scholarly journals Post-Impact Faulting of the Holfontein Granophyre Dike of the Vredefort Impact Structure, South Africa, Inferred from Remote Sensing, Geophysics, and Geochemistry

Geosciences ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 96
Author(s):  
Martin D. Clark ◽  
Elizaveta Kovaleva ◽  
Matthew S. Huber ◽  
Francois Fourie ◽  
Chris Harris
Keyword(s):  
South Africa ◽  
Remote Sensing ◽  
Crater Floor ◽  
Impact Structure ◽  
Impact Melt ◽  
Geophysical Modeling ◽  
Tectonic Events ◽  
Post Impact ◽  
The Impact

Better characterization features borne from long-term crustal modification processes is essential for understanding the dynamics of large basin-forming impact structures on Earth. Within the deeply eroded 2.02 Ga Vredefort Impact Structure in South Africa, impact melt dikes are exposed at the surface. In this study, we utilized a combination of field, remote sensing, electrical resistivity, magnetic, petrographical, and geochemical techniques to characterize one such impact melt dike, namely, the Holfontein Granophyre Dike (HGD), along with the host granites. The HGD is split into two seemingly disconnected segments. Geophysical modeling of both segments suggests that the melt rock does not penetrate below the modern surface deeper than 5 m, which was confirmed by a later transecting construction trench. Even though the textures and clast content are different in two segments, the major element, trace element, and O isotope compositions of each segment are indistinguishable. Structural measurements of the tectonic foliations in the granites, as well as the spatial expression of the dike, suggest that the dike was segmented by an ENE–WSW trending sinistral strike-slip fault zone. Such an offset must have occurred after the dike solidified. However, the Vredefort structure has not been affected by any major tectonic events after the impact occurred. Therefore, the inferred segmentation of the HGD is consistent with long-term crustal processes occurring in the post-impact environment. These crustal processes may have involved progressive uplift of the crater floor, which is consistent with post-impact long-term crustal adjustment that has been inferred for craters on the Moon.

scholarly journals Ocean resurge-induced impact melt dynamics on the peak-ring of the Chicxulub impact structure, Mexico

2021 ◽  
Author(s):  
Felix M. Schulte ◽  
◽  
Axel Wittmann ◽  
Stefan Jung ◽  
Joanna V. Morgan ◽  
...  
Keyword(s):  
Impact Structure ◽  
Physical Processes ◽  
Hydrothermal Conditions ◽  
Chemical Examination ◽  
Impact Melt ◽  
Target Rock ◽  
Single Process ◽  
Chicxulub Impact ◽  
The Impact

AbstractCore from Hole M0077 from IODP/ICDP Expedition 364 provides unprecedented evidence for the physical processes in effect during the interaction of impact melt with rock-debris-laden seawater, following a large meteorite impact into waters of the Yucatán shelf. Evidence for this interaction is based on petrographic, microstructural and chemical examination of the 46.37-m-thick impact melt rock sequence, which overlies shocked granitoid target rock of the peak ring of the Chicxulub impact structure. The melt rock sequence consists of two visually distinct phases, one is black and the other is green in colour. The black phase is aphanitic and trachyandesitic in composition and similar to melt rock from other sites within the impact structure. The green phase consists chiefly of clay minerals and sparitic calcite, which likely formed from a solidified water–rock debris mixture under hydrothermal conditions. We suggest that the layering and internal structure of the melt rock sequence resulted from a single process, i.e., violent contact of initially superheated silicate impact melt with the ocean resurge-induced water–rock mixture overriding the impact melt. Differences in density, temperature, viscosity, and velocity of this mixture and impact melt triggered Kelvin–Helmholtz and Rayleigh–Taylor instabilities at their phase boundary. As a consequence, shearing at the boundary perturbed and, thus, mingled both immiscible phases, and was accompanied by phreatomagmatic processes. These processes led to the brecciation at the top of the impact melt rock sequence. Quenching of this breccia by the seawater prevented reworking of the solidified breccia layers upon subsequent deposition of suevite. Solid-state deformation, notably in the uppermost brecciated impact melt rock layers, attests to long-term gravitational settling of the peak ring.

