Localization of layer-parallel faults in San Rafael swell, Utah and other monoclinal folds

The subdivision of sedimentary rock layers by the formation of layer-parallel faults in a developing fold can be explained by a combination of intralayer weaknesses and internal layer-parallel shear stresses generated by bending. An analysis of stress equilibrium of layer-parallel shear stresses and fiber normal stresses indicates that the maximum shear stress increases with the square of the thickness and the layer-parallel derivative of curvature, termed here coil, evaluated at middepth of the bed. The maximum shear stress is at middepth in the layer, so a laminated bed, as it bends, will tend to fault at middepth along pre-existing layer-parallel weaknesses and thereby subdivide the single mechanical layer into two mechanical layers of half the original thickness. Using the method of computation of coil outlined here, we are able to show that the coil predicts the location and sense of shear of layer-parallel faults in several field examples. The method illustrates how a few careful field measurements of layer geometry can provide valuable information about conditions of folding.