Evolution of compactive shear deformation bands: Numerical models and geological data

Networks of localized compactive and isovolumetric shear bands were generated in 2-D finite-difference models. They reproduce the most striking feature of the evolution of natural deformation bands characterized by compactive (or non-dilatant) inelastic volumetric deformation, i.e., the band thickening from the flanks by incorporation of the initially intact host rock. Such a thickening was obtained in the models where the hardening modulus h grew with inelastic deformation. This growth causes strengthening of the material within the initial bands resulting from deformation bifurcation. The strengthening above a certain level causes band widening due to the accretion to its edges of a not yet deformed material as it becomes involved in compactive shearing. The inelastic deformation is therefore the most rapid along the band flanks, while the band core part mainly undergoes elastic unloading and thickens due to the incorporation of new strands of sheared and compacted material. The initial band spacing depends on the initial h value and grows with it in accordance with predictions from bifurcation theory. During the post bifurcation deformation, the spacing reduces due to both the formation of new and propagation of the existent bands. The band pattern in a layer is also shown to be dependent on the properties and thickness of the adjacent layers, causing acceleration of deformation localization, reduction of the band spacing, and a periodic clustering of the bands of different orientation groups. The band patterns generated resemble the natural band networks. The increase of h imposed in the models appears thus as both an important and realistic property. Therefore the adjustment/calibration of the constitutive models based on the reproduction of natural deformation patterns in numerical simulations represents an important tool. (C) 2011 Elsevier B.V. All rights reserved.