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Geological Society of America Bulletin






Reverse-drag folds are often used to infer subsurface fault geometry in extended terrains, yet details of how these folds form in association with slip on normal fault systems are poorly understood. Detailed structural mapping and global positioning system (GPS) surveying of the Frog Fault and Lone Mountain Monocline in the western Grand Canyon demonstrate a systematic relationship between elements of the normal fault system and fold geometry. The Lone Mountain Monocline, which parallels the Frog Fault, is made up of two half-monoclinal flexures: a hanging-wall fold in which dips gradually increase toward the fault over ~1.5 km reaching a maximum dip of 25° and a footwall fold in which dips decrease away from the fault over ~0.5 km from a maximum of 12°. The highest dips associated with folding are found where throw on the Frog Fault and a synthetic fault are at a maximum. Lower dips are found where there is less throw on the Frog Fault and antithetic faults are present. This relationship between fault and fold geometry suggests that the folding is associated with Basin and Range extension rather than Laramide contraction. Mechanical models of normal fault-related deformation predict similar patterns of folding over planar faults of finite extent and corroborate the important role of subsidiary fault geometry in the overall pattern of deformation. Application of these models in an inverse sense yields ~1 km estimates of down-dip extent, a result that may indicate fault confinement within the sedimentary section or weakening effects associated with folding layered strata. A general analysis illustrates that reverse-drag folds of moderate dip are expected to form in association with slip on planar faults of finite extent—a result that has the potential to impact our estimates of hydrocarbon volumes, crustal extension, and earthquake hazards associated with continental normal faults.