Journal cover Journal topic
Solid Earth An interactive open-access journal of the European Geosciences Union
Journal topic

Journal metrics

Journal metrics

  • IF value: 2.380 IF 2.380
  • IF 5-year value: 3.147 IF 5-year
    3.147
  • CiteScore value: 3.06 CiteScore
    3.06
  • SNIP value: 1.335 SNIP 1.335
  • IPP value: 2.81 IPP 2.81
  • SJR value: 0.779 SJR 0.779
  • Scimago H <br class='hide-on-tablet hide-on-mobile'>index value: 32 Scimago H
    index 32
  • h5-index value: 31 h5-index 31
Volume 7, issue 5
Solid Earth, 7, 1331-1347, 2016
https://doi.org/10.5194/se-7-1331-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Solid Earth, 7, 1331-1347, 2016
https://doi.org/10.5194/se-7-1331-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 16 Sep 2016

Research article | 16 Sep 2016

Mid-crustal shear zone development under retrograde conditions: pressure–temperature–fluid constraints from the Kuckaus Mylonite Zone, Namibia

Johann F. A. Diener1, Åke Fagereng1,*, and Sukey A. J. Thomas1 Johann F. A. Diener et al.
  • 1Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
  • *present address: School of Earth & Ocean Sciences, Cardiff University, Cardiff CF10 3AT, UK

Abstract. The Kuckaus Mylonite Zone (KMZ) forms part of the larger Marshall Rocks–Pofadder shear zone system, a 550km-long, crustal-scale strike-slip shear zone system that is localized in high-grade granitoid gneisses and migmatites of the Namaqua Metamorphic Complex. Shearing along the KMZ occurred ca. 40Ma after peak granulite-facies metamorphism during a discrete tectonic event and affected the granulites that had remained at depth since peak metamorphism. Isolated lenses of metamafic rocks within the shear zone allow the PT–fluid conditions under which shearing occurred to be quantified. These lenses consist of an unsheared core that preserves relict granulite-facies textures and is mantled by a schistose collar and mylonitic envelope that formed during shearing. All three metamafic textural varieties contain the same amphibolite-facies mineral assemblage, from which calculated pseudosections constrain the PT conditions of deformation at 2.7–4.2kbar and 450–480°C, indicating that deformation occurred at mid-crustal depths through predominantly viscous flow. Calculated TMH2O diagrams show that the mineral assemblages were fluid saturated and that lithologies within the KMZ must have been rehydrated from an external source and retrogressed during shearing. Given that the KMZ is localized in strongly dehydrated granulites, the fluid must have been derived from an external source, with fluid flow allowed by local dilation and increased permeability within the shear zone. The absence of pervasive hydrothermal fractures or precipitates indicates that, even though the KMZ was fluid bearing, the fluid/rock ratio and fluid pressure remained low. In addition, the fluid could not have contributed to shear zone initiation, as an existing zone of enhanced permeability is required for fluid infiltration. We propose that, following initiation, fluid infiltration caused a positive feedback that allowed weakening and continued strain localization. Therefore, the main contribution of the fluid was to produce retrograde mineral phases and facilitate grain-size reduction. Features such as tectonic tremor, which are observed on active faults under similar conditions as described here, may not require high fluid pressure, but could be explained by reaction weakening under hydrostatic fluid pressure conditions.

Publications Copernicus
Download
Citation
Share