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

Abstract

AbstractUnderstanding 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.