REACTIVATION OF BASEMENT STRUCTURES IN SE NEBRASKA REVEALED BY USING LINEAMENT ANALYSIS AND REMOTE SENSING

SE Nebraska is underlain by the southern extent of the main Mid-Continent Rift System (MRS) and the northern extent of the Nemaha Uplift (NU). The MRS dates from 1.1 Ga, during the formation of Rodinia and has been reactivated with both extensional and compressional senses and different times. Reactivation of the MRS is associated with the Ancestral Rocky Mountain Orogeny, 300Ma, the same event that is responsible for the NU. There are indications that these tectonic entities have also been uplifted and reactivated during Laramide time, but the exact nature of this reactivation and the relationship of the MRS and northern extent of the NU is unknown. This study aims to document surface fracture data in SE Nebraska, in order to illuminate these relationships.

Surface lineaments were mapped using ArcGIS, and used to identify potential field sites with measurable fracture sets. Fracture orientation data was then collected from these sites, close to mapped lineaments. Outcrop sites consist of Pennsylvanian and Permian limestone and shale cyclothems, probably of the Absaroka Group. These field measurements were then compared to existing fault maps and previous work in the region.

Surface fracture sets in this regions show one prominent orientations, namely striking northwest to southeast, and dipping close to vertical. Although the Humboldt Fault is present in SE Nebraska and continues southwest into Kansas, existing fault maps show a southwest/northeast orientation in the region where data was collected. This prominent fracture orientation is thus interpreted to correspond to the Humboldt Fault which serves as a structural barrier between the Nemaha Structural Zone and the Forest City Basin to the east. The appearance of these known basement trends in the surface fracture orientation dataset strongly indicates a period of reactivation that post-dates formation of these units.