Strain localization in homogeneous granite near the brittle-ductile transition: A case study of the Kellyland fault zone, Maine, USA

We present a detailed case study of a strike-slip shear zone that cut homogeneous granite near the brittle-ductile transition. This zone contains three distinct strain facies: (1) a 2-3-km-wide belt of foliated granite, (2) a 100-300-m-wide belt of small localized shear zones, and (3) a 200-400-m-wide belt of ultramylonite that includes both homogeneous and local pinstripe ultramylonites. The foliated granite formed via dislocation creep of quartz and minor dissolution-precipitation creep and dislocation creep of feldspar under amphibolite-facies conditions shortly after granite crystallization. The localized shear zones and homogeneous ultramylonites formed directly from pseudotachylyte and cataclasite, and they deformed by granular flow-grain-boundary sliding coupled with dissolution-precipitation creep. These deformation features reveal a three-phase rheologic evolution in granite near the brittle-ductile transition. Phase 1 is recorded by the foliated granite, and the rheology was governed by dislocation creep of quartz. Phase 2 was a transient period of brittle deformation and represents a temporal strength maxima in the fault zone. Phase 3 was a long-lived period of ductile deformation, and the rheology was governed by the granular flow mechanism. Strain weakening in this zone was a direct result of grain size reduction and phase mixing during transient brittle deformation.