Geological Society, London, Special Publications, 2005
... of Geological 1 and Statistical 2 Sciences, University of Cape Town, Rondebosch 7701, South A... more ... of Geological 1 and Statistical 2 Sciences, University of Cape Town, Rondebosch 7701, South Africa (e-maik [email protected] ... of the Bushveld Complex that intruded the centre of this craton in the Palaeoproterozoic (2.05-2.06 Ga; Cawthorne & Walraven 1998; Eglington ...
Amalgamation of a number of continental fragments during the Late Neoproterozoic resulted in a un... more Amalgamation of a number of continental fragments during the Late Neoproterozoic resulted in a united Gondwana continent. The time period in question, at the end of the Precambrian, spans about 250 million years between ∼800 and 550 Ma. Geological activity focused along orogenic belts in Africa during that time period is generally referred to as “Pan African.” We identify three age-related classes
Interpretation of the detailed patterns of ocean-floor transforms revealed by satellite altimetry... more Interpretation of the detailed patterns of ocean-floor transforms revealed by satellite altimetry enables the creation of the Indian Ocean to be described quantitatively as four consecutive plate-tectonic regimes separated at 200, 136, 89 and 43 Ma. Each regime is reversed in turn by keeping transform termini coincident and colinear until conjugate points on the margins of pre-existing plates regain their pre-regime integrity. Progressive elimination of the Indian Ocean, demonstrable as a smooth computer animation (http://www.kartoweb.itc.nl/gondwana), leads to a refined re-assembly of the continental fragments of central Gondwana that is substantiated by new geological data. A sequence of Euler interval poles that describes the dispersal of the Gondwana fragments, time-calibrated against available magnetic anomaly data, is given. The model requires a mid-Cretaceous position for India’s southern tip about 1000 km south of Madagascar, prior to India’s rapid northward migration.
Southern Africa's topography is distinctive. An inland plateau of low relief and high average ele... more Southern Africa's topography is distinctive. An inland plateau of low relief and high average elevation is separated from a coastal plane of high relief and low average elevation by a steeply dipping escarpment. The origin and evolution of this topography is poorly understood because, unlike high plateaus elsewhere, its development cannot be easily linked to present day compressional plate boundary processes. Understanding the development of this regional landscape since the break-up of Gondwana is a first order step towards resolving regional epeirogenesis. We present data that quantifies the timing and extent of exhumation across the southern Cape escarpment and coastal plane, using apatite fission track analysis (AFTA) of 25 outcrop samples and 31 samples from three deep boreholes (KW1/67, SA1/66, CR1/68). Outcrop fission track (AFT) ages are Cretaceous and are significantly younger than the stratigraphic ages of their host rocks, indicating that the samples have experienced elevated paleotemperatures. Mean track lengths vary from 11.86 to 14.23 μm. The lack of Cenozoic apatite ages suggests that major cooling was over by the end Cretaceous. The results for three boreholes, situated seaward (south) of the escarpment, indicate an episode of increased denudation in the mid-late Cretaceous (100-80 Ma). An earlier episode of increased denudation (140-120 Ma) is identified from a borehole north of the escarpment. Thermal modelling indicates a history involving 2.5-3.5 km of denudation in the mid-late Cretaceous (100-80 Ma) at a rate of 175 to 125 m/Ma. The AFT data suggest that less than 1 km of overburden has been eroded regionally since the late Cretaceous (b80 Ma) at a rate of 10 to 15 m/Ma, but do not discount the possibility of minor (in relative amplitude) episodes of uplift and river incision through the Cenozoic. The reasons for rapid denudation in these early and mid-Cretaceous episodes are less clear, but may be related to epeirogenic uplift associated with an increase in mantle buoyancy as reflected in two punctuated episodes of alkaline intrusions (e.g. kimberlites) across southern Africa and contemporaneous formation of two large mafic igneous provinces (~130 and 90 Ma) flanking its continental margins. Because Cenozoic denudation rates are relatively minimal, epeirogenic uplift of southern Africa and its distinct topography cannot be primarily related to Cenozoic mantle processes, consistent with the lack of any significant igneous activity across this region during that time. (J. Tinker), [email protected] (M. de Wit), [email protected] (R. Brown).
