Browsing by Author "Diener, Johann"

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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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    High-temperature metamorphism in the western Namaqua-Natal Metamorphic Province (South Africa): implications for low-pressure granulite terranes
    (2019) Schorn, Simon; Diener, Johann
    The interpretation of pressure (P)-temperature (T)-time (t) data is key to reconstruct the geologic evolution of ancient exhumed orogens. Of these, temperature is regarded as proxy for the heat energy available during orogenic metamorphism. Heat - the ultimate driving force of metamorphism - is consumed by a number of processes occurring during orogeny. Metamorphic rocks evolve through chemical reactions that consume energy in order to advance. As long as strongly energy-intensive processes advance in the crust, the temperature that is effectively attained is controlled by the interplay between the rate of heat energy supplied to the site of reaction and the rate of heat consumed by the process(es). Melting reactions in particular are strongly endothermic and consume a substantial proportion of the orogenic heat budget. Fertile metapelites are volumetrically minor but petrologically significant, as P-T-t-deformation constraints and burial-exhumation paths are preferentially derived from this lithology, whereas refractory granites sensu lato compose the bulk of orogenic crust. Metapelites are affected by a number of heat-consuming reactions that cause a near-isothermal state as long as they advance, however granites intersect few endothermic melt-producing reactions during orogenic metamorphism. Granitoids therefore may effectively reach a higher temperature compared to metapelites exposed to the same heat input. Temperature determined from metapelites may therefore not represent the true thermal maximum experienced by a portion of metamorphic crust - especially in granulite terranes where partial melting is widespread. Because temperature is an effect rather than the cause of heat energy transferred into the crust, it may not represent the most geodynamically relevant parameter to describe metamorphism. In this study a suite of supracrustal litholgies from the Bushmanland Subprovince (BSP) of the Namaqua-Natal Metamorphic Province (NNMP) of southern Africa are investigated via thermodynamic modelling and zircon-monazite U-Pb in-situ geochonology. Regional P-T-t distribution reveals a complex polyphasic evolution with two major tectonometamorphic episodes at ∼1.2-1.1 and 1.04-1.0 Ga, respectively. The older event is temporally linked to the emplacement of the felsic pre- to syn-tectonic Little Namaqualand Suite that caused widespread greenschist- to amphibolite facies contact metamorphism. This is recorded in garnet-cordierite-sillimanite gneisses exposed at the northernmost locality investigated here and as cm-sized porphyroblasts (e.g. andalusite) hosted in rare Mg-Al-rich gneisses. The younger event is characterised by granulite facies metamorphism peaking at 1040{1000 Ma in the entire BSP, with pelitic granulites recording variable apparent temperatures of between ∼760 and <900 ◦C at 5-6 kbar. Coarse-grained porphyroblasts of andalusite in an example of Mg-Al-rich gneiss were replaced by symplectites of high-grade phases during heating at the granulite event. At upper-greenschist facies conditions the immobility of Al caused the formation of monomineralic coronae at the expense of andalusite, effectively isolating the porphyroblasts from the matrix. Diffusion of Fe + Mg along respective gradients of µFe and µMg with simultaneous immobility of Al + Si led to the breakdown of andalusite to symplectites of cordierite + spinel during near-isobaric heating to peak conditions. Monomineralic coronae of sapphirine developed during near-isobaric cooling at the expense of previously-formed symplectites. Detailed investigation of a conformable sequence of sedimentary and mafic granulites from the locality Hytkoras in central Bushmanland reveal a disparity of some 60-70 ◦C in estimated peak metamorphic temperature. Aluminous metapelites equilibrated at ∼770-790 -C whereas two-pyroxene granulite and garnet-orthopyroxene-biotite gneiss record distinctly higher conditions of ∼830-850 -C. Semipelite and Mg-Al-rich gneisses yield poorly-constrained estimates that span the range derived from other lithologies. All samples record peak pressure of ∼5-6 kbar, and followed a roughly isobaric heating path from andalusite-greenschist / lower-amphibolite facies conditions through a tight clockwise loop at near-peak conditions, followed by near-isobaric cooling. The disparity in peak temperatures appears to be robust, as the low-variance assemblages in all samples reflect well-known melting reactions that only occur over narrow temperature intervals. The stable coexistence of both products and reactants of these melting reactions indicates that they did not go to completion before metamorphism waned. Calculated pressure-enthalpy diagrams show that the melting reactions are strongly endothermic and therefore buffer temperature while heat is consumed by melting. Because the respective reactions occur at distinct P-T conditions and have different reactant assemblages, individual lithologies are thermally buffered at different temperatures and to different degrees, depending on the occurrence and abundance of reactant minerals. If little to no thermal communication is assumed, this implies that lithology exerts a first-order control over the heating path and the peak temperature that can be attained for a specific heat budget. Calculations show that all lithologies received essentially the same suprasolidus heat budget of 19 ± 1 kJ.mol−1, which led to manifestation of lower peak temperatures in the more fertile and strongly buffered aluminous metapelites compared to more refractory rock types. The heat source responsible for near isochronous high-grade metamorphism at a regional scale most likely was a mixture of mafic mantle underplating combined with radiogenic heating in a slowly buried juvenile crust. Heat transfer to higher levels of the crust was aided by advective heating via the syn-metamorphic emplacement of the post-tectonic Spektakel Suite that locally led to near-ultrahigh temperature (UHT) conditions recorded by proximal pelitic granulites. The overall characteristics of the BSP, namely (i) heating from shallow low-grade conditions to granulite facies temperatures at a maximum depth of ∼20 km along a roughly isobaric heating path with minor concomitant burial, (ii) seemingly coeval peak metamorphism recorded at a regional scale that was accompanied/preceded by voluminous felsic magmatism (iii) that was followed by (iv) slow, largely isobaric cooling to an ambient geotherm with no significant attending exhumation, are consistent with a long-lived evolution in a continental back-arc mobile belt setting.
