The effect of foliation orientation on the inferred rotation axes and rotation angles of rotated porphyroblasts

Abstract

The rotation axes and rotation angles inferred from the intersection of inclusion fabrics in rotated porphyroblasts and the external foliation are apparent axes and apparent angles whose orientation and magnitude are strongly influenced by the initial orientation of the foliation with respect to the geometry of the deformation. Spherical porphyroblasts may show apparent angles of rotation consistently smaller than the corresponding true rotation angles and apparent rotation axes oriented at a distinct angle with the true axis of rotation. For nonspherical porphyroblasts this effect may lead to a scatter of apparent rotation axes. Sections of a rock intersecting the foliation through the resulting sector of apparent axes will show apparent forward and apparent backward rotations, even when the foliation remains stationary throughout the deformation and all porphyroblasts rotate with the same sense of true rotation. This effect holds the key to one possible explanation of apparent reversals observed in some paracrystalline rotated porphyroblasts.