Mechanisms of dilatant en echelon crack formation in bedded chalk

This study focuses on various styles of dilatant out-of-plane en echelon cracks, which are associated with bed restricted burial joints, that cut slightly deformed sediments in the Beer Sheva syncline. There is a positive linear fit of en echelon crack length to layer thickness (R-2=0.928) in fringes which were formed by discontinuous breakdowns in joints. Lithological properties influence the breakdown process. The breakdown of joints into en echelon cracks in chalk beds is influenced by both local conditions and remote stresses. There are two distinct cases of opposite senses of segment rotation in a single fringe: first, in a curved fringe; and, second, in a straight fringe. The former can best be assigned to local deviations of the minimum principal direction from perpendicularity to the parent joint, whereas the latter appears to reflect changes in remote stresses. Two of the three sets of en echelon segmentation which were distinguished in the Beer Sheva syncline are correlated with two distinct tectonic associations. The K-III/K-I ratio increases from the layer centre towards the layer boundaries, and this is the driving motivation for both the gradual increase of barb overlap from the centre towards the layer boundaries, and the en echelon segmentation along the layer boundaries. (C) 1997 Elsevier Science Ltd.