COMPARING METHODS OF STRAIN CHARACTERIZATION IN THE CHESTER GNEISS DOME

The Chester dome in southeast Vermont is one of many gneiss domes in the Appalachian orogen, and is composed of predominantly gneiss and schist with widely varying mineralogy and apparent strain. Therefore this dome provides a unique opportunity to study strain in naturally deformed rocks, and to relate strain and rheology to mineralogy. There are a number of methods used to characterize strain including shape-preferred orientation (SPO), S-C fabrics, and crystallographic preferred orientation (CPO), but these methods are rarely directly compared to one another. The focus of this study is to characterize the SPO (aspect ratio and long axis orientation) as well as the S-C fabric when present in thin sections using measurements made using photomicrographs. We will relate SPO and S-C fabric to one another and to mineralogy. We expect high mica content to correlate to higher strain because micas are weak and localize strain. We will also investigate how SPO relates to strain within similar rocks by measuring samples from different locations within the dome that have different strain. We will relate our analogue determinations of SPO and S-C fabrics with electron backscatter diffraction determinations of crystallographic preferred orientation. Preliminary results from two samples are consistent with the hypothesis that mica has an influence on strain. The two samples have ~40% and ~20% mica by volume, quartz aspect ratios of 4.2±2.1 and 3.2±1.8, and biotite aspect ratios of 10.5±4.4 and 6.8±4.6, respectively. EBSD-derived CPO strength (ODF-J) values for quartz are 1.3 and 1.2, and for biotite are 17.7 and 7.4. Grain aspect ratio and ODF-J are both higher in the sample with more mica suggesting that aspect ratio and CPO strength may similarly track strain in mica-rich rocks. We will further characterize this relationship between CPO strength and aspect ratio, and whether a similar relationship holds for rocks rich in amphibole.