The Yukon-Tanana composite terrane, northern Canada and eastern Alaska, is divisible into two distinct terranes on the basis of metamorphic cooling ages and inferred structural level. The Nisling terrane records Early Cretaceous cooling ages, whereas the Teslin-Taylor Mountain terrane records pre-Late Jurassic cooling. The Nisling terrane, present at the lowest structural levels within the Yukon-Tanana terrane, represents North American continental-margin crust that was partly subducted beneath the accretionary prism of a southwest-dipping (present-day coordinates) early Mesozoic subduction complex, represented by the Teslin-Taylor Mountain and Slide Mountain terranes. The Nisling is exposed as a result of mid-Cretaceous crustal extension subsequent to Jurassic crustal thickening related to Teslin-Taylor Mountain-Slide Mountain emplacement. Although the Nisling and Teslin-Taylor Mountain terranes have many petrologic and tectonic differences, their mutual relations provide a unified view of early to middle Mesozoic tectonics of the western North American margin, a view that dispels the notion that large tracts of the northern Cordillera are "suspect."

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