Relations between jointing and faulting based on fracture-spacing ratios and fault-slip profiles: A new method to estimate strain in layered rocks

By using fault-slip profiles and fracture-spacing ratios (FSRs), a direct link is established between slip variations across a normal fault zone and anomalous joint densities in adjacent wall rock. Faulting in bedded chalk near Beer Sheva, Israel, was impeded by competent cherts, requiring the development of subsidiary faults to accommodate unrelieved strain. The composite slip profile for the entire normal-fault zone is smooth and uniform except for a sharp localized decrease near one bed, resulting in a steep slip gradient. This bed displays unusually closely spaced joints in the hanging wall (FSR = 2.2), as opposed to typical joint spacings (FSR = 1.5) in the footwall. The slip deficit may reflect a surplus of elastic strain stored in that bed. which was released in the form of additional joints during subsequent jointing. This study reveals that joint densities in wall rock can vary as a function of fault geometry and slip distribution, and the FSR may be used as an effective tool to estimate strain around local structures.