DEFORMATION HISTORY OF THE SOUTHERN YUKON-TANANA TERRANE FROM TWO SITES IN THE EASTERN ALASKA RANGE

This study evaluates the deformation history of two sites in the relatively unstudied Yukon-Tanana terrane of the eastern Alaska Range using new mapping data from the Alaska Geological and Geophysical Survey. Both sites are dominated by Devonian (and older?) meta-siliciclastics of the Yukon-Tanana terrane along with variable amounts of sill-like bodies of gneiss derived from Late Devonian to Early Mississippian porphyritic granite to gabbro bodies. The western site, located near Wood River west of Delta Junction, has Mississippian-Devonian age meta-volcanic and meta-volcaniclastic rocks. The eastern site, near the town of Tok, has amphibolites with a geochemical signature compatible with being basaltic flows (or the intrusive equivalents). The highest grade of metamorphism at these two sites affects the Devonian and Mississippian rocks. Locally, greenschist retrograde metamorphism occurs associated with Cretaceous plutons. The metamorphic grade is highest in the eastern site with garnet amphibolite grade metamorphism, whereas the western site underwent lower greenschist grade metamorphism.

All of the penetrative ductile deformation in these two areas is pre-Cretaceous. Both areas preserve evidence for an early phase of syn-metamorphic ductile deformation of approximately north-south compression. A second ductile deformation and associated metamorphism overprints the first deformation with NW-SE compression. The resulting metamorphic rocks are L-S tectonites with strong NE-trending lineations in the western site and NE- and WNW-trending lineations in the eastern site. The timing of these events is poorly constrained between the Early Mississippian and the late Early Cretaceous and may be related to regionally observed terrane accretion.

Cretaceous and younger deformation occurs as local discrete shear zones and faults. Several episodes of more brittle deformation affect Cretaceous and younger rocks. The timing constraints for this younger brittle deformation are as of yet not well defined. A preliminary interpretation of the eastern site has Late Cretaceous top to the south extensional(?) deformation, Paleocene right-lateral transpressional deformation, Eocene transtension and Quaternary N-S trending compression.