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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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    Mineral Equilibrium Constraints on the Feasibility of Diffusively-Fluxed Melting in the Continental Crust
    (2019) Tafur, Lorena Andrea; Diener, Johann
    Generation of granitic magma predominantly occurs by melting through the breakdown of hydrous minerals. However, melting due to the influx of water has been recognised in anatectic amphibolite-facies composite grey gneisses, metagreywackes and low-pressure metapelites, and has consequently been proposed as an alternative mechanism for granite generation and crustal differentiation. Water-fluxed melting is recognised by voluminous melt production at relatively low temperature, where hydrous minerals are stable and anhydrous minerals are preferentially consumed during melting. Mineral equilibrium modelling to determine the P–T conditions, melt volumes, melting reactions and viable fluid sources reveal that water-present melting in all target lithologies is confined to the wet solidus and does not extend to temperatures higher than 700–710 ◦C. Melting at suprasolidus conditions does not involve the mechanical flow of a free water phase. Instead, the process is driven by diffusion of H2O along chemical potential gradients and is therefore more appropriately described as diffusively-fluxed melting. Diffusively-fluxed melting is not restricted to specific compositions or P–T conditions, although it is more efficient at lower pressure and in lithologies with a low hydrous mineral content. Melting reactions in all lithologies primarily consume quartz and feldspars to yield 5–6 mol.% melt for each mol.% of H2O added. aH2O remains constant at ∼0.70–0.77 during progressive melting as long as alkali feldspar is present. Once alkali feldspar is exhausted, plagioclase becomes the main reagent, producing more tonalitic melt compositions with gradually higher aH2O. Melting will initiate and proceed as long as a µH2O gradient exists between the fluid source and target lithology. Our calculations show that an ordinary magma, such as an I-type magma with typical, undersaturated H2O content, has a µH2O high enough to be a viable fluid source, allowing diffusively-fluxed melting to produce melt volumes and fertility comparable to that of dehydration melting. However, voluminous melt production requires a considerable volume of H2O, which necessitates a focussed fluid source such as a magma conduit or melt-bearing shear zone. Any other magmatic fluid source will undergo a similar amount of crystallisation as the melt fraction produced in the target rock, such that there will be no net melt production. Considering that shear-zone hosted magma conduits are relatively rare, diffusively-fluxed melting appears to only be viable in a small fraction of the anatectic orogenic crust. Therefore, whereas it may play a significant role in locally raising melt volumes and modifying magma chemistry through mingling and hybridisation, it does not appear to, of itself, be able to meaningfully contribute to crustal differentiation.
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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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    Processes of felsic melt migration through the mid-crust : evidence from field relations in the central zone of the Damara Belt, Namibia
    (2012) Faber, Carly; Diener, Johann
    The migration of granitic melt is the main mechanism that facilitates upward movement of heat and mass, and the chemical differentiation of the continental crust. Whereas the processes of melt segregation and emplacement are relatively well understood, melt ascent mechanisms are more speculative. Specific outstanding questions include the structure of melt conduits, the driving forces of melt ascent, and the timescales involved. The Central Zone of the Damara Belt presents a snapshot of melt migration through subsolidus, mid-crustal rocks. Outcrops selected for detailed investigation are representative of a variety of mestasedimentary rock types and strain environments, and all contain pervasive and interconnected leucosome networks representative of melt movement through, and emplacement into these rocks.
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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.