Vredefort Structure Research Articles

The auriferous Late Archaean Central Rand Group of South Africa: sea-level control of sedimentation?

Most earth scientists agree that either, or both, eustacy and tectonism play an important role in controlling sedimentation. In the case of the auriferous late Archaean Central Rand Group of South Africa, most early workers favoured a marine depositional environment for the gold placers. Transgressions and regressions, used to explain vertical alternations of coarse- and fine-grained deposits, were attributed solely to vertical movements of the basin floor, with no reference to eustatic variations. Tidal indicators are regarded as unequivocal evidence for a basin having been connected to the ocean, and their recognition is considered a preliminary requirement for investigation of sea-level fluctuations. Unambiguous recognition of sea-level fluctuations in the rock record is problematic in some cases, especially in non-fossiliferous deposits. To facilitate recognition in such deposits, the following criteria are proposed: (a) presence of thick, laterally continuous mudstones; (b) alternating coastal/marine and continental facies in the vertical profile; (c) onshore–offshore oscillating baylines; (d) marine facies unconformably overlain by fluvial facies; (e) turbidites, subaqueous debris flows and submarine canyons; (f) incised valleys. The Turffontein Subgroup of the Vredefort Structure, situated roughly at the centre of the preserved Witwatersrand Basin, is subdivided into five formations, Rt1 through Rt5. Tidal bundles in Rt3, some with mud-draped foresets, provide evidence for connection of the Witwatersrand Basin to the ocean during Central Rand times. Formation Rt4 is interpreted as being shallow marine in origin, and is unconformably overlain by a sandy fluvial succession, Rt5, thereby suggesting a regression and sea-level fall. In the Heidelberg area, in the northeastern part of the Witwatersrand Basin, the lower part of the Turffontein Subgroup consists predominantly of sandstone, with subordinate conglomerate, mudstone and diamictite. This succession also exhibits most of the characteristics, a to e, given above. Two deep scours that eliminate the rich Carbon Leader occur in parts of the Carletonville goldfield. The eastern scour has a fill comprising predominantly mudstone, and displays several additional characteristics of incised valley systems, such as a downstream increase in depth, and typical lateral and vertical facies arrangements. Texturally mature fluvial units occurring at different palaeotopographic levels indicate possible multiple episodes of incision and fill, suggesting lower-order sea-level fluctuations, superimposed upon a higher-order cycle. The results of the case studies presented provide clear evidence for sea-level control of deposition in the distal parts of the Witwatersrand Basin during Central Rand times. However, in proximal regions, sedimentation was probably governed by source area tectonism.

Age and isotopic evidence for the origin of the Archæan granitoid intrusives of the Johannesburg Dome, South Africa

Results of RbSr, PbPb and SmNd whole rock, Rbr biotite and PbPb zircon evaporation analyses are presented for certain granitoid rocks from the Johannesburg Dome. These data indicate that the granodiorite, granite and leucosome from migmatite were emplaced ∼ 3090 Ma ago, were genetically related and were derived primarily from a source between ∼ 3300 and ∼ 3500 Ma old. A portion of the granodiorite and granite might have been derived from a source between ∼ 4000 and ∼ 4300 Ma old. The tonalite was emplaced ∼ 3170 Ma ago and was derived from a source between 3.3 and 3.5 Ga old. RbSr biotite-whole rock ages, ranging between about ∼ 2614 and ∼ 2080 Ma, probably reflect complete resetting during differential uplift, erosion and cooling of the granitoid rocks in the Neoarchæan and Palæoproterozoic. If so, they apparently were not influenced by the emplacement of the ∼ 2060 Ma Bushveld Igneous Complex or the ∼ 2000 Ma Vredefort event. The granodiorite, granite and leucosome were emplaced coeval with and may be genetically related to compositionally similar plutonic and volcanic rocks in the Barberton area, Vredefort structure and Dominion Group.

