Other
Scientific paper
Mar 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008e%26psl.267..322t&link_type=abstract
Earth and Planetary Science Letters, Volume 267, Issue 1-2, p. 322-332.
Other
7
Scientific paper
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.
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