Jump to content

User:Nonabelian/Shock Fractures

From Wikipedia, the free encyclopedia

Shock Fractures

[edit]

Shock extension fractures,[1] shock fractures,[2] and vermicular (i.e., wormlike) microfractures[3]

Multiple studies have shown that using hydrofluoric acid (HF) to etch quartz grains can effectively highlight the difference between shock features filled with glass, caused by high-pressure impacts, and deformation features caused by tectonic forces, which do not contain glass.[4]

Examples

[edit]

Aorounga and Gweni Fada, Chad

[edit]
See also: List of impact structures in Africa

The Aorounga and Gweni Fada impact structures, located in Chad, are two of the few confirmed impact sites in Africa. There are only 15 confirmed impact structures on the continent, with Aorounga and Gweni Fada being the only such structures identified in Chad. Despite their significance, detailed geological mapping of these sites has not yet been conducted, largely due to the challenging conditions in northern Chad, including civil unrest. The limited availability of samples has further constrained detailed studies of these craters. However, the research conducted on the available samples has provided important insights into the shock metamorphism experienced by the rocks at these sites.[5]

Research conducted on Aorounga and Gweni Fada samples[a] provides valuable data on the conditions under which shock extension fractures form. These fractures are associated with low to moderate shock pressures and typically appear before more severe shock features like PDFs. The studies highlight the importance of identifying these subtle shock features in rocks that do not display more pronounced deformation, as they provide evidence of impact events even in the absence of more obvious markers like craters or extensive PDF networks.[6]

Aorounga Impact Structure

[edit]
Main article: Aorounga crater
Photo of Aorounga crater from space

The Aorounga structure, with an estimated diameter of 12.6 km, is believed to be part of a larger impact complex. Samples collected from the central area and the outer slopes of the inner ring structure have revealed a variety of lithologies, including quartzite and sandstone. These rocks exhibit varying degrees of shock metamorphism, ranging from minimal to moderate deformation.

  • Shock Extension Fractures: Several samples from Aorounga exhibited minor shock effects, particularly in the form of short, sub-planar fractures within quartz grains. These fractures, known as shock extension fractures, indicate that the rocks experienced compressive stress from the impact, but not to the extent required to form more intense shock features like Planar Deformation Features (PDFs).[7]
  • Variability in Shock Levels: The degree of shock metamorphism varied across the samples. While some showed only minor fractures, others displayed more intense shock features, including rare instances of PDFs. The presence of shock extension fractures in less deformed samples suggests that these fractures may develop at relatively low shock pressures, typically around 5 to 8 GPa.[7]
  • Influence of Rock Texture: The study noted that the texture of the rock, including grain size and arrangement, influenced the development of shock extension fractures. Larger quartz grains in some samples were more prone to these fractures, suggesting that the physical properties of the rock play a role in how shock pressures are expressed.[7]

Gweni Fada Impact Structure

[edit]
Main article: Gweni-Fada crater
Gweni-Fada crater viewed from the International Space Station

The Gweni Fada structure, approximately 14 km in diameter, is another significant impact site in Chad. Like Aorounga, it has not been extensively studied due to limited access to samples. The available samples, however, include quartzite and sandstone, some of which show evidence of shock deformation.

  • Limited Shock Deformation: Most samples from Gweni Fada showed minimal shock deformation, with some displaying minor fractures radiating from points where quartz grains contact each other. These features are consistent with shock extension fractures observed at lower pressure levels, typically around 5 GPa or less.
  • Relevance to Shock Extension Fractures: The study at Gweni Fada supports the view that shock extension fractures can occur at lower shock pressures. These fractures were often observed in samples lacking more severe shock features, indicating that they can serve as early indicators of impact-related stress before more extensive deformation occurs.

Cerro do Jarau, Brazil

[edit]
Main article: Cerro do Jarau crater

The Cerro do Jarau impact structure, located in Rio Grande do Sul, Brazil, is a prominent geological feature formed within the Paraná Basin. This structure, measuring approximately 13.5 km in diameter, is one of the few impact sites in the region and is believed to have formed in Jurassic-Cretaceous sedimentary and volcanic rocks, including basalt and sandstone.[8]

Recent studies have confirmed the impact origin of Cerro do Jarau, primarily through the identification of shock deformation features in quartz grains within the sandstone samples. Among these features are shock extension fractures (SEFs),[b] which are sub-planar fractures formed under the compressive stresses of an impact event.[8]

Santa Marta, Brazil

[edit]
Main article: Santa Marta crater

The Santa Marta structure in Brazil was confirmed as an impact crater through the identification of traditional shock metamorphic features, including shatter cones, PDFs, and shock extension fractures.[c][9]

Vredefort Dome, South Africa

[edit]
Main article: Vredefort impact structure

text.

