Origin and Evolution of Precambrian High-Grade Gneiss Terranes, with Special Emphasis on the Limpopo Complex of Southern Africa, 2011
The Geological Society of America Memoir 207 2011 Intracrustal radioactivity as an important heat... more The Geological Society of America Memoir 207 2011 Intracrustal radioactivity as an important heat source for Neoarchean metamorphism in the Central Zone of the Limpopo Complex Marco AG Andreoli South African Nuclear Energy Corporation, PO Box 582, Pretoria 0001, ...
Planar microdeformations in quartz from basement or collar rocks of the Vredefort Dome have been ... more Planar microdeformations in quartz from basement or collar rocks of the Vredefort Dome have been cited for years as the main microtextural evidence for shock metamorphism in this structure. In addition, Schreyer describes feldspar recrystallization in rocks from the center of the Dome as the result of transformation of diaplectic glass, and Lilly reported the sighting of mosaicism in quartz. These textural observations are widely believed to indicate either an impact or an internally produced shock origin for the Vredefort Dome. Two types of (mostly sub) planar microdeformations are displayed in quartz grains from Vredefort rocks: (1) fluid inclusion trails, and (2) straight optical discontinuities that sometimes resemble lamellae. Both types occur as single features or as single or multiple sets in quartz grains. Besides qualitative descriptions of cleavage and recrystallization in feldspar and kinkbands in mica, no further microtextural evidence for shock metamorphism at Vredefort has been reported to date. Some 150 thin sections of Vredefort basement rocks were re-examined for potential shock and other deformation effects in all rock-forming minerals. This included petrographic study of two drill cores from the immediate vicinity of the center of the Dome. Observations recorded throughout the granitic core are given along with conclusions.
40Ar/39Ar thermochronology applied to plagioclase from the Morokweng LL chondrite possibly sugges... more 40Ar/39Ar thermochronology applied to plagioclase from the Morokweng LL chondrite possibly suggests large asteroid collisional (breakup?) events at ~2Ga and ~0.7 Ga and records the age of the impact on Earth at ~145 Ma.
ABSTRACT Unravelling the brittle history of old crystalline basements is a challenging task due t... more ABSTRACT Unravelling the brittle history of old crystalline basements is a challenging task due to the long and complex nature of their geological evolution and to the common lack of clear time markers. Pre-existing ductile fabrics often play an important role in the localization of younger brittle features, which invariably reflect and result from multiple structural reactivations during the numerous orogenic events that controlled the growth and evolution of the crustal blocks. In spite of these complexities, the careful and systematic study of outcrops containing predominantly brittle structures (i.e. brittle deformation zones, faults, fractures and associated fault rocks) with the aim to document their character and kinematics may become a valuable tool towards the unravelling of these structural histories and can help refine our understanding of reactivation processes in the brittle field. The exceptionally well exposed crystalline basement of the ca. 1.03 Ga Namaqua Metamorphic Complex (NMC) in western South Africa is used as an example. Remote sensing analysis highlighted several sets of kilometric conjugate shear fractures. Their geometric arrangement and kinematics are constrained by the offset of lithological markers and by detailed field investigations that allowed gathering a significant dataset of fault-slip data. Striated fault planes were observed throughout the NMC and were documented by paying great attention to their geometry, kinematics, mineral coating and to the rock type they deform. Paleo-stress tensors were computed on internally homogeneous fault-slip datasets and a tentative relative geochronological succession of brittle deformation events was established. This was aided by separating in time faulting events through the usage of Cretaceous weathering horizons, silicified fluvial deposits, paleosols and 77-54 Ma olivine melilitite plugs as time markers. The oldest features recognized formed during four compressional episodes assigned to the Neoproterozoic Pan African evolution. This history is expressed by sub-vertical conjugate fracture sets and fits well the inferences derived from remote sensing. The greatest compressive direction rotated from NW-SE to NNE-SSW and finally to almost E-W. A subsequent ENE-WSW-oriented extensional episode is associated with the local effects of the opening of the Atlantic Ocean and was followed by a second, ca. E-W extensional episode, linked to the well-acknowledged Mid-Cretaceous (115-90 Ma) event of margin uplift. A late Santonian (85-83 Ma) NWSE compressive paleostress deformed the Late Cretaceous sequences and was in turn followed firstly by a renewed episode of NE-SW extension and later by ca. NNE-SSW Late Maastrichtian (69-65 Ma) shortening. The latter is broadly coeval with the emplacement of the Gamoep magmatic suite. A phase of WNW-ESE Cenozoic extension is assigned to the extensional phase recorded in the Okawango delta, interpreted as reflecting propagation of the East African Rift System into southern Africa. No stress tensor was computed for the present day "Wegener anomaly" stress field, oriented NW-SE. However, the available in situ stress measurements from the region were used to perform slip tendency analysis, which indicates that, under the currently existing stress conditions, WNW-ESE- and NNW-SSE-striking faults are critically stressed and are the most likely reactivated, in agreement with the present seismicity.
