Thermo‐mechanical interaction of a large impact melt sheet with adjacent target rock, Sudbury impact structure, Canada

2019 ◽  
Vol 54 (6) ◽  
pp. 1228-1245 ◽  
Author(s):  
Paul L. Göllner ◽  
Torben Wüstemann ◽  
Lisa Bendschneider ◽  
Sebastian Reimers ◽  
Martin D. Clark ◽  
...  
Keyword(s):  
Large Impact ◽  
Impact Structure ◽  
Mechanical Interaction ◽  
Impact Melt ◽  
Target Rock

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.

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 (<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 Supplemental Material: Inhomogeneous distribution of lithic clasts within the Daskop granophyre dike, Vredefort impact structure: Implications for emplacement of impact melt in large impact

2021 ◽  
Author(s):  
Matthew Huber ◽  
et al.
Keyword(s):  
Large Impact ◽  
Impact Structure ◽  
Inhomogeneous Distribution ◽  
Impact Melt ◽  
D Model

Video of the 3-D model of the Daskop granophyre dike generated from photogrammetric processing of aerial drone imagery

Inhomogeneous distribution of lithic clasts within the Daskop granophyre dike, Vredefort impact structure: Implications for emplacement of impact melt in large impact structures

2021 ◽  
Author(s):  
Matthew S. Huber ◽  
Elizaveta Kovaleva ◽  
Martin D. Clark ◽  
Stephen A. Prevec
Keyword(s):  
Turbulent Flow ◽  
Geophysical Data ◽  
Large Impact ◽  
High Abundance ◽  
Impact Structure ◽  
Inhomogeneous Distribution ◽  
Gravitational Settling ◽  
Field Mapping ◽  
Impact Melt ◽  
Thermally Driven

ABSTRACT The Vredefort granophyre dikes have long been recognized as being derived from the now-eroded Vredefort melt sheet. One dike, in particular, the Daskop granophyre dike, is notable for a high abundance of lithic clasts derived from various stratigraphic levels. In this study, we mapped the distribution of the clasts throughout the continuously exposed section of the dike using field mapping and aerial drone photography and attempted to constrain the emplacement mechanisms of the dike. We found that the clasts are not homogeneously spread but instead are distributed between clast-rich zones, which have up to 50% by area clasts, and clast-poor zones, which have 0–10% by area clasts. We examined three models to explain this distribution: gravitational settling of clasts, thermally driven local assimilation of clasts, and mechanical sorting of clasts due to turbulent flow. Of the three models, the gravitational settling cannot be supported based on our field and geophysical data. The assimilation of clasts and turbulent flow of clasts, however, can both potentially result in inhomogeneous clast distribution. Zones of fully assimilated clasts and nonassimilated clasts can occur from spatial temperature differences of 100 °C. Mechanical sorting driven by a turbulent flow can also generate zones of inhomogeneous clast distribution. Both local assimilation and mechanical sorting due to turbulent flow likely contributed to the observed distribution of clasts.

scholarly journals Supplemental Material: Inhomogeneous distribution of lithic clasts within the Daskop granophyre dike, Vredefort impact structure: Implications for emplacement of impact melt in large impact

2021 ◽  
Author(s):  
Matthew Huber ◽  
et al.
Keyword(s):  
Large Impact ◽  
Impact Structure ◽  
Inhomogeneous Distribution ◽  
Impact Melt ◽  
D Model

Video of the 3-D model of the Daskop granophyre dike generated from photogrammetric processing of aerial drone imagery

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, >66 wt% SiO2) granophyre variety, a more mafic (<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.

Where small can have a large impact: Structure and characterization of small-scale fisheries in Peru

2010 ◽  
Vol 106 (1) ◽  
pp. 8-17 ◽  
Author(s):  
Joanna Alfaro-Shigueto ◽  
Jeffrey C. Mangel ◽  
Mariela Pajuelo ◽  
Peter H. Dutton ◽  
Jeffrey A. Seminoff ◽  
...  
Keyword(s):  
Large Impact ◽  
Small Scale ◽  
Impact Structure ◽  
Small Scale Fisheries

Morokweng, South Africa: A large impact structure of Jurassic-Cretaceous boundary age

Geology ◽  
1997 ◽  
Vol 25 (8) ◽  
pp. 731 ◽  
Author(s):  
Christian Koeberl ◽  
Richard A. Armstrong ◽  
Wolf Uwe Reimold
Keyword(s):  
South Africa ◽  
Large Impact ◽  
Impact Structure
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