Zircon Microstructures Record Deformation History of Shock- and Tectonically-generated Pseudotachylites: a Case Study from the Vredefort Impact Structure, South Africa

2019 ◽  
Vol 60 (12) ◽  
pp. 2529-2546 ◽  
Author(s):  
E Kovaleva ◽  
M S Huber ◽  
G Habler ◽  
D A Zamyatin
Keyword(s):  
Tectonic Deformation ◽  
Impact Structure ◽  
Deformation Bands ◽  
Deformation History ◽  
Meteorite Impacts ◽  
History Of ◽  
Tectonic Settings ◽  
New Criterion

Abstract High-strain rate deformation can cause in situ melting of rocks, resulting in the formation of dark, micro- to nanocrystalline pseudotachylite veins. On Earth, pseudotachylite veins form during meteorite impacts, large landslides, and earthquakes. Within the Vredefort impact structure, both impact-generated and (pre-impact) tectonically-generated pseudotachylite veins have been described, but are challenging to distinguish. Here, we demonstrate a genetic distinction between two pseudotachylite veins from Vredefort by studying their petrography, degree of recrystallization and deformation, cross-cutting relationships and the deformation microstructures in associated zircon. We conclude that Vein 1 is pre-impact and tectonically-generated, and Vein 2 is impact-generated. In agreement, zircon microstructures in Vein 1 contain planar deformation bands (PDBs), attributed to tectonic deformation, whereas zircon microstructures in Vein 2 reveal microtwin lamellae, indisputable evidence of shock metamorphism. Thus, deformation microstructures in zircon may provide a new criterion for distinguishing the genetic origin of pseudotachylite veins. Zircons that have been removed from their context (i.e., alluvial or detrital zircon, zircon from Lunar breccia) should be interpreted with caution in terms of their deformation history. For example, zircon with PDBs cannot reliably be used as a marker for shock deformation, because this feature has been shown to form in purely tectonic settings.

scholarly journals Geologically Meaningful 40Ar/39Ar Ages of Altered Biotite from a Polyphase Deformed Shear Zone Obtained by in Vacuo Step-Heating Method: A Case Study of the Waziyü Detachment Fault, Northeast China

Minerals ◽  
2020 ◽  
Vol 10 (8) ◽  
pp. 648
Author(s):  
Wenbei Shi ◽  
Fei Wang ◽  
Lin Wu ◽  
Liekun Yang ◽  
Yinzhi Wang ◽  
...  
Keyword(s):  
Bulk Sample ◽  
White Mica ◽  
Detachment Fault ◽  
Fault Activity ◽  
Deformation History ◽  
Step Heating ◽  
History Of ◽  
Total Fusion

Discordant biotite 40Ar/39Ar age spectra are commonly reported in the literature. These can be caused by a number of processes related to in vacuo heating, homogenization of the argon distribution, and production of misleadingly flat age spectra. Problematic samples are typically derived from metamorphic belts; thermal overprinting and chloritization are two of the main known causes of disturbed age spectra. Biotite and muscovite of the Waziyü detachment fault, Yiwulüshan metamorphic core complex, Jinzhou, China, yield highly variable 40Ar/39Ar data that hinder reconstruction of their deformation history. We combined mineralogical studies with detailed 40Ar/39Ar dating of biotite, phengitic white mica, and K-feldspar augen from this fault. We infer that argon within the biotite was modified by hydrothermal fluids during fault activity and associated epidotization, chloritization, and muscovitization such that bulk sample step-heating, single grain total fusion, and in situ laser ablation of biotite produced mixed 40Ar/39Ar ages. However, detailed step-heating of biotite shows that this mineral records the ages of cooling and later alteration based on data from a coexisting rigid feldspar porphyroblast and neo-crystallized phengite that record two periods of fault activity at ~120–113 and 18–12 Ma. Our data reveal that the discordant biotite 40Ar/39Ar age spectra might represent a mixed age and that only detailed step-heating methods can extract meaningful geological details of the deformation history of a fault. Therefore, the mineral and the method must be carefully considered if metamorphic or deformed samples are dated.

scholarly journals Modern rural landscapes in contemporary heritage imaginaries: the case of Germany’s southern Oderbruch

2019 ◽  
Vol 63 ◽  
pp. 11002 ◽  
Author(s):  
Emily Bereskin
Keyword(s):  
Democratic Republic ◽  
Place Marketing ◽  
Current Application ◽  
Rural Landscapes ◽  
Heritage Sites ◽  
Discourse Production ◽  
History Of ◽  
Political Economic

