Browsing by Author "Fagereng, Åke"

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    Controls on the diversity of the fault slip styles at the brittle-ductile transition: examples from the Cape Fold Belt, Nuy Valley, South Africa
    (2019) de Carvalho, Antónia; Diener, Johann; Fagereng, Åke
    Crustal deformation models have a first-order rheological division, with pressure-dependent brittle deformation predominating at shallow depths, and temperature-dependent viscous deformation occurring in the deeper levels of the crust. The brittle-ductile transition zone separates these two regimes, it occurs at approximately 350°C for quartz and it is characterised by mixed-mode brittle and viscous deformation. Complex fault zones exhumed to the surface may preserve evidence that can explain the mechanics and the complex slip behaviour of faults. Fault rocks response to applied shear stress is affected by environmental conditions during deformation (such as temperature and pressure), composition of fault zone, fluid presence and strain rate. Thus, the interplay of these factors determines the slip style of a specific fault and may lead to multiple slip styles that overprint each other. The Nuy Valley area in Worcester, Western Cape, South Africa, exposes a section through the deeper parts of the Cape Fold Belt, where the Malmesbury Group schists experienced thrust faulting in response to crustal shortening. Individual thrust faults are manifested in different ways, with quartz-cemented breccias, limestone mylonites, abundant quartz veining and cataclasites attesting to faulting occurring by a diversity of slip style, which allows investigating how the interplay of the controlling factors lead to the observed diversity of fault rock. Through mineral equilibria modelling, the pressure-temperature conditions under which faulting occurred was determined to lie between 5 - 8 KPa and 250 - 420C, with fluid content lines indicating low amounts of dehydration during peak metamorphism. The exhumed fault being analysed in this study was active at 10 - 15 km deep at 25C.km-1 geothermal gradient. The temperature over this transition is relatively constant and short ranged throughout geological evolution of Worcester and the cyclic superposition of ductile and brittle deformation and change in slip styles along fault zones as found in Nuy Valley cannot be justified by ambient temperature and pressure oscillations. Lithotype and competency of wallrocks play an essential role in deformation partitioning by being crucial determinants of rheological properties, and accounts for the coexistence of brittle and ductile fabrics but not for cyclic overprint of slip styles. Fluid presence is evidenced by an intense network of quartz veins and hydraulic breccias and contributes to the weakening and strengthening of wallrock during deformation. Slip style diversity in the study area is considered to the result of the interplay of compositional variabilities, fluid flow and strain rate variations associated with the seismic cycle.
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    Mid-crustal shear zone development under retrograde conditions: pressure–temperature–fluid constraints from the Kuckaus Mylonite Zone, Namibia
    (2016) Diener, Johann F A; Fagereng, Åke; Thomas, Sukey A J
    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.
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    Neotectonic deformation features in plio-pleistocene coastal aeolianites: palaeoseismology and earthquake hazard implications for the Southern Cape, South Africa
    (2013) Hodge, Matthew Sotheron; Fagereng, Åke
    Deformation features within Plio-Pleistocene Bredasdorp Group coastal aeolianites within the Southern Cape, South Africa, have been studied to resolve the neotectonic history of the region, estimate the recent to current upper-crustal stress field, and infer seismic hazard. Previous studies indicate possible Quaternary faulting within the Southern Cape, major faults within the underlying geology, and a horizontal maximum compressive stress (SHmax) orientated WNWESE to NW-SE. This study utilised geological mapping, small-scale fieldwork, and remote sensing to investigate joints, faults, kink folds, cataclasites, and soft-sediment deformation features within the aeolianites. The majority of these deformation features are found proximal to the inferred traces of the Blomerus and Struisbaai faults. In places along the Blomerus Fault, the aeolianite cross-beds dip at angles of 40°-90 °, as opposed to dips of <37 ° (the angle of repose for unconsolidated aeolian sands) observed in the rest of the study area.
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    South African research in the Southern Ocean: New opportunities but serious challenges
    (2013) Treasure, Anne M; Moloney, Coleen L; Bester, Marthán N; McQuaid, Christopher D; Findlay, Ken P; Best, Peter B; Cowan, Don A; de Bruyn, P J Nico; Dorrington, Rosemary A; Fagereng, Åke; Froneman, P William; Grantham, Geoff H; Hunt, Brian P V; Meiklejohn, K Ian; Pakhomov, Evgeny A; Roychoudhury, Alakendra N; Ryan, Peter G; Smith, Valdon R; Chown, Steven L; Ansorge, Isabelle J
    South Africa has a long track record in Southern Ocean and Antarctic research and has recently invested considerable funds in acquiring new infrastructure for ongoing support of this research. This infrastructure includes a new base at Marion Island and a purpose-built ice capable research vessel, which greatly expand research opportunities. Despite this investment, South Africa's standing as a participant in this critical field is threatened by confusion, lack of funding, lack of consultation and lack of transparency. The research endeavour is presently bedevilled by political manoeuvring among groups with divergent interests that too often have little to do with science, while past and present contributors of research are excluded from discussions that aim to formulate research strategy. This state of affairs is detrimental to the country's aims of developing a leadership role in climate change and Antarctic research and squanders both financial and human capital.