Shock deformation of quartz influenced by grain size and shock direction: Observations on quartz-plagioclase rocks from the basement of the Ries Crater, Germany

Abstract

Planar elements in quartz, produced by shock induced plastic deformation, have been investigated in four quartz-plagioclase veins contained in an amphibolite from the crystalline basement of the Ries Crater from the drill hole ‘Nördlingen 1973’.

The crystallographic orientation of planar elements in quartz grains is similar in all four rocks ({10¯13} predominant, {0001} less frequent, {10¯12} and others still rarer), indicating an average shock pressure in the range between 150 and 200 kbar.

The spatial density of planar elements as measured by the number of systems per shocked grain, the number of individual elements per shocked grain, or as ratio shocked: unshocked grains increases with increasing grain size. This grain size effect is supposed to be primarily a consequence of the heterogeneity of the stress field which produced a random distribution of local stress maxima and locally restricted areas of plastic quartz deformation in the rock. The probability that planar elements develop within one individual grain increases, therefore, with increasing grain size.

In one leucosome in which the quartz grains were randomly oriented planar elements parallel to {10¯13} cluster in a stereographic projection within one belt. It is supposed that the pole of this belt indicates the direction in which the shock front passed through the rock.