T96 35th Annual Undergraduate Research Exhibition (Posters)

Monday, 16 October 2023
8:00 AM - 5:30 PM
Hall B

QUARTZ C-AXIS DEFORMATION THERMOMETRY CONSTRAINS RETROGRADE DEFORMATION WITHIN THE CHUNKY GAL MOUNTAIN FAULT, WESTERN NORTH CAROLINA

Paper number: T96-16
Program: Technical Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

Quartz C-axis opening angle thermometry is used to determine the temperature of deformation in metamorphic rocks, using an empirically derived linear relationship between C-axis slip orientation and deformation temperature. Calibration equations were derived from both experimental and natural data. A recently developed calibration extends the relationship to higher deformation temperatures and documents a shift in slope with a change in mechanism from subgrain rotation (SGR) to grain boundary migration (GBM) at temperatures near 650°C.

This study estimates deformation temperatures from quartz-rich domains in mylonitic samples from The Chunky Gal Mountain Fault, Central Blue Ridge Province, western North Carolina. The fault zone preserves a range of deformation temperatures. Peak metamorphism within the shear zone is estimated at ~800oC with evidence of ductile deformation of K-feldspar porphyroclasts. Muscovite fish swim in the mylonitic matrix and quartz microstructures include both SGR and GBM textures. All study samples (n=5) were collected from the same shear zone. Samples from other shear zones did not preserve a girdle pattern that could be used with this method. C-axis orientation data, collected using Electron Backscatter Diffraction (EBSD) data is plotted on stereonets, referenced to sample foliation and lineation. For samples with a c-axis girdle pattern, opening angles were measured from the net center between girdle limbs in each direction. We employed 3 different opening angle calibration equations.

Opening angles from shear zone samples range from 51° to 71°. Temperature estimates from these angles range from 400 °C to 580 °C and are similar for the 3 different calibrations. Deformation temperature estimates from the opening angle fall within the SGR-related calibration and are distinctly lower than peak metamorphic conditions. Our results may indicate that quartz fabrics document the late stages of shear zone movement.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-395374

© Copyright 2023 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.