Some considerations on the kinematics of crenulation cleavage

Abstract

The geometric evolution associated with the development of crenulation cleavage is analysed. The strong pre-existing anisotropy of the rocks in which this type of cleavage develops, together with the similar or sub-similar geometry of the microfolds, indicates that heterogeneous rotation shear is the main deformation type involved in their formation. In order to mitigate the high volume loss in the microfolds and the lack of a volume increase in the hinge zones that this mechanism entails, a volume correction is introduced, involving the occurrence of a constant stretching in the direction of the axial traces of the microfolds. In addition, a small component of flexural flow can act in the first stages of the microfolding. The analysis has been applied to a case of symmetric crenulation cleavage whose geometry has been fitted to two models. The most suitable of these involves heterogeneous rotation shear with a constant volume correction and implies a variable bulk shortening and volume loss in the different limbs of the microfolds. The values obtained range in most cases between 30 and 50% of bulk shortening and 10 and 30% of bulk volume loss.