Porphyroblast rotation and strain localization: debate settled!: comment

Abstract

This comment addresses two inferences used by Johnson (2009) to argue for porphyroblast rotation during bulk coaxial shortening. Firstly, he interprets that porphyroblast inclusion trails that are inclined (his figure 1) became inclined because the porphyroblast rotated during growth. He used orientation data from millipede microstructures from one hand sample and concluded on the basis of the total spread of inclusion trail orientations and porphyroblast axial ratios that porphyroblasts had rotated relative to one another during ductile deformation. Secondly, he presents a numerical model that indicates that asymmetrically shaped porphyroblasts rotated during coaxial deformation. However, I will show that the porphyroblasts did not rotate after they grew, in spite of localized shearing along the developing S3 that rotated visible matrix S2 in Johnson's figure 1.