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

Abstract

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.

Model for deformation-induced martensitic transformation in irradiated materials

“…The parameter δ characterizing the overall level of martensite volume fraction from shear band intersections is fitted to the data of the unirradiated case. The pre-existing nucleation site density Npre is approximated by quantitative stereology assuming the shear bands to be randomly oriented thin plates [20,63]. Note that the resulting Npre is close to the value used in a previous work [24].…”

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

“…Previous works have used a wide spectrum of rheologies in shear zone modeling, ranging from power‐law viscous (Fleitout & Froidevaux, 1980; Yuen et al., 1978; Yuen & Schubert, 1977) to complex visco‐elasto‐plastic rheology (Babeyko & Sobolev, 2008; Gerya & Yuen, 2007; Kaus, 2010). However, visco‐plastic and elasto‐plastic rheologies are perhaps the most commonly used rheological classes (Duretz et al., 2019; Roy, Saha, & Mandal, 2022; Roy, Roy, et al., 2022). Both of them have certain drawbacks for ductile shear zone modeling.…”

“…We develop 2D finite element (FE) models using material properties approximated to those of granitic rocks in the CGGC terrain. The FE modeling uses the open‐source, particle‐in‐cell, finite element code UNDERWORLD2 (http://www.underworldcode.org/) that solves the incompressible Stokes equations in combination with the energy conservation equation (Mansour et al., 2020; Moresi et al., 2007; Roy, Saha, & Mandal, 2022; Roy, Roy, et al., 2022). We choose a rheological model, represented by a combination of an elastic (spring), a plastic (frictional block) and a viscous (dashpot) element in series (Figure 7b).…”

“…Geological observations show that shear localization in rocks under crustal pressure‐temperature conditions accommodate a large amount of plastic creep in a heterogeneous manner and produce a complex architecture, characterized by arrays of multiple shear bands (Mancktelow & Pennacchioni, 2005; Meyer et al., 2017; Rogowitz et al., 2016; Scholz & Choi, 2021; Snyder & Kjarsgaard, 2013; Vauchez et al., 2012). These bands generally show a wide variation in their geometry, displaying narrow and sharp to wide and diffused boundaries (Pennacchioni & Mancktelow, 2007; Roy, Saha, & Mandal, 2022; Roy, Roy, et al., 2022). In a geological terrain they usually occur as a complex system with their varying spatial densities, for example, uniformly distributed numerous (Katz et al., 2004) to a few localized shear zones/bands (Fusseis & Handy, 2008).…”

“…The mechanics of band formation turns to be more complex in geological settings due to involvement of several additional factors, such as pore fluid pressure, synkinematic rheological transformations on geological time scales and time‐dependent rheological complexities. To deal with such geological problems, geoscientists have developed a range of numerical models on geological time scales (Beall et al., 2019; Gerya & Yuen, 2007; Meyer et al., 2017; Popov & Sobolev, 2008; Roy et al., 2021; Roy, Saha, & Mandal, 2022; Roy, Roy, et al., 2022; Willis et al., 2019). A comprehensive review of these studies is provided in Regenauer‐Lieb and Yuen (2003).…”