Post-Mississippian tectonic evolution of the Nemaha Tectonic Zone and Midcontinent Rift System, SE Nebraska and N Kansas

The geologic structures of the central Midcontinent of the USA are largely buried and known only from geophysical datasets, coupled with sparse well control and limited outcrop. Such unconstrained geophysical models preclude a deeper assessment of possible continental interior seismic hazards, which have the potential to cause appreciable damage. Within the study area in southeastern Nebraska and northeastern Kansas is an area of elevated seismic risk, with a spatial relationship to the Nemaha Tectonic Zone and the Midcontinent Rift System. Using sequential restorations of three published cross sections within Nebraska and Kansas this study demonstrates that the Nemaha Tectonic Zone and Midcontinent Rift System have each been reactivated several times since the end of the Mississippian (the details of deformation prior to the Mississippian are not considered). Our reconstructions indicate that in addition to major Pennsylvanian-Early Permian fault reactivation during the Ancestral Rocky Mountain orogeny there was also deformation both prior to the post-Mississippian unconformity associated with uplift on the Nemaha Tectonic Zone and after the deposition of late Early-early Late Cretaceous sediments in the study area, potentially due to the Laramide orogeny. Results also indicate that the magnitude of the far-field stresses is sufficient to cause seismogenic reactivation on favorably oriented pre-existing faults. This history of reactivation of geologic structures in the central Midcontinent suggests that seismic hazards in the region in the present cannot be ruled out. Though dangerous large earthquakes are uncommon in the continental interior, seismic activity along the structures in the study area would threaten several large population centers and the potential for this activity should not be ignored.