Calcite textures: examples from nappes with strain-path partitioning

Abstract

Strain-path partitioning (partitioning of deformation into components of pure and simple shear) in two Alpine nappes is qualitatively indicated by a variation in orientation of foliation, microkinematic criteria, quartz textures, and a comparison with strain-path partitioning in theoretical nappe models. The degree of non-coaxial deformation relative to the total deformation is quantified by using the asymmetry of textures in calcite tectonites. The texture interpretation is based on both polycrystal plasticity models and experimental data. Calcite tectonites in the Graz-Paleozoic thrust sheet (Austria) and the Morcles nappe (Switzerland) were deformed in a strain path with both pure and simple shear components. Texture asymmetry supports the strain path derived from geological observations. As an exception, a high-strain ultramylonite deformed nearly exclusively in pure shear. It is suggested that this represents a softening instability since deformation in pure shear is energetically more favorable than in simple shear. The incompatibilities caused by heterogeneous pure shear are accommodated by detachments and heterogeneous simple shear adjacent to the pure shear zone.