2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 19
Presentation Time: 8:00 AM-4:45 PM

Quantified Strains and Electron Backscatter Diffraction (EBSD) on Quartz Fabrics In the Seine Metaconglomerates

ANDERSON, Terra N., Geosciences, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201 and CZECK, Dyanna M., Geosciences, Univ of Wisconsin - Milwaukee, P.O. Box 413, Milwaukee, WI 53201, tna@uwm.edu

Several studies have documented quartz fabrics in the field and in the lab. However, none of these studies have focused on deformation under natural conditions where detailed three-dimensional strain was quantified and where deformation mechanisms were evaluated. In our study, we link quartz shape and crystallographic fabrics within deformed metaconglomerates across a quantified strain gradient. We used previously calculated strains on the Seine Metaconglomerates from the Rainy Lake region in northwestern Ontario as a framework for our fabric analysis. We utilized petrographic and Electron Backscattered Diffraction (EBSD) techniques to document quartz grain fabrics throughout the region.

We collected oriented quartzite clasts and quartz veins from several locations with different strain magnitudes. Thin sections were made from the samples in orientations parallel to lineation and perpendicular to foliation. Microstructural observations indicate that quartzite clasts and quartz veins deformed by dislocation creep with subgrain formation and recrystallization recovery mechanisms. Extensive undulose extinction and some subgrains exist in all samples. Grain size and the degree of undulose extinction within grains decreased with increasing strain in quartzite clasts.

Using a SEM-EBSD system and CHANNEL5 software produced by HKL Technology, an automatic map of the thin section was used to determine the crystallographic orientations of quartz grains. The map was constructed to maximize the area analyzed on the thin section with one measurement taken from each grain.

Preliminary results from the EBSD analyses show increased clustering of quartz c-axes crystallographic preferred orientation (CPO) within quartzite clasts with increasing strain magnitudes. Similarly, in most cases, there was an increase in CPO of quartz c-axes within quartz veins when strain magnitudes increased. These observations when taken with the microstructural observations are consistent with deformation by dislocation creep with recovery primarily by recrystallization.