Constraining kinematic rotation axes in high-strain zones: a potential microstructural method?

Abstract

AbstractThe correct determination of the kinematic rotation axis in high-strain zones is essential to the study of the tectonic evolution of the Earth’s crust. However, the common assumption that the kinematic rotation axis lies orthogonal to the XZ plane of the finite strain ellipse may be invalid in the case of general shear. Orientation data obtained by electron backscatter diffraction from calcite, deformed in the high-strain Gressoney Shear Zone of the Western Alps, has been investigated using orientation maps, bulk sample crystallographic orientation and misorientation analyses, and detailed intragrain misorientation and crystallographic dispersion analysis. The results demonstrate a strong geometrical coincidence amongst (1) the bulk macroscopic kinematic rotation axis, (2) the orientation of misorientation axes associated with low-angle boundaries, and (3) rotation axes associated with crystallographic dispersion at the intragrain scale. This coincidence is interpreted to reflect a geometric control of the kinematic framework of the high-strain zone on the activity of crystal slip systems. It is proposed that this relationship may be exploited as a new microstructural tool to determine the orientation of bulk kinematic rotation axes in high-strain zones without assuming a geometric link between kinematic rotation and XZ sections. Although further testing is required, application of the approach may lead to a significant advance in our understanding of natural general shear deformation.