USING CLEAVAGE REFRACTION, MICROSTRUCTURAL ANALYSIS, AND MINERALOGY TO LEARN ABOUT RHEOLOGY IN THE BARABOO QUARTZITE

The imposition of strain on layered rocks allows for possible insight into the relative rheologies of the layers. The contrasting structures between layers offer evidence for the different mechanical properties of different rock types during deformation. Within layered rocks, competence contrasts lead to deformation features such as folding, boudinage, and cleavage refraction. Previous research has suggested that cleavage refraction may be used to distinguish the relative viscosities of layers if the materials behaved in a Newtonian fashion, or it may be used to distinguish whether the layers behaved in a power-law fashion during deformation.

The Baraboo quartzites and interlayered phyllites in south-central Wisconsin were folded during the ca. 1650 Ma Mazatzal collision, during which they underwent low-grade (300-350°C) metamorphism. The folding resulted in pronounced cleavage refraction between the layers. Preliminary results show that the rheological contrast between the phyllite and quartzite, indicated by cleavage refraction, is not consistent throughout the region. The ratio differs by a factor of two, even within a small geographic region. The inconsistency of the cleavage relationship suggests that the rocks exhibit some degree of non-Newtonian behavior.

We have documented the variations in mineralogy between quartzites, phyllites, and transitional lithologies. We analyzed the microstructures within the layers and compared them to results from deformation experiments in quartzites in order to constrain deformation conditions. The microstructures show evidence of diffusional creep processes combined with some dislocation creep processes, especially in larger grains. The combination of quantified mineralogy, microstructural evidence, and competence contrasts exhibited by cleavage refraction allow us to link the rheological behavior of the rock to variations in mineralogy.