Coupled Brittle and Viscous Micromechanisms Produce Semibrittle Flow, Grain‐Boundary Sliding, and Anelasticity in Salt‐Rock
Coupled Brittle and Viscous Micromechanisms Produce Semibrittle Flow, Grain‐Boundary Sliding, and Anelasticity in Salt‐Rock
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DOI：
10.1029/2020jb021261
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发表时间：
2021-01
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影响因子：
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通讯作者：
Ji-hui Ding;F. Chester;J. Chester;Xianda Shen;C. Arson
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作者：
Ji-hui Ding;F. Chester;J. Chester;Xianda Shen;C. Arson
岩石半脆性变形过程中断裂、扩散和晶内塑性微机制的运作与理解地壳中脆塑性转变的力学行为直接相关。一个突出的问题是：(1)半脆性流动的微观机制是否可以被认为是独立运作的，就像典型的地壳强度曲线所代表的那样，跨越脆性到塑性的转变，或者(2)微观机制是耦合的，这样的转变就由依赖于有效压力、温度和应变率的独特流变学来代表。我们采用三轴应力循环实验来研究两种完全不同的单矿物、多晶、合成盐岩在半脆性流动中的弹塑性和粘弹性行为。在高差应力下的半脆流变过程中，颗粒状、低孔隙度、工作硬化的盐岩主要通过晶界滑动和翼裂张开变形，并伴有轻微的晶内位错滑动。相比之下，完全退火、接近零孔隙度的盐岩在较低的差应力下以晶内位错滑移的方式流动，同时伴有晶界滑动和打开。在高应变率的半脆流动过程中，晶界滑动是摩擦的，但伴随的水从流体包裹体沿晶界扩散可以在低应变率下激活流体辅助扩散滑动。半脆性流动中弹性性能的变化主要反映了沿封闭晶界滑动的激活。观察到的微观结构，半脆性流动后循环应力过程中明显的滞后和非弹性，以及应力松弛行为表明微观机制的耦合作用导致了不同的流变（上述假设2）。
The operation of fracture, diffusion, and intracrystalline‐plastic micromechanisms during semibrittle deformation of rock is directly relevant to understanding mechanical behavior across the brittle‐plastic transition in the crust. An outstanding question is whether (1) the micromechanisms of semibrittle flow can be considered to operate independently, as represented in typical crustal strength profiles across the brittle to plastic transition, or (2) the micromechanisms are coupled such that the transition is represented by a distinct rheology with dependency on effective pressure, temperature, and strain rate. We employ triaxial stress‐cycling experiments to investigate elastic‐plastic and viscoelastic behaviors during semibrittle flow in two distinctly different monomineralic, polycrystalline, synthetic salt‐rocks. During semibrittle flow at high differential stress, granular, low‐porosity, work‐hardened salt‐rocks deform predominantly by grain‐boundary sliding and wing‐crack opening accompanied by minor intragranular dislocation glide. In contrast, fully annealed, near‐zero porosity salt‐rocks flow at lower differential stress by intragranular dislocation glide accompanied by grain‐boundary sliding and opening. Grain‐boundary sliding is frictional during semibrittle flow at higher strain rates, but the associated dispersal of water from fluid inclusions along boundaries can activate fluid‐assisted diffusional sliding at lower strain rates. Changes in elastic properties with semibrittle flow largely reflect activation of sliding along closed grain boundaries. Observed microstructures, pronounced hysteresis and anelasticity during cyclic stressing after semibrittle flow, and stress relaxation behaviors indicate coupled operation of micromechanisms leading to a distinct rheology (hypothesis 2 above).