Browsing by Author "Fagereng, Ake"

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    Metamorphic and melt-migration history of midcrustal migmatitic gneisses from Nupskapa, The Maud Belt, Antarctica
    (2014) Thomas, Sukey Anna Jay; Diener, Johann; Fagereng, Ake
    Melt migration is an important process in the crust that causes significant mass transport, as well as differentiation and stabilisation of continental crust. Melt migration near the source occurs pervasively, through interconnected networks of melt-bearing structures. This style is restricted to the suprasolidus mid- to lower crust, while focused migration and ascent of magma occurs in isolated dykelike structures under subsolidus conditions, generally in the upper crust where brittle fracturing of rocks can occur. The details of how and when melt migration changes from a pervasive to focused style are poorly understood, particularly the temperature, pressure and deformation conditions which allow the transition to occur. The Nupskapa nunatak, in Dronning Maud Land of East Antarctica, exposes large cliffs that record evidence of multiple episodes of melt movement, in the form of pervasive leucogranite vein networks cross-cut by larger leucogranite dykes. Mineral equilibria modelling with THERMOCALC and comparison of results with previous work indicates that the Nupskapa nunatak records both Grenvillian and Pan-African metamorphism. Coarse-grained peak assemblages in samples from the Nupskapa area record conditions of 820-880 C at 9.5-11.6 kbar, while post-tectonic retrograde assemblages record late Pan-African conditions of 555- 595 C at 3.2{4.8 kbar. These later conditions lie between the wet solidus and the brittle-viscous transition and are inferred to represent the conditions of intrusion for post-tectonic composite dykes. Small-scale leucosomes predominantly lie parallel to the gneissic host rock fabric and define a pervasive network across the Nupskapa cliff. These leucosomes exhibit diffuse feathery boundaries and are inferred to represent in situ melting and melt segregation during M1 granulite facies peak metamorphism. Composite leucogranitic dykes cross-cut both the early leucosome phase and Pan-African shear zones in the field area. These north-trending, subvertical dykes are neariii orthogonal to the gneissic fabric. They are 0.5-2 m wide and spaced ~10-20 m apart but not interconnected except where two dykes coalesce. The dykes show almost no shear displacement, indicating that they formed via tensile fracture. This indicates that their intrusion occurred during extensional or strike-slip deformation, under conditions of low differential stress, probably coupled to high melt pressure. The composite dykes resulted from the far-field transport of melt from a source 5 to 15 km below the Nupskapa outcrop. Although individually they are discrete and focused structures, they are numerous across the field area and closely spaced, so together they do not represent a wholly focused melt transfer system. The style of melt migration displayed by the composite dykes is an example of the transition from pervasive to focused migration, occurring in the mid-crust at subsolidus conditions. This transition involved a network of smaller melt-filled fractures gradually coalescing into larger ones with decreasing depth. If pervasive migration becomes focused via this gradual transition, melt accumulation and mixing need not occur solely in the source or final emplacement structure, but rather occurs throughout transport of the magma.
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    Microseismic observations in the Ceres-Tulbagh aftershock zone, Western Cape, South Africa, and their tectonic implications
    (2013) Smit, Louis; Fagereng, Ake
    South Africa is considered a stable continental region where earthquakes from a tectonic source are usually of small to moderate magnitude. In September 1969 a local magnitude (ML) 6.3 sinistral strike-slip earthquake occurred near the towns of Ceres and Tulbagh in the Western Cape, South Africa. This is still the largest earthquake in instrumental history in South Africa. During 2012, a temporary seismic array of 15 surface stations was deployed for a 3 month period in the aftershock zone of the 1969 Ceres earthquake. A total of 168 microseismic events could be located within the boundaries of the array, roughly 30 km x 40 km. Travel times of these events were used for coupled hypocenter-velocity inversion to produce a one dimensional velocity model with station corrections. The hypocenters of recorded events were relocated using the velocity model and local magnitude was empirically derived for all 168 events. P- and S-wave velocity ranges from 4.9 km/s to 6.4 km/s and 2.8 km/s to 4.7 km/s, respectively, from the surface down to 12 km depth. The magnitude of the microseisms ranges from -2.2 < ML < 1.6 with a magnitude of completeness of Mc -1.5, and follow a Gutenberg-Richter distribution with a b -value of 0.9. The microseismic events occurred down to a depth of 15 km within a sub-vertical fault zone roughly 4 km wide, striking SE-NW and passing below the towns of Ceres and Tulbagh. Seismic events appear to occur in two clusters ranging from 0 to 5 km and 8 to 12 km depth, respectively, separated by a 4 km along-strike discontinuity in seismic activity. There is good agreement between the orientations of the strike of the surface trace of the 1969 aftershock plane and the strike of the surface trace of the microseismic plane. Microseismic activity is attributed to the reactivation in basement structures of either the Malmesbury Group, or the Namaqua Natal Metamorphic complex from far field stress transfer from the Southwest Indian Ridge. It is proposed, albeit on speculation, that the presence of microseismic activity along the vertically oriented fault zone could be the manifestation of an incipient plate boundary formation.
