Deformation structures in the Hovin Group schists in the Hommelvik-Hell region (Norway)

Abstract

Measurement of wavelength/thickness ratios of buckle-folded veins in metasediments of the Upper Hovin Group showed that the ratios are rather constant within individual exposures although there are considerable variations from exposure to exposure. Straight and pinching and swelling veins usually make low angles with the cleavage, while veins showing the maximum degree of folding usually make high angles with the cleavage. Veins of an intermediate orientation show a moderate degree of folding. Such a relationship suggests that the cleavage is parallel to the plane of maximum extension in these rocks. The phenomenon of cleavage refraction is explained by variation in the nature of strain in beds of unlike competency. The attitude of cleavage depends chiefly upon three factors, viz.: 1. (1) layer-parallel compressive strain; 2. (2) shear strain generated during buckling; and 3. (3) rigid rotation of the competent layers. The diverging and converging attitudes of cleavages can be explained by the relative importance of these factors. From occurrence of stretched pebbles at an angle to cleavage, from presence of boudinage at hinges of folds, and from recognition of rotated tension gashes, it is suggested that the beds have undergone a period of stretching before the present system of folds had developed.