STRUCTURAL ANALYSIS OF EOCENE DIKE SWARMS IN AND NEAR THE DUNCAN HILL PLUTON, NORTH CASCADES, WASHINGTON

Eocene dike swarms in the North Cascades are thought to provide insights into the regional strain field during postulated ridge subduction in the Pacific Northwest. One of the most voluminous swarms is spatially associated with the elongate (NW-SE), ~46 Ma granodioritic Duncan Hill pluton. This pluton is tilted, and dikes are most abundant near its shallow southern end. Previous work and new field observations from this investigation indicate that dominantly rhyolite, granite porphyry, and mafic (“lamprophyres”) dikes intrude host rock gneisses and tonalites of the Cretaceous Chelan Complex, and that mafic dikes are typically farther from the pluton. Textures range from porphyritic to aplitic and some dikes are highly hydrothermally altered. The ~80 km² study area is divided into a northern domain, near the deeper part of the Duncan Hill pluton, and a southern domain that contains the roof of the pluton and was studied in much more detail. Over the whole study area, ~250 dikes were measured (north = 19; south= 233). Dikes have dominantly steep dips, but vary widely in strike. In the northern domain, dikes trend NW (303-339°), whereas in the southern domain they strike NE (005-044°). Dikes closest to the Duncan Hill pluton strike ENE, and with distance away from pluton they rotate to a more NE strike. Extension magnitudes in two well-exposed transects (270 m and 290 m long) in the southern domain are ~6% and 3.8%, percentages which likely are representative of extension across the study area. The average extension direction from the dike strikes in the southern domain is ~295-115°, which is oblique to the average strike (~320°) of the North Cascades orogen. Regionally, dike orientations in the southern domain are broadly consistent with those elsewhere, including directly SW of the Duncan Hill pluton (strike ~025-058°; Sylva, 2014); in the voluminous Teanaway swarm, ~60 km to the SW (strike ~019-036°; Doran, 2009); and adjacent to the Eocene Cooper Mountain batholith, ~ 20 km east of the study area (strike ~005°; Raviola, 1988). The heterogeneity of dike trends in the study area indicates a complex strain field, which may reflect temporal changes and/or local controls such as the emplacement of the Duncan Hill pluton.