Dyanna M. Czeck1, Carol J. Ormand2
1. Department of Geosciences, University of Wisconsin - Milwaukee, P.O. Box 413, Milwaukee, WI 53201 USA
2. Department of Geology and Geophysics, University of Wisconsin Madison, Madison, WI 53706 USA
Deformation experiments have greatly enhanced our understanding of the rheology of natural materials. However, it is only through expanding the information known of naturally deformed rocks and pairing that knowledge with appropriate experimentally derived knowledge that we may hope to gain the most from these experiments.
Many studies have extrapolated rheologic parameters in naturally deformed rocks from microstructural comparisons with experimental deformation of relatively simple rocks. There are also many other possible ways to gain rheologic information from natural deformation structures including 1) wavelength/ thickness ratios of boudinage (Smith, 1975, 1977), 2) wavelength/ thickness ratios of folds (Smith, 1975, 1977), 3) fold shapes (Hudleston & Holst, 1984; Hudleston & Lan, 1993), 4) shapes of boudins and mullions (Ramsay, 1967; Hudleston & Lan, 1995), 5) differences in strain between various deformed conglomerate clasts (Lisle et al., 1983; Treagus & Treagus, 2002), and 6) cleavage refraction across deformed layers (Treagus, 1983, 1999). As a way to expand upon these studies, we have chosen to study simple rocks that can 1) readily be compared to experimental deformation, and 2) have a variety of structures from which we can glean rheologic information.
The rocks in the region of Baraboo, Wisconsin, USA are primarily super mature quartzites interlayered with phyllites. The rocks have an ideal simple mineralogy for our study. Prior to metamorphism, the rocks were layers of pure quartz sand with minor layers of clay and sandy clay in varying proportions. The rocks were folded and metamorphosed to greenschist facies during the Mazatzal Orogeny approximately 1.65 Ga (Holm et al., 1998), resulting in a large syncline on the order of 10 km across. The resultant metamorphic mineralogy consists of large quartz grains derived from the sand and small quartz and mica derived from the clay. Most of the layers are almost pure quartz, in excess of 95%. In these rocks, we analyzed microstructures and look at meter scale features such as fold shape, fold to layer thickness ratios, boudinage to layer thickness ratios, boudinage shape, and cleavage refraction to see how each of these features may elucidate the rheology.
We analyzed the rocksą microstructures under a polarizing microscope. Microstructures include solution seams that indicate diffusive mass transfer (DMT) in the form of stress-induced solution transfer. There is also shape-preferred orientation (SPO) of quartz grains, most likely caused by some form of DMT. Additionally, the quartz displays features indicative of dislocation creep (regime 2 with low strain accumulation) including sweeping undulatory extinction, subgrain formation, and irregular grain boundaries (Hirth & Tullis, 1992). Relatively few triple junctions indicate deformation accomplished by dislocation creep with grain boundary migration. From metamorphic evidence, the temperatures were low (300-350°C), so it seems that the strain rate must have been very slow. The combination of microstructures from both a linear flow law (DMT) and power-law flow law (dislocation creep), suggests an overall nonlinear rheology based on the microstructural evidence.
Cleavage refraction across layers can determine effective viscosity contrasts and linear versus nonlinear behavior (Treagus, 1983, 1999; Groome & Johnson, 2006). We applied the equations from Treagus (1983, 1999) and the sensitivity analysis of Groome & Johnson (2006) to cleavage refraction measurements at layer boundaries in order to determine effective viscosity ratios. We intentionally did not use data with clearly localized shear at phyllite boundary or more complicated cleavages (e.g. crenulations). Most results indicate the effective viscosity ratios are within the range of what we would expect in rocks based on previous work where ratios <10 (summarized by Treagus, 1999). The results also show that the effective viscosity ratios between the quartzite and phyllite are not constant throughout the fold.
A more detailed refraction analysis was conducted on transects across layers with variable amounts of quartz and mica (gradational lithologies between quartzite and phyllite). For each section with refracted cleavage, we analyzed thin sections cut at an orientation perpendicular to cleavage and bedding and did systematic point-counting of mineralogies. Generally, our results indicate that there is a positive relationship between increased ratio of quartz percentage and increased viscosity ratio between layers. However, in detail, there is an inconsistent viscosity ratio relationship between layers with essentially identical mineralogies. This observation supports a nonlinear rheology or perhaps a temporal or spatial variability in rheology due to metamorphism and the development of fabrics.
Preliminary work on other small-scale structures also supports a hypothesis of non-linear rheology. Localized pinch and swell style boudinage on the south limb of the syncline indicates nonlinear rheology (Hudleston & Lan, 1995). We are continuing fold shape, fold thickness to wavelength ratios, and boudinage thickness to wavelength ratio analyses.
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