Micro-shear zones in experimentally deformed octachloropropane

Localisation of deformation in rocks is known to be important at all scales in ductily deforming rocks. However, relatively little is known of the sig nificance of shear localisation at small scales ( < mm-cm), where ongoing d eformation and recovery may obliterate any traces of such localisation. We investigated localisation of deformation in a < 100 mu m thick sample of the rock analogue octachloropropane in a transparent torsional deformation cell with a circular shear tolls in which arbitrarily high values of simpl e shear call be achieved. Photomicrographs of the deforming sample were tak en every 1-2 min over eight and five minute intervals, while the sample was deforming at a bulk shear-strain rate of 4.6x10(-4) s(-1), after a steady- state microstructure was established. The distribution of deformation was d etermined 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 mu m). These shears le ft no clear indicative microstructures. The micro-shear zones shifted throu gh the material, partly along with migrating grain boundaries, causing a ho mogenisation of accumulated strain. The existence of such micro-shear zones can therefore not be determined easily by microstructural analysis of defo rmed material, even though in these experiments the micro-shear zones accom modated up to 75% of the total deformation. A second form of localisation, which occurs at the inside and outside of th e sheer zone and which is often observed in this type of experiment, was al so noted, and can be accounted for by the friction between the sample and t he confining glass plates. (C) 1999 Elsevier Science Ltd. All rights reserv ed.