Microscale deformation of a Proterozoic Granite near the nexus of the Mid-Continent Rift and Nemaha Uplift, SE Nebraska, USA

Deformation microstructures can be used to decipher multiple stages of deformation in ancient terranes through the assessment of classical high-temperature minerals, fabrics, and textures (e.g., alteration minerals, reduction of grain size) as well as low-temperature cross-cutting features (e.g., brittle fracture).  Microstructural studies are typically necessary, in conjunction with hand-sample and/or outcrop work, to fully characterize the spectrum of deformation events and mechanisms in a system and to understand the impact that future deformation events may have on similar rock masses.  

In this study, we focus on samples from SE Nebraska, where crystalline basement, chiefly the Central Plains orogeny, dates 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”, one of exceedingly few basement cores in SE Nebraska, was drilled in 1887 as part of an unsuccessful search for rock salt or brine. Pieces of it were distributed as souvenirs, then painstakingly reclaimed and rearticulated by the state geologist. We present a first-ever textural and microstructural analysis of the basement rock from this historic core.

Preliminary thin section petrography indicates that the groundmass of the basement rock is primarily quartz, twinned feldspar, mica and some opaques (zircon?) with remnant intergrowths and textures consistent with cooling from a melt; therefore, we consider the basement in this area to be granitic. Under crossed polars, we observe undulose extinction in the quartz grains, implying high strain. Quartz grains also show subgrain development at grain boundaries. Thin sections and hand samples also reveal the development of phyllosilicate shear zones, altered from the precursor micas that are folded and kinked themselves. 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, including possible epidote.

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, but other two phases of deformation occurred earlier, probably during the Cambrian to Mississippian, an interval about which little in terms of regional tectonism and deformation is known.  Our work highlights the importance of cratonic uplift events in the fracture of rock masses, even on the microscopic scale, during basement evolution.  It also portends important insights from continued investigation.