Mechanisms of Shear Band Formation in Heterogeneous Materials Under Compression: The Role of Pre‐Existing Mechanical Flaws

Abstract

AbstractShear bands critically govern the shear failure processes and associated geophysical phenomena, for example, 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.