Experiments in the cleavage-parallel compression of slate demonstrate important changes in the mode of deformation with increasing confining pressure. At pressures up to 3 kb shear fractures are dominant. In the range 3.5 to 7.24 kb the rock yields through the development of an intersecting network of narrow kink-bands. The transition is reflected in a marked change in the slope of the Mohr envelope at a confining pressure of 3 kb, indicating that different failure criteria are relevant to the two types of deformation, that also give stress-strain curves of different shape.

The geometrical relationship between shear fractures and kink-bands is described in detail. The shear fractures make a much smaller angle (mean value 30.7°) with the direction of maximum compression than do the kink-bands (mean value 55.4°).

At the highest confining pressures employed (about 7 kb), there is evidence of kink-band growth by lateral migration and separation of the bounding kink-planes. At all lower pressures the length of the rotating, kinked, segment of foliation is determined at inception.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.