scholarly journals Clarketal_2021.zip

2021 ◽  
Author(s):  
Martin Clark
Keyword(s):  
South Africa ◽  
Remote Sensing ◽  
Impact Structure ◽  
Aerial Photo ◽  
Special Issue ◽  
Still Images ◽  
Spatial Expression ◽  
Post Impact

This dataset is supplementary to a submitted manuscript to MDPI: Geosciences, "Making an Impact" special issue.<div><br></div><div>Manuscript title: Post-impact faulting of the Holfontein Granophyre Dike of the Vredefort Impact Structure, South Africa, inferred from remote sensing, geophysics, and geochemistry.</div><div>Authors: Clark, M.D., Kovaleva, E., Huber, M.S., Fourie, F., Harris, C.<br><div><br></div><div>The file contains an aerial photo and videos acquired via a drone which document the spatial expression of the geological dike.</div></div><div><br></div><div>Resolution of video is 4k (3840x2160 pix). Resolution of still images is 5472x3648 pix.</div>

Emplacement of sharp-walled sulfide veins during the formation and reactivation of impact-related structures at the Broken Hammer Mine, Sudbury, Ontario

2020 ◽  
Vol 57 (10) ◽  
pp. 1149-1166 ◽  
Author(s):  
M.F. Hall ◽  
B. Lafrance ◽  
H.L. Gibson
Keyword(s):  
Massive Sulfide ◽  
Hydrothermal Fluids ◽  
Crater Floor ◽  
Sudbury Igneous Complex ◽  
Tectonic Events ◽  
Brittle Ductile Transition ◽  
Basement Rocks ◽  
Post Impact ◽  
Platinum Group ◽  
The Impact

Broken Hammer is a hybrid Cu–Ni–Platinum Group Element (PGE) footwall deposit located in Archean basement rocks below the impact-induced Sudbury Igneous Complex (SIC), Canada. The deposit consists of massive chalcopyrite veins surrounded by thin epidote, actinolite, and quartz selvedges and low-sulfide, high-PGE mineralization consisting of disseminated chalcopyrite (<5%) and platinum group minerals, associated with Ni-bearing chlorite overprinting alteration patches of epidote, actinolite, and quartz. The veins are grouped into five steeply-dipping sets, striking northeast-, southwest-, southeast-, south-, and east–west, which were emplaced along impact-related fractures that were reactivated multiple times during stabilization of the crater floor. Early reactivation of the fractures created pathways for the migration of hydrothermal fluids from which quartz and chlorite precipitated sealing the fractures. Renewed slip shattered the quartz–chlorite veins into fragments that were incorporated in massive sulfide veins that crystallized from fractionated sulfide melts or from high temperature (400–500 °C) hydrothermal fluids, which migrated outward into the basement rocks from a cooling and crystallizing SIC melt sheet. Hydrothermal fluids syn-genetic with the epidote–actinolite–quartz alteration distributed the PGE into the footwall rocks, or late hydrothermal fluids associated with the Ni-bearing chlorite leached Ni and PGMs from the sulfide veins and redistributed them to form low-sulfide, high-PGE zones in the footwall rocks. During post-impact tectonic events, slip at temperatures below the brittle–ductile transition for chalcopyrite (<200 °C to 250 °C) produced striations along the vein margins. The Broken Hammer deposit exemplifies how Cu–Ni–PGE footwall deposits formed by the reactivation of impact-related fractures that provided conduits for the migration of melts and hydrothermal fluids.

scholarly journals Clarketal_2021.zip

2021 ◽  
Author(s):  
Martin Clark
Keyword(s):  
South Africa ◽  
Remote Sensing ◽  
Impact Structure ◽  
Aerial Photo ◽  
Special Issue ◽  
Still Images ◽  
Spatial Expression ◽  
Post Impact