The geomorphic origin and evolution of the tectonically unique interior highland of southern Afri... more The geomorphic origin and evolution of the tectonically unique interior highland of southern Africa, the Kalahari Plateau, and its flanking low-lying coastal planes, remain largely unresolved because of a lack of regional quantitative analyses of its uplift and erosion history. Here we focus on the southern Cape, South Africa and link onshore denudation, based on new apatite fission track thermochronology results, to offshore sediment accumulation, using abundant well data and a seismic reflection profile. We attempt to relate source and sink in order to resolve some first order issues concerning timing of the exhumation and development of the topographic features of southern Africa. The volume of sediment accumulated off South Africa's south coast is calculated using 173 wells and a seismic reflection profile. A total, uncompacted, sediment volume of 268,500 km 3 accumulated off South Africa's south coasts since ∼136 Ma, in the Outeniqua and Southern Outeniqua Basins. Accumulation volumes and rates were highest in the early Cretaceous (48,800 × 10 4 km 3 at ∼8150 km 3 /Ma from ∼136 to 130 Ma, and 57,500 × 10 4 km 3 at 5750 km 3 /Ma from ∼130 to 120 Ma) and midlate Cretaceous (83,700 × 10 4 km 3 at 3200 km 3 /Ma from ∼93 to 67 Ma). Volumes and accumulation rates were lowest for the early-mid-Cretaceous (47,400 × 10 4 km 3 at 1750 km 3 /Ma from ∼120 to 93 Ma) and the Cenozoic (31,200 × 10 4 km 3 at 450 km 3 /Ma from ∼67 to 0 Ma). Although our analysis shows that the accumulated volume of offshore sediments does not match the calculated volume of onshore erosion, as quantified through apatite fission track thermochronology (e.g. Tinker, J.H., de Wit, M.J., Brown, R., 2008. Mesozoic exhumation of the 439 southern Cape, South Africa, quantified using apatite fission track thermochronology. Tectonophysics, ), the timing of increased sediment accumulation closely matches the timing of increased onshore denudation. This suggests that the greatest volumes of material were transported from source to sink during two distinct Cretaceous episodes, and that the processes driving onshore denudation decreased by an order of magnitude during the Cenozoic. (J. Tinker), [email protected] (M. de Wit), [email protected] (R. Brown).
Well-preserved pillow lavas in the uppermost part of the Early Archean volcanic sequence of the H... more Well-preserved pillow lavas in the uppermost part of the Early Archean volcanic sequence of the Hooggenoeg Formation in the Barberton Greenstone Belt exhibit pronounced flow banding. The banding is defined by mm to several cm thick alternations of pale green and a dark green, conspicuously variolitic variety of aphyric metabasalt. Concentrations of relatively immobile TiO2, Al2O3 and Cr in both varieties of lava are basaltic. Compositional differences between bands and variations in the lavas in general have been modified by alteration, but indicate mingling of two different basalts, one richer in TiO2, Al2O3, MgO, FeOt and probably Ni and Cr than the other, as the cause of the banding. The occurrence in certain pillows of blebs of dark metabasalt enclosed in pale green metabasalt, as well as cores of faintly banded or massive dark metabasalt, suggest that breakup into drops and slugs in the feeder channel to the lava flow initiated mingling. The inhomogeneous mixture was subsequently stretched and folded together during laminar shear flow through tubular pillows, while diffusion between bands led to partial homogenisation. The most common internal pattern defined by the flow banding in pillows is concentric. In some pillows the banding defines curious mushroom-like structures, commonly cored by dark, variolitic metabasalt, which we interpret as the result of secondary lateral flow due to counter-rotating, transverse (Dean) vortices induced by the axial flow of lava towards the flow front through bends, generally downward, in the tubular pillows. Other pillows exhibit weakly-banded or massive, dark, variolitic cores that are continuous with wedge-shaped apophyses and veins that intrude the flow banded carapace. These cores represent the flow of hotter and less viscous slugs of the dark lava type into cooled and stiffened pillows.
... a Centre for Geobiology and Department of Earth Science, University of Bergen, Allegt. 41, 50... more ... a Centre for Geobiology and Department of Earth Science, University of Bergen, Allegt. 41, 5007 Bergen, Norway. b Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada N6A 5B7. c Scripps ...
Exceptionally well-preserved pillow lavas and inter-pillow hyaloclastites from the Barberton Gree... more Exceptionally well-preserved pillow lavas and inter-pillow hyaloclastites from the Barberton Greenstone Belt in South Africa contain textural, geochemical, and isotopic biomarkers indicative of microbially mediated alteration of basaltic glass in the Archean. The textures are micrometer-scale tubular structures interpreted to have originally formed during microbial etching of glass along fractures. Textures of similar size, morphology, and distribution have been attributed to microbial activity and are commonly observed in the glassy margins of pillow lavas from in situ oceanic crust and young ophiolites. The tubes from the Barberton Greenstone Belt were preserved by precipitation of fine-grained titanite during greenschist facies metamorphism associated with seafloor hydrothermal alteration. The presence of organic carbon along the margins of the tubes and low d 13 C values of bulk-rock carbonate in formerly glassy samples support a biogenic origin for the tubes. Overprinting relationships of secondary minerals observed in thin section indicate the tubular structures are pre-metamorphic. Overlapping metamorphic and igneous crystallization ages thus imply the microbes colonized these rocks 3.4-3.5 Ga. Although, the search for traces of early life on Earth has recently intensified, research has largely been confined to sedimentary rocks. Subaqueous volcanic rocks represent a new geological setting in the search for early life that may preserve a largely unexplored Archean biomass. D
Exceptionally well-preserved pillowed and massive phenocryst-free metabasaltic lava flows in the ... more Exceptionally well-preserved pillowed and massive phenocryst-free metabasaltic lava flows in the uppermost part of the Palaeoarchaean Hooggenoeg Complex of the Barberton Greenstone Belt exhibit both flow banding and large leucocratic varioles. The flow banding is defined by blebs and bands of pale and dark green metabasalt and was the result of mingling of two types of basalt (Robins et al. in Bull Volcanol 72:579-592, 2010a). Varioles occur exclusively in the dark chlorite-, MgO- and FeO-rich metabasalt. Varioles are absent in the outermost rinds of pillows and increase in both abundance and size towards the centres of pillows. In the central parts of some pillows, they impinge to form homogeneous pale patches, bands or almost homogenous cores. Individual varioles consist essentially of radially orientated or outwardly branching dendritic crystals of albite. Many varioles exhibit concentric zones and finer-grained rims. Some varioles seem to have grown around tiny vesicles and vesicles appear to have been trapped in others between a core and a finer-grained rim. The matrix surrounding the ocelli contains acicular pseudomorphs of actinolite and chlorite after chain-like, skeletal Ca-rich pyroxenes that are partly overgrown by the margins of varioles. Varioles are enriched in the chemical constituents of feldspar but contain concentrations of immobile TiO2, Cr, Zr and REE that are similar to the host metabasalts. The shape, distribution, texture and composition of the varioles exclude liquid immiscibility and support an origin by spherulitic crystallisation of plagioclase from severely undercooled basalt melt and glass. Nucleation of plagioclase was strongly inhibited and took place on vesicles, on the bases of drainage cavities and along early fractures. Eruption in deep water and retention of relatively high concentrations of volatiles in the melt may be the principal cause of spherulitic crystallisation in the interiors of pillows rather than only in their margins as in younger submarine flows.
