Strain localization and fluid pathways in mylonite: inferences from in situ deformation of a water-bearing quartz analogue (norcamphor)

Abstract

Simple shearing of polycrystalline norcamphor containing 10–15 vol% water, at near-atmospheric pore fluid pressure and a range of constant temperatures (3–35°C) and shear strain rates (5×10 −5–4×10 −4 s −1), induces localization of both strain and fluid flow. Prior to deformation, the water is located at grain triple junctions and pockets along grain boundaries. It forms an average dihedral angle of 46° with the surrounding norcamphor grains. During initial shearing ( γ≤1), grain boundaries oriented subparallel to the principal shortening direction dilate and fill with water. At 1< γ<2, these open grain boundaries interconnect to form water-filled dilatant shear surfaces at low angles (10–15°) to the shear zone boundary. These surfaces resemble shear bands or C′ surfaces in mylonitic rock and, depending on the temperature, accommodate displacement by cataclasis ( T<15°C) or dislocation creep ( T>15°C). The tips of the shear surfaces propagate alternately by intracrystalline plasticity and subcritical fracturing, concomitant with dynamic recrystallization in the rest of the sample. The episodic interconnection of dilatant shear surfaces is associated with short-term increases in displacement rate parallel to the surfaces. These surfaces coalesce to form a high-strain, fluid-filled network subparallel to the experimental shear zone. However, this network never spans the entire length of the shear zone at any given time, even after shearing to γ=8.5. The deformation is more homogeneous and fewer dilatant shear surfaces develop at higher deformational temperatures and/or lower strain rates due to the increased activity of dislocation creep. Prolonged stress annealing removes most microstructural evidence of the syntectonic fluid pathways. Dilatant shear surfaces in norcamphor resemble relics of mica-filled, synmylonitic fractures in dynamically recrystallized quartz from greenschist facies mylonite, suggesting that fluid played a similar mechanical role in nature and experiment. The coalescence of dilatant, fluid-filled shear surfaces represents a strain-dependent increase in pore connectivity within mylonitic shear zones. The experiments indicate that fluid flow along deep crustal mylonitic shear zones is probably limited by the rate at which the tips of the dilatant shear surfaces propagate subparallel to the shearing plane.