This dataset is supplementary to a submitted manuscript to MDPI: Geosciences, "Making an Impact" special issue.<div><br></div><div>Manuscript title: Post-impact faulting of the Holfontein Granophyre Dike of the Vredefort Impact Structure, South Africa, inferred from remote sensing, geophysics, and geochemistry.</div><div>Authors: Clark, M.D., Kovaleva, E., Huber, M.S., Fourie, F., Harris, C.<br><div><br></div><div>The file contains an aerial photo and videos acquired via a drone which document the spatial expression of the geological dike.</div></div><div><br></div><div>Resolution of video is 4k (3840x2160 pix). Resolution of still images is 5472x3648 pix.</div>

scholarly journals Clarketal_2021.zip

2021 ◽  
Author(s):  
Martin Clark
Keyword(s):  
South Africa ◽  
Remote Sensing ◽  
Impact Structure ◽  
Aerial Photo ◽  
Special Issue ◽  
Still Images ◽  
Spatial Expression ◽  
Post Impact

This dataset is supplementary to a submitted manuscript to MDPI: Geosciences, "Making an Impact" special issue.<div><br></div><div>Manuscript title: Post-impact faulting of the Holfontein Granophyre Dike of the Vredefort Impact Structure, South Africa, inferred from remote sensing, geophysics, and geochemistry.</div><div>Authors: Clark, M.D., Kovaleva, E., Huber, M.S., Fourie, F., Harris, C.<br><div><br></div><div>The file contains an aerial photo and videos acquired via a drone which document the spatial expression of the geological dike.</div></div><div><br></div><div>Resolution of video is 4k (3840x2160 pix). Resolution of still images is 5472x3648 pix.</div>

Granular zircon from Vredefort granophyre (South Africa) confirms the deep injection model for impact melt in large impact structures

Geology ◽  
2019 ◽  
Vol 47 (8) ◽  
pp. 691-694 ◽  
Author(s):  
Elizaveta Kovaleva ◽  
Dmitry A. Zamyatin ◽  
Gerlinde Habler
Keyword(s):  
South Africa ◽  
Large Impact ◽  
Crater Floor ◽  
Pressure Shock ◽  
Impact Melt ◽  
Basement Rocks ◽  
High Shock ◽  
Injection Model ◽  
History Of ◽  
The Impact

Abstract The Vredefort impact structure, South Africa, is a 2.02 Ga deeply eroded meteorite scar that provides an opportunity to study large impact craters at their lower stratigraphic levels. A series of anomalous granophyre dikes in the core of the structure are believed to be composed of an impact melt, which intruded downwards from the crater floor, exploiting fractures in basement rocks. However, the melt emplacement mechanisms and timing are not constrained. The granophyre dikes contain supracrustal xenoliths captured at higher levels, presently eroded. By studying these clasts and shocked minerals within, we can better understand the nature of dikes, magnitude of impact melt movement, conditions that affected target rocks near the impacted surface, and erosional rates. We report “former reidite in granular neoblastic” (FRIGN) zircon within a granite clast enclosed in the granophyre. High-pressure zircon transformation to reidite (ZrSiO4) and reversion to zircon resulted in zircon grains composed of fine neoblasts (∼0.5–3 µm) with two or three orthogonal orientations. Our finding provides new independent constraints on the emplacement history of Vredefort granophyre dikes. Based on the environment, where other FRIGN zircons are found (impact glasses and melts), the clast was possibly captured near the top of the impact melt sheet and transported to the lowermost levels of the structure, traveling some 8–10 km. Our finding not only provides the highest-pressure shock estimates thus far discovered in the Vredefort structure (≥30 GPa), but also shows that microscopic evidence of high shock pressures can be found within large eroded craters at their lowest stratigraphic levels.

Genesis of the mafic granophyre of the Vredefort impact structure (South Africa): Implications of new geochemical and Se and Re-Os isotope data

2021 ◽  
Author(s):  
Wolf Uwe Reimold ◽  
Toni Schulz ◽  
Stephan König ◽  
Christian Koeberl ◽  
Natalia Hauser ◽  
...  
Keyword(s):  
South Africa ◽  
Country Rock ◽  
Order Estimate ◽  
Impact Structure ◽  
Impact Melt ◽  
Highly Siderophile Elements ◽  
Isotope Data ◽  
Rock Types ◽  
Siderophile Elements ◽  
Os Isotope

