Under upper crustal conditions, deformations are primarily brittle (i.e., localized) and accommodated by frictional mechanisms. At greater depth, deformations are ductile (i.e., distributed) and accommodated by crystal plasticity, diffusion mass transfer or cataclastic flow. The transition from the brittle to the ductile domain is not associated with a critical depth, but rather varies in time and space. One main parameter controlling the variation of this transition is the pore fluid pressure. On the one hand, a pore fluid pressure increase reduces the effective stresses and possibly increases the strain rate, bringing the system closer to brittle conditions. On the other hand, pore fluid can favor ductile mechanisms, mostly via chemical effects, by facilitating intra‐crystalline plasticity, enhancing fluid‐solid diffusion and fracture healing/sealing. We report triaxial laboratory experiments that investigated the effect of pore fluid pressure increase during the ductile deformation of Tavel limestone. Three injection rates were tested: 1, 5, and 10 MPa/min. We demonstrate that: (1) Under initially ductile conditions pore fluid pressure increase immediately turns the system from compaction to dilation. (2) Dilation is due to the development of localized shear fractures. However, the macroscopic localization of the deformation is not instantaneous when the ductile to brittle transition is surpassed; a transient creeping phase is first needed. (3) To reach macroscopic brittle failure of initially ductile samples, a critical dilatancy is required. (4) Injection rate controls the final fracture distribution. We demonstrate that pore pressure build‐up in a rock undergoing ductile deformation can induce shear fracturing of the system.

中文翻译：

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孔隙流体压力累积引起的球墨岩脆性断层

在上地壳条件下，变形主要是脆性的（即局部的），并通过摩擦机制来适应。在更大的深度处，变形是延性的（即分布的），并通过晶体可塑性，扩散质量传递或碎裂流来适应。从脆性区域到延性区域的过渡与临界深度无关，而是在时间和空间上变化。控制这一转变变化的一个主要参数是孔隙流体压力。一方面，孔隙流体压力的增加会减小有效应力，并可能会增加应变率，使系统更接近脆性条件。另一方面，孔隙液可通过促进晶体内可塑性来促进塑性机制，主要是通过化学作用，增强流固扩散和骨折愈合/密封。我们报告了三轴实验室实验，该实验研究了塔维尔石灰石延性变形过程中孔隙流体压力增加的影响。测试了三种注入速率：1、5和10 MPa / min。我们证明：（1）在最初的延性条件下，孔隙流体压力的增加立即使系统从压实变为膨胀。（2）膨胀是由于局部剪切裂缝的发展所致。但是，当超过韧性到脆性转变时，形变的宏观局部化不是瞬时的。首先需要一个瞬态蠕变阶段。（3）为了达到最初延展性样品的宏观脆性破坏，需要一个临界膨胀率。（4）注入速率控制最终的裂缝分布。