Date of Award


Document Type


Degree Name

Master of Science (MS)



First Advisor

T.M. Harrison


Field, structural, and isotopic studies in the southeastern Eagle Quadrangle reveal a complex Mesozoic history involving major metamorphism, fold development, thrusting, and igneous intrusion. These events may be related to the accretion of a terrane, possibly Stikine, to the North American continental margin.
40Ar/39Ar incremental heating experiments on metamorphic minerals from amphibolite facies rocks reveal Late Triassic to Middle Jurassic plateau ages interpreted as the age of major metamorphism. Large scale northeast trending folds within these rocks formed after the major metamorphism and before the intrusion of an undeformed, unmetamorphosed cross-cutting dike in Middle Jurassic. These folds probably developed in response to northwest-southeast compression related to terrane accretion.
A major thrust system which separates the amphibolite facies metamorphic rocks on the south from lower grade rocks to the north is characterized by complex thrust melanges, distinct faults, and shear zones between and within the two groups of rocks. There are distinct differences in the structural character of rocks on either side of this fault zone as indicated by differences in fold style and orientation of major fold generations. Thrusting involved ophiolitic material as suggested by the presence of serpentinized ultramafic rocks and silica-carbonate rocks within the thrust zone. Ar/Ar data from a sample within the fault zone indicates that thrusting occurred after major metamorphism and before 187 ± 2 Ma.
40Ar/39Ar data also show that a major batholith (Taylor Mountain) within the southern terrane was intruded just after the peak of metamorphism at about 207 ± 2 Ma and cooled at an average rate of 10ºC/Ma with the metamorphic rocks.
The Taylor Mountain batholith may be related to the Klotassin suite of northwestern Canada and the metamorphic terrane may be the extension of the Stikine block into east-central Alaska.


Cushing, G.W., 1984. The tectonic evolution of the eastern Yukon Tanana Upland, Alaska.
Unpublished MSc. thesis, State University of New York at Albany. 255 pp., +vi
University at Albany Science Library call number: SCIENCE Oversize (*) QE 40 Z899 1985 C88