ABSTRACT This contribution is concerned with the debated origin of the impact melt rock in the central uplift of the world’s largest confirmed impact structure—Vredefort (South Africa). New major- and trace-element abundances, including those of selected highly siderophile elements (HSEs), Re-Os isotope data, as well as the first Se isotope and Se-Te elemental systematics are presented for the felsic and mafic varieties of Vredefort impact melt rock known as “Vredefort Granophyre.” In addition to the long-recognized “normal” (i.e., felsic, &gt;66 wt% SiO2) granophyre variety, a more mafic (&lt;66 wt% SiO2) impact melt variety from Vredefort has been discussed for several years. The hypothesis that the mafic granophyre was formed from felsic granophyre through admixture (assimilation) of a mafic country rock component that then was melted and assimilated into the superheated impact melt has been pursued here by analysis of the two granophyre varieties, of the Dominion Group lava (actually metalava), and of epidiorite mafic country rock types. Chemical compositions, including high-precision isotope dilution–derived concentrations of selected highly siderophile elements (Re, Os, Ir, Pt, Se, Te), and Re-Os and Se isotope data support this hypothesis. A first-order estimate, based on these data, suggests that some mafic granophyre may have resulted from a significant admixture (assimilation) of epidiorite to felsic granophyre. This is in accordance with the findings of an earlier investigation using conventional isotope (Sr-Nd-Pb) data. Moreover, these outcomes are in contrast to a two-stage emplacement model for Vredefort Granophyre, whereby a mafic phase of impact melt, derived by differentiation of a crater-filling impact melt sheet, would have been emplaced into earlier-deposited felsic granophyre. Instead, all chemical and isotopic evidence so far favors formation of mafic granophyre by local assimilation of mafic country rock—most likely epidiorite—by a single intrusive impact melt phase, which is represented by the regionally homogeneous felsic granophyre.

Anisotropy of magnetic susceptibility (AMS) of impact melt breccia and target rocks from the Dhala impact structure, India

2021 ◽  
Author(s):  
Anuj Kumar Singh ◽  
Jayanta Kumar Pati ◽  
Shiva Kumar Patil ◽  
Wolf Uwe Reimold ◽  
Arun Kumar Rao ◽  
...  
Keyword(s):  
Magnetic Susceptibility ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Impact Structure ◽  
Magnetic Phases ◽  
Impact Melt ◽  
Rock Samples ◽  
Average Value ◽  
Magnetic Fabrics ◽  
Target Rock ◽  
The Impact

ABSTRACT The ~11-km-wide, Paleoproterozoic Dhala impact structure in north-central India comprises voluminous exposures of impact melt breccia. These outcrops are discontinuously spread over a length of ~6 km in a semicircular pattern along the northern, inner limit of the monomict breccia ring around the central elevated area. This study of the magnetic fabrics of impact breccias and target rocks from the Dhala impact structure identified a weak preferred magnetic orientation for pre-impact crystalline target rocks. The pre- and synimpact rocks from Dhala have magnetite and ilmenite as common magnetic phases. The distributions of magnetic vectors are random for most impact melt breccia samples, but some do indicate a preferred orientation. Our anisotropy of magnetic susceptibility (AMS) data demonstrate that the shape of susceptibility ellipsoids for the target rocks varies from prolate to oblate, and most impact melt breccia samples display both shapes, with a slight bias toward the oblate geometry. The average value for the corrected degree of anisotropy of impact melt rock (P′ = 1.009) is lower than that for the target rocks (P′ = 1.091). The present study also shows that both impact melt breccia and target rock samples of the Dhala structure have undergone minor postimpact alteration, and have similar compositions in terms of magnetic phases and high viscosity. Fine-grained iron oxide or hydroxide is the main alteration phase in impact melt rocks. Impact melt rocks gave a narrow range of mean magnetic susceptibility (Km) and P′ values, in contrast to the target rock samples, which gave Km = 0.05–12.9 × 10−3 standard international units (SI) and P′ = 1.036–1.283. This suggests similar viscosity of the source magma, and limited difference in the degrees of recorded deformation. Between Pagra and Maniar villages, the Km value of impact melt breccias gradually decreases in a clockwise direction, with a maximum value observed near Pagra (Km = 1.67 × 10−3 SI). The poor grouping of magnetic fabrics for most impact melt rock samples implies local turbulence in rapidly cooled impact melt at the front of the melt flow immediately after the impact. The mean K1 for most impact melt samples suggests subhorizontal (&lt;5°) flow in various directions. The average value of Km for the target rocks (4.41 × 10−3 SI) is much higher compared to the value for melt breccias (1.09 × 10−3 SI). The results of this study suggest that the melt breccias were likely part of a sheet-like body of sizeable extent. Our magnetic fabric data are also supported by earlier core drilling information from ~70 locations, with coring depths reaching to −500 m. Our extensive field observations combined with available widespread subsurface data imply that the impact melt sheet could have covered as much as 12 km2 in the Dhala structure, with an estimated minimum melt volume of ~2.4 km3.

