NEBRASKA PORTION OF THE MIDCONTINENT RIFT SYSTEM; FROM ANCIENT AULACOGEN TO MODERN METROPLEX

The Midcontinent Rift System (MRS) formed at 1.1 Ga, during the assembly of Rodina. The Nebraska portion of the MRS, which underlies the Lincoln-Omaha-Council Bluffs metroplex and its 1.2 million residents, is largely uninvestigated. The historically mapped Union Fault (UF), which parallels the Lincoln-Omaha axis to the south, is the WSW extension of the Thurman-Redfield Fault Zone (TRFZ) from Iowa. This WSW extension is based on offsets interpreted from boreholes correlated across several kilometers with little quality control, and the exact nature of geologic structure is unknown.

The UF/TRFZ represents Phanerozoic reactivation along the southern boundary of the MRS. Epicenters of at least five post-1850s earthquakes (MMI I-IV) in Nebraska plot in the vicinity of the UF and the southern margin of the MRS. A fault zone at the northern boundary of the MRS in Nebraska has never been formally recognized, yet four additional earthquake (MMI I-VII) epicenters plot within 12 km of the northern limit of rift sediments (mapped on the basis of sparse borehole data).

Lineament analysis, additional geophysical data and field measurements indicate that the MRS is now associated with a basement high, the Nemaha Uplift, which is oriented N-S and connects the offset zones of the pre-existing rift template in Nebraska and Kansas. The Nemaha Uplift is an Ancestral Rocky Mountain uplift, formed as the supercontinent of Pangea was being assembled in the Carboniferous, by the collision of Gondwana with Laurentia. Surface analysis of associated fault trends indicates that the faults bounding the Nemaha Uplift may also have been reactivated during later compression.

Despite the presence of flood basalt, the MRS remained an aulacogen in the Proterozoic, rather than evolving to a true spreading center. Rift closure and initial uplift occurred during the later stages of the Proterozoic Grenville orogen; collision between Laurentia and Amazonia. The 1.1 billion-year history of the MRS and associated reactivated features demonstrates that pre-existing weaknesses within continental crust provide a locus for subsequent deformation, and that these weak zones may persist through several supercontinent cycles.