The tonalitic orthogneiss-dominated Central Skagit Gneiss Complex of the Cretaceous to Eocene North Cascades crystalline core consists of four units, differentiated on field and petrographic observations. New U/Pb zircon analyses yield crystallization ages for the orthogneisses between ∼66 and 48 Ma. Microstructures indicate medium- to high-temperature (400-650°C) deformation. Much deformation in the adjacent dextral Gabriel Peak tectonic belt, a segment of the 500-km-long Ross Lake fault system, predates crystallization of ca. 49.3 Ma orthogneisses. Lineations in the Eocene gneisses dominantly record subhorizontal NNE stretching discordant to the NW regional trend of the Cascades core and the tectonic belt. This discordance may reflect a change in the Eocene strain field, which is best recorded in rocks that were hot and ductile at the time. Thus, structures in the Central Skagit Gneiss Complex probably record some of the latest ductile deformation in the core.
|Advisor:||Miller, Robert B.|
|School:||San Jose State University|
|School Location:||United States -- California|
|Source:||MAI 47/05M, Masters Abstracts International|
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