Formation of parallel joint sets and shear band/fracture networks in physical models

• Published in: Tectonophysics 2012-12, Vol.581, p.84-92
• Main Authors: Jorand, C., Chemenda, A.I., Petit, J.-P.
• Format: Article
• Language: English
• Subjects: Bands; Compressive strength; Conjugates; Crack initiation; Deformation bands; Dilation; Earth Sciences; Environmental Sciences; Extension tests; Fracture mechanics; Geophysics; Global Changes; Instability; Joints; Mathematical models; Networks; Physics; Rock mechanics; Sciences of the Universe; Slip bands
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/j.tecto.2011.11.021

Both oedometric and plane-strain tests were performed with parallelepipedic samples made of synthetic granular, cohesive, frictional and dilatant rock analogue material GRAM2. For the first time parallel sets of fractures that have all the characteristics of natural joints were reproduced in the laboratory. The fractures are regularly spaced, normal to σ3, and have plumose morphology very similar to that of natural joints. These fractures can form at tensile stress σ3 much smaller in magnitude than the tensile strength of material and even at slightly compressive σ3. When mean stress σ exceeds a certain value, the fractures become oblique to σ1 (the obliquity increases with σ), forming networks of conjugate shear bands/fractures. These results of plane-strain experiments are in good agreement with those of better controlled conventional axisymmetric tests on a similar material in Chemenda et al. (2011b) and are closer to real geological situations. Both types of experiments are complementary. Their results lead to the conclusion that at least certain categories of natural fractures (including joints, and conjugate shear fractures/bands) were initiated as deformation localization bands. The band orientation is defined by the constitutive properties/parameters (notably the dilatancy factor) that are sensitive to σ. ► Synthetic rock analogue material was fractured under plane-strain extension. ► Fracture mechanism and fractography depend on the effective mean stress P. ► With P increase the mechanisms are: mode I cracking, dilatancy, and shear banding.