scholarly journals The impact of minimum wages for domestic workers in Bloemfontein, South Africa

2010 ◽  
Vol 8 (1) ◽  
Author(s):  
Phillip F. Blaauw ◽  
Louis J. Bothma
Keyword(s):  
South Africa ◽  
Minimum Wage ◽  
Structural Changes ◽  
Domestic Workers ◽  
Real Wages ◽  
Employment Effects ◽  
Course Of Action ◽  
Minimum Wage Legislation ◽  
The Impact

Orientation: The number of domestic workers in South Africa has decreased in the last decade, seemingly corresponding with efforts by government to increase regulation.Research purpose: The purpose of this study was to investigate possible structural changes in this labour market over the last decade, as well as the possible employment effects of the latest minimum wage provisions.Motivation for the study: Previous studies on the topic were carried out either prior to, or just after, the implementation of the minimum wage legislation for domestic workers. Now, five years after implementation, the conclusions and predictions of these studies need to be evaluated.Research design, approach and method: The study utilised a repeat survey in the suburb of Langenhoven Park in Bloemfontein, where two previous microstudies had been conducted. Structural interviews were conducted with a sample of 132 respondents and the data analysed.Main findings: There are now fewer domestic workers working for more employers, than there were ten years ago. In contrast to predictions from the literature, these changes mostly occurred before the implementation of the legislation. Real wages and legislative compliance improved for those who remained employed.Practical implications: The task of balancing the improvement of the lives of domestic workers and the possible adverse consequences of the legislation, in the form of job losses, remains as daunting as it was ten years ago.Contribution: Literature predicts changes in the market for domestic workers to be long term. This study shows that most changes took place before the implementation of the legislation as employers decided on their course of action.

Paleomagnetism and rock magnetism of East and West Clearwater Lake impact structures

2019 ◽  
Vol 56 (9) ◽  
pp. 983-993
Author(s):  
Jérôme Gattacceca ◽  
William Zylberman ◽  
Adam B. Coulter ◽  
François Demory ◽  
Yoann Quesnel ◽  
...  
Keyword(s):  
Rock Magnetism ◽  
Drill Hole ◽  
Hydrothermal Activity ◽  
Radiometric Dating ◽  
Impact Structure ◽  
The West ◽  
Impact Melt ◽  
Basement Rocks ◽  
East And West ◽  
The Impact

The East and West Cleawater Lake impact structures (Wiyâshâkimî Lake, Québec), ∼26 and 32 km in diameter, respectively, have been proposed to represent an impact doublet. We investigated their paleomagnetism to contribute to this debate. The paleomagnetic directions of the impact melt rocks and impact melt-bearing breccias from the West Clearwater structure are compatible with the radiometric age of 280–290 Ma previously determined for this structure and indicate that the impact occurred during a reverse polarity interval of the geomagnetic field. A similar remagnetization direction is found in the basement within 10 km of the structure center, whereas basement farther away from the center has escaped remagnetization by the impact. Samples for the East Clearwater structure come from two holes drilled in 1963 and 1964. Unfortunately, the drill hole through the melt rocks is tilted by 30° from the vertical with an unknown azimuth. The paleomagnetic inclination of these melt rocks cannot be constrained to better than between −28° and +32°. This is, however, distinct from the inclination of the melt rocks of the West Clearwater Lake impact structure (−27.8° ± 3.7°), suggesting that the two structures do not represent an impact doublet, in agreement with recent radiometric dating. The basement rocks and the melt rocks within 10 km of the center of the West Clearwater Lake impact structure show a magnetic signature of titanohematite that crystallized during postimpact hydrothermal activity under oxidizing conditions. This is not observed in the basement or the melt rocks from the East Clearwater Lake impact structure.

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