STRUCTURE AND TECTONIC ORIGIN OF THE SWAKANE GNEISS IN THE WENATCHEE BLOCK, NORTH CASCADES, WASHINGTON

The North Cascades crystalline core is an ideal natural laboratory to study processes associated with deep burial and rapid exhumation of clastic sediments in magmatic arcs. The Swakane Biotite Gneiss (SBG), the youngest and structurally deepest meta-supracrustal unit in the Cascades core, was metamorphosed at 8-12 kb between ~73-68 Ma. Detrital zircons reveal Mesozoic and Proterozoic age peaks, and the Late Cretaceous (~76 Ma) maximum depositional age indicates rapid burial. These features are similar to the Pelona–Rand–Orocopia schists, which structurally underlie the Sierra Nevada batholith in southern California. The SBG is divided by the Entiat fault into two NW-trending belts: the Chelan block in the NE and Wenatchee block in the SW. We mapped the SBG in the much less studied Wenatchee block where it is exposed in three domains: a southern domain, which is an inlier in an Eocene basin; a large central domain; and a small isolated northern domain. All domains are dominantly biotite gneiss, and in the central and northern domains, the gneiss is locally interfingered with 1.5-10 m thick and <2.5 km long concordant lenses of amphibolite. Three thin (<2 m) lenses of quartzite are localized with the amphibolite, and the interpreted basalt and chert protoliths may represent slices of oceanic crust and pelagic sediment imbricated with the clastic protoliths of the biotite gneiss. Thin (3-10 cm thick) metaperidotite pods, which are restricted to the northern domain, may have been serpentinites imbricated with the sediments. All of the domains are intruded by ~73–68 Ma leucocratic sheets, and in the northern domain, the SBG is migmatitic.

A strong moderate to steeply dipping foliation occurs in all domains. A well-developed lineation plunges gently in the southern and central domains, but is much weaker and steeper in the northern domain. Foliation, lineation, and associated kinematic indicators in the SBG in the Chelan block record pervasive top-to-N shear during Eocene exhumation. The southern and central Swakane domains of the Wenatchee block record the same top-to-N shear, albeit less pervasive. In contrast, fabrics in the northern domain appear to reflect mostly pure shear.