Strain measurements from deformed quartz grains in the slaty rocks from the ardennes and the northern Eifel

Abstract

Methods of linear correlation of strain axes have been used to determine the two-dimensional shapes of the deformed elliptical quartz grains in thin sections of the slaty rocks from the Hercynian tectonic belt in the Ardennes and the northern Eifel. It is found that in each sample the quartz grains from a particular lithological band can be considered to come from a single population having a certain axial ratio and a preferred orientation of the long axes parallel to the slaty-cleavage trace. It is reasoned that such a population could only be derived from an original population of nearly circular grains. Natural octahedral unit shear, a derivative of it ε_s, and the Lode's parameter have been used to describe the shapes of the three-dimensional strain ellipsoids. Throughout this tectonic belt the maximum compressive strain is more or less uniform in each rock type; the values of the other two principal strains show larger variations. In each rock type ε_s has a remarkably constant value, but the Lode's parameter ranges in value from 0 to nearly 1. All the ellipsoids fall in the constriction field.