Vredefort shatter cones revisited

Shatter cones have been described from a number of circular and polygonal structures worldwide, the origin of which has been alternatively ascribed to the impacts of large extraterrestrial projectiles or to catastrophic endogenic processes. Despite their association with enigmatic, catastrophic processes, the nature of shatter cones and the physics involved in their formation have not been comprehensively researched. Results of detailed field and laboratory studies of shatter cones from three areas in the collar of the Vredefort Dome in South Africa are presented. Vredefort shatter cones are directly related to a widely displayed fracture phenomenon, termed “multiply striated joint sets (MSJS)”. MSJs are planar to curviplanar fractures occuring at spacings of < 1 to several millimeters. The joint sets have a fractal character. When a new measurement protocol is used in the field, involving study of all joint surfaces and all steps and striae exposed on these surfaces, new information is gained on the genesis and significance of the MSJS and on their relationship to striated conical fractures. The internal constitution of a rock specimen with MSJS was examined in detail, by documenting the precise geometry of many fractures in a suite of parallel thin sections transecting the specimen. The steps and striae on shatter cone surfaces have the characteristics of displacement fractures (microfaults), along which evidence of melting is observed. Shatter cone and MSJS surfaces are often covered with glassy films; we evaluate whether these fracture phenomena are linked to the formation of pseudotachylitic (friction) melt. Our field and petrographic observations can be interpreted as consistent with the generation of shatter cones/MSJS relatively late in the formation of the Vredefort structure. This scenario contrasts sharply with the widely held view that shatter cones are formed during the early “compression” phase of a shock event that affected horizontal strata.

Archean age for the granulite facies metamorphism near the center of the Vredefort structure, South Africa

Granulite facies metamorphic assemblages in rocks exposed near the center of the 2.02 Ga Vredefort impact structure previously have been interpreted either as Early Proterozoic, genetically related to the 2060 Ma Bushveld Complex, or as Archean, and representative of lower crust that rebounded to upper crustal levels following an impact event. Zircon and monazite recovered from the granulite facies rocks record high-grade metamorphism at 3107 ± 9 Ma and a primary age of ≥ 3425 Ma for detrital zircon. A shock-deformed, but otherwise pristine, dolerite dike that intrudes the granulite terrane yields a U-Pb zircon age of ≥ 2560 Ma, providing a minimum age for the metamorphism. These isotopic age data are difficult to reconcile with a regional high-grade metamorphic event in the crust beneath Vredefort at 2060 Ma. Instead, the preimpact, high-temperature metamorphic history observed in the Vredefort lower crustal rocks indicates an enigmatic high-temperature event during the stabilization of diamondiferous Archean tectosphere.

Inaugeral lecture - Meteorite impacts on Earth and on the Earth Sciences

Plate tectonics has become established during the last thirty years as a coherent explanation for the major features of the Earth today. The formation of new crust at mid-ocean ridges, the movement of relatively rigid lithospheric plates over the softer underlying asthenosphere, and the collision of plates throwing up the Earth's major mountain ranges, are familiar concepts to many people. However, recent discoveries in Earth sciences challenge us to a newer concept of the Earth's evolution. These discoveries indicate that large meteorite impacts have had enormous effects on the Earth, perhaps most dramatically illustrated 65 million years ago, when an impact at Chicxulub in the Caribbean may have been responsible for mass extinctions (including the dinosaurs) on a global scale. The evidence for these catastrophic events is surprisingly enigmastic in the geological record, and has been the subject of intense scientific debate and disagreement. Some of the major types of direct evidence for meteorite impacts include the craters formed by impact, and the effects of impact on rocks, including melting the formation of shatter cones, and a whole variety of small-scale features that are revealed under the microscope. Perhaps paradoxically, these microstructures are the most unambiguous evidence for meteorite impacts, because they formed under radically greater stresses, temperatures and strain rates than those of plate tectonic processes, and are therefore quite distinct from plate tectonic microstructures. These features of meteorite impacts on Earth are illustrated by examples from several known impact sites, including the Vredefort structure in South Africa and a possible impact structure at Highbury south of Mhangura in Zimbabwe. Large meteorite impacts on Earth have significantly augmented Earth's mineral resources since the formation of the Earth, by creating diamonds in the ultra-high pressure conditions of impact, by providing structures that trap petroleum, and possibly by creating base metal deposits. The likely environmental consequences of impacts include a searing heat wave followed by global cooling, causing episodes of mass extinction that may occur in cycles. There is some controversial evidence for the theory that the first life on Earth itself may have been transported here on meteorites from Mars. The possibility of a major meteorite impact on Earth in the near future emphasizes the dramatic nature of these recent discoveries, which are having deep impacts in the Earth sciences, possibly even constituting a scientific revolution. Journal of Applied Science in Southern Africa Vol.5, No.1 pp. 63-80

