STRUCTURE OF THE NORTHERN SKAGIT GNEISS COMPLEX, NORTH CASCADES, WASHINGTON

The crystalline core of the North Cascades is part of a thick (>55 km) ~96-45 Ma continental magmatic arc. The highest-grade part of the arc is the Skagit Gneiss Complex (SGC) composed of partially migmatitic amphibolite-facies orthogneiss, banded biotite gneiss, and paragneiss. The northern SGC experienced isothermal decompression at >650°C from 8-10 to 3-5 kbar (Whitney, 1992) and protracted migmatization from 68-51 Ma (Gordon et al., 2007) during the change from regional transpression to transtension at ~57 Ma. The northern SGC, which has received little previous detailed structural analysis, is the ideal place to study the shift in the strain regime at deep levels of the arc.

Foliations in the complex strike NW, parallel to the trend of the orogen, and dip moderately to steeply to the NE and SW. Lineations mostly plunge gently SE. Microstructures record ductile deformation at relatively low T (300-400°C) and high T (>450°C); low-T and high-T deformations are focused in different km-scale NW-striking zones. Early subisoclinal to isoclinal folds of foliation have wavelengths of 2-22 cm. Axial planes are inclined to recumbent and strike NE; variably oriented hinge lines plunge 2°-51° mostly SE. More widespread open to gentle folds refold the earlier folds forming type 2-3 interference patterns. The late folds have wavelengths of 1 cm-7 km, axial planes are upright to gently inclined and strike NW, and hinge lines plunge 17° on average and mostly SE. Foliation measurements (n=223) refine the geometry of a previously mapped NW-SE-trending regional antiform. After restoration of foliation to a gentle orientation prior to upright folding, kinematic indicators record top-to-the-SE and top-to-the-NW shear in discrete km-scale NW-striking zones that correlate with high-T (>450°C) and lower-T (300-400°C) deformation, respectively. The top-to-the-NW-shear most likely occurred during regional transtension. The most recent ductile deformation is marked by strong subhorizontal L>>S fabrics in granite and dacite dikes, which intrude all major units and structures at ~46 Ma in a 4.4 km-wide zone that is more extensive than previously documented. The SGC thus preserves a complicated history of variable orogen-parallel flow and folding at deep arc levels.