CONSTRAINTS ON QUARTZ DEFORMATION CONDITIONS USING EBSD, PALEOPIEZOMETRY, AND THIN SECTION MICROSTRUCTURES IN THE CHUNKY GAL MOUNTAIN FAULT, SOUTHWESTERN NORTH CAROLINA

Measurements of recrystallized grain size in fault rocks are directly correlated to differential stress during deformation. A common tool used to estimate these deformation stresses is piezometry. Located in the Blue Ridge Province in Southwestern North Carolina, the Chunky Gal Mountain Fault (CGMF) separates two tectonic terranes. This fault zone includes several distinct shear zones near the fault zone boundary that display evidence of variable deformation intensity in the form of mylonites and protomylonites. Oriented mylonite and ultramylonite samples with quartz-rich domains were collected from several shear zones within the CGMF. MATLAB code published with a recent piezometer calibration by Cross was used to determine relict versus recrystallized quartz grains from several shear zone samples. Samples from three different shear zones with increasing distance from the main fault zone display average recrystallized grain sizes ranging from as little as 16.7 microns to as large as 44.5 microns. Average differential stress estimates using two different piezometers ranged from 29.3 to 74.2 MPa. Quartz microstructures from most of these samples display both subgrain rotation (SGR) and grain boundary migration (GBM) textures, with some dominated one or the other. Generally, samples dominated by GBM have larger grain sizes and lower differential stress and we see evidence of higher differential stress nearer the main fault boundary. However, the SGR/GBM boundary for these natural samples is at larger average grain sizes than the experimental regime 2/3 boundary. Recent calibrations of natural samples to experimental show that both dislocation creep regimes and microstructures are very similar, but natural samples show the SGR/GBM boundary about 200°C lower and higher grain sizes than experimental samples with the natural SGR/GBM boundary falling around 500°C.