STRUCTURE OF THE SOUTH-CENTRAL SKAGIT GNEISS COMPLEX: IMPLICATIONS FOR TRANSIENT WEAKNESS IN THE MID-CRUST OF A CONTINENTAL MAGMATIC ARC

The Skagit Gneiss Complex (SGC) in the Cretaceous – Eocene North Cascades crystalline core is composed of tonalitic to granodioritic orthogneiss intruded into paragneiss that is partially migmatitic. Peak P-T conditions in the northern portion of the SGC record the highest-grade metamorphism in the North Cascades at >650˚C and ~8-10 kbar. Geologic mapping, structural and microstructural analysis, and U/Pb geochronology of a portion of the south-central SGC allowed for comparison of strain among rocks of different ages during Cretaceous –Eocene NW-SE stretching. Contacts of sub-units of the SGC are subparallel to the stretching direction. Orthogneiss emplaced from ~79-73 Ma is exposed in the middle of the study area and is migmatitic to the west where it intrudes paragneiss. Younger, ~60 Ma orthogneiss is exposed to the east. Small amounts of orthogneiss with ~50-48 Ma crystallization ages intrude all units. Medium-temperature sub-grain rotation and low-temperature bulging microstructures in the ~79-73 gneiss are associated with weak to moderately well-developed foliation and lineation. One to two meter-wide, E-W striking, sinistral oblique mylonitic shear zones are preferentially developed in ~79-73 Ma gneiss. Migmatitic orthogneisses display high-temperature grain boundary migration recrystallization of quartz. Mesoscale folds are most common in migmatite, particularly in paragneiss. Hinges of folds in paragneiss range from perpendicular to parallel to the NW-SE stretching direction. Orthogneiss formed at ~60 Ma or from ~50-48 Ma have strong constrictional fabrics; lineations plunge gently SE or less commonly NW. Quartz in ~60 Ma and younger orthogneiss displays grain boundary migration overprinted by lower-temperature sub-grain rotation and bulging microstructures. Sub-solidus fabrics locally overprint parallel magmatic fabrics which all strike NW. Map-scale folds with SE trending hinges are most abundant in ~60 Ma and younger orthogneisses. Kinematic indicators throughout the SGC dominantly reflect top-to-NW shear, but locally show top-to-SE or pure shear. These observations suggest that strain was preferentially accommodated in gneiss emplaced ~60 Ma and later.