Radiometric dating of melt rocks at impact craters has revealed that some giant impacts appear to... more Radiometric dating of melt rocks at impact craters has revealed that some giant impacts appear to overlap in time with major boundaries in Earth history [e.g., the Cretaceous–Tertiary (K/T) and Jurassic–Cretaceous (J/K) boundaries]. The Morokweng impact crater in South Africa is coincident in age with the J/K boundary. However, the types of objects that generate large craters are poorly known because it is difficult to unambiguously identify the projectile from the signature it imparts into the impact rocks. Meteorites are highly enriched in the platinum-group elements (PGE), which have been widely used as a tool for identifying the presence of a meteorite signature. Here we present new PGE analyses from the Morokweng impact melt sheet. Our data reveal high PGE concentrations and high degree of PGE correlation through the melt sheet. Regression analysis was used to determine the projectile PGE signature and constrain input from the terrestrial target rocks. The closest match to Morokweng is the PGE signature of ordinary (L or LL) chondrite meteorites, which is broadly in agreement with the results of an earlier Cr isotope study. The results of these independent studies provide strong evidence that a large, ordinary chondrite projectile struck the area of Morokweng in the late Jurassic.
Radiometric dating of melt rocks at impact craters has revealed that some giant impacts appear to... more Radiometric dating of melt rocks at impact craters has revealed that some giant impacts appear to overlap in time with major boundaries in Earth history [e.g., the Cretaceous-Tertiary (K/T) and Jurassic-Cretaceous (J/K) boundaries]. The Morokweng impact crater in South Africa is coincident in age with the J/K boundary. However, the types of objects that generate large craters are poorly known because it is difficult to unambiguously identify the projectile from the signature it imparts into the impact rocks. Meteorites are highly enriched in the platinum-group elements (PGE), which have been widely used as a tool for identifying the presence of a meteorite signature. Here we present new PGE analyses from the Morokweng impact melt sheet. Our data reveal high PGE concentrations and high degree of PGE correlation through the melt sheet. Regression analysis was used to determine the projectile PGE signature and constrain input from the terrestrial target rocks. The closest match to Morokweng is the PGE signature of ordinary (L or LL) chondrite meteorites, which is broadly in agreement with the results of an earlier Cr isotope study. The results of these independent studies provide strong evidence that a large, ordinary chondrite projectile struck the area of Morokweng in the late Jurassic.
Egu General Assembly Conference Abstracts, Apr 1, 2013
To mitigate the uncertainties in assessing the geohazards and rock conditions that affect the nuc... more To mitigate the uncertainties in assessing the geohazards and rock conditions that affect the nuclear, mining (including hydrocarbon extraction) and civil engineering activities in South Africa, the authors are working to improve the data coverage concerning the present day stress field. In principle, this implies constraining the principal compressive stresses (σ1>σ2>σ3) or at least the maximum horizontal compressive stress (σH) because knowledge of these parameters may determine the reactivation potential of known faults, or the behaviour of large excavations and wells. By contrast, much of the subcontinent is under-represented in the World Stress Map database. For this reason we have taken a number of steps, firstly by installing a compact Trillium seismic sensor at Stofkloof (Namaqualand; adjacent to the Vaalputs low and intermediate level radioactive waste disposal facility) and 1-sec sensors at Aggeneys and Koffiemeul (Bushmanland). All stations are equipped with Reftek data loggers and powered by solar panels. The data from these stations will be integrated with data from the national network to obtain focal mechanism solutions for seismic events in the Northern Cape - southernmost Namibia region (also known as the Grootvloer cluster). These neotectonic stress tensors are then combined with σH parameters obtained from calliper logs of off-shore wells and from the geometry of joints, faults and sheared fractures in palaeosols (Bushmanland), soils and calcrete (NW Free State) and aeolianites (southern Cape). We also include underground rock engineering phenomenological observations and measurements, and data in the public domain. Our data consistently indicate a NNW-SSE oriented σH (Wegener Stress Anomaly or WSA) that prevails across most of central, southern and western South Africa, Namibia up to the Ruacana hydroelectric power plant at the Angola border. However, in the Congo basin, a few earthquake focal mechanisms suggest rotation of the regional σH to an E-W direction. Geological units affected by the WSA include the Cretaceous oceanic lithosphere (Walvis Ridge), the southern Angola-Kasai craton, the offshore Outeniqua and Orange Basins, the Cape Fold Belt, the Namaqualand metamorphic complex, and the Archaean Craton up to the Witwatersrand basin and the Witbank coal field. In contrast, σH azimuths in the NE-SW quadrants seem prevalent in E Mpumalanga, N Natal, and northern Limpopo. Whereas the origin of these latter stress azimuths are probably linked to the propagation of the E African Rift System, the strike-slip to transpressional character of much of the WSA remains unexplained. Similarly puzzling are a 3-fold increase in seismic events (proxy for strain rate) over the past 20 years in the Grootvloer cluster, and evidence that the WSA is the last of at least 7 successive tectonic regimes to leave their brittle imprints along the SE Atlantic seaboard since the break-up of W Gondwana.