Ultramafic rocks at the center of the Vredefort structure: Further evidence for the crust on edge model

Ultramafic rocks from the center of the Vredefort structure previously have been interpreted either as Early Proterozoic igneous rocks genetically related to the Bushveld complex, or as Archean upper mantle rocks. Re-Os isotopic data from these rocks display very low 187 Os/ 188 Os and 187 Re/ 188 Os, suggesting that they are not magma chamber cumulates, but rather the residues of melt extraction, similar in Re-Os systematics to subcontinental lithospheric mantle. Whole-rock Re depletion model ages for two samples are 3.3 and 3.5 Ga, indicating that the postulated melt extraction event must have occurred in the Archean. On the basis of these data we conclude that the ultramafic rocks are not related to the Bushveld event, but rather represent upper mantle that rebounded to crustal levels following an impact event. This conclusion strengthens evidence for a model whereby the rocks exposed in the core of the Vredefort structure represent the lower parts of a crust on edge profile that possibly contains a paleo-Moho very near the Earth9s surface.

Constraints on the size of the Vredefort impact crater from numerical modeling

Abstract— The Vredefort structure in South Africa was created by a meteorite impact about two billion years ago. Since that time, the crater has been deeply eroded; so to estimate its original size, researchers have had to rely heavily upon comparison to other terrestrial impact structures. Recent estimates of the original crater diameter range from 160 km to as much as 400 km. In this study, we combined the capabilities of both hydrocode and finite‐element modeling, using the former to predict where the pressure of an impact‐generated shock wave would have been high enough to form planar deformation features (PDFs) and shatter cones and the latter to follow the subsequent displacement of these shock isobars during the collapse of the crater. We established constraints on the sizes of the projectile and the transient crater (and, thus, on the size of the final crater) by comparing the observed locations of PDFs around Vredefort to the results of our simulations of impacts by projectiles of various sizes. These simulations indicate that a rocky projectile with a diameter of ∼10 km, impacting vertically at a velocity of 20 km/s generates shock pressures that are consistent with the distribution of PDFs around Vredefort. These projectile parameters correspond to a transient crater ∼80 km in diameter or a final crater ∼120–160 km in diameter. Allowing for uncertainties in our modeling procedures, we consider final craters 120 to 200 km in diameter to be consistent with the observed locations of PDFs at Vredefort. The shock pressure contour corresponding to the formation of shatter cones is almost horizontal near the surface, making the locations of these features less useful constraints on the crater size. However, they may provide a constraint on the amount of erosion that has occurred since the impact.

Integrated geophysical modelling of a giant, complex impact structure: anatomy of the Vredefort Structure, South Africa

Only three very large, confirmed impact structures are known on Earth: the Chicxulub Crater (Mexico), 65 Ma, ca. 180 km wide; the Sudbury Structure (Canada), 1.85 Ga, 200 km in diameter, and the Vredefort Structure in South Africa, 2.02 Ga. While extensive data on large impact structures have been obtained by remote sensing studies of such features on other planetary bodies, only this small number of large terrestrial impact structures can provide data crucial to understanding these catastrophic impact processes on Earth. Integrated modelling of gravity and magnetic data, constrained by geological as well as refraction and reflection seismic data, accomplished the reconstruction of the Vredefort impact structure in South Africa, approximately 250 km wide. The original Vredefort impact structure covered the whole extent of the Archaean Witwatersrand Basin, distinguished by enormous gold resources, as it is structurally preserved today. In fact, it is clear that the preservation of vast volumes of economically important Witwatersrand strata is the direct result of the formation of the ring basin around the central uplift (the Vredefort Dome) of the impact structure. This study is the first attempt to create an integrated and geophysically well-constrained model of this very large, complex impact structure.

