Times are displayed in (UTC-05:00) Central Time (US & Canada) Change

Protracted Thermal History for the Chunky Gal Mountain Fault, Western North Carolina Constrained by Quartz C- and A-axis Opening Angle Thermometry

Session: 37th Annual Undergraduate Research Exhibition Sponsored by Sigma Gamma Epsilon (Posters)

Presenting Author:

Grayson Houghton

Authors:

Houghton, Grayson K.¹, Peterson, Virginia L.², Rahl, Jeffrey M.³

(1) Department of Earth and Environmental Geoscience, Washington and Lee University, Lexington, VA, USA, (2) Department of Geology, Grand Valley State University, Allendale, MI, USA, (3) Department of Earth and Environmental Geoscience, Washington and Lee University, Lexington, VA, USA,

Abstract:

The regionally significant Chunky Gal Mountain Fault (CGMF) in southwestern North Carolina preserves a complex structural and metamorphic history and may have accommodated emplacement of the Buck Creek Ultramafic complex during Appalachian orogenesis. The Buck Creek complex and nearby Winding Stair Gap preserve lower crustal granulite-facies conditions (~800 °C and 1.0-1.4 GPa) and petrologic data documents CGMF deformation progressing from ~825°C, 0.9 GPa to retrograde closure temperatures of ~600°C and 0.75 GPa. At the macro scale, several narrow steeply dipping, ~1 m-wide shear zones along and near the main CGMF contact show apparent normal deflection of the dominant foliation. Protomylonitic and mylontic rocks with well-developed kinematics at the micro scale are associated with a subhorizontal lineation. Some microstructural evidence, such as lineated sillimanite, plastically-deformed feldspars, and mica fish, indicate high deformation temperatures. However, quartz recrystallization occurred predominantly by subgrain rotation, implying recrystallization at lower temperatures than the peak metamorphic conditions. We collected crystallographic orientation data on quartz and feldspar using electron backscatter diffraction (EBSD). Deformation temperatures were estimated using quartz opening angle thermometry, utilizing a MATLAB routine to measure c- and a-axis opening angles. We find opening angles of 51-58°, corresponding to deformation temperatures in the range of 400-450°C; lower than the retrograde closure temperatures. Together, these observations suggest a protracted deformation as the rocks cooled from ~800 to 400°C. The sub-horizontal shear associated with these fabrics contrasts with the apparent normal-sense motion on the Chunky Gal Mountain Fault, suggesting a significant change in geometry during the later stages of motion along the structure.