Earth's early evolution and the conditions under which life first emerged are highly controversia... more Earth's early evolution and the conditions under which life first emerged are highly controversial topics. Did Venus-type tectonics dominate the early Earth, with vast landscapes of hot spot volcanoes belching lava and gases into a tenuous carbon dioxide/methane-rich atmosphere, or was plate tectonics already in operation? Debates rage about when water first became dominant across Earth's surface, the temperatures and depths of early oceans, and when the first protocontinents began to form. These large uncertainties confound scientists' understanding of ecological niches available for the emergence of simple life, and a quantifiable image of the early Archean world is still missing.
Well-preserved pillow lavas in the uppermost part of the Early Archean volcanic sequence of the H... more Well-preserved pillow lavas in the uppermost part of the Early Archean volcanic sequence of the Hooggenoeg Formation in the Barberton Greenstone Belt exhibit pronounced flow banding. The banding is defined by mm to several cm thick alternations of pale green and a dark green, conspicuously variolitic variety of aphyric metabasalt. Concentrations of relatively immobile TiO2, Al2O3 and Cr in both varieties of lava are basaltic. Compositional differences between bands and variations in the lavas in general have been modified by alteration, but indicate mingling of two different basalts, one richer in TiO2, Al2O3, MgO, FeOt and probably Ni and Cr than the other, as the cause of the banding. The occurrence in certain pillows of blebs of dark metabasalt enclosed in pale green metabasalt, as well as cores of faintly banded or massive dark metabasalt, suggest that breakup into drops and slugs in the feeder channel to the lava flow initiated mingling. The inhomogeneous mixture was subsequently stretched and folded together during laminar shear flow through tubular pillows, while diffusion between bands led to partial homogenisation. The most common internal pattern defined by the flow banding in pillows is concentric. In some pillows the banding defines curious mushroom-like structures, commonly cored by dark, variolitic metabasalt, which we interpret as the result of secondary lateral flow due to counter-rotating, transverse (Dean) vortices induced by the axial flow of lava towards the flow front through bends, generally downward, in the tubular pillows. Other pillows exhibit weakly-banded or massive, dark, variolitic cores that are continuous with wedge-shaped apophyses and veins that intrude the flow banded carapace. These cores represent the flow of hotter and less viscous slugs of the dark lava type into cooled and stiffened pillows.
The mafic-ultramafic units of the ∼ 3.8 Ga Isua supracrustal belt (ISB) in Greenland occur in a t... more The mafic-ultramafic units of the ∼ 3.8 Ga Isua supracrustal belt (ISB) in Greenland occur in a two-armed arcuate zone (eastern and western arms) and are grouped into two major tectonostratigraphic units based on their lithological and geochemical characteristics: (1) Undifferentiated amphibolites (UA), and (2) Garbenschiefer amphibolites (GA). The UA contains all major lithological units of a typical Penrosetype complete ophiolite sequence. The GA is composed dominantly of volcaniclastic and volcanic rocks, commonly found in immature island arcs. The available geochemical data from UA and GA show distinct differences between the two units. Compared with the geochemical evolution of some of the well known Phanerozoic ophiolites, the pillow lavas and associated dikes of the UA show a compositional range that is similar to typical MORB-type Ligurian ophiolites in the Western Alps-Apennines and those displayed by LIPtype Caribbean ophiolites. The GA is characterized by island arc tholeiite (IAT) to boninite-like rocks and defines a magmatic evolution that is comparable to that of suprasubduction zone (SSZ) ophiolites in the Mediterranean region. Our proposed geodynamic model for the ISB suggests that the UA was built by primary to differentiated, mantle-generated melts during seafloor spreading, little to moderately affected by subduction processes, and that the IAT to boninitic-like rocks of the GA formed at a later stage by melting from a strongly subduction-affected, depleted and hydrated mantle. Our interpretation of the ISB is that the UA and GA represent early and late stages, respectively, in the formation of a SSZ ophiolite. This implies that Phanerozoic-type plate tectonic processes, such as seafloor spreading and subduction, were operating by 3.8 Ga in the Palaeoarchean. Please cite this article as: Furnes, H., et al., Isua supracrustal belt (Greenland)-A vestige of a 3.8 Ga suprasubduction zone ophiolite, and the implications for Archean geology, Lithos (2009), Please cite this article as: Furnes, H., et al., Isua supracrustal belt (Greenland)-A vestige of a 3.8 Ga suprasubduction zone ophiolite, and the implications for Archean geology, Lithos (2009),
This chapter summarizes research undertaken over the past 15 years upon the microbial alteration ... more This chapter summarizes research undertaken over the past 15 years upon the microbial alteration of originally glassy basaltic rocks from submarine environments. We report textural, chemical and isotopic results from the youngest to the oldest in-situ oceanic crust and compare these to data obtained from ophiolite and greenstone belts dating back to c. 3.8 Ga. Petrographic descriptions of the granular and tubular microbial alteration textures found in (meta)-volcanic glasses from pillow lavas and volcanic breccias are provided and contrasted with textures produced by abiotic alteration (palagonitization). The geological setting in particular the degree of deformation and metamorphism experienced by each study site is documented in outcrop photographs, geological maps and stratigraphic columns (where possible). In addition, X-ray mapping evidence and carbon isotopic data that are consistent with a biogenic origin for these alteration textures is explained and a model for their formation is presented. Lastly, the petrographic observations and direct radiometric dating techniques that have been used to establish the antiquity and syngenicity of these microbial alteration textures are reviewed.
