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.