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    Pre-rift evolution of Malawian high-grade basement rocks
    (2017) Huang, Leslie; Diener, Johann; Fagereng, Ake
    There is some controversy in terms of the basement geology of Malawi which ultimately stems from the overall lack of metamorphic studies conducted in the area. The geological complexity of Malawi comes from that fact that it sits at the intersection of three major orogenic belts: The Palaeoproterozoic Ubendian Belt, Mesoproterozoic Kibaran/Irumide Belt, and Pan African Mozambique Belt. Its complexity makes it difficult to unravel, especially in terms of identifying features of older orogenic events which have already experienced multiple metamorphic overprinting from subsequent events. This thesis provides a more detailed pre-rift evolution of the Malawian basement rocks by reporting ages and P-T conditions from four localities surrounding Lake Malawi, namely Chilumba, Mlowe, Maganga, and Mangochi. Results reveal that at 1985-1974 Ma, garnet-cordierite granulites were equilibrated under conditions of 760°C at 4.5-5 kbar possibly as a result of subduction-related magmatism. Subsequently, at 1100 Ma, charnockites were emplaced and metamorphosed under peak conditions of 770-780°C at 4.3-6 kbar due to Kibaran-age magmatic underplating. Remnants of the Irumide/Kibaran Orogeny is relatively scarce throughout Malawi and although the Mangochi charnockites were emplaced during Kibaran-age tectonism, it also experienced at least two different metamorphic events thereafter. The first occurred either during early stages of the East African orogen or Rodinia break-up at 900-800 Ma while the second occurred during the late stages of the East African orogen at 650-600 Ma. Possible remnants of the Kuunga Orogeny are recorded in Chilumba and Maganga as an amphibolite facies metamorphic event which took place around 570 Ma under peak conditions of roughly 660-670°C at 6-8 kbar. Findings of this study have not only provided a more detailed metamorphic history of Malawi but also paved way for future studies in the area to further explore why similar rocks found in such close proximity to each other preserve vastly different tectonic environments.
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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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    A structural and geochemical traverse across the NW outcrop of the Colenso Fault Zone, Saldanha, South Africa
    (2016) Hamel, Kaylan; Fagereng, Ake; Harris, Chris
    The Colenso Fault Zone marks the boundary between the Tygerberg and Swartland Terranes in the Pan-African Saldania Orogenic Belt. The fault zone comprises several discrete shear discontinuities that in places cross-cut relatively undeformed granites of the Cape Granite Suite. It is NW-SE striking, ~150 km long, and ~7km wide, but poorly exposed except in coastal exposures at the NW end. The deformation sequence can be divided into 2 sequences: The oldest deformation started with the emplacement of the G1, G2 and then the G3 of the Cape Granite Suite, followed by formation of aplite veins and strike-slip faulting generating cataclasites. The initial strike-slip sense of movement along the fault was sinistral, followed by dextral strike-slip shearing and finally late stage jointing. The Colenso Fault Zone is host to 3 large zones of cataclasis that are in the order of up to a few hundred metres in exposed down-dip and along-strike lengths. The cataclasites are composed of quartz and plagioclase clasts, in a phyllosilicate matrix. Both the wide cataclasites (several metres) and small cataclasite zones (tens of centimeters) show a decrease in shear intensity away from the core of the fault zone. The cataclasites have the same bulk chemical composition as the surrounding granite.
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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.
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    Tectono-metamorphic history of the re-worked, high-grade Maud Belt at central-Eastern H.U. Sverdrupfjella, Antarctica
    (2015) Byrnes, Gregory; Diener, Johann; Fagereng, Ake
    The reworking of granulites by amphibolite- to granulite-facies metamorphism can complicate the interpretation of their geological history because the event that reached higher peak P-T conditions will either completely overprint earlier peak assemblages or prevent the formation of new 'peak' minerals. The extent of reworking in granulites is controlled by three main factors, namely: (1) the pressures and temperatures reached in earlier and later metamorphic events, (2) the extent of deformation during subsequent events, and (3) the amount of fluid influx into the system during subsequent metamorphic events. Extensive reworking will occur if the peak temperature of the later event exceeds that of the earlier event, but if it does not, reworking will be less pervasive, and restricted to areas of deformation and/or fluid influx. The Salknappen nunatak in central-Eastern H.U. Sverdrupfjella, Antarctica forms a part of the highgrade Maud Belt that was formed by a granulite facies Grenvillian orogeny and was variably overprinted by high-grade metamorphism (eclogite to amphibolite facies) during the PanAfrican orogeny. The degree of reworking during the Pan-African has been a contentious issue for some time, with early workers assigning the metamorphic peak to the Grenvillian, whereas others assigned it to the Pan-African. Mineral assemblages and textures preserved in metapelitic and metamafic rocks preserve evidence of only one prograde to retrograde metamorphic cycle with peak mineral assemblages that are characteristic of granulites. Sillimanite in metapelitic