Browsing by Author "Diener, JFA"

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    The metamorphic evolution of an ancient accretionary prism in the southern zone of the Damara Belt in Namibia
    (2013) Cross, Clayton Basil; Diener, JFA; Fagereng, A
    The Southern Zone of the Damara Belt in central Namibia has an apparent stratigraphic thickness that exceeds 100 km and is comprised of highly strained, metamorphosed clastic sedimentary rocks that are intercalated with slices of metamorphosed basalt and gabbro. One of the only modern geological environments in which such vast amounts of sediments can be accumulated, with intercalated mafic rocks, is in an accretionary prism above a subduction zone, where the accreted material is also subjected to high pressure – low temperature metamorphic conditions and deformation to high strains. This has been suggested as the origin of the Southern Zone. Samples collected from two localities within and immediately adjacent to the Southern Zone include representative rocks of both metapelitic and metamafic compositions. The inferred peak mineral assemblage in the metapelitic rocks consists of weak to moderately zoned garnet, staurolite and in some cases, kyanite porphyroblasts set in a fine-grained matrix of chlorite, biotite, muscovite, paragonite, epidote, ilmenite and quartz. The matrix exhibits a penetrative foliation that is defined by the alignment of the micaceous minerals. The garnet, staurolite and kyanite porphyroblasts overprint this fabric. By contrast, the metamafic rocks are very fine-grained with an inferred peak mineral assemblage of zoned amphibole, epidote, rutile, quartz, biotite and in some cases, chlorite and sphene. The amphibole and micaceous minerals define a distinct fabric. None of the minerals occur as porphyroblasts that overprint this fabric.
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    Shear zones of the Maud Belt, Antarctica : kinetics and deformation mechanisms
    (2014) McGibbon, David; Fagereng, Ake; Diener, JFA
    The rocks of the Maud Belt, western Dronning Maud Land, Antarctica, have experienced at least two deformation phases related to the Grenvillian and Pan- African orogenies. Deformation is heterogeneous and strain is commonly localized within shear zones. The two study areas H.U. Sverdrupfjella and Neumayerskarvet mainly consist of paragneisses and orthogneisses and in places migmatites and granite intrusions. The orthogneisses and paragneisses mainly consist of coarse to medium grained quartz, feldspar and biotite and in places hornblende, garnet and epidote. The rock types only differ in the proportion of these minerals, the paragneisses having a higher proportion of biotite than the orthogneisses. Both study areas contain well developed lineations, defined by elongated quartz grains and in places hornblende and epidote, and a S₁+₂ foliation defined by biotite. The foliation is related to the shear zones in the region which are commonly sub-horizontal but locally sub-vertical in eastern Neumayerskarvet. In eastern Neumayerskarvet the paragneisses wrap around the competent orthogneiss units, resulting in sub-vertical strike-slip shear zones alongside the competent orthogniess units. Two differently orientated lineations are found in the study areas, a weak, shallow plunging, E-trending lineation that occurs within the host rock and always alongside a well developed, shallow plunging, SE-trending lineation. Within the shear zones only the SE-trending lineation is found. The presence of only the SE-trending lineation in the shear zones implies that the SE-trending lineation is associated with a more recent deformation phase, D₂, and that the weak Etrending lineation is associated with an older deformation phase, D₁. Two major collisional events affected the region, the Grenvillian (~1300 Ma to ~900 Ma) and the Pan-African (~600 Ma to ~450 Ma). D₂ is therefore likely associated with the Pan-African orogeny and D₁ with the older Grenvillian orogeny. Evidence for D₁ is distributed broadly within the host rock and is absent from the shear zones. If D₁ localized shear zones did exist, they have been overprinted by D₂. The presence of only D₂ in the shear zones implies that strain in D₂ was localized. The strain partitioning into narrow shear zones during the more recent deformation phase could be due to pre-existing fabrics from an earlier deformation phase. Superposition of later deformation into zones of pre-existing fabrics could be typical of areas that have experienced multiple deformation phases.
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    The structural evolution of an ancient accretionary prism in the Damara Belt, Namibia
    (2015) Hartnady, Michael Ian Hay; Fagereng, Ake; Diener, JFA
    The Southern Marginal Zone (SMZ) of the Damara Belt, exposed in the Gaub Canyon in central Namibia, consists of fourteen lithotectonic units of high strain amphiholite facies rock with pelagic, hemi-pelagic and clastic sedimentary protoliths. These rocks are intercalated With lenses of metabasite. Regional high-pressure - low-temperature metamorphic conditions (~1O kbar and ~600°C) dominate the Southern and Southern Marginal Zones of the Damara Belt, leading to the interpretation that these tectonostratigraphic terranes formed in an accretionary prism along an ancient subduction margin. The structures in the SMZ are the result of progressive deformation, inferred to have initiated under low-grade metamorphic conditions (D₁) and evolved through prograde to peak metamorphism (D₂), ending in relatively low-temperature retrograde conditions (D₃). Each of the deformation phases is characterised by a foliation. D₁ is associated With pure shear dominated layer-parallel extension characterised by disrupted lithological layering and hedding-parallel foliation S₀+₁. D₃ is defined as deformation related to the formation of an axial-planar S₂ caused by folding of S₀+₁ around F₂ hinge lines. Widespread isoclinal recumbent folding resulted in transposition of these fahrics and the general foliation is thus termed S₀+₁+₂. This composite foliation contains a down-dip stretching lineation L₂. Folding was contemporaneous With top-to-the-SE directed thrusting in D₂ faults and shear zones that are seen to displace D₁ fabric. Fold hinge lines parallel to L₂ suggest D₂ is characterised by non-ideal simple shear. D₃ is defined by a crenulation cleavage S₃, at near right angles to S₀+₁+₂ foliation resulting from NW-SE pure shear shortening. This phase of deformation is also associated with retrograde, reverse faulting that is localised along some of the D₂ shear zones.