Strain distribution within a km-scale, mid-crustal shear zone: The Kuckaus Mylonite Zone, Namibia

Abstract

The subvertical Kuckaus Mylonite Zone (KMZ) is a km-wide, crustal-scale, Proterozoic, dextral strike-slip shear zone in the Aus granulite terrain, SW Namibia. The KMZ was active under retrograde, amphibolite to greenschist facies conditions, and deformed felsic (and minor mafic) gneisses which had previously experienced granulite facies metamorphism during the Namaqua Orogeny. Lenses of pre- to syn-tectonic leucogranite bodies are also deformed in the shear zone. Pre-KMZ deformation (D1) is preserved as moderately dipping gneissic foliations and tightly folded migmatitic layering. Shear strain within the KMZ is heterogeneous, and the shear zone comprises anastomosing high strain ultramylonite zones wrapping around less deformed to nearly undeformed lozenges. Strain is localized along the edge of leucogranites and between gneissic lozenges preserving D1 migmatitic foliations. Strain localization appears controlled by pre-existing foliations, grain size, and compositional anisotropy between leucogranite and granulite. The local presence of retrograde minerals indicate that fluid infiltration occurred in places, but most ultramylonite in the KMZ is free of retrograde minerals. In particular, rock composition and D1 fabric heterogeneity are highlighted as major contributors to the strain distribution in time and space, with deformation localization along planes of rheological contrast and along pre-existing foliations. Therefore, the spatial distribution of strain in crustal-scale ductile shear zones may be highly dependent on lithology and the orientation of pre-existing fabric elements. In addition, foliation development and grain size reduction in high strain zones further localizes strain during progressive shear, maintaining the anastomosing shear zone network established by the pre-existing heterogeneity.