Ultramafic rocks in the centre of the Vredefort structure (South Africa): geochemical affinity to Bushveld rocks

The geochemistry and mineral chemistry of ultramafic rocks in the centre of the Vredefort structure have been reinterpreted. A detailed examination of previously published major element, trace and REE analyses as well as mineral analyses strongly suggests a magmatic origin in contrast to previously postulated tectonic emplacement of mantle material. A strong geochemical affinity to ultramafic rocks from the Bushveld Complex is evident. The magmatic nature of the ultramafic rocks in the centre of the Vredefort structure has implications for the discussion of the metamorphic history of the Vredefprt structure which shows evidence of unusually high, and as yet not satisfactorily explained, geothermal gradients during peak metamorphism.

A 2.023 Ga age for the Vredefort impact event and a first report of shock metamorphosed zircons in pseudotachylitic breccias and Granophyre

UPb isotope systematics of shock metamorphosed zircon grains from pseudotachylitic breccias and Granophyre from the controversial Vredefort Structure, South Africa, provide new and compelling evidence for an impact origin for this structure. Zircon grains from these rocks exhibit planar microstructures and polycrystalline textures similar to those from the Chicxulub crater breccia, K/T boundary ejecta, and rocks from the Sudbury Structure. A concordant 2023 ± 4 Ma (2σ) age for newly crystallized, unshocked zircon grains from recrystallized pseudotachylitic breccia from the central part of the Vredefort Structure provides a good approximation of the time of impact. This age indicates that the impact post-dates Bushveld magmatism by at least 30 m.y. UPb isotopic results for individual, pre-impact zircon grains indicate crystallization ages from about 3060 to 3300 Ma and Pb loss at ca. 2000 Ma. Data for high U grains plot below a discordia line from 3060 to 2023 Ma and indicate both impact- and post-impact related Pb loss. The data and granular morphology of a zircon grain from the Granophyre indicates probable ca. 2.0 Ga and ca. 1.0 Ga Pb loss. Although planar microstructures in zircon are ubiquitous, there are also some unshocked, low-U grains, and these record a ca. 3.1 Ga primary age. The older ca. 3.1–3.3 Ga ages for shocked zircons reflect formation and modification of granitoid crust in the region of the Vredefort Structure prior to and during a metamorphic event at about 3080 Ma.The resilience of zircon shock features to post-impact alteration and annealing, in combination with precise UPb dating of individual shocked grains provide a valuable method for indentifying ancient, metamorphosed and tectonically modified impact structures.

Geology and evolution of the Vredefort impact structure, South Africa

The importance of large impact events for the evolution of all bodies in the Solar System has been recognised by a large proportion of the geological community only in the past 15 years. More than 150 impact structures are now known on the Earth's surface. The origin of the Vredefort dome, a ca 70 kilometre wide structural uplift terrane in the centre of the economically important Witwatersrand basin, by either endogenic or impact processes, has long been controversial. Detailed microdeformation studies have recently proven that the dome represents the central uplift of one of the largest (original diameter: ca 300 kilometres) and oldest (2023 ± 4 Ma) known terrestrial impact structures. The Vredefort structure is not only of importance because of its controversial origin and particular setting in the centre of the economically important Witwatersrand basin. This structure is also the type locality for pseudotachylitic breccia and well-known for its abundant shatter cones and the enigmatic Vredefort granophyre. A wealth of new data on the Vredefort structure have become available in recent years: detailed microdeformation studies have revealed the presence of bona fide shock metamorphic effects in quartz and zircon; UPb dating of single zircons from pseudotachylitic breccia have provided reliable age information regarding the time of impact and have permitted temporal separation of the Vredefort impact event from the emplacement of the 2050 Ma Bushveld complex. This achievement has major implications for the understanding of the impact-related thermal and hydrothermal effects in the Witwatersrand basin as a whole and the regional metamorphic evolution. As continued study of the Vredefort structure has the potential to contribute much more to the understanding of large impact cratering events and their potential geological and economic effects, besides contributing to the general geological knowledge of the evolution of the Kaapvaal craton, this paper reviews the current knowledge on the Vredefort structure and to emphasise those areas in need of future investigation.