Using the Southern Oderbruch as a case study, this paper investigates the presence and representation of the modern rural landscapes of the German Democratic Republic within the region’s contemporary heritage and tourism landscape. Following an analysis of extant discourse production in place marketing materials and heritage sites (primarily local museums), the paper argues that although the unique landscapes developed in concert with the collective farms (landwirtschaftliche Produktionsgenossenschaften) of the GDR remain very much in situ, they remain largely invisible in the heritage and touristic representation of the Oderbruch, which tends to focus on more traditional manifestations of “pastoral beauty” and on historical events preceding the founding of the GDR. This paper hypothesizes several reasons for this conspicuous absence, arguing that the history of the LPG defies local will to narrativise due to its ongoing social, legal, and economic reverberations in everyday life. The second half of the paper reviews the current application effort fora European Cultural Heritage designation for the Oderbruch. The paper highlights the complexity of the situational landscape surrounding the production of heritage, in terms of political, economic, social, and symbolic factors and argues for similar analyses as a comparative path of investigation for the MODSCAPES project.

scholarly journals Properties of Impact-Related Pseudotachylite and Associated Shocked Zircon and Monazite in the Upper Levels of a Large Impact Basin: a Case Study From the Vredefort Impact Structure

Minerals ◽  
2020 ◽  
Vol 10 (12) ◽  
pp. 1053
Author(s):  
Elizaveta Kovaleva ◽  
Roger Dixon
Keyword(s):  
Impact Crater ◽  
Shock Pressure ◽  
Initial Structure ◽  
Impact Structure ◽  
Near Surface ◽  
X Ray ◽  
Host Rocks ◽  
The Impact

The Vredefort impact structure in South Africa is deeply eroded to its lowermost levels. However, granophyre (impact melt) dykes in such structures preserve clasts of supracrustal rocks, transported down from the uppermost levels of the initial structure. Studying these clasts is the only way to understand the properties of already eroded impactites. One such lithic clast from the Vredefort impact structure contains a thin pseudotachylite vein and is shown to be derived from the near-surface environment of the impact crater. Traditionally, impact pseudotachylites are referred to as in situ melt rocks with the same chemical and isotopic composition as their host rocks. The composition of the sampled pseudotachylite vein is not identical to its host rock, as shown by the micro-X-ray fluorescence (µXRF) and energy-dispersive X-ray (EDX) spectrometry mapping. Mapping shows that the melt transfer and material mixing within pseudotachylites may have commonly occurred at the upper levels of the structure. The vein is spatially related to shocked zircon and monazite crystals in the sample. Granular zircons with small granules are concentrated within and around the vein (not farther than 6–7 mm from the vein). Zircons with planar fractures and shock microtwins occur farther from the vein (6–12 mm). Zircons with microtwins (65°/{112}) are also found inside the vein, and twinned monazite (180°/[101]) is found very close to the vein. These spatial relationships point to elevated shock pressure and shear stress, concentrated along the vein’s plane during impact.

scholarly journals Remediation of Historical Photographs in Mobile Augmented Reality

2021 ◽  
Vol 7 (1) ◽  
pp. 29-40
Author(s):  
Espen Johnsen Bøe
Keyword(s):  
3D Models ◽  
Unexpected Result ◽  
3D Animation ◽  
Museum Exhibits ◽  
Historical Significance ◽  
History Of ◽  
3D Representation ◽  
Historical Photographs

Being present at a location of historical significance, often demands imagination to understand the full scope of the area. An approach to spark one’s imagination is to present a mediated simulation of a historic location in situ. As an application example, we used the sitsim AR platform to develop a simulation that conveys the history of fishermen in the historic fishing village of Storvågan in Lofoten, Norway. The study presents a rendition of the sitsim AR platform’s functionality for engaging presentations of historical photographs. This functionality is enhanced from solely representing buildings in a historical photograph into also representing animated human characters. In Storvågan, a museum (Lofotmuseet) occupies historically significant buildings amid the historic surroundings. This museum exhibits a historical photograph of fishermen that also shows how the area once looked. This photograph is remediated into a 3D animation, presented as a real-time generated simulation, at the location where the photograph was originally photographed. The study documents a design experiment including the modelling and animation of a 3D representation depicting the photograph. The functionality is evaluated based on user feedback from a case study of a beta version on location in Lofoten. Users reported that the animated fishermen contribute to an engaging experience and a feeling of being “part of the history.” The majority of users perceived the 3D representation as credible. An analysis of the modelled characters concludes that the 3D-models lack perceptual validity; hence, the case study’s positive results were somewhat unexpected. Three theories are presented as conceivable explanations for the unexpected result. Ultimately, the study provides a method for modelling and animation of people from a historical photograph, and showcases how the animation of human characters in a sitsim may be applied to convey cultural heritage in an engaging way. 

