CLEAVAGE REFRACTION AS A MEASURE OF VISCOSITY CONTRAST: NEW INSIGHTS FROM THE BARABOO SYNCLINE

Layered strata provide great opportunities to study the mechanical properties of rocks during deformation. Since each layer has a unique strain history, the contrasting structural features of the individual layers offer evidence of the gradients of mechanical properties of rocks during deformation. Even where it is not possible to quantify rheological properties from structural fabrics, it may well be possible to identify potential flow laws that governed the deformation. This information can and should be used, in conjunction with laboratory measurements of rheology, to refine models of crustal deformation.

The Baraboo Syncline, in south-central Wisconsin, is a region of deformed layered strata where we can gain rheological insights. Interbedded quartzites and phyllites in the Baraboo Syncline display structures resulting from competence contrasts between the layers: boudinage, small scale folding, and cleavage refraction. The competence contrast is partially controlled by differences in effective viscosity between the layers. Theoretically, the existence of boudinage is only possible for nonlinear viscous behavior. However, the prevalence of microstructures that imply deformation by diffusional processes in the Baraboo quartzites and phyllites suggests a dominant linear viscous behavior. We assessed viscosity contrast for these rocks at several different outcrops to determine whether the layers within the Baraboo Syncline deformed by linear or nonlinear viscous flow.

We estimated viscosity ratios for lithological boundaries using cleavage refraction, a suitable proxy for strain refraction. Preliminary estimates suggest that viscosity ratios vary by at least a factor of two, even within geographically limited regions. We therefore conclude that the Baraboo rocks deformed by nonlinear viscous flow, in spite of the inferred diffusional deformation mechanisms. This has broad implications for the rheological properties of the deforming Laurentian crust during the low-grade (350-500 degree Celsius) metamorphism these rocks experienced in the Mazatzal Orogeny.