A TEST OF GENERAL SHEAR VERSUS FOLDING FOR THE ORIGIN OF A CRYPTIC STRUCTURAL FEATURE, NASON TERRANE, WASHINGTON

The tectonostratigraphic Nason Terrane of Washington has been the topic of study due to its importance in 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 a major shear zone or superposed folding. Distinguishing fold-controlled lineations from shear zone produced lineations can be a complex, but important problem. This study provides 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 NW with subhorizontal lineations. Previous observations revealed a unique zone following strike ~1 km wide where fold hinges and mineral lineations are typically subvertical with mineral lineations clustering tightly and trending NE. Fold hinge lines are more dispersed along the strike 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. 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 of Magloughlin (1990).

Electron backscatter diffraction analysis reveals biotite and amphibole possess a lattice preferred orientation which 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, pointing to a zone of displacement best characterized by northeast-side-up general shear.