CORDILLERAN TERRANES REVISITED: THE DOSSIER ON SOME LONG-SUSPECTED RELATIONSHIPS

The terrane concept was first introduced nearly 25 years ago to describe the major components of the orogenic collage that constitutes the North American Cordillera. In the original definition, terranes are fault-bounded crustal blocks which preserve a geological record distinct from that of adjacent terranes, and whose paleogeographic position with respect to each other and to the North American craton are uncertain. This concept has inspired an intense campaign of regional tectonic studies of the internal frameworks and external relationships of Cordilleran terranes. A number of these studies have identified linkages between adjacent terranes, and thus exceptions to their assumed fault-bounded nature. In the original development of the terrane concept, linking assemblages were limited to late, post-tectonic or post-amalgamation overlaps. In the last 25 years, work by ourselves and others show inter-terrane linkages and shared elements that can range as old as the oldest rocks in one of them. These include: (1) Cambrian stratigraphic linkages between Kootenay terrane and ancestral North America; (2) the onlap of Slide Mountain terrane onto the Kootenay terrane in southern B.C.; (3) Early Permian depositional ties and stratigraphic overlap between Yukon-Tanana and Slide Mountain terranes in Yukon; (4) Yukon-Tanana terrane as basement to Quesnellia; (5) late Paleozoic and Triassic ties between Stikinia and Yukon-Tanana terrane; (6) an Early Jurassic plutonic suite shared by Stikinia, Yukon-Tanana and Quesnellia; and (7) a Pennsylvanian pluton that links the Alexander and Wrangellia terranes. Such observed commonalities effectively eliminate some of the paleogeographic uncertainties that were previously inferred between adjacent terranes (although not necessarily with respect to North America) and highlight their shared history. This emerging pattern of relationships between established terranes requires either a wholesale revision of some of the major Cordilleran terranes and their boundaries, or a redefinition of the term terrane and its underlying concept such that it more accurately reflects the complex geodynamic setting in which many of these terranes formed.