A TEM investigation of shock metamorphism in quartz from the Sudbury impact structure (Canada)

The Sudbury basin in Canada is an elliptical feature (approx. 60 × 27 km in size) that is now widely believed to be part of a large (approx. 200 km diameter) meteorite impact structure which formed about 1.85 Ga ago and was subsequently deformed and metamorphosed. Despite prolonged debate over the origin and original size of the Sudbury structure, strong evidence for meteorite impact has been provided by the discovery in its rocks of a wide range of shock-metamorphic features, especially the optically observable traces (fluid inclusion arrays) of former Planar Deformation Features (PDFs) in quartz parallel to {10 1 3} planes. In this new examination of Sudbury samples, no preserved original PDFs (glassy lamellae) were observed optically in quartz grains from basement rock fragments in the Onaping Formation, a unit interpreted as a ‘fallback breccia’ deposited within the original crater. However, TEM revealed other thin lamellar features preserved in the quartz; these are identified as Brazil twin lamellae parallel to the basal plane (0001). These lamellae are 15–200 nm thick, show a typical spacing of 30 nm to several microns apart, and are occasionally decorated with fluid inclusions (≤ 0.5 μm in size), which probably formed during post-shock alteration and annealing. Numerous subgrain boundaries (SGBs) were also detected, many of them oriented roughly parallel to the basal plane (0001). The SGBs apparently formed during post-shock recrystallization, leading to the local disappearance of Brazil twin lamellae. Experimental evidence suggests that such basal Brazil twins are the unique product of high-pressure shock waves. The features in the Sudbury samples are identical to those observed in the Vredefort structure, South Africa, which is also widely accepted as an ancient impact structure about 2.0 Ga old. The recognition of basal Brazil twins in quartz at Sudbury provides additional evidence for meteorite impact origin and also emphasizes the high value of these durable shock features for identifying old and tectonically metamorphosed impact structures.

Re-Os isotope and geochemical study of the Vredefort Granophyre: Clues to the origin of the Vredefort structure, South Africa

Research Article| October 01, 1996 Re-Os isotope and geochemical study of the Vredefort Granophyre: Clues to the origin of the Vredefort structure, South Africa Christian Koeberl; Christian Koeberl 1Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria Search for other works by this author on: GSW Google Scholar Wolf Uwe Reimold; Wolf Uwe Reimold 2Department of Geology, University of the Witwatersrand, Johannesburg 2050, South Africa Search for other works by this author on: GSW Google Scholar Steven B. Shirey Steven B. Shirey 3Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, D.C. 20015 Search for other works by this author on: GSW Google Scholar Author and Article Information Christian Koeberl 1Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria Wolf Uwe Reimold 2Department of Geology, University of the Witwatersrand, Johannesburg 2050, South Africa Steven B. Shirey 3Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, D.C. 20015 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1996) 24 (10): 913–916. https://doi.org/10.1130/0091-7613(1996)024<0913:ROIAGS>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Christian Koeberl, Wolf Uwe Reimold, Steven B. Shirey; Re-Os isotope and geochemical study of the Vredefort Granophyre: Clues to the origin of the Vredefort structure, South Africa. Geology 1996;; 24 (10): 913–916. doi: https://doi.org/10.1130/0091-7613(1996)024<0913:ROIAGS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The origin of granophyric rock dikes (the Vredefort Granophyre) at the 2 Ga Vredefort structure in South Africa has been controversial. They have been interpreted as either impact melt or igneous intrusions. Most Vredefort Granophyre samples have considerably higher Os contents than the country rocks and 187Re/188Os and 187Os/188Os ratios that scatter about a 2 Ga isochron. Their initial 187Os/188Os ratios (at 2 Ga) overlap the meteoritic data range, indicating that all the granophyre samples contain some meteoritic Os. The Re-Os isotopic composition of the granophyre is significantly different from that of local precursor rocks and indicates that up to 0.2% of a chondritic component could be contained in the granophyre. Our results confirm the interpretation that the Vredefort Granophyre represents an impact melt rock. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Laser probe argon‐40/argon‐39 dating of coesite‐ and stishovite‐bearing pseudotachylytes and the age of the Vredefort impact event

