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.

中文翻译：

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纳米晶体和非晶材料的形成导致地壳岩石平行的脆性-粘性流动：石英-长石聚集体的实验

岩石圈中的脆性粘性转变发生在许多大地震成核的区域。为了研究这种转变，我们剪切了具有不同比例的石英和钾长石的双矿物骨料。我们 在恒定位移速率或恒定载荷边界条件下，在温度T  = 750°C和压力P _c = 800 MPa的条件下，在固体介质变形仪中对样品进行变形。在恒定的位移速率下，样品达到高剪切应力（τ  = 0.4-1 GPa，取决于矿物质的比例），然后减弱。在恒定负载下，应变率对低于τ≈400  MPa的应力表现出较低的敏感性，随后具有较高的应力敏感性（应力指数，n = 9–13）在更高的应力下，与矿物比例无关。应变沿着“滑动区域”以C和C'方向定位。滑移区的材料显示出极小的晶粒尺寸减小和流动特征。在峰值强度下，样品的1-2 vol％由直的和短的滑移区组成。随着应变的增加，滑移区变为吻合和分支，并占到9 vol％。这种发展伴随着样品的应变减弱。滑移区划定了较大的碎裂晶状体，形成了较弱的叶片。我们的结果表明，应变局部化导致滑移区岩石从微晶固体转变为类似流体的材料。