Strain distribution within a km-scale,mid-crustal shear zone: The Kuckaus Mylonite Zone,Namibia |
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| Affiliation: | 1. Department of Geology, Gombe State University, Nigeria;2. Department of Physics, Ahmadu Bello University, Zaria, Nigeria;3. Center for Energy Research and Training, Ahmadu Bello University, Zaria, Nigeria;1. School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China;2. Yunnan Geological Survey, Kunming 650051, China;1. State Key Laboratory for Mineral Deposits Research, Institute of Continental Geodynamics, Department of Earth Sciences, Nanjing University, Nanjing 210093, China;2. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;3. Département des Génies Civil, Géologique et des Mines, École Polytechnique de Montréal, Montréal, Québec, H3C 3A7, Canada;4. Development and Research Center of China Geological Survey, Beijing 100037, China;1. Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, SO14 3ZH, UK;2. Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany;3. GNS Science, Private Bag 1930, Dunedin 9054, New Zealand;4. Department of Geology, University of Otago, Dunedin 9016, New Zealand;1. Department of Geodynamics and Sedimentology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria;2. OMV Exploration and Production GmbH, Trabrennstrasse 6-8, 1020 Vienna, Austria |
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| 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. |
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| Keywords: | Shear zones Strain localization Ductile deformation Rheology High strain zones Mylonites |
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