Epidote is a common hydrous mineral present in subduction zones subject to greenschist metamorphic conditions – and potentially an important control on the fault stability-instability transition observed under greenschist facies. We explore controls on this transition through shear experiments on simulated epidote gouge at temperatures of 100–500 °C, effective normal stresses of 100–300 MPa and pore fluid pressures of 30–75 MPa. We use rate-and-state friction to define these controls of temperature, effective stress and pore fluid pressure on gouge stability. Experimental results indicate that the epidote gouge is frictionally strong (μ ∼ 0.73) and the frictional strength is insensitive to variations in temperature or pressure. With increasing temperature, the epidote gouge exhibits a first transition from velocity-strengthening to velocity-weakening at sub-greenschist conditions (T < 100 °C) before transitioning to velocity-strengthening under greenschist metamorphic conditions (T > 300 °C). Elevating the pore fluid pressure or decreasing the effective stress promotes unstable sliding. The transition in gouge rheology at varied temperatures and pressures is explained by the competition between granular flow-induced gouge dilation and pressure solution-induced gouge compaction. Our results demonstrate that the rate-and-state frictional stability of epidote gouges support the potential for a fault stability-instability-stability transition for subduction under greenschist metamorphic conditions.

中文翻译：

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热液条件下绿帘石凿槽的速率-状态摩擦及其对绿片岩变质条件下俯冲断层稳定性的影响


绿帘石是一种常见的含水矿物，存在于受绿片岩变质条件影响的俯冲带中，并且可能是在绿片岩相下观察到的断层稳定性-不稳定性转变的重要控制。我们通过在 100-500 °C 的温度、100-300 MPa 的有效法向应力和 30-75 MPa 的孔隙流体压力下对模拟绿帘石气凿进行剪切实验来探索对这种转变的控制。我们使用速率和状态摩擦来定义温度、有效应力和孔隙流体压力对气刨稳定性的控制。实验结果表明，绿帘石凿孔具有摩擦强度 （μ ∼ 0.73），并且摩擦强度对温度或压力的变化不敏感。随着温度的升高，绿帘石凿孔在亚格林片岩条件下 （T < 100 °C） 首次表现出从速度增强到速度减弱的转变，然后在格林片岩变质条件下 （T > 300 °C） 过渡到速度增强。提高孔隙流体压力或降低有效应力会促进不稳定的滑动。不同温度和压力下气刨流变学的转变可以通过颗粒流诱导的气刨膨胀和压力溶液诱导的气刨压实之间的竞争来解释。我们的结果表明，绿帘石凿孔的速率和状态摩擦稳定性支持在格林片岩变质条件下俯冲的断层稳定性-不稳定-稳定性过渡的可能性。