Geological indicators for impact: The anomalous case of the Vredefort structure, South Africa

The Vredefort Dome is located within and almost central to the Witwatersrand basin in its presently known extent. It exposes a central Archean granite core which is surrounded by a collar of supracrustal rocks. These collar rocks outline a strong polygonal geometry. The Archean core is comprised of two concentric zones, the Outer Granite Gneiss (OGG), and the core central Inlandsee Leucogranofels (ILG). The rocks of the inner core display granulite facies metamorphism, while the OGG is in amphibolite facies. The inner core is believed from recent drill hole information to be underlain by mafic and ultramafic gneisses, the extent of which cannot be assessed at present. A fairly broad zone of charnockites separates the OGG and ILG domains. This zone is characterized by a high concentration of pseudotachylite and ductile shearing. Whereas a number of other domical structures are located within or surrounding the Witwatersrand basin, the Vredefort structure is anomalous, in that it has: a partly polygonal geometry; extensive alkali intrusives in the northwestern sector; granophyre dykes (ring-dykes peripheral to the contact collar-basement and NW-SE or NE-SW trending dykes within the Archean basement); contact metamorphism of the collar supracrustal rocks; the overturning of collar supracrustals in the northern sectors; deformation phenomena widely regarded as representing shock metamorphism (pseudotachylite, (sub)planar microdeformation features in quartz, shatter cones and occurrences of high-P quartz polymorphs); a positive 30 mgal gravity anomaly; and high amplitude magnetic anomalies. Recent geophysical, structural and petrological evidence pertinent for the identification of the processes that led to the formation of the Vredefort structure are summarized

Microdeformation in Vredefort rocks; evidence for shock metamorphism

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

A quasi-Hertzian stress field from an internal source: A possible working model for the Vredefort structure

The Vredefort structure is a large domal feature about 110 km southeast of Johannesburg, South Africa, situated within and almost central to the large intracratonic Witwatersrand Basin. This structure consists of an Archean core of ca. 45 km in diameter, consisting largely of granitic gneiss, surrounded by a collar of metasedimentary and metavolcanic supracrustal rocks of the Dominian Group, Witwatersrand and Ventersdorp Supergroups, and Transvaal Sequence. The interpretation of images of the gravity and magnetic fields over Vredefort has permitted the delineation of several important features of the structure and of its environment. The outline of the collar strata is a prominent feature of both the gravity and the magnetic fields. The Vredefort structure shares this distinctive geometry with other structures (e.g., Manicouagan, Decaturville, Sierra Madera) of debated impact origin. In all these, successively older strata with steep outward dips are encountered while traversing inward to the center of the structure. A further attribute of these structures is the shortening of the outcrop of a particular stratigraphic unit compared to the original perimeter of that unit. To account for the geometric attributes of the Vredefort structure a mechanical scheme is required where there is radial movement of horizontal strata toward, with uplift in, the center of the Vredefort structure. Two models can be proposed: (1) one in which there is a rapid rise and violent disruption of cover rocks in response to expansion of a fluid accumulation; and (2) one in which there is, in contrast, a nonexplosive, quasi-Hertzian stress field resulting from a diapiric process. Both models can accommodate the geometry and structural components of Vredefort

Atomic ambitions : a role-theoretic analysis of Brazilian and Indian quests for nuclear capability

Ankara : The Department of International Relations İhsan Doğramacı Bilkent University, 2014.Thesis (Master's) -- Bilkent University, 2014.Includes bibliographical references leaves 197-219.This thesis uses role theory to investigate the motivations of national leaders considering the acquisition of nuclear weapons. The correlation between expressed roles and proliferation decisionmaking is examined in the cases of Brazil and India within the framework of a most similar systems design. The material and ideational sources of Brazilian and Indian national role conceptions are traced using a model developed by Marijke Breuning (2011). In the Brazilian case, the expressed national role conceptions are found to be inconsistent with acquisition of nuclear weaponry, whereas in the Indian case, nuclear explosives were seen as powerful symbols consistent with the role conceptions of national iv leaders. The insights yielded by role theory in these cases provide a valuable addition to the existing nonproliferation literature.Reimold, Benjamin AM.S

The Strategic Context of the UAE’s Nuclear Project: A Model for the Region?

Cataloged from PDF version of article.When the government of the United Arab Emirates (UAE) issued a policy statement in April 20081 indicating that the country was seriously considering developing a civilian nuclear-power program, it set the region and the world speculating as to the possible motivations behind such a move at that time. Since that date, the UAE has aggressively forged ahead, signing bilateral agreements with nuclear-supplier countries while increasing cooperation with the International Atomic Energy Agency (IAEA) in support of its bid to add nuclear power to its national energy portfolio. Most recently, the UAE has actually broken ground on its first and second reactors, in 2012 and 2013. At a time when the nuclear industry's "renaissance" has slowed or even faltered2 as a result of the 2011 Fukushima Daiichi accident, the global dynamics of the industry seemed to have little effect on the bold and confident pace of the UAE's nuclear plans. Worldwide, the future looks dimmer for nuclear than it did a few short years ago: Germany has pursued a policy of early decommissioning of its nuclear-power capacity;3 the Netherlands has adopted a "wait and see" attitude with respect to new nuclear plants;4 and new plants already under construction in China, France and Finland have experienced delays and cost overruns.5 Even in the face of these signs of a slowdown in the industry, however, the UAE continues to press ahead in its pursuit of nuclear power. Yet why should an oil-rich country like the UAE pursue a civilian nuclear power program, especially at a time when the future of nuclear-power around the world is uncertain

