A test of general shear versus folding for the origin of a cryptic structural feature, Nason Terrane, Washington

The Nason terrane of the Cascades Crystalline Core is a complex tectonostratigraphic terrane that has been the topic of much study due to its long deformational history and importance in the debate surrounding the Baja B.C. hypothesis. Structural patterns along Nason Ridge in the central part of the terrane have been interpreted as the result of either the presence of a major shear zone or the development of fold interference patterns. Distinguishing fold-controlled lineations from shear zone produced lineations can be a complex, but important problem. This study aims to provide a better understanding of high-temperature deformation which may be obscured due to extensive recrystallization.;Outside of the cryptic zone the foliation is steeply dipping and striking to the NW with lineations which are subhorizontal. Previous observations revealed a structural zone ∼1 km wide striking NW-SE. Within the zone, fold hinges and mineral lineations are typically subvertical with mineral lineations clustering tightly and trending NE, indicating that this zone has a fundamentally different structural fabric than the surrounding terrane. Fold hinge lines are more dispersed along the trend of the zone, possibly reflecting incomplete rotation of pre-existing fold hinges into a near vertical orientation.;Anisotropy of magnetic susceptibility analysis indicates the magnetic fabric correlates well with lineations and foliations measured in the field, and lineations likewise shallow to near horizontal outside the zone. Quantification of the strength of the AMS fabric reveals an oblate spheroid, which is interpreted as a proxy for an oblate strain ellipsoid, indicating the presence of a component of pure shear. Asymmetrical microstructural features present include tailed porphyroclasts, mineral fish, and domino clasts. Sense of shear indicators agree with the hypothesized NE-side-up motion (Magloughlin, 1990).;Electron backscatter diffraction analysis reveals biotite and amphibole possess a lattice preferred orientation (LPO), whereas quartz and plagioclase have a nearly random crystallographic fabric. The biotite and amphibole LPO was used to determine NE-side-up sense of shear in thin sections from the cryptic zone.;It is clear from the agreement between outcrop scale structures, magnetic fabric, microstructures, and crystallographic fabric that a significantly different structural fabric is tightly localized on the zone. The highly focused nature of the zone and the lack of structures typically created by fold interference indicates that superposed folding is unlikely. These data, coupled with geothermobarometric constraints, point to a zone of displacement best characterized by northeast-side-up general shear.