Models predicting the size of the Vredefort Crater vary considerably due to the fact that so litt... more Models predicting the size of the Vredefort Crater vary considerably due to the fact that so little of the original crater is exposed. In this study we combine geological observation with magnetic and gravity imaging to visualize the original crater.
In the present study we have reported our detailed petrographic observations on the type of shock... more In the present study we have reported our detailed petrographic observations on the type of shocked deformations on two boreholes drilled at ~18-20 km distance to the southwest and northwest of the center of the Morokweng impact crater, South Africa.
The paper reviews more than 20 years of structural, stratigraphic and seismic monitoring studies ... more The paper reviews more than 20 years of structural, stratigraphic and seismic monitoring studies focused on the Vaalputs radioactive waste disposal site, 100 km SSE of Springbok, in Namaqualand. Our finds, supported by the recordings of two 3-components seismometers, show that the frequency of seismic events in this region may be slowly increasing over time, that the predicted Mmax is
Origin and Evolution of Precambrian High-Grade Gneiss Terranes, with Special Emphasis on the Limpopo Complex of Southern Africa, 2011
The Geological Society of America Memoir 207 2011 Intracrustal radioactivity as an important heat... more The Geological Society of America Memoir 207 2011 Intracrustal radioactivity as an important heat source for Neoarchean metamorphism in the Central Zone of the Limpopo Complex Marco AG Andreoli South African Nuclear Energy Corporation, PO Box 582, Pretoria 0001, ...
Planar microdeformations in quartz from basement or collar rocks of the Vredefort Dome have been ... more Planar microdeformations in quartz from basement or collar rocks of the Vredefort Dome have been cited for years as the main microtextural evidence for shock metamorphism in this structure. In addition, Schreyer describes feldspar recrystallization in rocks from the center of the Dome as the result of transformation of diaplectic glass, and Lilly reported the sighting of mosaicism in quartz. These textural observations are widely believed to indicate either an impact or an internally produced shock origin for the Vredefort Dome. Two types of (mostly sub) planar microdeformations are displayed in quartz grains from Vredefort rocks: (1) fluid inclusion trails, and (2) straight optical discontinuities that sometimes resemble lamellae. Both types occur as single features or as single or multiple sets in quartz grains. Besides qualitative descriptions of cleavage and recrystallization in feldspar and kinkbands in mica, no further microtextural evidence for shock metamorphism at Vredefort has been reported to date. Some 150 thin sections of Vredefort basement rocks were re-examined for potential shock and other deformation effects in all rock-forming minerals. This included petrographic study of two drill cores from the immediate vicinity of the center of the Dome. Observations recorded throughout the granitic core are given along with conclusions.
40Ar/39Ar thermochronology applied to plagioclase from the Morokweng LL chondrite possibly sugges... more 40Ar/39Ar thermochronology applied to plagioclase from the Morokweng LL chondrite possibly suggests large asteroid collisional (breakup?) events at ~2Ga and ~0.7 Ga and records the age of the impact on Earth at ~145 Ma.
