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Mortensen, J.K., 1983

Age and evolution of the Yukon Tanana Terrane - southeastern Yukon Territory

Bibliographic Reference

Mortensen, J.K., 1983, Age and evolution of the Yukon Tanana Terrane - southeastern Yukon Territory: University of California, Santa Barbara, Ph.D. dissertation.


Recent mapping and isotopic dating of the portion of the Yukon-Tanana Terrane (YTT) northeast of the Tintina fault in southeastern Yukon Territory has yielded new insights into the nature of this exotic terrane. The YTT in the study area consists of a regionally mappable structural sequence, interpreted to be original stratigraphy, consisting of metasedimentary rocks mixed with abundant mafic to felsic metavolcanic rocks and invaded by widespread metamorphosed granitic intrusions. This middle and upper Paleozoic (and perhaps older) sequence was highly metamorphosed and deformed, then uplifted and eroded by Late Triassic time, when sedimentary rocks containing clasts of the metamorphic basement were deposited upon it. Structural imbrication followed, with the emplacement of sheared greenstone and mafic to ultramafic plutonic rock along thrust surfaces. Early Jurassic diorite and monzonite intrusive into the metamorphic sequence appears to be involved in the thrust faulting. Final stages of evolution of the YTT included intrusion of mid-Cretaceous granitic bodies, open folding, eruption of Cretaceous and Tertiary(?) volcanic rocks, and offset along the Tintina fault, separating this area from the more widespread exposures of YTT in western Yukon and east-central Alaska. Forty-eight new U-Pb analyses and 18 Rb-Sr analyses provide constraints on the timing of formation of portions of the terrane and its tectonism. The YTT has previously been interpreted to consist mainly of sheared trench melange. It now appears more likely that the YTT originated as a mid-Paleozoic volcanic-plutonic arc built on continental crust. This early history has been largely obscured by later tectonism and plutonism.

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