Evidence for coeval Late Triassic terrestrial impacts from the Rochechouart (France) meteorite crater

High temperature impact melt breccias from the Rochechouart (France) meteorite crater record magnetization component with antipodal, normal and reverse polarities. The corresponding paleomagnetic pole for this component lies between the 220 Ma and 210 Ma reference poles on the Eurasian apparent polar wander path, consistent with the 214 $\pm$ 8 Ma 40Ar/39Ar age of the crater. Late Triassic tectonic reconstructions of the Eurasian and North American plates place this pole within 95% confidence limits of the paleomagnetic pole from the Manicouagan (Canada) meteorite impact crater, which is dated at 214 $\pm$ 1 Ma. Together, these observations reinforce the hypothesis of a Late Triassic, multiple meteorite impact event on Earth

Expanding the role of impurity spectroscopy for investigating the physics of high-Z dissipative divertors

New techniques that attempt to more fully exploit spectroscopic diagnostics in the divertor and pedestal region during highly dissipative scenarios are demonstrated using experimental results from recent low-Z seeding experiments on Alcator C-Mod, JET and ASDEX Upgrade. To exhaust power at high parallel heat flux, q ‖ > 1 GW/m 2 , while minimizing erosion, reactors with solid, high-Z plasma facing components (PFCs) are expected to use extrinsic impurity seeding. Due to transport and atomic physics processes which impact impurity ionization balance, so-called ‘non-coronal’ effects, we do not accurately know and have yet to demonstrate the maximum q ‖ which can be mitigated in a tokamak. Radiation enhancement for nitrogen is shown to arise primarily from changes in Li- and Be-like charge states on open field lines, but also through transport-driven enhancement of H- and He-like charge states in the pedestal region. Measurements are presented from nitrogen seeded H-mode and L-mode plasmas where emission from N 1+ through N 6+ are observed. Active charge exchange spectroscopy of partially ionized low-Z impuri- ties in the plasma edge is explored to measure N 5+ and N 6+ within the confined plasma, while passive UV and visible spectroscopy is used to measure N 1+ -N 4+ in the boundary. Examples from recent JET and Alcator C-Mod experiments which employ nitrogen seeding highlight how improving spectroscopic cov- erage can be used to gain empirical insight and provide more data to validate boundary simulations.EURATOM 63305

Discerning primary versus diagenetic signals in carbonate carbon and oxygen isotope records: An example from the Permian-Triassic boundary of Iran

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordSedimentary successions across the Permian-Triassic boundary (PTB) are marked by a prominent negative carbon isotope excursion. This excursion, found in both fossil (e.g., brachiopod) and bulk carbonate at many sites around the world, is generally considered to be related to a global carbon cycle perturbation. Oxygen isotopes also show a negative excursion across the PTB, but because δ18O is more prone to diagenetic overprint (especially in bulk carbonate), these data are often not used in palaeoenvironmental analyses. In the present study, bulk-rock and brachiopod δ13C and δ18O, as well as conodont δ18O, were analyzed in PTB successions at Kuh-e-Ali Bashi and Zal (NW Iran) in order to evaluate diagenetic overprints on primary marine isotopic signals. The results show that the use of paired C-O isotopes and Mn-Sr concentrations is not sufficient to identify diagenetic alteration in bulk materials, because δ13C-δ18O covariation can be due to environmental factors rather than diagenesis, and Sr/Ca and Mn/Ca ratios can vary as a function of bulk-rock lithology. Comparison of δ13C profiles shows that all bulk carbonate is altered to some degree, although the general bulk-rock trend mimics that of the brachiopod data with a systematic offset of -1.2(±0.4)‰. This suggests that the first-order δ13C trend in bulk carbonate is generally robust but that the significance of small-scale carbon isotope fluctuations is uncertain, especially when such fluctuations are linked to lithologic variation. The PTB interval, which is marked by a low-carbonate 'Boundary Clay' in the study sections, may be especially prone to diagenetic alteration, e.g., via late-stage dolomitization. Comparison of oxygen-isotope profiles for bulk rock and well-preserved fossils (both brachiopods and conodonts) shows that the former are offset by -2.1(±0.4)‰. Diagenetic modeling suggests that these offsets were the product mainly of early diagenesis at burial temperatures of ~50-80°C and water/rock ratios of <10. Authigenic carbonates precipitated during early diagenesis represent a potentially major sink for isotopically light carbon at a global scale that has received relatively little attention to date.TJA thanks the Sedimentary Geology and Paleobiology program of the U.S. National Science Foundation (NSF EAR-1053449), the NASA Exobiology program (NNX13AJ1IG), and the China University of Geosciences—Wuhan (SKL-GPMR program GPMR201301, and SKL-BGEG programBGL21407) for their support. This study was funded by Deutsche Forschungsgemeinschaft (DFG; projects KO1829/12-1, KO1829/12-2 and KO2011/8-1)