Publication Date

Spring 2024

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

Advisor

Robert Miller; Ellen Metzger; Jonathan Miller

Abstract

The Skagit Gneiss Complex in the North Cascades magmatic arc consists of Late Cretaceous to Eocene orthogneiss and partially migmatitic paragneiss. Orthogneiss protoliths intruded during two magmatic flare-ups, making the Skagit Gneiss an ideal location to study the relationship between flare-ups and deformation. Geologic mapping, microstructural analysis, and U-Pb dating of zircon were used to construct a timeline of magmatism, metamorphism and deformation in a portion of the Skagit Gneiss. A tonalitic gneiss intruded between ~79-73 at the beginning of the older flare-up and an ~60 Ma, tonalitic to granodioritic, leucocratic orthogneiss intruded at the end of this flare-up. NW-SE trending, medium-temperature foliation and lineation are weakly to moderately well developed in high structural levels of the ~79-73 Ma orthogneiss. The deepest portions of the orthogneiss have strong fabrics that reflect high-temperature deformation and migmatization. An ~60 Ma metamorphic age from the tonalitic gneiss is co-magmatic to crystallization of the ~60 Ma leucocratic gneiss. Microstructures in the leucocratic gneiss indicate high temperatures during deformation. Strongly linear fabrics are concordant with those in the older tonalitic gneiss, and are consistent with regional dextral transpression at 60 Ma. Shear in all rocks is dominantly top-to-the-northwest. These interpretations suggest that deformation peaked with temperature in the south-central Skagit Gneiss at the end of a flare-up. These conclusions suggest flare-ups raise the geothermal gradient in an arc and enable migmatization and dynamic recrystallization of plagioclase, resulting in temporary crustal weakening.

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