Proterozoic accretionary tectonics at the southern margin of the Archean Wyoming craton

Abstract

The Proterozoic Cheyenne belt, at the southern margin of the Archean Wyoming craton, consists of strongly deformed lithotectonic blocks bound by major mylonite zones. From north to south and structurally lowest to highest, these blocks include (1) Archean crystalline basement and associated early Proterozoic miogeoclinal rocks; (2) an amphibolite-grade orthogneiss terrane of unknown age containing probable rift-related mafic intrusive rocks; (3) a 1750–1790 Ma, marginal basin(?) succession consisting of upper amphibolite-grade pelitic and volcano-genie schist, associated peraluminous granite, and minor ultramafic rocks; and (4) a 1750–1790 Ma intermediate to mafic plutonic-metamorphic complex interpreted as the deep roots of an island-arc system. The earliest deformation, D1, produced a synmetamorphic, penetrative, horizontal transposition foliation and associated recumbent folds. Tectonic blocks were juxtaposed along major mylonite zones during D2. Macroscopic anil microscopic structures associated with D2 indicate northward thrusting along low-angle mylonite zones of successively deeper crustal blocks over supracrustal rocks of the Wyoming craton at about 1750 Ma. Thrusting occurred at minimum temperatures of 475 °C and produced an inverted metamorphic gradient in pelitic rocks north of the belt. Mylonite zones were subsequently steepened and reactivated under greenschist-facies conditions during a period of dextral strike slip (D3) between 1750 and 1400 Ma. Cataclasis (D4) and brecciation associated with the Laramide orogeny (D5) locally overprint earlier structures. Oblique convergence between an island arc and the Archean continental nucleus may explain accretion, crustal shortening, and subsequent strike slip. The Phanerozoic accretionary events of eastern and western North America provide an analog for this model. Presence of similar lithologies and shear zones south of the Cheyenne belt suggest that the southern margin of the Wyoming craton may have been a long-lived zone of crustal accretion.