga. This block does not exhibit a decre asing metamorphic gradient away from the mylonite zone, but is at unif ormly high grade. Thus, rather than a thick-skinned thrust-ramp model for uplift, we invoke vertical uplift along a high-angle reverse fault without rotation. This style of uplift may be a tectonic response to transpression, underplating, or tectonic interfingering at depth durin g collisional orogeny, and it may be a more common feature of basement uplifts within foreland fold-and-thrust belts than has been previousl y recognized.