STRUCTURAL GEOLOGY OF THE SKAGIT GNEISS COMPLEX (SGC), NORTH CASCADES, WASHINGTON

The crystalline core of the North Cascades (Cascades core) is part of a >60 km thick, exhumed Cretaceous to Eocene continental magmatic arc (e.g., Miller and Paterson, 2001). The Skagit Gneiss Complex (SGC), composed of NW-striking, partly migmatitic ortho- and paragneiss, comprises the highest grade and likely the most deeply exhumed rocks of the arc. Eocene crystallization ages for protoliths of some Skagit orthogneisses permit the study of younger structures in the Cascades core. Limited structural work has been conducted in the SGC, and a synthesis of foliation and lineation patterns for the entire SGC is lacking. We concentrated our synthesis on regional-scale structures using our own and previously published data; early isoclinal folds and other mesoscale structures are present but are not discussed further.

Foliation and lineation orientations within the southern SGC show several dramatic swings over relatively short distances. Moving northward, foliation strike changes from WNW (300º) to NNW (350º) to NE (025º) and back to NW (330º) in <50 km along strike. Dips are typically moderately steep. Lineation trends similarly rotate from ESE (120º) to NW (330º) to NNE (010º) to NW (340º)-SE (160º) with gentle plunges.

Regional folds with gentle plunges within the southern SGC are generally symmetric and open to gentle (interlimb angles of 110°-135°) with wavelengths typically of 8-10 km, and locally down to 3 km. Hinge lines rotate from 300º to 340º in <20 km. In the northern SGC, the entire unit has a broadly antiformal geometry (Tabor et al., 2003). When combined, foliations, lineations, and regional fold hinge lines in the Skagit show a late, previously unrecognized steeply plunging fold that encompasses much of the central and southern part of the complex.

Our synthesis suggests a much more complicated structural pattern and deformation history for the central and southern SGC than was previously recognized. Geochronology and field relations imply that these regional folds were formed over a relatively short timespan and raise several important questions, including; what is the age range of the dominant fabrics in the Skagit Gneiss Complex, and what was the cause of Eocene shortening?