‘Brittle’ shear zones in experimentally deformed quartz single crystals

Intracrystalline shear zones developed in a quartz single crystal that was experimentally deformed to 26% finite strain under conditions favourable for macroscopically ductile deformation. The crystal was uniaxially compressed parallel to 〈c〉, in the presence of 1 vol.% of added water, at a temperature of 800 °C, a confining pressure of 1200 MPa and a strain rate of 10−6 s−1. We investigated how the shear zones initiated and how their structure evolved with ongoing deformation. Detailed crystallographic orientation analyses were carried out with the help of the electron backscatter diffraction (EBSD) technique and complemented by transmission electron microscopy (TEM). The shear zones appeared to develop along planar microcracks oriented parallel to crystallographic rhomb planes. Further microcracking resulted in cataclastic shear zone development. New grains developed by rotation of fracture fragments. The rotation of the fragments in the shear direction was probably facilitated by the high water pressure and the relatively high porosity in the shear zone. Shear zones finally exhibit a strong crystallographic preferred orientation (CPO). Our results suggest that cataclastic deformation processes may lead to shear zone development. The CPO developed within the shear zones through dynamic recrystallisation, i.e. new grains with 〈c〉 oriented approximately perpendicular to the shear zone boundary grew at the cost of other new grains.