Exceptionally well-preserved pillowed and massive phenocryst-free metabasaltic lava flows in the ... more Exceptionally well-preserved pillowed and massive phenocryst-free metabasaltic lava flows in the uppermost part of the Palaeoarchaean Hooggenoeg Complex of the Barberton Greenstone Belt exhibit both flow banding and large leucocratic varioles. The flow banding is defined by blebs and bands of pale and dark green metabasalt and was the result of mingling of two types of basalt (Robins et al. in Bull Volcanol 72:579–592, 2010a). Varioles occur exclusively in the dark chlorite-, MgO- and FeO-rich metabasalt. Varioles are absent in the outermost rinds of pillows and increase in both abundance and size towards the centres of pillows. In the central parts of some pillows, they impinge to form homogeneous pale patches, bands or almost homogenous cores. Individual varioles consist essentially of radially orientated or outwardly branching dendritic crystals of albite. Many varioles exhibit concentric zones and finer-grained rims. Some varioles seem to have grown around tiny vesicles and vesicles appear to have been trapped in others between a core and a finer-grained rim. The matrix surrounding the ocelli contains acicular pseudomorphs of actinolite and chlorite after chain-like, skeletal Ca-rich pyroxenes that are partly overgrown by the margins of varioles. Varioles are enriched in the chemical constituents of feldspar but contain concentrations of immobile TiO2, Cr, Zr and REE that are similar to the host metabasalts. The shape, distribution, texture and composition of the varioles exclude liquid immiscibility and support an origin by spherulitic crystallisation of plagioclase from severely undercooled basalt melt and glass. Nucleation of plagioclase was strongly inhibited and took place on vesicles, on the bases of drainage cavities and along early fractures. Eruption in deep water and retention of relatively high concentrations of volatiles in the melt may be the principal cause of spherulitic crystallisation in the interiors of pillows rather than only in their margins as in younger submarine flows.
Microbial bioerosion of volcanic glass produces conspicuous ichnofossils in oceanic crusts that a... more Microbial bioerosion of volcanic glass produces conspicuous ichnofossils in oceanic crusts that are a valuable tracer of sub-surface microorganisms. Two morphologically distinct granular and tubular ichnofossils are produced. The ‘Granular form’ consists of individual or coalescing, spherical bodies with average diameters of ~0.4 μm. The ‘Tubular form’ are straight, sometimes ranched, to curving and spiralled tubes with average diameters of 1-2 μm and lengths of up to ~200μm. A biogenic origin for these structures is confirmed by: the concentration of DNA that binds to biological stains in recent examples; enrichments in C, N and P along their margins in both recent and ancient examples; and systematic C isotope shifts measured upon disseminated carbonate in the surrounding glass. The constructing microorganisms are thought to include heterotrophs and chemolithoautotrophs that may utilise Fe and Mn from basaltic glass as electron donors and derive carbon sources and electron acceptors from circulating fluids. These microbial ichnofossils are found at depths of up to 550 metres in the oceanic crust in the glassy rims of pillow basalts and interpillow breccias. A diverse spectrum of ichnofabrics is created by overlapping phases of granular and tubular bioerosion; banded abiotic dissolution; and the precipitation of phyllosilicates, zeolites and iron-oxy-hydroxides. The resulting ichnofabrics have been documented from in situ oceanic crust spanning the youngest to the oldest oceanic basins (0 to 170 Ma). Their geological record extends to include meta-volcanic glass in oceanic crustal fragments from Phanerozoic to Proterozoic ophiolites. Examples infilled by the mineral titanite (CaTiSiO4) have also been found in Palaeo- to Mesoarchaean pillow basalts from the Barberton Greenstone Belt of South Africa and the East Pilbara Terrane of Western Australia. Direct 206Pb / 238U radiometric dating of the Australia examples has confirmed their Archaean age and thus they represent the oldest candidate ichnofossils on Earth.
