Blunt-ended dyke segments

Two examples of blunt-ended dykes from the Rooi Rand dyke swarm in South Africa are examined in order to determine the mechanism by which such features form. Although other interpretations of blunt-ended dykes have been proposed, evidence in the Rooi Rand examples suggests that dilation was transferred along a zone of shear at the dyke tip oriented at a high angle to the dyke plane. Microscopic analysis of samples from blunt-ended tip regions reveals cataclasis and mineral straining in the dyke walls in the zone of dyke linkage. The indication is that adjacent, blunt-ended, en échelon dyke segments dilate along a shear zone, producing cataclasis of the host rock. Both segments dilate in this manner and are blunt-ended prior to linkage. Horns may develop at the outer corners of the blunt tips so that, subsequent to linkage, the overall geometry resembles that predicted by the conventional model of bridge failure between overlapped en échelon dykes. However, permanently strained bridges predicted by that model are not necessary for the model described here. In addition, blunt-ended dykes that dilate along a cross-linking shear zone do not need to overlap in order to link together, in contrast to existing model predictions. Dilation adjacent to a blunt-ended dyke may also be accommodated by intrusion of magma into shear zone fractures that vary in orientation with respect to the main dyke. Near the dyke, the neartip stress field overrides the remote stress field and generates magma-filled shear-related fractures at high angles to the dyke plane. With increasing distance from the dyke, the remote stress field becomes dominant and resultant shear-related fractures are oriented at successively lesser angles to the dyke plane.