THE OBSOLETE WINDY-MCKINLEY TERRANE OF WESTERN YUKON AND EASTERN ALASKA: NEW CORRELATIONS AS TERRANE ANALYSIS RETURNS TO ITS ROOTS

Arguably, the Cordilleran terrane concept was first demonstrated in south-central Alaska. Data collected there in the 70’s and 80’s defined stratigraphically and structurally distinct, fault-bound crustal blocks which came to be referred to as ‘terranes’. Paleontological and paleomagnetic data showed some terranes to be far-traveled with respect to their neighbours and to the North American craton, proof that the potential inherent in plate tectonic processes for the long-distance dispersal of rock masses from distant and diverse paleogeographic origins had been realized in their assembly into the Cordilleran orogen.

Although the terrane concept has since been applied and refined throughout the Cordillera, some of the original terranes defined in Alaska have not, until recently, been re-evaluated. New data from rocks in western Yukon previously assigned to the Windy-McKinley terrane form the basis for new correlations. Siluro-Devonian psammitic schist and Devonian carbonaceous schist and felsic meta-igneous rocks underlying much of the area west of the White River resemble, and are continuous with, parautochthonous Laurentian rocks of east-central Alaska. Locally occurring in the area are less deformed and metamorphosed clastic rocks which resemble Triassic formations deposited on Laurentian and peri-Laurentian terranes elsewhere. Middle to Late Triassic gabbro intrusions into the parautochthonous assemblage form the southeastern end of a belt of intrusions that, together with the parautochthonous rocks, extends westwardly into the Alaska Range. Southeast of the parautochthonous rocks is a belt of mantle ultramafic rocks spatially associated with Early Permian diabase and gabbro, chert, argillite and mafic volcanic rocks. With its composition and position between parautochthonous rocks and allochthonous rocks of Yukon-Tanana terrane to the southeast, this belt of oceanic rocks correlates with the Slide Mountain terrane. Owing to these correlations, the use of ‘Windy-McKinley’ should be discontinued.

The occurrence of Laurentian parautochthonous rocks and Slide Mountain terrane SW of rocks of Yukon-Tanana terrane adds another layer of complexity to kinematic models to explain the enclosure of the Panthalassic Cache Creek terrane between terranes of peri-Laurentian affinity.