On the origin of shear-band network patterns in ductile shear zones

Ductile yielding of rocks and similar solids localize shear zones, which often show complex internal structures due to the networking of their secondary shear bands. Combining observations from naturally deformed rocks and numerical modelling, this study addresses the following crucial question: What dictates the internal shear bands to network during the evolution of an initially homogeneous ductile shear zone? Natural shear zones, observed in the Chotonagpur Granite Gneiss Complex of the Precambrian craton of Eastern India, show characteristic patterns of their internal shear band structures, classified broadly into three categories: type I (network of antithetic low-angle Riedel (R) and synthetic P-bands), type II (network of shear-parallel C and P/R bands) and type III (distributed shear domains containing isolated undeformed masses). Considering strain-softening rheology, our two-dimensional viscoplastic models reproduce these three types, allowing us to predict the condition of shear band growth with a specific network pattern as a function of the following parameters: normalized shear zone thickness, bulk shear rate and bulk viscosity. This study suggests that complex anastomosing shear-band structures can evolve under simple shear kinematics in the absence of any pure shear component.

High-strain zones: laboratory perspectives on strain softening during ductile deformation

Deformation in the Earth’s outer shell is mostly localized into narrow high-strain zones. Because they can have displacements up to several hundreds or thousands of kilometres, they can affect the entire lithosphere. The properties of high-strain zones control the kinematics and dynamics of our planet, and are therefore of key importance for an understanding of plate tectonics, stress accumulation and release (e.g. earthquakes), mountain building, etc.

One of the requirements of shear zone formation in ductile rocks is localized strain softening (Hobbs et al. 1990). In this paper we review the strain softening mechanisms that were identified and proposed 25 years ago and analyse their relevance in light of recent experimental results conducted to large strains. For this purpose, some of the newer developments in experimental deformation techniques that permit high strain in torsion are summarized and recent results are reviewed. Using these results we discuss mechanisms, processes and conditions that lead to localization.