Analysis of joints in two monoclines in Israel

Areal and detailed studies of joints in the Hathira and Hazera monoclines of the northern Negev, Israel, pose several questions concerning relations between jointing and folding in this area. The joints can be divided into five areal sets: three of the sets have become slickensided by shear displacements and might be called small faults, and the other two sets appear to be extension fractures. Use of the co-appearance matrix, a method introduced here, indicates that members of a conjugate set of joints intersect at a nearly constant angle throughout the area but that the acute bisector of the conjugate set trends in different directions in different areas. This suggests that areal sets of joint sets should be defined in terms of angular relations among members rather than in terms of trends. The patterns of the joints seem to bear no simple relationship to the Hazera and Hathira monoclines. One might expect the acute bisector of conjugate sets to trend normal to axes of the monoclines; however, the bisector in the areas studied deviates 30° to 45° from normal to the axes. Average strain due to strike-slip displacement along steeply dipping joints or small faults was measured by detailed mapping of one area. The maximum shortening accommodated by slip along joints was 1.5 percent. The axis of maximum shortening calculated from strain analysis is horizontal and trends to N78°E, whereas the normal to the axes of the monoclines is in a direction of about N45°W. Thus, direction of deformation accommodated by slip along joints and direction of deformation due to monocline formation deviate markedly.