scholarly journals On the Radial and Longitudinal Variation of a Magnetic Cloud: ACE, Wind, ARTEMIS and Juno Observations

Solar Physics ◽  
2020 ◽  
Vol 295 (11) ◽  
Author(s):  
Emma E. Davies ◽  
Robert J. Forsyth ◽  
Simon W. Good ◽  
Emilia K. J. Kilpua
Keyword(s):  
Axial Magnetic Field ◽  
Magnetic Cloud ◽  
Flux Rope ◽  
Time History ◽  
Minimum Variance ◽  
Magnetic Clouds ◽  
Analysis Method ◽  
History Of

AbstractWe present observations of the same magnetic cloud made near Earth by the Advance Composition Explorer (ACE), Wind, and the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) mission comprising the Time History of Events and Macroscale Interactions during Substorms (THEMIS) B and THEMIS C spacecraft, and later by Juno at a distance of 1.2 AU. The spacecraft were close to radial alignment throughout the event, with a longitudinal separation of $3.6^{\circ}$ 3.6 ∘ between Juno and the spacecraft near Earth. The magnetic cloud likely originated from a filament eruption on 22 October 2011 at 00:05 UT, and caused a strong geomagnetic storm at Earth commencing on 24 October. Observations of the magnetic cloud at each spacecraft have been analysed using minimum variance analysis and two flux rope fitting models, Lundquist and Gold–Hoyle, to give the orientation of the flux rope axis. We explore the effect different trailing edge boundaries have on the results of each analysis method, and find a clear difference between the orientations of the flux rope axis at the near-Earth spacecraft and Juno, independent of the analysis method. The axial magnetic field strength and the radial width of the flux rope are calculated using both observations and fitting parameters and their relationship with heliocentric distance is investigated. Differences in results between the near-Earth spacecraft and Juno are attributed not only to the radial separation, but to the small longitudinal separation which resulted in a surprisingly large difference in the in situ observations between the spacecraft. This case study demonstrates the utility of Juno cruise data as a new opportunity to study magnetic clouds beyond 1 AU, and the need for caution in future radial alignment studies.

Timing constraints on deformation history of the Sudbury Impact Structure

1994 ◽  
Vol 31 (11) ◽  
pp. 1654-1660 ◽  
Author(s):  
Jianjun Wu ◽  
Bernd Milkereit ◽  
David Boerner
Keyword(s):  
Timing Constraints ◽  
Impact Structure ◽  
Relative Timing ◽  
Deformation History ◽  
Initial Shape ◽  
Igneous Complex ◽  
Sudbury Igneous Complex ◽  
Sudbury Structure ◽  
History Of ◽  
Seismic Images

Herein, we present new high-resolution seismic images of the Sudbury Impact Structure, acquired across the Sudbury Igneous Complex and its environs, which provide evidence for the relative timing of the deformation events that reshaped the initial Sudbury Structure. The seismic images show that the lower unit of the Sudbury basinal fill sediments, the Onwatin argillite, is penetrated by a set of blind, imbricated thrusts, whereas the overlying Chelmsford turbidites are unaffected by faulting. We interpret this observation to mean that the deposition of the Chelmsford sediments postdates the latest major deformation of the Sudbury Structure, suggesting that the uniform paleocurrent trends observed in the Chelmsford turbidites are not related to the initial shape of the Sudbury Structure.

Freeze-etch TEM of aqueous polymer gel network morphology

1986 ◽  
Vol 44 ◽  
pp. 796-797
Author(s):  
J. A. N. Zasadzinski ◽  
R. K. Prud'homme
Keyword(s):  
Metal Ion ◽  
Polymer Network ◽  
Polymer Gels ◽  
Polymer Gel ◽  
Deformation History ◽  
Crosslinked Polymer ◽  
Aqueous Polymer ◽  
History Of ◽  
Gel Network ◽  
Network Morphology

The rheological and mechanical properties of crosslinked polymer gels arise from the structure of the gel network. In turn, the structure of the gel network results from: thermodynamically determined interactions between the polymer chain segments, the interactions of the crosslinking metal ion with the polymer, and the deformation history of the network. Interpretations of mechanical and rheological measurements on polymer gels invariably begin with a conceptual model of,the microstructure of the gel network derived from polymer kinetic theory. In the present work, we use freeze-etch replication TEM to image the polymer network morphology of titanium crosslinked hydroxypropyl guars in an attempt to directly relate macroscopic phenomena with network structure.

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