Microstructural variation of an axial plane cleavage around a fold — a H.V.E.M. study

Abstract

A detailed microstructural analysis of a divergent slaty cleavage fan developed in a fine grained, phyllosilicate rich lithology is reported. The presence of crenulations in the central region of the fan where cleavage lamellae are absent, and modified crenulations in the regions with initial cleavage lamellae strongly suggests that a crenulation fabric developed throughout the fan before the cleavage lamellae. In the limbs of the fold mechanical rotation, grain boundary sliding and possibly the removal of material are the important mechanisms of initial cleavage lamellae development. However, the syn-tectonic growth of phyllosilicates along the axial planes of crenulations is the dominant mechanism of cleavage lamellae development in the hinge of the fold and during the later stages of cleavage development in the fold limbs. It is suggested that only in restricted areas of the fold hinge is the cleavage developed parallel to the XY plane of the finite strain ellipsoid on a microstructural scale.