ABSTRACT Unravelling the brittle history of old crystalline basements is a challenging task due t... more ABSTRACT Unravelling the brittle history of old crystalline basements is a challenging task due to the long and complex nature of their geological evolution and to the common lack of clear time markers. Pre-existing ductile fabrics often play an important role in the localization of younger brittle features, which invariably reflect and result from multiple structural reactivations during the numerous orogenic events that controlled the growth and evolution of the crustal blocks. In spite of these complexities, the careful and systematic study of outcrops containing predominantly brittle structures (i.e. brittle deformation zones, faults, fractures and associated fault rocks) with the aim to document their character and kinematics may become a valuable tool towards the unravelling of these structural histories and can help refine our understanding of reactivation processes in the brittle field. The exceptionally well exposed crystalline basement of the ca. 1.03 Ga Namaqua Metamorphic Complex (NMC) in western South Africa is used as an example. Remote sensing analysis highlighted several sets of kilometric conjugate shear fractures. Their geometric arrangement and kinematics are constrained by the offset of lithological markers and by detailed field investigations that allowed gathering a significant dataset of fault-slip data. Striated fault planes were observed throughout the NMC and were documented by paying great attention to their geometry, kinematics, mineral coating and to the rock type they deform. Paleo-stress tensors were computed on internally homogeneous fault-slip datasets and a tentative relative geochronological succession of brittle deformation events was established. This was aided by separating in time faulting events through the usage of Cretaceous weathering horizons, silicified fluvial deposits, paleosols and 77-54 Ma olivine melilitite plugs as time markers. The oldest features recognized formed during four compressional episodes assigned to the Neoproterozoic Pan African evolution. This history is expressed by sub-vertical conjugate fracture sets and fits well the inferences derived from remote sensing. The greatest compressive direction rotated from NW-SE to NNE-SSW and finally to almost E-W. A subsequent ENE-WSW-oriented extensional episode is associated with the local effects of the opening of the Atlantic Ocean and was followed by a second, ca. E-W extensional episode, linked to the well-acknowledged Mid-Cretaceous (115-90 Ma) event of margin uplift. A late Santonian (85-83 Ma) NWSE compressive paleostress deformed the Late Cretaceous sequences and was in turn followed firstly by a renewed episode of NE-SW extension and later by ca. NNE-SSW Late Maastrichtian (69-65 Ma) shortening. The latter is broadly coeval with the emplacement of the Gamoep magmatic suite. A phase of WNW-ESE Cenozoic extension is assigned to the extensional phase recorded in the Okawango delta, interpreted as reflecting propagation of the East African Rift System into southern Africa. No stress tensor was computed for the present day "Wegener anomaly" stress field, oriented NW-SE. However, the available in situ stress measurements from the region were used to perform slip tendency analysis, which indicates that, under the currently existing stress conditions, WNW-ESE- and NNW-SSE-striking faults are critically stressed and are the most likely reactivated, in agreement with the present seismicity.
Radiometric dating of melt rocks at impact craters has revealed that some giant impacts appear to... more Radiometric dating of melt rocks at impact craters has revealed that some giant impacts appear to overlap in time with major boundaries in Earth history [e.g., the Cretaceous–Tertiary (K/T) and Jurassic–Cretaceous (J/K) boundaries]. The Morokweng impact crater in South Africa is coincident in age with the J/K boundary. However, the types of objects that generate large craters are poorly known because it is difficult to unambiguously identify the projectile from the signature it imparts into the impact rocks. Meteorites are highly enriched in the platinum-group elements (PGE), which have been widely used as a tool for identifying the presence of a meteorite signature. Here we present new PGE analyses from the Morokweng impact melt sheet. Our data reveal high PGE concentrations and high degree of PGE correlation through the melt sheet. Regression analysis was used to determine the projectile PGE signature and constrain input from the terrestrial target rocks. The closest match to Morokweng is the PGE signature of ordinary (L or LL) chondrite meteorites, which is broadly in agreement with the results of an earlier Cr isotope study. The results of these independent studies provide strong evidence that a large, ordinary chondrite projectile struck the area of Morokweng in the late Jurassic.