Geological Society, London, Special Publications, 2005
... of Geological 1 and Statistical 2 Sciences, University of Cape Town, Rondebosch 7701, South A... more ... of Geological 1 and Statistical 2 Sciences, University of Cape Town, Rondebosch 7701, South Africa (e-maik [email protected] ... of the Bushveld Complex that intruded the centre of this craton in the Palaeoproterozoic (2.05-2.06 Ga; Cawthorne & Walraven 1998; Eglington ...
Amalgamation of a number of continental fragments during the Late Neoproterozoic resulted in a un... more Amalgamation of a number of continental fragments during the Late Neoproterozoic resulted in a united Gondwana continent. The time period in question, at the end of the Precambrian, spans about 250 million years between ∼800 and 550 Ma. Geological activity focused along orogenic belts in Africa during that time period is generally referred to as “Pan African.” We identify three age-related classes
Interpretation of the detailed patterns of ocean-floor transforms revealed by satellite altimetry... more Interpretation of the detailed patterns of ocean-floor transforms revealed by satellite altimetry enables the creation of the Indian Ocean to be described quantitatively as four consecutive plate-tectonic regimes separated at 200, 136, 89 and 43 Ma. Each regime is reversed in turn by keeping transform termini coincident and colinear until conjugate points on the margins of pre-existing plates regain their pre-regime integrity. Progressive elimination of the Indian Ocean, demonstrable as a smooth computer animation (http://www.kartoweb.itc.nl/gondwana), leads to a refined re-assembly of the continental fragments of central Gondwana that is substantiated by new geological data. A sequence of Euler interval poles that describes the dispersal of the Gondwana fragments, time-calibrated against available magnetic anomaly data, is given. The model requires a mid-Cretaceous position for India’s southern tip about 1000 km south of Madagascar, prior to India’s rapid northward migration.
Southern Africa's topography is distinctive. An inland plateau of low relief and high average ele... more Southern Africa's topography is distinctive. An inland plateau of low relief and high average elevation is separated from a coastal plane of high relief and low average elevation by a steeply dipping escarpment. The origin and evolution of this topography is poorly understood because, unlike high plateaus elsewhere, its development cannot be easily linked to present day compressional plate boundary processes. Understanding the development of this regional landscape since the break-up of Gondwana is a first order step towards resolving regional epeirogenesis. We present data that quantifies the timing and extent of exhumation across the southern Cape escarpment and coastal plane, using apatite fission track analysis (AFTA) of 25 outcrop samples and 31 samples from three deep boreholes (KW1/67, SA1/66, CR1/68). Outcrop fission track (AFT) ages are Cretaceous and are significantly younger than the stratigraphic ages of their host rocks, indicating that the samples have experienced elevated paleotemperatures. Mean track lengths vary from 11.86 to 14.23 μm. The lack of Cenozoic apatite ages suggests that major cooling was over by the end Cretaceous. The results for three boreholes, situated seaward (south) of the escarpment, indicate an episode of increased denudation in the mid-late Cretaceous (100-80 Ma). An earlier episode of increased denudation (140-120 Ma) is identified from a borehole north of the escarpment. Thermal modelling indicates a history involving 2.5-3.5 km of denudation in the mid-late Cretaceous (100-80 Ma) at a rate of 175 to 125 m/Ma. The AFT data suggest that less than 1 km of overburden has been eroded regionally since the late Cretaceous (b80 Ma) at a rate of 10 to 15 m/Ma, but do not discount the possibility of minor (in relative amplitude) episodes of uplift and river incision through the Cenozoic. The reasons for rapid denudation in these early and mid-Cretaceous episodes are less clear, but may be related to epeirogenic uplift associated with an increase in mantle buoyancy as reflected in two punctuated episodes of alkaline intrusions (e.g. kimberlites) across southern Africa and contemporaneous formation of two large mafic igneous provinces (~130 and 90 Ma) flanking its continental margins. Because Cenozoic denudation rates are relatively minimal, epeirogenic uplift of southern Africa and its distinct topography cannot be primarily related to Cenozoic mantle processes, consistent with the lack of any significant igneous activity across this region during that time. (J. Tinker), [email protected] (M. de Wit), [email protected] (R. Brown).
