STRUCTURES OF THE CENTRAL PART OF THE SKAGIT GNEISS COMPLEX, NORTH CASCADES, WASHINGTON

The Skagit Gneiss Complex forms an antiformal part of the crystalline core of the North Cascades, a thick continental magmatic arc, and contains perhaps the deepest and highest-grade metamorphic rocks recording the youngest cooling ages in the core. Plutonism in the Skagit Gneiss Complex was coeval with ductile deformation and amphibolite-facies metamorphism that began by ~90 Ma and continued into the Eocene. This study focuses on a 12 km-wide swath through the locally migmatitic central Skagit Gneiss Complex extending eastward from the Eocene (~46 Ma) Railroad Creek pluton to the dextral Ross Lake fault zone. Variably recrystallized leucocratic orthogneisses comprise > 95 % of the rocks. Plutonic protoliths to orthogneisses in the complex as a whole have crystallization ages from ca. 90 Ma to 49 Ma, and preliminary U-Pb data from the study area and broadly similar rocks to the south suggest that many of the bodies are ca. 50 Ma. Map units are elongate, varying from < 1 km to ~ 4 km in width, and trend parallel to the NW regional structural trend. Multiple generations of NE-trending cm-to m-scale sheets of trondhjemite and tonalite, and rafts of supracrustal host rock, are common within units and are ubiquitous along the eastern margin of the study area. Cross-cutting relationships indicate that sheeted units young to the west and are truncated by a late distinctive granodiorite with sphene-cored plagioclase phenocrysts. All units are cut by E–W-striking cm-to m-scale dikes. Previous reconnaissance (Adams, 1961) indicated NNE-striking foliation oriented at significant angles to the regional NW strike and truncated by the Ross Lake fault zone. Our work has expanded the size of the domain of anomalously oriented structures to ~ 130 km2, and has documented up to ~50º discordance between foliation and the regional structural trend. Foliation strikes N–S to N20E and dips 30º-60º E, and mineral lineation plunges an average of ~13º NNE. The extension direction (N–S) defined by average dike orientation is compatible with that of the stretching lineation. The structural and geochronologic data suggest that the N–S stretching is Eocene in age and represents some of the youngest strain in the Skagit Gneiss Complex and Cascades core. The cause of this late strain at a high angle to the regional strain field remains enigmatic.