Micro-shear zones in experimentally deformed octachloropropane

Abstract

Localisation of deformation in rocks is known to be important at all scales in ductily deforming rocks. However, relatively little is known of the significance of shear localisation at small scales (<mm–cm), where ongoing deformation and recovery may obliterate any traces of such localisation. We investigated localisation of deformation in a <100 μm thick sample of the rock analogue octachloropropane in a transparent torsional deformation cell with a circular shear zone in which arbitrarily high values of simple shear can be achieved. Photomicrographs of the deforming sample were taken every 1–2 min over eight and five minute intervals, while the sample was deforming at a bulk shear-strain rate of 4.6×10 −4 s −1, after a steady-state microstructure was established. The distribution of deformation was determined using pattern matching on digitised photomicrographs. Localisation of deformation was observed in the form of anastomosing micro-shear zones on the grain and sub-grain scale (10–100 μm). These shears left no clear indicative microstructures. The micro-shear zones shifted through the material, partly along with migrating grain boundaries, causing a homogenisation of accumulated strain. The existence of such micro-shear zones can therefore not be determined easily by microstructural analysis of deformed material, even though in these experiments the micro-shear zones accommodated up to 75% of the total deformation. A second form of localisation, which occurs at the inside and outside of the shear zone and which is often observed in this type of experiment, was also noted, and can be accounted for by the friction between the sample and the confining glass plates.