The geomorphic origin and evolution of the tectonically unique interior highland of southern Afri... more The geomorphic origin and evolution of the tectonically unique interior highland of southern Africa, the Kalahari Plateau, and its flanking low-lying coastal planes, remain largely unresolved because of a lack of regional quantitative analyses of its uplift and erosion history. Here we focus on the southern Cape, South Africa and link onshore denudation, based on new apatite fission track thermochronology results, to offshore sediment accumulation, using abundant well data and a seismic reflection profile. We attempt to relate source and sink in order to resolve some first order issues concerning timing of the exhumation and development of the topographic features of southern Africa. The volume of sediment accumulated off South Africa's south coast is calculated using 173 wells and a seismic reflection profile. A total, uncompacted, sediment volume of 268,500 km 3 accumulated off South Africa's south coasts since ∼136 Ma, in the Outeniqua and Southern Outeniqua Basins. Accumulation volumes and rates were highest in the early Cretaceous (48,800 × 10 4 km 3 at ∼8150 km 3 /Ma from ∼136 to 130 Ma, and 57,500 × 10 4 km 3 at 5750 km 3 /Ma from ∼130 to 120 Ma) and midlate Cretaceous (83,700 × 10 4 km 3 at 3200 km 3 /Ma from ∼93 to 67 Ma). Volumes and accumulation rates were lowest for the early-mid-Cretaceous (47,400 × 10 4 km 3 at 1750 km 3 /Ma from ∼120 to 93 Ma) and the Cenozoic (31,200 × 10 4 km 3 at 450 km 3 /Ma from ∼67 to 0 Ma). Although our analysis shows that the accumulated volume of offshore sediments does not match the calculated volume of onshore erosion, as quantified through apatite fission track thermochronology (e.g. Tinker, J.H., de Wit, M.J., Brown, R., 2008. Mesozoic exhumation of the 439 southern Cape, South Africa, quantified using apatite fission track thermochronology. Tectonophysics, ), the timing of increased sediment accumulation closely matches the timing of increased onshore denudation. This suggests that the greatest volumes of material were transported from source to sink during two distinct Cretaceous episodes, and that the processes driving onshore denudation decreased by an order of magnitude during the Cenozoic. (J. Tinker), [email protected] (M. de Wit), [email protected] (R. Brown).
Well-preserved pillow lavas in the uppermost part of the Early Archean volcanic sequence of the H... more Well-preserved pillow lavas in the uppermost part of the Early Archean volcanic sequence of the Hooggenoeg Formation in the Barberton Greenstone Belt exhibit pronounced flow banding. The banding is defined by mm to several cm thick alternations of pale green and a dark green, conspicuously variolitic variety of aphyric metabasalt. Concentrations of relatively immobile TiO2, Al2O3 and Cr in both varieties of lava are basaltic. Compositional differences between bands and variations in the lavas in general have been modified by alteration, but indicate mingling of two different basalts, one richer in TiO2, Al2O3, MgO, FeOt and probably Ni and Cr than the other, as the cause of the banding. The occurrence in certain pillows of blebs of dark metabasalt enclosed in pale green metabasalt, as well as cores of faintly banded or massive dark metabasalt, suggest that breakup into drops and slugs in the feeder channel to the lava flow initiated mingling. The inhomogeneous mixture was subsequently stretched and folded together during laminar shear flow through tubular pillows, while diffusion between bands led to partial homogenisation. The most common internal pattern defined by the flow banding in pillows is concentric. In some pillows the banding defines curious mushroom-like structures, commonly cored by dark, variolitic metabasalt, which we interpret as the result of secondary lateral flow due to counter-rotating, transverse (Dean) vortices induced by the axial flow of lava towards the flow front through bends, generally downward, in the tubular pillows. Other pillows exhibit weakly-banded or massive, dark, variolitic cores that are continuous with wedge-shaped apophyses and veins that intrude the flow banded carapace. These cores represent the flow of hotter and less viscous slugs of the dark lava type into cooled and stiffened pillows.
... a Centre for Geobiology and Department of Earth Science, University of Bergen, Allegt. 41, 50... more ... a Centre for Geobiology and Department of Earth Science, University of Bergen, Allegt. 41, 5007 Bergen, Norway. b Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada N6A 5B7. c Scripps ...
Exceptionally well-preserved pillow lavas and inter-pillow hyaloclastites from the Barberton Gree... more Exceptionally well-preserved pillow lavas and inter-pillow hyaloclastites from the Barberton Greenstone Belt in South Africa contain textural, geochemical, and isotopic biomarkers indicative of microbially mediated alteration of basaltic glass in the Archean. The textures are micrometer-scale tubular structures interpreted to have originally formed during microbial etching of glass along fractures. Textures of similar size, morphology, and distribution have been attributed to microbial activity and are commonly observed in the glassy margins of pillow lavas from in situ oceanic crust and young ophiolites. The tubes from the Barberton Greenstone Belt were preserved by precipitation of fine-grained titanite during greenschist facies metamorphism associated with seafloor hydrothermal alteration. The presence of organic carbon along the margins of the tubes and low d 13 C values of bulk-rock carbonate in formerly glassy samples support a biogenic origin for the tubes. Overprinting relationships of secondary minerals observed in thin section indicate the tubular structures are pre-metamorphic. Overlapping metamorphic and igneous crystallization ages thus imply the microbes colonized these rocks 3.4-3.5 Ga. Although, the search for traces of early life on Earth has recently intensified, research has largely been confined to sedimentary rocks. Subaqueous volcanic rocks represent a new geological setting in the search for early life that may preserve a largely unexplored Archean biomass. D
Exceptionally well-preserved pillowed and massive phenocryst-free metabasaltic lava flows in the ... more Exceptionally well-preserved pillowed and massive phenocryst-free metabasaltic lava flows in the uppermost part of the Palaeoarchaean Hooggenoeg Complex of the Barberton Greenstone Belt exhibit both flow banding and large leucocratic varioles. The flow banding is defined by blebs and bands of pale and dark green metabasalt and was the result of mingling of two types of basalt (Robins et al. in Bull Volcanol 72:579-592, 2010a). Varioles occur exclusively in the dark chlorite-, MgO- and FeO-rich metabasalt. Varioles are absent in the outermost rinds of pillows and increase in both abundance and size towards the centres of pillows. In the central parts of some pillows, they impinge to form homogeneous pale patches, bands or almost homogenous cores. Individual varioles consist essentially of radially orientated or outwardly branching dendritic crystals of albite. Many varioles exhibit concentric zones and finer-grained rims. Some varioles seem to have grown around tiny vesicles and vesicles appear to have been trapped in others between a core and a finer-grained rim. The matrix surrounding the ocelli contains acicular pseudomorphs of actinolite and chlorite after chain-like, skeletal Ca-rich pyroxenes that are partly overgrown by the margins of varioles. Varioles are enriched in the chemical constituents of feldspar but contain concentrations of immobile TiO2, Cr, Zr and REE that are similar to the host metabasalts. The shape, distribution, texture and composition of the varioles exclude liquid immiscibility and support an origin by spherulitic crystallisation of plagioclase from severely undercooled basalt melt and glass. Nucleation of plagioclase was strongly inhibited and took place on vesicles, on the bases of drainage cavities and along early fractures. Eruption in deep water and retention of relatively high concentrations of volatiles in the melt may be the principal cause of spherulitic crystallisation in the interiors of pillows rather than only in their margins as in younger submarine flows.
