Abstract
Abstract. The Kuckaus Mylonite Zone (KMZ) forms part of the larger Marshall Rocks–Pofadder shear zone system, a 550 km-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. 40 Ma 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 P–T–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 P–T conditions of deformation at 2.7–4.2 kbar and 450–480 °C, indicating that deformation occurred at mid-crustal depths through predominantly viscous flow. Calculated T–MH2O 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.
Highlights
Crustal-scale deformation is commonly localized into major faults, in the upper crust and ductile shear zones in the lower crust (e.g. Savage and Burford, 1973; Kirby, 1985; Zoback et al, 1985; Scholz, 1988; Wittlinger et al, 1998)
It is not clear what weakening mechanisms remain active through the life of a retrograde shear zone, as elevated fluid pressures are hard to maintain in an open system, but retrograde reactions may lead to the growth of fine-grained products and the formation of new interconnected weak fabrics. We address these questions here by investigating the retrograde metamorphic history of an exhumed, crustal-scale strike-slip shear zone, the Kuckaus Mylonite Zone in southern Namibia, to constrain its pressure– temperature–fluid history and discuss the implications of our results for weakening mechanisms and strain localization
The observed mineral assemblages lead us to conclude that the Kuckaus Mylonite Zone (KMZ) was fluid bearing during deformation, but the general absence of hydrofractures and hydrothermal precipitates indicate that fluid pressures and the fluid/rock ratio remained low
Summary
Crustal-scale deformation is commonly localized into major faults, in the upper crust and ductile shear zones in the lower crust (e.g. Savage and Burford, 1973; Kirby, 1985; Zoback et al, 1985; Scholz, 1988; Wittlinger et al, 1998). If a shear zone develops within dry, previously migmatized crust under retrograde conditions, there are unlikely to be brittle discontinuities on which a shear zone can initiate (Rennie et al, 2013), unless fine-grained pseudotachylytes have formed through propagation of earthquake ruptures into the viscous regime (Sibson, 1980; Moecher and Steltenpohl, 2009) or local brittle structures formed through fluid-absent, high-stress failure (Andersen et al, 2008) or acceleration within crystal-plastic shear zones (White, 2012) Under retrograde conditions, it is, not intuitive to envision how initial shear zone weakening occurs, as an external fluid is required for reaction weakening, but requires deformation to be introduced to the site of initiation.
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