A POSSIBLE NATURAL EXAMPLE OF BRITTLE-TO-VISCOUS STRAIN LOCALIZATION UNDER NEAR-CONSTANT-STRESS CONDITIONS

The NE-striking Kellyland fault zone (KFZ) is part of the crustal-scale, strike-slip Norumbega fault system in Maine, USA. Granite cut by the KFZ in our study area records a range of fault processes and deformation mechanisms across the brittle-viscous transition. Solid-state deformation began shortly after granite crystallization and was localized across a series of mappable domains during cooling. From SE to NW across strike these domains are: (1) a 2–3-km-wide band of variably foliated granite, (2) a 50–300-m-wide band of pulverized foliated granite hosting cm-scale mylonitic shear zones and coeval brittle faults, and (3) a >100-m-wide band of granite-derived ultramylonite. The early foliation in domain 1 is defined by elongated quartz grains, and quartz dislocation creep was the rate-controlling mechanism during this early phase. Seismogenic deformation overprints the early foliation in domain 2. Seismogenesis initiated when flow stresses recorded by recrystallized quartz reached 96–104 MPa at temperatures of 400–450 °C, and it is recorded by pervasive pulverization and coeval formation of breccia and minor pseudotachylyte along the margins of domain 3. Interseismic viscous creep at ~100 MPa flow stresses is recorded by mutual cross-cutting relationships between breccia-hosted mylonitic shear zones, brittle fractures, and pseudotachylyte. Field and microstructural observations indicate that breccia-hosted shear zones are low-strain equivalents of the domain-3 ultramylonite, and seismogenic deformation abated as ultramylonite formed. The rheology of domain 3 ultramylonites was governed by grain-size-sensitive (GSS) creep at 112–124-MPa flow stresses. We interpret these higher flow stresses as the time-averaged equivalent of differential stress during seismic cycles. Hence, the KFZ was likely a constant-stress system from the onset of seismogenic deformation wherein the rate-controlling mechanism shifted from episodic seismogenic slip and interseismic viscous creep to steady state GSS creep in ultramylonite derived from brittle fault rocks. Flow stresses recorded by these rocks also imply that the whole KFZ was relatively weak if the brittle-viscous transition and uppermost viscous zone are the strongest part of the crust.