Shock effects in quartz; compression versus shear deformation; an example from the Rochechouart impact structure, France

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doi: 10.1016/j.epsl.2007.11.035
Authors:Trepmann, C. A.
Author Affiliations:Primary:
Ruhr-University Bocham, Institute for Geology, Mineralogy and Geophysics, Bochum, Federal Republic of Germany
Volume Title:Earth and Planetary Science Letters
Source:Earth and Planetary Science Letters, 267(1-2), p.322-332. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X
Publication Date:2008
Note:In English. 45 refs.; illus., incl. geol. sketch map
Summary:Different quartz microstructures from Rochechouart impact breccias, indicative of shock-induced compression and shear deformation are compiled, to obtain information on the mean stress and the deviatoric components of the shock wave-associated stress. Annealed rhombohedral planar deformation features (PDFs) are widespread in quartz from Rochechouart impact breccias, as analysed by optical microscopy, scanning electron and transmission electron microscopy. Shocked quartz can show mosaicism with domains of <200 nm in diameter that are misoriented to each other. This is interpreted as the result of post-shock annealing of a high density of rhombohedral PDFs crosscutting each other. Pockets of newly crystallised quartz aggregates, interpreted as annealed diaplectic glass, are surrounded by optically almost isotropic quartz and occur in intensely shocked quartz. These areas are proposed to represent gradually increasing shock intensities from the host quartz grain to the newly crystallised quartz aggregate due to local pressure and temperature variations on the mm-scale. These microstructures indicate compression at high shock pressures (20-35 GPa) and they show no evidence of shear deformation at high differential stress. Annealing of these microstructures probably took place shortly after shock when post-shock temperatures were still high. Quartz microstructures in the Rochechouart impact breccias that record shock-induced shear deformation are mechanical Brazil twins and planar fractures that are supposed to represent microfaults. These shock effects indicate high differential stress on the order of a few GPa. Quartz that contains Brazil twins and planar fractures shows no or few rhombohedral PDFs, indicating relatively low shock pressures (<15 GPa). In general, shock effects indicating high differential stress do not occur together with those indicative of high shock pressure. These findings reveal that high shock pressures (>20 GPa) are apparently not accompanied by high differential stresses. Only at attenuated shock pressure (<15 GPa), differential stresses are effective to cause shear deformation. As a result of deformation during shock compression, the mean stress decreases with time and distance from the point of impact, whereas the deviatoric components can be expected to increase due to the high anisotropy of rocks and minerals. [G.L.B.]
Sections:Meteorites and tektites; Physical properties of rocks and minerals
Mineral Groups:Framework silicates
Subjects:Annealing; Compression; Deformation; Framework silicates; Impact craters; Impact features; Metamorphism; Microstructure; Planar deformation features; Quartz; Shear; Shock metamorphism; Shock waves; Silica minerals; Silicates; Transmission electron microscopy; Twinning; Europe; France; Haute-Vienne France; Rochechouart France; Western Europe; Brazil twins; Diaplectic glass; Mosaicism; Planar fractures; Rochechouart Structure
Coordinates:N454900 N454900 E0005000 E0005000
Abstract Numbers:08M/1231
Record ID:2008109569
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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