Abstract— Age determinations have been made on pseudotachylytic rocks from the controversial Vredefort structure of South Africa using the laser microprobe 40Ar/39Ar dating technique. Coesite‐ and stishovitebearing veins in a quartzite from the Central Rand Group of the collar rocks were dated using a 10‐μm diameter focused ultra‐violet laser beam. These yielded a weighted mean age of 2027 ± 18 Ma (2σ). Six pseudotachylytes, sampled from four different locations within the Outer Granite Gneiss of the core, were dated using an 50–100‐μm diameter focused infrared laser beam. These pseudotachylytes exhibit altered vein margins with apparent ages considerably younger than ages obtained from the fresher centres of veins. The best weighted mean pseudotachylyte matrix age obtained was 2018 ± 14 Ma (2σ). Most of the clasts within the pseudotachylyte matrices retain significantly older (e.g., Archean) ages, indicative of their parent rock history. Our results show that five of the seven dated samples possess matrix ages of ∼2000 Ma, similar to the age of the Granophyre (Walraven et al., 1990), a supposed impact melt rock (French and Nielsen, 1990). The dating of coesite‐ and stishovite‐bearing veins equates the shock event with pseudotachylyte formation, generation of the Granophyre and creation of the Vredefort structure. The results affirm that the Vredefort Dome is a meteorite impact structure and show that it formed at 2018 ± 14 Ma (2σ).

Magnetic anomaly near the center of the Vredefort structure: Implications for impact-related magnetic signatures

A strong magnetic anomaly near the center of the ancient and deeply eroded Vredefort structure is attributed to remanent magnetization caused by a large meteorite impact at ∼2.0 Ga. The rocks underlying the anomaly are Archean gneisses thought to represent mid-crustal depths that were uplifted to the surface during the postulated impact event. Measurements of the remanent magnetization of the basement rocks yielded consistent vectors of declination = 25° , inclination = 56° , k = 18, α = 16 that correspond to the paleomagnetic pole position at time of impact. Petrologic studies indicate that during impact, large volumes of rock were heated enough to cause thermal remagnetization in the ambient field. Thermal effects of all large impacts on cratons may induce a remanent magnetization of sufficient intensity to cause anomalies in the geomagnetic field that are detectable even by satellites.

Magnetic anomaly near the center of the Vredefort structure: Implications for impact-related magnetic signatures: Comment and Reply

Magnetic anomaly near the center of the Vredefort structure: Implications for impact-related magnetic signatures: Comment and Reply

A TEM investigation of shock metamorphism in quartz from the Vredefort dome, South Africa

The origin of the Vredefort structure in South Africa is still debated. Several causes have been discussed, namely asteroid impact, internal gas explosion or tectonic processes. Evidence of dynamic rock deformation is pervasive in the form of planar features in quartz grains, shatter cones, veins of pseudotachylite and occurrence of coesite and stishovite (high-pressure quartz polymorphs). A number of these characteristics is widely believed to support an impact origin. However, the planar features in quartz, which are generally considered as one of the strongest indicators of impact, are in the Vredefort case considered as anomalous when compared with those from accepted impact structures. We have investigated by optical and transmission electron microscopy (TEM) the defect microstructures in quartz grains from different lithologies sampled at various places at the Vredefort structure. Whatever the locality, only thin mechanical Brazil twin lamellae in the basal plane are observed by TEM. So far, such defects have only been found in quartz from impact sites, but always associated with sets of thin glass lamellae in rhombohedral planes 10−1 n with n = 1, 2, 3, and 4. At the scale of the optical microscope, Brazil twins in (0001) are easily detected in Vredefort quartz grains because of the numerous tiny fluid inclusions which decorate them. Similar alignments of tiny fluid inclusions parallel to other planes are also detected optically, but at the TEM scale no specific shock defects are detected along their traces. If these inclusion alignments initially were shock features, they are now so severely weathered that they can no longer be recognized as unambiguous shock lamellae. Fine-grained coesite was detected in the vicinity of narrow pseudotachylite veinlets in a quartzite specimen, but stishovite was not found, even in areas where its occurrence was previously reported. Finally, definite evidence of high-temperature annealing was observed in all the samples. These observations lead us to the conclusion that our findings regarding microdeformation in quartz are consistent with an impact origin for the Vredefort structure. Most of the original shock defects are now overprinted by an intense post-shock annealing episode. Only the thin mechanical twin lamellae in the basal plane have survived.

