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The Kuckaus Mylonite Zone (KMZ) forms part of the larger MarshallRocks–Pofadder shear zone system, a 550 km-long, crustal-scale strike-slipshear zone system that is localized in high-grade granitoid gneisses andmigmatites of the Namaqua Metamorphic Complex. Shearing along the KMZoccurred ca. 40 Ma after peak granulite-facies metamorphism during adiscrete tectonic event and affected the granulites that had remained atdepth since peak metamorphism. Isolated lenses of metamafic rocks within theshear zone allow the P–T–fluid conditions under which shearing occurredto be quantified. These lenses consist of an unsheared core that preservesrelict granulite-facies textures and is mantled by a schistose collar andmylonitic envelope that formed during shearing. All three metamafic texturalvarieties contain the same amphibolite-facies mineral assemblage, from whichcalculated pseudosections constrain the P–T conditions of deformation at2.7–4.2 kbar and 450–480 °C, indicating that deformation occurredat mid-crustal depths through predominantly viscous flow. CalculatedT–MH2O diagrams show that the mineral assemblages werefluid saturated and that lithologies within the KMZ must have beenrehydrated from an external source and retrogressed during shearing. Giventhat the KMZ is localized in strongly dehydrated granulites, the fluid musthave been derived from an external source, with fluid flow allowed by localdilation and increased permeability within the shear zone. The absence ofpervasive hydrothermal fractures or precipitates indicates that, even thoughthe KMZ was fluid bearing, the fluid/rock ratio and fluid pressure remainedlow. In addition, the fluid could not have contributed to shear zoneinitiation, as an existing zone of enhanced permeability is required forfluid infiltration. We propose that, following initiation, fluid infiltrationcaused a positive feedback that allowed weakening and continued strainlocalization. Therefore, the main contribution of the fluid was to produceretrograde mineral phases and facilitate grain-size reduction. Features suchas tectonic tremor, which are observed on active faults under similarconditions as described here, may not require high fluid pressure, but couldbe explained by reaction weakening under hydrostatic fluid pressureconditions

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