Joint intensity in layered rocks: The unsaturated, saturated, supersaturated, and clustered classes

We derive here a new model for joint intensity in brittle, layered rock. According to this model, joint intensity depends on the tectonic stresses, the friction between the layers, and the tensile strength of the brittle layers. For typical geological settings of layered sedimentary rocks, the joint intensity is expected to achieve saturation when the ratio of the host layer thicknesses to the joint spacing ranges from 1 to 3. On the other hand, field measurements in the western margins of the Dead Sea basin, as well as experimental observations, reveal that this ratio ranges from 0.1 to 35. Consequently, we classify the observed range of intensity into four groups: unsaturated, saturated, supersaturated, and clustered. While our model, as well as previous models, can explain the first two groups, alternative mechanisms are required to explain the second two groups. It is shown, based on our recent field and experimental analyses, that fast propagating, dynamic fractures can produce the tightly spaced patterns of the supersaturated and clustered cases.