A Review of the Geology of and Deformation Related to the Vredefort Structure, South Africa

The origin of the Vredefort Structure in South Africa has been for decades the subject of vigorous controversy between two schools that either favor an origin by bolide impact or by internal processes, namely an internal gas explosion or tectonic forces. The question at the center of this debate is whether Vredefort's enigmatic rock defamation phenomena, exceptional pseudotachylite occurrences, shatter cones, microdeformation features in quartz, as well as occurrences of coesite and stishovite, are of impact-diagnostic value. This article reviews the geology of the Vredefort Structure and evaluates the various hypotheses about its origin.

LASER step-heating [formula omitted] age spectra from early Archean (~3.5 Ga) Barberton greenstone belt sediments: A technique for detecting cryptic tectono-thermal events

Samples from sediments of the Fig Tree Group located in the central part of the circa 3.2 to 3.5 Ga Barberton greenstone belt (BGB) have been analyzed by the 40Ar 39Ar laser step-heating technique. This technique has enabled previously cryptic thermal overprints to be detected in various sedimentary units which include: reworked chemical sediments (barite), clastic sediments (sandstone/shale), and one stromatolite. In most cases the apparent age plateaux can be identified by corresponding Ca K and Cl K ratio plots as belonging to separate mineral phases. Most of the samples exhibit simple Ar diffusion loss during the low temperature part of the experiments while occasionally showing excess Ar, or possible 39Ar recoil, effects. The various sediments analyzed show evidence for distinct overprinting between 2025 and 2090 Ma. One barite sample gave T 0 (original blocking age) = 2673 ± 3 Ma (1σ) which is close to a calculated model Ar diffusion-loss age for the same sample of 2688 ± 3 Ma (1σ). This age is interpreted as representing final granitoid activity adjacent to the BGB, and/or craton-scale tectonism associated with the Limpopo Orogeny. Two samples gave T 0 ages of ∼2350–2400 Ma which may reflect increased thermal gradients associated with the formation of the thick Transvaal sedimentary basin that may once have covered the BGB. The dominant apparent age plateaux together with modelled diffusive Ar loss ages of 2025–2090 Ma, are thought to represent regional thermal anomalies related to large-scale tectono-thermal activity in the Kaapvaal craton, of which the Bushveld Complex (which covers a surface area of ~ 60,000 km 2) and formation of the Vredefort Structure are obvious manifestations. The strong thermal overprinting recorded by the sediments has effectively removed any ancient atmosphere (i.e., 40Ar 36Ar ratios < 295.5) signatures.

Aspects of the dynamic and thermal metamorphic history of the Vredefort cryptoexplosion structure: implications for its origin

The Vredefort structure is the oldest and the largest known cryptoexplosion structure on earth. An approximately 36 km deep section through the Archean sialic crust and the overlying Precambrian strata of the Kaapvaal craton is exposed in the core of the structure. The geology presented in the exposed section includes all the principal metamorphic facies in the crust and records a long and complex thermo-tectonic history which dates back to at least 3.5 Ga. The petrographie and geological observations in the basement rocks indicate that there is a complex interrelationship between the Archean geology and the 2.0 Ga dynamic and thermal metamorphic overprint (some of which are postulated to be indicative of impact processes). The dynamic and thermal metamorphic effects do not increase progressively towards the centre of the structure as found at known impact structures. In particular, dynamic deformation phenomena such as pseudotachylite and planar features in quartz reach maximum intensity in the rocks close to the Vredefort discontinuity, a brittle-ductile shear zone which separates upper crustal amphibolite facies rocks from lower crustal granulites. In certain other lithological zones, deformation phenomena are noticeably absent or diminished. We suggest that changes in the physical and chemical properties of the rocks from margin to centre of the basement may account for the variation in the intensity of the 2.0 Ga thermal and dynamic metamorphic effects observed at Vredefort. In conclusion, our overall impression of the Vredefort structure is that it is a relic of an ancient meteorite impact crater, but that there were thermo-tectonic events which occurred both prior to and after the postulated impact event, which complicates the interpretation of its origin.