A model for terrane accretion: Yukon‐Tanana and Slide Mountain Terranes, northwest North America

Abstract

To date, many North American Cordilleran geologists employing the concept of suspect terranes have emphasized differences in lithology, metamorphism, styles of deformation, and timing of these “distinct” tectonic units, arguing that each evolved separately from its neighbors. This practice of focusing on dissimilarity, and nomenclature used in terrane analysis, may be detrimental to our understanding of the evolutionary relationships between terranes. I present a model in which the Yukon‐Tanana (YT) and Slide Mountain (SM) terranes, which do exhibit certain lithotectonic disparities, are correlated within a unifying tectonic framework in which each terrane represents a different crustal level of the same late Paleozoic‐early Mesozoic magmatic arc and west dipping subduction complex formed during plate convergence outboard of western North America. The SM represents the upper crustal level of the arc and accretionary complex, and the heterogeneously metamorphosed and ductilely deformed YT represents the deeper crustal level. The Teslin suture zone (TSZ), the high‐P/T portion of the YT, formed the décollement between the YT‐SM composite terrane hanging wall and the descending North American plate; TSZ structural relations thus provide insight into the kinematic interaction of this composite terrane with North America prior to its late‐stage emplacement onto North America. The present‐day “collage” of suspect terranes clearly comprises a multitude of structurally dismembered and dissimilar lithotectonic assemblages. Nonetheless, it is possible to see order in the chaos, to reassemble the pieces in a tectonically meaningful fashion. The YT‐TSZ‐SM association in Yukon may serve as a model for tectonically coherent continental margin expansion.