Using cleavage refraction and microstructural evidence to determine the relative rheology of naturally deformed quartzites and phyllites from Baraboo, Wisconsin
Jolene T. Traut and Dyanna M. Czeck
Department of Geosciences, University of Wisconsin–Milwaukee, P.O. Box 413, Milwaukee, WI 53201
Understanding the rheology of rocks is a first-order question in tectonic studies where we aim to understand how rocks flow in response to stress. Rock rheology is largely studied through experimental work, but cannot be easily applied to natural deformation because of the long time-scales involved and the complications derived from rocks containing multiple minerals, all with different strengths. The Baraboo syncline near Baraboo, WI is an ideal test case to compare natural deformation to rheologic experimental results conducted on quartz because the main lithology at Baraboo is a quartzite derived from a clean quartz arenite. Within the Baraboo syncline, the variation in lithology (quartzite and phyllite) results in a difference in how these rocks responded to deformation and metamorphism during the regional contraction from the Mazatzal orogeny.
We conducted a detailed analysis of the cleavage refraction through a graded contact between the quartzite and phyllite layers in several areas around the syncline (Figure 1) and combined those results with mineralogy determined by X-ray diffraction (XRD). The angle between bedding and cleavage varies between approximately 89° in the most competent quartzite layers (approximately 99% quartz and 1% pyrophyllite) and 15° in the least competent phyllite layers (approximately 82% quartz and 13% pyrophyllite). Due to the increase in pyrophyllite, cleavage readily forms in the phyllite causing a smaller cleavage/bedding angle than what can be seen in the more competent quartzite. Where possible, we determined effective viscosity ratios using the Treagus (1999) cleavage refraction method. Preliminary cleavage/bedding angle data suggest that the effective viscosity ratios range between 1.0 – 3.7, which are values comparable to other studies.
Microstructural evidence indicates that varying degrees of recrystallization of quartz occurred during deformation, and it is possible that the amount of smaller, recrystallized grains respond differently than the original depositional grains. By combining point counting methods and the total quartz abundance determined through XRD, we will be able to extract whether these recrystallized grains impact the overall rheology.
Treagus, S., 1999. Are viscosity ratios of rocks measurable from cleavage refraction? Journal of Structural Geology, 21, 895-901.