Earth's early evolution and the conditions under which life first emerged are highly controversia... more Earth's early evolution and the conditions under which life first emerged are highly controversial topics. Did Venus-type tectonics dominate the early Earth, with vast landscapes of hot spot volcanoes belching lava and gases into a tenuous carbon dioxide/methane-rich atmosphere, or was plate tectonics already in operation? Debates rage about when water first became dominant across Earth's surface, the temperatures and depths of early oceans, and when the first protocontinents began to form. These large uncertainties confound scientists' understanding of ecological niches available for the emergence of simple life, and a quantifiable image of the early Archean world is still missing.
Well-preserved pillow lavas in the uppermost part of the Early Archean volcanic sequence of the H... more Well-preserved pillow lavas in the uppermost part of the Early Archean volcanic sequence of the Hooggenoeg Formation in the Barberton Greenstone Belt exhibit pronounced flow banding. The banding is defined by mm to several cm thick alternations of pale green and a dark green, conspicuously variolitic variety of aphyric metabasalt. Concentrations of relatively immobile TiO2, Al2O3 and Cr in both varieties of lava are basaltic. Compositional differences between bands and variations in the lavas in general have been modified by alteration, but indicate mingling of two different basalts, one richer in TiO2, Al2O3, MgO, FeOt and probably Ni and Cr than the other, as the cause of the banding. The occurrence in certain pillows of blebs of dark metabasalt enclosed in pale green metabasalt, as well as cores of faintly banded or massive dark metabasalt, suggest that breakup into drops and slugs in the feeder channel to the lava flow initiated mingling. The inhomogeneous mixture was subsequently stretched and folded together during laminar shear flow through tubular pillows, while diffusion between bands led to partial homogenisation. The most common internal pattern defined by the flow banding in pillows is concentric. In some pillows the banding defines curious mushroom-like structures, commonly cored by dark, variolitic metabasalt, which we interpret as the result of secondary lateral flow due to counter-rotating, transverse (Dean) vortices induced by the axial flow of lava towards the flow front through bends, generally downward, in the tubular pillows. Other pillows exhibit weakly-banded or massive, dark, variolitic cores that are continuous with wedge-shaped apophyses and veins that intrude the flow banded carapace. These cores represent the flow of hotter and less viscous slugs of the dark lava type into cooled and stiffened pillows.
The mafic-ultramafic units of the ∼ 3.8 Ga Isua supracrustal belt (ISB) in Greenland occur in a t... more The mafic-ultramafic units of the ∼ 3.8 Ga Isua supracrustal belt (ISB) in Greenland occur in a two-armed arcuate zone (eastern and western arms) and are grouped into two major tectonostratigraphic units based on their lithological and geochemical characteristics: (1) Undifferentiated amphibolites (UA), and (2) Garbenschiefer amphibolites (GA). The UA contains all major lithological units of a typical Penrosetype complete ophiolite sequence. The GA is composed dominantly of volcaniclastic and volcanic rocks, commonly found in immature island arcs. The available geochemical data from UA and GA show distinct differences between the two units. Compared with the geochemical evolution of some of the well known Phanerozoic ophiolites, the pillow lavas and associated dikes of the UA show a compositional range that is similar to typical MORB-type Ligurian ophiolites in the Western Alps-Apennines and those displayed by LIPtype Caribbean ophiolites. The GA is characterized by island arc tholeiite (IAT) to boninite-like rocks and defines a magmatic evolution that is comparable to that of suprasubduction zone (SSZ) ophiolites in the Mediterranean region. Our proposed geodynamic model for the ISB suggests that the UA was built by primary to differentiated, mantle-generated melts during seafloor spreading, little to moderately affected by subduction processes, and that the IAT to boninitic-like rocks of the GA formed at a later stage by melting from a strongly subduction-affected, depleted and hydrated mantle. Our interpretation of the ISB is that the UA and GA represent early and late stages, respectively, in the formation of a SSZ ophiolite. This implies that Phanerozoic-type plate tectonic processes, such as seafloor spreading and subduction, were operating by 3.8 Ga in the Palaeoarchean. Please cite this article as: Furnes, H., et al., Isua supracrustal belt (Greenland)-A vestige of a 3.8 Ga suprasubduction zone ophiolite, and the implications for Archean geology, Lithos (2009), Please cite this article as: Furnes, H., et al., Isua supracrustal belt (Greenland)-A vestige of a 3.8 Ga suprasubduction zone ophiolite, and the implications for Archean geology, Lithos (2009),
This chapter summarizes research undertaken over the past 15 years upon the microbial alteration ... more This chapter summarizes research undertaken over the past 15 years upon the microbial alteration of originally glassy basaltic rocks from submarine environments. We report textural, chemical and isotopic results from the youngest to the oldest in-situ oceanic crust and compare these to data obtained from ophiolite and greenstone belts dating back to c. 3.8 Ga. Petrographic descriptions of the granular and tubular microbial alteration textures found in (meta)-volcanic glasses from pillow lavas and volcanic breccias are provided and contrasted with textures produced by abiotic alteration (palagonitization). The geological setting in particular the degree of deformation and metamorphism experienced by each study site is documented in outcrop photographs, geological maps and stratigraphic columns (where possible). In addition, X-ray mapping evidence and carbon isotopic data that are consistent with a biogenic origin for these alteration textures is explained and a model for their formation is presented. Lastly, the petrographic observations and direct radiometric dating techniques that have been used to establish the antiquity and syngenicity of these microbial alteration textures are reviewed.