Meteor Crater, Arizona, United States

[edit]

text.

Czech Republic

[edit]

text.

Chiemgau impact, Germany

[edit]

text.

Ries Crater impact, Germany

[edit]

text.

Azuara Structure, Spain

[edit]

text.

Rubielos de la Cérida Impact Structures, Spain

[edit]

text.

Trinity

[edit]

Notes

[edit]
  1. ^ Relevant samples from Koeberl et al. (2005) displaying SEF's include:
    Aorounga Impact Structure
    • OR-4: Numerous grains display short and dense, often divergent shock extension fractures. These fractures are indicative of shock pressures less than 8 GPa.
    • OR-7A: Contains a large number of grains with densely spaced and short extension fractures, produced in ~5 GPa shock experiments with quartzite.
    • OR-9A: A few grains with shock extension fracture arrays were observed, associated with pressures around 5 GPa.
    • OR-9B: Abundant grains contain dense and closely spaced arrays of short, generally slightly curved and often divergent extension fractures, estimated at 6–8 GPa.
    • OR-9D: Contains grains with subplanar fractures, with some featuring densely spaced arrays, associated with pressures around 8 GPa.
    • OR-10: Several grains exhibit single or multiple sets of short extension fractures, along with more extensive irregular fracturing.
    Gweni Fada Impact Structure
    • GF-12: This sample contains many grains with short shock extension fracture arrays. These fractures are indicative of moderate shock pressures (~8–10 GPa).
    The pressure estimates for samples containing shock extension fractures are primarily derived from the following studies by Huffman & Reimold (1996) and Stöffler & Langenhorst (1994).
  2. ^ From the Reimold et al. (2019) study several samples in Table 1 exhibited shock extension fractures (SEFs):
    Cerro do Jarau structure
    • P19: This polymict breccia sample showed cataclasis and was noted to contain SEFs along with planar fractures (PFs).
    • P75A: This sandstone sample had some sections displaying SEFs, along with planar fractures and other shock features.
    • CJ19A: This strongly crushed sandstone with basalt clasts exhibited SEFs along with other shock deformation features.
    • CJ48A: This sample of strongly crushed, heterogranular sandstone was noted for having significant cataclasis and SEFs.
    • CJ190: This sample exhibited abundant grains with SEFs, alongside other shock deformation features.
  3. ^ From the de Oliveira et al. (2014) study, several samples in Table 1 exhibited shock extension fractures (SEFs):
    Santa Marta impact structure
    • SM-01: This groundmass-supported sandstone showed evidence of shock metamorphism, including undulatory extinction, reduced birefringence of quartz, and one or two sets of planar fractures (PFs). Shock extension fractures were also noted in this sample.
    • SM-4: Similar to SM-01, this sample contained a 0.6 cm wide clast of sericitic shale with quartz grains displaying one set of PDFs and/or PFs. Shock extension fractures were identified alongside these features.(Fig. 11.a)
    • E-19: A fine-grained sandstone with approximately 15 vol% reddish groundmass, this sample frequently exhibited PFs and FFs in larger quartz grains. Shock extension fractures were noted in two grains.

Citations

[edit]
  1. ^ Koeberl et al. (2005); Kowitz et al. (2013); Reimold et al. (2019); Reimold & Koeberl (2014)
  2. ^ Gratz (1984); Kowitz et al. (2016)
  3. ^ Kowitz et al. (2013); Reimold & Koeberl (2014); Buchanan & Reimold (2002)
  4. ^ Gratz (1984); Gratz, Fisler & Bohor (1996); Blenkinsop (2002), pp. 80–89; Bohor, Betterton & Krogh (1993); Bohor, Fisler & Gratz (1995); Hamers & Drury (2011)
  5. ^ Koeberl et al. (2005), p. 1455.
  6. ^ Koeberl et al. (2005), p. 1463-65.
  7. ^ a b c Koeberl et al. (2005), p. 1465.
  8. ^ a b Reimold et al. (2019).
  9. ^ de Oliveira et al. (2014).

Bibliography

[edit]

Journal Articles

[edit]

Conference Papers

[edit]

Books

[edit]
Retrieved from "https://en.wikipedia.org/w/index.php?title=User:Nonabelian/Shock_Fractures&oldid=1240978721"