Sandbox models of downward-steepening normal faults

Two deformational styles resulted from sandbox experiments examining the hanging-wall deformation above a downward-steepening normal-fault bend comprised of two planar segments dipping at 40 and 60 degrees. In the first, hanging-wall layer-parallel extension dominated, with synthetic normal faults forming an upward-widening fault-bound wedge above the fault bend. Early forming, arcuate, vertical-to-reverse faults bound the wedge on its down-thrown side and were followed by a planar, large-displacement synthetic normal fault bounding the wedge on the upthrown side. This later fault accommodated almost all of the subsequent displacement that occurred on the underlying 60 degrees fault. Slip variably occurred along the 40 degrees segment of the fault. Good analogs in nature exist for this deformation geometry. In the second deformational style, hanging-wall layer-parallel compression dominated, forming a monocline with antithetic hinge surfaces dipping downward to the fault-bend surface. Antithetic reverse faults formed in the limb of the monocline. The upper hinge of the monocline intersected the footwall at the fault bend, whereas its lower hinge intersected the 60 degrees fault below the fault bend. The lower hinge was older and formed near the location of the fault bend prior to its being translated down the 60 degrees master fault as part of the hanging wall. Elements of this geometry correlate well with the results of some geometric-based numerical models, but strong analogs in nature are elusive.