Exceptionally well-preserved pillowed and massive phenocryst-free metabasaltic lava flows in the ... more Exceptionally well-preserved pillowed and massive phenocryst-free metabasaltic lava flows in the uppermost part of the Palaeoarchaean Hooggenoeg Complex of the Barberton Greenstone Belt exhibit both flow banding and large leucocratic varioles. The flow banding is defined by blebs and bands of pale and dark green metabasalt and was the result of mingling of two types of basalt (Robins et al. in Bull Volcanol 72:579–592, 2010a). Varioles occur exclusively in the dark chlorite-, MgO- and FeO-rich metabasalt. Varioles are absent in the outermost rinds of pillows and increase in both abundance and size towards the centres of pillows. In the central parts of some pillows, they impinge to form homogeneous pale patches, bands or almost homogenous cores. Individual varioles consist essentially of radially orientated or outwardly branching dendritic crystals of albite. Many varioles exhibit concentric zones and finer-grained rims. Some varioles seem to have grown around tiny vesicles and vesicles appear to have been trapped in others between a core and a finer-grained rim. The matrix surrounding the ocelli contains acicular pseudomorphs of actinolite and chlorite after chain-like, skeletal Ca-rich pyroxenes that are partly overgrown by the margins of varioles. Varioles are enriched in the chemical constituents of feldspar but contain concentrations of immobile TiO2, Cr, Zr and REE that are similar to the host metabasalts. The shape, distribution, texture and composition of the varioles exclude liquid immiscibility and support an origin by spherulitic crystallisation of plagioclase from severely undercooled basalt melt and glass. Nucleation of plagioclase was strongly inhibited and took place on vesicles, on the bases of drainage cavities and along early fractures. Eruption in deep water and retention of relatively high concentrations of volatiles in the melt may be the principal cause of spherulitic crystallisation in the interiors of pillows rather than only in their margins as in younger submarine flows.
Microbial bioerosion of volcanic glass produces conspicuous ichnofossils in oceanic crusts that a... more Microbial bioerosion of volcanic glass produces conspicuous ichnofossils in oceanic crusts that are a valuable tracer of sub-surface microorganisms. Two morphologically distinct granular and tubular ichnofossils are produced. The ‘Granular form’ consists of individual or coalescing, spherical bodies with average diameters of ~0.4 μm. The ‘Tubular form’ are straight, sometimes ranched, to curving and spiralled tubes with average diameters of 1-2 μm and lengths of up to ~200μm. A biogenic origin for these structures is confirmed by: the concentration of DNA that binds to biological stains in recent examples; enrichments in C, N and P along their margins in both recent and ancient examples; and systematic C isotope shifts measured upon disseminated carbonate in the surrounding glass. The constructing microorganisms are thought to include heterotrophs and chemolithoautotrophs that may utilise Fe and Mn from basaltic glass as electron donors and derive carbon sources and electron acceptors from circulating fluids. These microbial ichnofossils are found at depths of up to 550 metres in the oceanic crust in the glassy rims of pillow basalts and interpillow breccias. A diverse spectrum of ichnofabrics is created by overlapping phases of granular and tubular bioerosion; banded abiotic dissolution; and the precipitation of phyllosilicates, zeolites and iron-oxy-hydroxides. The resulting ichnofabrics have been documented from in situ oceanic crust spanning the youngest to the oldest oceanic basins (0 to 170 Ma). Their geological record extends to include meta-volcanic glass in oceanic crustal fragments from Phanerozoic to Proterozoic ophiolites. Examples infilled by the mineral titanite (CaTiSiO4) have also been found in Palaeo- to Mesoarchaean pillow basalts from the Barberton Greenstone Belt of South Africa and the East Pilbara Terrane of Western Australia. Direct 206Pb / 238U radiometric dating of the Australia examples has confirmed their Archaean age and thus they represent the oldest candidate ichnofossils on Earth.
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