Structure of the Skagit Gneiss Complex in Diablo Lake area, North Cascades, WA

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, composed mostly of partially migmatitic amphibolite-facies orthogneiss, banded biotite gneiss, and paragneiss. The typically NW-striking foliation and mostly gently SE-plunging lineation formed dominantly between 69 and 51 Ma. Four-fold generations are recorded in the study area, some of which formed from 51 to 46 Ma. The prominent upright km-scale folds are similar to Eocene folds in the southern part of the Skagit Gneiss Complex and suggest at least a short interval of regional shortening during an extended period of overall transtension. Latest ductile deformation is marked by strong subhorizontal constrictional fabrics in granodiorite, which intrudes all other major units and structures at 46-45 Ma. Microstructures record relatively low-temperature (300-400º C) and medium- to high-temperature (>450º C) ductile deformation, which are focused in different km-scale domains. Orientations, sequences, and timing of structures are similar in the northern and southern portions of the Skagit Gneiss Complex, but structures are different in orientation and apparently do not record the switch in direction of non-coaxial shear in the central portion.