Normal Faults and Their Hanging-Wall Deformation: An Experimental Study

Abstract

We have used clay models to study the effects of fault shape and displacement distribution on deformation patterns in the hanging wall of a master normal fault. The experimental results show that fault shape influences the style of secondary faulting and folding. Mostly antithetic normal faults form above concave-upward fault bends, whereas mostly synthetic normal faults form above low-angle fault segments and convex-upward fault bends. Beds dip toward the master normal fault above concave-upward fault bends and away from the master normal fault above low-angle fault segments and convex-upward fault bends. Generally, secondary faulting and folding are youngest at fault bends and become progressively older past fault bends. Hanging-wall deformation patterns differ significantly when a basal plastic sheet imposes a constant-magnitude displacement distribution on the master normal fault. In models without a plastic sheet, numerous secondary normal faults form in the hanging wall of the master normal fault. Most secondary normal faults propagate upward and, consequently, have greater displacement at depth. In models with a plastic sheet, few visible secondary normal faults develop. Most of these faults propagate downward and, consequently, have less displacement at depth. Hanging-wall folding is wider and bedding dips are gentler in models without a plastic sheet than in identical models with a plastic sheet. The observed particle paths, displacement distributions, bedding dips, and orientations of the principal-strain axes in our physical models with and without a basal plastic sheet are compatible with the assumption that homogeneous, inclined simple shear accommodates the hanging-wall deformation. Not all of our modeling observations, however, are consistent with this assumption. Specifically, the observed variability with depth of the distribution and intensity of deformation is incompatible with homogeneous, inclined simple shear as the hanging-wall deformation mechanism.