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, 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. Plain Language Summary: In Earth's lithosphere, tectonic deformations often concentrate preferentially along narrow, linear regions to form shear bands/zones, which trigger a range of geophysical phenomena, like earthquakes. Understanding the origin of shear bands is thus a subject of great concern in geophysics. Earlier studies mainly addressed the problem of band formation in homogeneous media. However, geological terrains are usually heterogeneous due to various inherent entities, like fractures, melt pockets and foreign objects. We present a set of field examples to highlight different types of shear bands produced in such heterogeneous geological settings. From laboratory experiments on analogue materials our study demonstrates the role of pre‐existing weak bodies in determining the modes of band formation under compressive stresses. The experimental results enable us to explain band growth with simple as well as complex internal structures. The deformation rate in the heterogeneous system is found as another critical factor to decide the band structures, where increasing strain rate results in transformation from simple to complex internal patterns. We include real‐scale numerical simulations to show the suitability of our experimental material for reproducing geological band structures under laboratory conditions. These two parallel lines of investigations lead us to a generalized conclusion of our study. Key Points: Control of inherent mechanical heterogeneity on shear strain localizationRole of global strain rate on the modes of shear band formationDiverse types of bands in natural shear zones inferred from plane‐strain compression tests on heterogeneous models [ABSTRACT FROM AUTHOR]