The relationship between principal stress directions and the geometry of kinks in foliated rocks

Abstract

This paper describes the results of a series of experiments carried out on card decks and foliated rock in order to study the nucleation and growth of kinks. Parameters varied in the experiments were confining pressure and the angle of inclination between the foliation and the direction of maximum shortening. The results showed that most kinks initiated as symmetric or monoclinal buckles which rapidly changed to an angular, stable form similar to that observed in nature. The amount of strain required to set up this stable geometry is small. The effect of increasing confining pressure is to diminish the size of the kinks while that of changing the angle of inclination of the foliation to the direction of maximum shortening is to modify the style of deformation from the development of a set of conjugate kinks at small angles (0 – 5°), to a single set of kinks at larger angles (5 – 15°), and, finally, to a combination of sliding along the foliation and kinking at the largest angles (15 – 30°) investigated. During this change in mode of deformation, the angular relationships between the kinks and the directions of the applied loads change systematically and graphs are presented which may be of use in relating the geometry of natural kinks to the principal stress directions.