Mechanisms of shear band formation in heterogeneous materials under compression: the role of pre-existing mechanical flaws

Shear bands critically govern the shear failure processes and associated geophysical phenomena, e.g., faulting, in Earth’s crust. Earlier studies on homogeneous materials recognized temperature and strain rate as principal factors controlling the band growth. However, how inherent mechanical heterogeneities can influence their growth mechanisms and complex internal structures needs further investigations. The present article revisits this problem by combining field observations and laboratory experiments, supported by numerical simulations considering geological strain-rates. Field studies in the Chotanagpur Granite Gneissic Complex of eastern India reveal varied types of macro-scale shear zone localization. We conducted plane-strain compression experiments on rock-analogue models to investigate their origin. The experiments show that mechanical heterogeneities develop wide bands, localized preferentially in their neighborhood, unlike uniformly distributed, conjugate sets of closely spaced narrow bands in a homogeneous system. The wide bands eventually attain a composite structure, containing a core of densely packed band-parallel sharp secondary bands, flanked by linear zones of closely spaced, narrow bands. We also demonstrate the effects of global strain-rate on the band evolution in heterogeneous systems. Decreasing strain-rates replace the composite bands by well-defined homogeneous shear bands, containing a core of uniform shear, bordered by weakly sheared narrow zones, grading into almost undeformed walls. The experimental results are complemented with numerical models based on visco-elasto-plastic rheology, which establish appropriate scaling of the rock analogue to mid-crustal deformations. This integrated approach finally leads us to conclude that inherent heterogeneities can result in large variations of shear band structures in geological terrains.

剪切带（Shear bands）是控制地壳内剪切破裂过程及相关地球物理现象（如断层作用）的关键要素。早期针对均质材料的研究认为，温度与应变速率是控制剪切带生长的核心影响因素。然而，固有力学非均质性如何作用于剪切带的生长机制与复杂内部结构，仍有待进一步探究。
本文结合野外观测与室内实验，并辅以考虑地质应变速率的数值模拟，重新探讨了这一科学问题。对印度东部乔塔纳格普尔花岗片麻岩杂岩（Chotanagpur Granite Gneissic Complex）的野外调查显示，该区域存在多种宏观尺度的剪切带局部化类型。我们通过开展岩石类比模型的平面应变压缩（plane-strain compression）实验，对剪切带的成因展开研究。实验结果表明，力学非均质性会催生宽幅剪切带，且剪切带优先在非均质性邻域内局部发育，这与均质体系中均匀分布、共轭排列的密集窄剪切带特征截然不同。
宽幅剪切带最终会形成复合结构：其核心区域为密集排布、与带体平行的尖锐次级剪切带，两侧则环绕着由密集窄剪切带构成的线性弱变形区。此外，我们还验证了全局应变速率对非均质体系中剪切带演化的调控作用：当应变速率降低时，复合剪切带会转变为特征清晰的均质剪切带，这类剪切带以均匀剪切核心为主体，边缘被弱剪切窄带包裹，并逐渐过渡至几乎未发生变形的围岩。
实验结果辅以基于黏弹塑性流变学（visco-elasto-plastic rheology）的数值模型，该模型实现了岩石类比模型与地壳中部变形的合理尺度匹配。这一综合研究方法最终证实，固有力学非均质性可导致地质区域内剪切带结构产生显著差异。