MICROSCALE DEFORMATION OF SE NEBRASKA BASEMENT: INSIGHTS INTO THE PRECAMBRIAN EVOLUTION OF THE MIDCONTINENT, USA

The recently named Pinware-Baraboo-Picuris (PBP) orogen provides a new paradigm for interpreting deformation microstructures in enigmatic crystalline basement samples from SE Nebraska whose history is only now being deciphered. Microstructural studies, in conjunction with hand-sample and/or outcrop observations where possible, can characterize the spectrum of deformation events and mechanisms in an orogen, as well as foreshadowing the impact that future deformation events may have on similar rock masses. In the Midcontinent, USA, a few deep cores provide the basis for a powerful reassessment of basement tectonics.

Our crystalline basement samples date from the Proterozoic assembly of the central craton of North America. This basement was deformed by the evolution of two major features: (1) the 1.1 Ga Mid-Continent Rift System (MCRS) and (2) the >300 Ma Nemaha Uplift (NU). Due to the uplift on the flanks of the MCRS and the NU, ~60 m of these basement rocks were recovered in a fragmented small-diameter core. This so-called “Capitol/Capital Beach core” was drilled in 1887 as part of an unsuccessful search for rock salt or brine. We present ongoing work from the first-ever textural and microstructural analysis of the basement rock from this historic core.

Thin section and microprobe analysis indicates that the groundmass of the basement rock is primarily felsic, with mica, some opaques, and textures consistent with cooling from a melt; therefore, we consider the basement in this area to be granitic. This finding further warrants reassessment of the local structure of the Midcontinent Rift. Thin sections also reveal the development of folded phyllosilicate shear zones. Furthermore, we note at least two generations of brittle fracture in quartz and feldspar grains, some of which propagate through and displace the phyllosilicate shear zones and are filled with alteration minerals.

Our microstructural data implies that the granitic basement has undergone no less than four discrete phases of deformation since accretion. We associate two of these phases with the evolution of the MRCS and NU evolution, however one of the other deformation phases may date to the PBP orogen. We opine that microscopic analyses of basement rocks can reveal a rich, albeit incomplete, history of rock-mass fracture in cratonic basement.