STRAIN LOCALIZATION IN HOMOGENOUS GRANITIC ROCKS AT THE BRITTLE-PLASTIC TRANSITION: A CASE STUDY OF THE KELLYLAND FAULT ZONE, WASHINGTON COUNTY, MAINE

Understanding the strength of the crust at the brittle-plastic transition is key to developing models of plate-scale behavior. To this end, we present a case study of the Kellyland fault zone. In this zone feldspar was not altered and major element concentrations remained constant. The Kellyland fault zone contains an outer belt of localized cm–dm-scale ultramylonite zones cutting foliated granite and a central ultramylonite zone. These zones contain two ultramylonite facies—pinstripe ultramylonites and homogeneous ultramylonites. Homogenous ultramylonites contain an ultra-fine-grained (<5 mm) feldspar + biotite matrix, feldspar porphyroclasts, and boudinaged and folded quartz bodies. Feldspar porphyroclasts have recrystallized tails that grade into the matrix, and quartz is 30–100% recrystallized. Pinstripe ultramylonites contain 0.1–5-mm-wide feldspar, quartz, and biotite ribbons. Feldspar in pinstripe ultramylonites is 30–95% recrystallized, and quartz is ~100% recrystallized. Highly recrystallized feldspar ribbons are not boudinaged. In all rocks, feldspar records grain boundary bulging and quartz records subgrain-rotation recrystallization. The localized shear zones primarily contain homogeneous ultramylonite. Their contacts often truncate individual mineral grains, and they occasionally root into discrete fractures. The central zone contains both ultramylonite types interlayered at the cm–m scale with gradational contacts.

We interpret the homogeneous ultramylonite as recrystallized cataclasite +/- pseudotachylyte, but we see no evidence for polyphase brittle overprinting. Therefore, formation of homogeneous ultramylonite was catalyzed by brittle deformation, but brittle deformation ceased once the matrix formed. The pinstripe ultramylonite probably formed after deformation was localized in the homogeneous ultramylonites. The ultra-fine-grained feldspar + biotite matrix is the weakest component in the system. Both quartz and feldspar deformed plastically at greenschist-facies conditions, and there is little apparent strength difference between recrystallized aggregates of the two phases. These results indicate that the quartz dislocation creep flow law is not a good approximation of the strength of granitic rocks at the brittle-plastic transition.