UNDERSTANDING THE REACTIVATION OF BASEMENT STRUCTURES IN SE NEBRASKA USING ANALOG MODELS

SE Nebraska and NE Kansas are underlain by the Precambrian Mid-Continental Rift (MCR) and the Pennsylvanian-Permian Nemaha Uplift (NU). The MCR is dated to approximately 1.1 Ga and extends from Lake Superior to Oklahoma. The Nemaha Uplift was created during the Ancestral Rocky Mountain Orogeny around 300 Ma. This orogeny reactivated pre-existing weak zones probably related to the MCR. Both fault systems may have been reactivated in subsequent tectonic regimes given the orientations of known surface faults in the area. This study aims to investigate the correlation between known basement and surface faults or lineaments, and make predictions about the locations of additional large-scale structures in the subsurface in a portion of SE Nebraska-NE Kansas.

Remote sensing of surface lineaments from Landsat data reveals four main lineament trends. Trends oriented at 035 and 055 are believed to correlate to the MCR and the NU respectively. There are two other prominent directions, one around 088 and another around 003 with no known associated basement faults.

From this data a series of analog models were constructed to test basement fault geometries that may have influenced the lineament orientations. Each model tested a different scenario of potential basement faults, increasing in complexity, with a final model investigating the effects of multiphase deformation. Coarse sand was used to represent the basement and fine sand to represent the cover. Pre-existing faults were made by a cut in the sand using a knife. The sand pack was compressed to about 15% bulk shortening, consistent with the regional tectonic history. Photographs were taken every hour, before the model was sliced to create cross sections showing the internal deformation.

All models show a strong correlation between modification of the surface faults and the underlying basement features. Single-deformation-phase models also showed reactivation of the basement faults, similar to the structures found on published cross-sections of the region. The multi-phase deformation model indicates that the basement fault reactivation can occur even through thick sediment packages, as found in the region. Thus, these models indicate that reactivation of pre-existing structures is a reasonable explanation for the complex fracturing found in SE Nebraska and NE Kansas.