Formation of Nanocrystalline and Amorphous Materials Causes Parallel Brittle‐Viscous Flow of Crustal Rocks: Experiments on Quartz‐Feldspar Aggregates

The brittle‐viscous transition in the lithosphere occurs in a region where many large earthquakes nucleate. To study this transition, we sheared bimineralic aggregates with varying ratio of quartz and potassium feldspar. We deformed the samples in a solid‐medium deformation apparatus at temperature, T  = 750°C and pressure, P c  = 800 MPa under either constant displacement rate or constant load boundary conditions. Under constant displacement rate, samples reach high shear stress ( τ  = 0.4–1 GPa depending on mineral ratio) and then weaken. Under constant load, the strain rate shows low sensitivity to stress below τ  ≈ 400 MPa, followed by a higher stress sensitivity (stress exponent, n  = 9–13) at higher stresses irrespective of mineral ratio. Strain is localized along “slip zones” in a C and C′ orientation. The material in the slip zones shows extreme grain size reduction and flow features. At peak strength, 1–2 vol% of the sample is composed of slip zones that are straight and short. With increasing strain, the slip zones become anastomosing and branching and occupy up to 9 vol%; this development is concomitant with strain‐weakening of the sample. Slip zones delimit larger cataclastic lenses, which develop a weak foliation. Our results suggest that strain localization leads to microstructural transformation of the rocks from a crystalline solid to a fluid‐like material in the slip zones. The measured rheological response is a combination of viscous flow in the slip zones and cataclastic flow in coarser‐grained lenses and can be modeled as a frictional slider coupled in parallel with a viscous dashpot.