Arcuate hinge cleavage associated with welded contacts: an example

Abstract

Arcuate hinge cleavage (a.h.c.) shows a near bedding-parallel, concentric, arcuate development within the inner arcs of hinge zones. It is favoured by alternating layers of marked viscosity contrasts with little layer-parallel shortening prior to parallel folding. A field example of a.h.c. from a greywacke/pelite sequence of the Variscan of Central Europe is presented. The a.h.c. is developed as a slaty cleavage in the inner-arc hinge zones of pelite beds close to and welded to the outer arc of greywacke layers. Microscopically it is defined by the alignment of platy minerals and oblate quartz grains. The a.h.c. (S a) is cut by a divergently fanning crenulation cleavage (S b) which, in turn, is cut by slip surfaces parallel to bedding. The slip surfaces are cut by a fracture cleavage which is the macroscopically observed axial surface cleavage (S c). This sequence of deformational increments implies the onset of bedding slip after the formation of a.h.c. (S a) and (S b). We therefore suspect inhibited bedding slip by welded contacts to favour the development of a.h.c.