Studying Rheology Using Cleavage Refraction in Quartz-Rich Layered Rocks from Baraboo Wisconsin
Mentor 1
Dyanna Czeck
Location
Union 250
Start Date
28-4-2017 12:40 PM
Description
In metamorphic rocks forming at depth in tectonic regions, cleavage forms by the planar alignment of minerals. In layered metamorphic rocks, this cleavage will develop in different orientations based on the strength of the various rock types comprising the layers, causing cleavage refraction across layer boundaries. We can use this cleavage refraction to estimate the strength differences (in terms of effective viscosity ratios) between layers, a critical parameter that is poorly constrained in naturally-deformed rocks. Effective viscosity ratios can be used to evaluate whether rocks deformed by a simple linear-viscous model or whether a more complex rheologic model is needed. Layered quartzites and phyllites exposed near Baraboo, Wisconsin provide an ideal natural laboratory for study. The rocks contain varying proportions of quartz (strong phase) and pyrophyllite (weak phase) with only minor amounts of hematite and other minerals. Previously collected cleavage orientation and mineralogy data are used from those layers to estimate effective viscosity ratios and relate them to mineralogy between not only adjacent layers, as has been previously reported, but nonadjacent layers as well. Within a particular outcrop, the qualitative relationship between quartz content and cleavage orientation is clear; the angle between cleavage and bedding is smaller in layers with lesser amounts of quartz (and conversely more pyrophyllite). Graphs comparing the effective viscosity ratios and volume of strong phase (quartz) show an exponential relationship that fits within theoretical end-member curves of two phase mixtures with strong and weak phases. However, the data plot closer to the curve for strong inclusions in a weak matrix rather than the curve for weak inclusions in a strong matrix, which is contrary to what we expect from the mineralogy and microstructural evidence. Nevertheless, the data demonstrate that we can link mineralogy to effective viscosity ratios in naturally deformed rocks.
Studying Rheology Using Cleavage Refraction in Quartz-Rich Layered Rocks from Baraboo Wisconsin
Union 250
In metamorphic rocks forming at depth in tectonic regions, cleavage forms by the planar alignment of minerals. In layered metamorphic rocks, this cleavage will develop in different orientations based on the strength of the various rock types comprising the layers, causing cleavage refraction across layer boundaries. We can use this cleavage refraction to estimate the strength differences (in terms of effective viscosity ratios) between layers, a critical parameter that is poorly constrained in naturally-deformed rocks. Effective viscosity ratios can be used to evaluate whether rocks deformed by a simple linear-viscous model or whether a more complex rheologic model is needed. Layered quartzites and phyllites exposed near Baraboo, Wisconsin provide an ideal natural laboratory for study. The rocks contain varying proportions of quartz (strong phase) and pyrophyllite (weak phase) with only minor amounts of hematite and other minerals. Previously collected cleavage orientation and mineralogy data are used from those layers to estimate effective viscosity ratios and relate them to mineralogy between not only adjacent layers, as has been previously reported, but nonadjacent layers as well. Within a particular outcrop, the qualitative relationship between quartz content and cleavage orientation is clear; the angle between cleavage and bedding is smaller in layers with lesser amounts of quartz (and conversely more pyrophyllite). Graphs comparing the effective viscosity ratios and volume of strong phase (quartz) show an exponential relationship that fits within theoretical end-member curves of two phase mixtures with strong and weak phases. However, the data plot closer to the curve for strong inclusions in a weak matrix rather than the curve for weak inclusions in a strong matrix, which is contrary to what we expect from the mineralogy and microstructural evidence. Nevertheless, the data demonstrate that we can link mineralogy to effective viscosity ratios in naturally deformed rocks.