Radiometric dating of melt rocks at impact craters has revealed that some giant impacts appear to... more Radiometric dating of melt rocks at impact craters has revealed that some giant impacts appear to overlap in time with major boundaries in Earth history [e.g., the Cretaceous-Tertiary (K/T) and Jurassic-Cretaceous (J/K) boundaries]. The Morokweng impact crater in South Africa is coincident in age with the J/K boundary. However, the types of objects that generate large craters are poorly known because it is difficult to unambiguously identify the projectile from the signature it imparts into the impact rocks. Meteorites are highly enriched in the platinum-group elements (PGE), which have been widely used as a tool for identifying the presence of a meteorite signature. Here we present new PGE analyses from the Morokweng impact melt sheet. Our data reveal high PGE concentrations and high degree of PGE correlation through the melt sheet. Regression analysis was used to determine the projectile PGE signature and constrain input from the terrestrial target rocks. The closest match to Morokweng is the PGE signature of ordinary (L or LL) chondrite meteorites, which is broadly in agreement with the results of an earlier Cr isotope study. The results of these independent studies provide strong evidence that a large, ordinary chondrite projectile struck the area of Morokweng in the late Jurassic.
Egu General Assembly Conference Abstracts, Apr 1, 2013
To mitigate the uncertainties in assessing the geohazards and rock conditions that affect the nuc... more To mitigate the uncertainties in assessing the geohazards and rock conditions that affect the nuclear, mining (including hydrocarbon extraction) and civil engineering activities in South Africa, the authors are working to improve the data coverage concerning the present day stress field. In principle, this implies constraining the principal compressive stresses (σ1>σ2>σ3) or at least the maximum horizontal compressive stress (σH) because knowledge of these parameters may determine the reactivation potential of known faults, or the behaviour of large excavations and wells. By contrast, much of the subcontinent is under-represented in the World Stress Map database. For this reason we have taken a number of steps, firstly by installing a compact Trillium seismic sensor at Stofkloof (Namaqualand; adjacent to the Vaalputs low and intermediate level radioactive waste disposal facility) and 1-sec sensors at Aggeneys and Koffiemeul (Bushmanland). All stations are equipped with Reftek data loggers and powered by solar panels. The data from these stations will be integrated with data from the national network to obtain focal mechanism solutions for seismic events in the Northern Cape - southernmost Namibia region (also known as the Grootvloer cluster). These neotectonic stress tensors are then combined with σH parameters obtained from calliper logs of off-shore wells and from the geometry of joints, faults and sheared fractures in palaeosols (Bushmanland), soils and calcrete (NW Free State) and aeolianites (southern Cape). We also include underground rock engineering phenomenological observations and measurements, and data in the public domain. Our data consistently indicate a NNW-SSE oriented σH (Wegener Stress Anomaly or WSA) that prevails across most of central, southern and western South Africa, Namibia up to the Ruacana hydroelectric power plant at the Angola border. However, in the Congo basin, a few earthquake focal mechanisms suggest rotation of the regional σH to an E-W direction. Geological units affected by the WSA include the Cretaceous oceanic lithosphere (Walvis Ridge), the southern Angola-Kasai craton, the offshore Outeniqua and Orange Basins, the Cape Fold Belt, the Namaqualand metamorphic complex, and the Archaean Craton up to the Witwatersrand basin and the Witbank coal field. In contrast, σH azimuths in the NE-SW quadrants seem prevalent in E Mpumalanga, N Natal, and northern Limpopo. Whereas the origin of these latter stress azimuths are probably linked to the propagation of the E African Rift System, the strike-slip to transpressional character of much of the WSA remains unexplained. Similarly puzzling are a 3-fold increase in seismic events (proxy for strain rate) over the past 20 years in the Grootvloer cluster, and evidence that the WSA is the last of at least 7 successive tectonic regimes to leave their brittle imprints along the SE Atlantic seaboard since the break-up of W Gondwana.
Models predicting the size of the Vredefort Crater vary considerably due to the fact that so litt... more Models predicting the size of the Vredefort Crater vary considerably due to the fact that so little of the original crater is exposed. In this study we combine geological observation with magnetic and gravity imaging to visualize the original crater.
In the present study we have reported our detailed petrographic observations on the type of shock... more In the present study we have reported our detailed petrographic observations on the type of shocked deformations on two boreholes drilled at ~18-20 km distance to the southwest and northwest of the center of the Morokweng impact crater, South Africa.
The paper reviews more than 20 years of structural, stratigraphic and seismic monitoring studies ... more The paper reviews more than 20 years of structural, stratigraphic and seismic monitoring studies focused on the Vaalputs radioactive waste disposal site, 100 km SSE of Springbok, in Namaqualand. Our finds, supported by the recordings of two 3-components seismometers, show that the frequency of seismic events in this region may be slowly increasing over time, that the predicted Mmax is
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