What can Microstructural Analysis and the Elk Creek Earth MRI Survey tell us about Upper Crustal Deformation in SE Nebraska?

Caroline M Burberry, Department of Earth and Atmospheric Sciences, Bessey Hall, University of Nebraska-Lincoln 68588-0340, USA
R Matt Joeckel, Conservation and Survey Division, School of Natural Resources, Hardin Hall, University of Nebraska-Lincoln 68583-0996, USA; University of Nebraska State Museum, Morrill Hall, Lincoln, NE 68588, USA
Michele M Waszgis, Conservation and Survey Division, School of Natural Resources, Hardin Hall, University of Nebraska-Lincoln 68583-0996, USA
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Abstract

The recent release of the Elk Creek airborne magnetic and radiometric survey by the U.S. Geological Survey (USGS) Earth Mapping Resources Initiative (Earth MRI) allows an unprecedented glimpse into the upper crustal deformation patterns of the southeastern region of Nebraska. This area was deformed by the evolution of two major features: (1) the 1.1 Ga Midcontinent Rift System (MRS) and (2) the >300 Ma Nemaha Uplift (NU). Due to the uplift and inversion of the MRS and the NU, basement rocks have been recovered in a series of cores across the region, including the fragmented, small diameter Capitol/Capital Beach core. We present ongoing work from the first-ever textural and microstructural analysis of the basement rock from this historic core. We also present similar data from cores KC 81-1, Schroeder 1, PH 1 (from the flanks of the NU), and Radenslaben (from the MRS). Lastly, further study will integrate the recently released Elk Creek survey data with existing regional core analysis of the Elk Creek Carbonatite Complex and surrounding area to develop a more comprehensive characterization of the structural features.

Thin section analysis indicates that the groundmass of the Capitol/Capital Beach basement rock is primarily felsic, with mica, some opaques, and textures consistent with cooling from a melt; therefore, we consider this rock to be granitic. Thin sections of the Capitol Beach granite reveal the development of folded phyllosilicate shear zones, and at least two generations of brittle fracturing in quartz and feldspar grains, some of which propagate through and displace the phyllosilicate shear zones and are filled with alteration minerals. Large clasts from KC 81-1 and Schroeder 1 are also granitic in nature and show limited weathering and several generations of fracturing. Microstructural data implies that the regional basement has undergone no less than four discrete phases of deformation since its accretion ~1.8 Ga. We further note that the basaltic basement is also heavily fractured. We associate two deformation phases with the evolution of the MRS and NU, however one of the other deformation phases may date to the 1.52-1.34 Ga Pinware-Baraboo-Picuris orogen.

High-resolution geophysical survey data warrants reexamination of previously interpreted aspects of the regional structural framework, including compositional classifications, fault geometry, and structural features. Recent microstructural analysis from the study area has highlighted the history of rock mass fracture systems in the cratonic basement and illuminated the deformation mechanisms of the upper crust.

Session

Deformation in the upper crust