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 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 samples solid-medium deformation apparatus at temperature, T = 750°C pressure, Pc 800 MPa under either constant displacement rate or load boundary conditions. Under rate, reach high shear stress (τ 0.4–1 GPa depending on mineral ratio) then weaken. load, strain shows low sensitivity to below τ ≈ 400 MPa, followed by higher (stress exponent, n 9–13) stresses irrespective ratio. Strain is localized along “slip zones” C C′ orientation. material slip zones extreme grain size reduction flow features. At peak strength, 1–2 vol% sample composed that are straight short. With increasing strain, become anastomosing branching occupy up 9 vol%; development concomitant strain-weakening sample. Slip delimit larger cataclastic lenses, which develop weak foliation. Our results suggest localization leads microstructural transformation rocks from crystalline solid fluid-like zones. measured rheological response combination viscous coarser-grained lenses can be modeled as frictional slider coupled parallel dashpot.