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This report presents four U/Pb zircon dates from the Kootenay arc and Cariboo Mountains of southeastern British Columbia. When combined with existing biochronological data, two of these dates tightly constrain the age of shortening of the sedimentary basin between the North American continental margin and the Triassic-Jurassic Quesnellia volcanic arc to mid-Toarcian (late Early Jurassic, ca. 187–185 Ma). The other two dates affirm a Mesozoic age of the prominent phase of southwest-vergent folding in the Omineca crystalline belt and permit correlation of these structures with similar folds of the mid-Toarcian Quesnellia–North America terrane boundary. Previously published dates and geological relationships show that this latter southwest-vergent folding was finished by the end of the Aalenian (earliest Middle Jurassic, 174 Ma). Taken together, these data show that the earliest two “phases” of Jurassic deformation of the western edge of North America occurred in a ca. 10-million year interval spanning the Early to Middle Jurassic boundary. Although this deformation consists of two phases, given the short time interval, it is likely that they are part of the same continuous episode of shortening of the continental margin.

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