Frictional melting of peridotite and seismic slip
Frictional melting of peridotite and seismic slip
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DOI：
10.1029/2008jb005990
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发表时间：
2009-06-13
影响因子：
3.9
通讯作者：
Cavallo, A.
中科院分区：
文献类型：
--
作者：
Del Gaudio, P.;Di Toro, G.;Cavallo, A.
地震滑动速率（约 1 m/s）下沿断层的摩擦强度的演变是控制地震力学的关键因素。在地幔深处，摩擦引起的熔化和熔化润滑可能会影响地震滑移和地震数据。我们报告了旨在研究地幔岩石动态断层强度和摩擦熔融过程的实验室实验。我们在高速旋转剪切装置和圆柱形样品（直径 21.8 mm）中对 Balmuccia 橄榄岩进行了 20 次实验，实验范围广泛，包括法向应力 (5.4-16.1 MPa)、滑移率 (0.23-1.14 m/s) 和位移 (1.5-71 m)。在实验过程中，剪切应力随着五个主要阶段（阶段 1-5）的累积位移而变化。在第一阶段（第一次强化），摩擦系数μ增加到0.4-0.7（摩擦的第一个峰值）。在第 2 阶段（突然第一次减弱），mu 下降至约 0.25-0.40。在第 3 阶段（逐渐第二次强化），剪切应力向摩擦力的第二个峰值增加（mu = 0.30-0.40）。在第 4 阶段（逐渐第二次减弱），剪切应力下降至稳态值（第 5 阶段），mu = 0.15。第一阶段和第二阶段的持续时间太短，无法用当前的实验配置进行详细研究。通过中断第 3、4 和 5 阶段的实验，对滑移区的微观结构（场发射扫描电子显微镜）和地球化学（电子探针显微分析仪和能量色散 X 射线光谱）分析表明，第二次强化（第 3 阶段）与晶粒支撑的贫熔层的产生有关，而第二次弱化（第 4 阶段）和稳态（第 5 阶段）与连续富熔层的形成有关。估计温度高达 1780 摄氏度。实验过程中形成的微观结构与天然超镁铁质拟速晶石中发现的微观结构非常相似。通过在不同的法向应力和滑移率下进行实验，(1) 从强度的第一个峰值达到稳定状态的“热”（因为它包括热激活的第一和第二次弱化）滑移距离随着法向应力和滑移率的增加而减小，(2) 稳态剪应力随着法向应力的增加而略有增加，并且对于给定的法向应力，随着滑移率的增加而减小。剪应力与法向应力的比率约为 0.15，远低于岩石的典型摩擦系数(0.6-0.8)。通过熔体润滑的本构方程描述了稳态剪切应力与法向应力的依赖性。与天然假速晶岩中发现的微观结构相似的存在以及描述实验数据的本构方程的确定允许将实验观察结果外推到自然条件和地幔岩石破裂动力学的研究中。
The evolution of the frictional strength along a fault at seismic slip rates (about 1 m/s) is a key factor controlling earthquake mechanics. At mantle depths, friction-induced melting and melt lubrication may influence earthquake slip and seismological data. We report on laboratory experiments designed to investigate dynamic fault strength and frictional melting processes in mantle rocks. We performed 20 experiments with Balmuccia peridotite in a high-velocity rotary shear apparatus and cylindrical samples (21.8 mm in diameter) over a wide range of normal stresses (5.4-16.1 MPa), slip rates (0.23-1.14 m/s), and displacements (1.5-71 m). During the experiments, shear stress evolved with cumulative displacement in five main stages ( stages 1-5). In stage 1 ( first strengthening), the coefficient of friction mu increased up to 0.4-0.7 ( first peak in friction). In stage 2 ( abrupt first weakening), mu decreased to about 0.25-0.40. In stage 3 ( gradual second strengthening), shear stress increased toward a second peak in friction (mu = 0.30-0.40). In stage 4 ( gradual second weakening), the shear stress decreased toward a steady state value ( stage 5) with mu = 0.15. Stages 1 and 2 are of too short duration to be investigated in detail with the current experimental configuration. By interrupting the experiments during stages 3, 4, and 5, microstructural ( Field Emission Scanning Electron Microscope) and geochemical ( Electron Probe Micro-Analyzer and Energy Dispersive X-Ray Spectroscopy) analysis of the slipping zone suggest that second strengthening ( stage 3) is associated with the production of a grain-supported melt-poor layer, while second weakening ( stage 4) and steady state ( stage 5) are associated with the formation of a continuous melt-rich layer with an estimated temperature up to 1780 degrees C. Microstructures formed during the experiments were very similar to those found in natural ultramafic pseudotachylytes. By performing experiments at different normal stresses and slip rates, ( 1) the "thermal'' ( as it includes the thermally activated first and second weakening) slip distance to achieve steady state from the first peak in strength decreased with increasing normal stress and slip rate and ( 2) the steady state shear stress slightly increased with increasing normal stress and, for a given normal stress, decreased with increasing slip rate. The ratio of shear stress versus normal stress was about 0.15, well below the typical friction coefficient of rocks (0.6-0.8). The dependence of steady state shear stress with normal stress was described by means of a constitutive equation for melt lubrication. The presence of microstructures similar to those found in natural pseudotachylytes and the determination of a constitutive equation that describes the experimental data allows extrapolation of the experimental observations to natural conditions and to the study of rupture dynamics in mantle rocks.