Garnet rotation and the development of axial plane crenulation cleavage

Abstract

Rotated helicitic garnets, syntectonically overgrowing axial plane crenulation cleavage on the limbs of folds, show the opposite sense of rotation to that undergone by the folded foliation during the development of the fold. The garnet rotation is inconsistent with simple minded shear models, for the development of the crenulation cleavage, and it is useful to consider this problem in order to understand the development of crenulation cleavage better. Various models are considered in an attempt to explain both garnet rotation and cleavage morphology. Earlier models were based on the assumption that garnet rotation is driven by shear parallel to S 2 . They were mostly unsatisfactory, but one, involving migration of the crenulation hinges, is viable when combined with other models described here. The new models proposed here are based on the assumption that garnet rotation is driven by shear on the crenulated surface ( S 1 ); the new foliation ( S 2 ) is considered to be a passive structure, at least in the early stages of its development. These models satisfactorily explain garnet rotation and development of the cleavage, including differentiation and they can be used to predict the morphological evolution of the cleavage microstructure as it should be preserved in garnets. Comparison of observed and predicted helicitic structures strongly supports the proposed models. Evidence is presented to show that where a garnet has an S e that is a schistosity and an S i that is a crenulation cleavage, the schistosity can have developed by unfolding of the crenulations.