Electron backscatter diffraction data for mylonites from the Kuckaus Mylonite Zone, Namibia

Electron backscatter diffraction was used on thin sections of three mylonites (protomylonite, mylonite, ultramylonite) from the Kuckaus Mylonite Zone, Namibia, cut parallel to lineation and perpendicular to foliation. The dataset includes four Channel 5 sample sets (.cpr and .crc), one for each mylonite and one for a porphyroclast pressure shadow from the protomylonite. The raw data were used in the analysis for the publication mentioned below in order to obtain crystallographic preferred orientations, internal misorientations of grains, and grain size data. Minerals analysed include quartz, K-feldspar, and plagioclase. The data were collected using a Zeiss Sigma HD Field Emission Gun Analytical Scanning Electron Microscope at the Electron Microbeam facility in the School of Earth and Ocean Sciences at Cardiff University.

The data were used in the following publication:

DOI: 10.1029/2019GL083388

http://doi.org/10.1029/2019GL083388

Title:  Weaker than weakest: on the strength of shear zones

Abstract:

Thin, laterally continuous ultramylonites within kilometer-scale ductile shear zones may control mid-lower crustal strength where deformation is localised. Interconnected phyllosilicate networks are commonly suggested to be the weakest geometry a shear zone can reach, yet fine-grained polyphase mixtures are commonly found in the cores of high strain zones. We study a continental strike-slip shear zone which deformed granulite facies quartzofeldspathic migmatitic gneisses at retrograde amphibolite- to greenschist facies conditions. A brittle feldspar framework and interconnected phyllosilicate networks control the strength of the lower strain protomylonites and mylonites respectively, whereas the ultramylonites comprise a fine-grained mixture of the host rock minerals. The progressive localisation of strain into ultramylonites demonstrates how fine-grained polyphase mixtures can become weaker than, and supersede, interconnected phyllosilicate networks with increasing shear strain. This contradicts the common assumption that interconnected layers of phyllosilicates is the weakest state a shear zone can reach.