rocks forms pseudomorphs after kyanite whereas garnet breakdown microstructures and in both metapelitic and metamafic rocks formed as a result of near-isothermal decompression. Garnet and hornblende display retrograde zoning profiles whereas retrograde cummingtonite, hornblende, plagioclase and ilmenite in metamafic rocks moderately constrain retrograde conditions. Pseudosection modelling with THERMOCALC on peak mineral assemblages from metapelitic and metamafic samples collected at Salknappen provides a robust peak P-T estimate (M1) of 760 – 790 ºC at 8.5 – 10 kbar. Phase diagram modelling of more subtle retrograde assemblages constrain retrograde metamorphic conditions (M2) to between ~550 – 750 °C and ~2 – 5 kbar. Both M1 and M2 likely occurred during the Grenvillian in a single orogenic cycle along a clockwise metamorphic path, where peak metamorphism was followed by near-isothermal decompression of ~5 kbar. Recrystallised quartz in melt leucosomes confirms that retrogression (M2) occurred after peak metamorphism. M2 was followed by the intrusion of megacrystic leucogranite dykes that most likely formed during the Pan-African in response to iii melt migration as a result of melting deeper in the crust. These dykes and earlier gneisses were intruded by the Dalmation granites at c. 470 Ma, at which point the Salknappen nunatak was at crustal conditions approximating the brittle-ductile transition. The study area in central-Eastern H.U. Sverdrupfjella preserves the peak and retrograde metamorphic assemblages from the Grenvillian orogeny and does not display evidence of reworking by a later granulite facies event. Salknappen does not display evidence of reworking during the Pan-African because peak metamorphism did not exceed peak temperatures attained during the Grenvillian orogeny and also did not form discrete, localised deformation zones with a significant influx of fluid during the Pan-African orogeny. This study presents a case where the effects of mid-crustal reworking by a high-grade metamorphic event are not shown due to the lack of rehydration, pervasive deformation and an elevated residuum solidus as a result of higher peak temperatures in an earlier granulite facies metamorphic event. When working with polymetamorphic terranes that have been subjected to more than one granulite facies orogenic cycle, the interpretation of the geological history of such an area should be done with caution and P-T estimates should be done with methods that are less affected by the long retrograde histories.
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    Tectonometamorphic evolution of Medium-P granulites of the Namaqua Metamorphic Province at the Gordonia Subprovince marginal zone, southern Namibia
    (2017) Sebetlela, Teboho; Diener Johann Ferdinand, August; Fagereng, Ake
    The western Namaqua Metamorphic Complex (NMC) is a Mesoproterozoic mediumto low-pressure, high temperature metamorphic belt that is characterised by a complex, polyphase Mesoproterozoic metamorphic history. Using an integrated approach, the P-T-t evolution of a portion of the marginal zone between two major tectonic domains in the western NMC: the Richtersveld and Gordonia Subprovices is investigated, with the aim of resolving a P-T-t path for the peak to retrograde evolution of a major regional thrust, that will in turn help to elucidate its geodynamic significance. The Kum Kum Klippe represents a structural outlier of the Gordonia Subprovince, located at the frontal zone of the Grunau Terrane, where the Grunau Terrane (hangingwall), Pella Terrane (footwall) and the bounding Lower Fish River Thrust Zone (LFRTZ) are all well exposed. The Pella Terrane in this region shows signi ficant lithological heterogeneity and is constituted by felsic orthogneisses, metama fic rocks and leucogranite intrusives, while the Grunau Terrane is dominated by pelitic granulites. The LFRTZ along the southwestern margin of the Kum Kum Klippe is a ~30-40 m wide tabular zone primarily localised in the Pella Terrane. Phase equilibria modelling of pelitic granulites from the hangingwall constrains peak metamorphic P-T conditions of ~5.2-5.9 kbar and ~790-815°C which are correlated to a monazite growth event at c. 1262-1184 Ma. Sheared pelitic granulites constrain the P-T conditions for early stage shearing in the LFRTZ to ~2.8-5.0 kbar and ~640-785°C which likely occurred shortly after peak metamorphism. Pella amphibolite samples from the footwall and LFRTZ constrain the conditions of tectonic juxtaposition to ~2.7 kbar and ~542°C which must have occurred subsequent to cooling from suprasolidus conditions and is thus constrained at <1184 Ma. The constrained peak to retrograde P-T-t path is characterised by a short segment of substantial decompression subsequent to peak metamorphic conditions at c. 1262-1184 Ma. This is followed by a period dominated by cooling with only a small component of concomitant decompression, which together with the early decompressive segment likely correspond to the period of retrograde shearing, which ultimately led to tectonic juxtaposition in the mid-amphibolite facies at c. <1184 Ma. The P-T path presented suggests that the LFRTZ does not represent a terrane bounadry that juxtasposes crustal entities which converged and collided as the result of Wilson cycle subduction to collision tectonics. Contrastingly the metamorphism at highly elevated geotherms and largely cooling dominated retrograde trajectory determined in this study are more compatible with a continental backarc setting, which has recently been proposed as an alternative model. Thus the P-T constraints presented in this study contradict the collisional model and terrane concept that has long been widely accepted for the western NMC.