'Fracture cleavage' in the Duluth Complex, northeastern Minnesota ( USA).

An unusual cleavage is developed in troctolitic rocks of the late Precambrian Duluth Complex in NE Minnesota. It is localised in a zone approx 250 m wide, and it is parallel to the axes of gentle folds in rhythmic banding. The rocks show no textural signs of ductile deformation, and the cleavage is defined by fractures that are fairly straight and evenly spaced, but on a fine scale, somewhat irregular, multiple, anastomosing, and locally crosscutting. Localization of the cleavage seems to be due to the presence of a structure, probably a dike, below the layered intrusive rocks. This could have served to localize stresses and reactive fluids at high pressure during a late stage of continental rifting.-from Authors