Factors Determining Shear‐Parallel Versus Low‐Angle Shear Band Localization in Shear Deformations: Laboratory Experiments and Numerical Simulations

Understanding the mechanics of shear localization in materials is a key to interpreting a wide range of failure-assisted geophysical processes. This paper addresses a crucial problem of shear band formation in ductile shear zones undergoing simple shear movement. We studied ductile shear zones in two geological terrains of Eastern India and observed extensive micro- to macro-scale shear-parallel deformation bands (C-bands) in them. However, laboratory-scale simple shear experiments on wet sand (Coulomb) and putty (viscoplastic) failed to produce C-bands. They developed shear bands in conjugate sets: low-angle R1 (10°–15°) and high-angle R2 (65°–75°) and single sets of low-angle shear bands (LSB), respectively. These contrasting findings between field and experiments motivate us to address the question- under what conditions can shear bands localize parallel to the bulk shear direction? With the help of 2D numerical models, based on viscoplastic rheology, we demonstrate C-band and LSB formation as two competing mechanisms of shear localization, mediated by a combined effect of the shear zone parameters: geometric (shear zone thickness), kinematic (bulk shear rate) and rheological (bulk viscosity). Finally, two non-dimensional factors: (a) dynamic (Ω: global shear stress /cohesive strength [Ci]) and (b) geometric (δ: effective thickness [Wc]/total shear zone thickness [W]) are recognized to show the fields of LSB and C-bands formation. We also provide a theoretical analysis of the shear band orientation as a function of the dynamic and geometric parameters.