164 D19. Recent Advances in Structural Geology and Tectonics (Posters)

Tuesday, 17 October 2023
8:00 AM - 5:30 PM
Hall B

THE EFFECTS OF GRAIN SIZE ON RF-PHI ANALYSIS: RESULTS FROM NATURALLY AND DIGITALLY DEFORMED ROCK

Paper number: 164-26
Program: Technical Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

Strain analysis conducted on deformed orthogneiss, metaconglomerate, and conglomerate from the

southern Appalachians demonstrate a positive correlation between primary grain size and bulk strain

(Rs) calculated using Rf-Phi analysis. For example, deformed igneous feldspar in numerous samples of

the amphibolite facies Kowaliga Gneiss (eastern Blue Ridge, Alabama) consistently yielded higher Rs

values for larger crystals (e.g. analyzed in hand sample) than smaller crystals (e.g. analyzed in thin

section). Rf-Phi analysis of deformed primary quartz clasts from the lower greenschist facies Cheaha

Quartzite (Talladega belt, Alabama) yielded systematically higher Rs values for clasts ranging from very

fine sand to granules in size. Rf-Phi analysis on a weakly deformed conglomerate in the Colvin Mountain

sandstone (Appalachian foreland, Georgia) showed similar results for clasts ranging from very fine sand

to medium pebbles in size. In order to determine whether or not this increase in calculated Rs with grain

size for naturally deformed samples likely results from strain partitioning (i.e. mechanical behavior) or

might be a function of fundamental anisotropies in the undeformed rock (e.g. clast orientation, aspect

ratio), we collected a number of undeformed conglomerates from the southern Appalachians of

Alabama for this project. Each sample was cut in two perpendicular planes, with a wide range of large

and small clasts (n>450) analyzed for trends in orientation (Phi) and aspect ratio (Rf). Sample faces were

photographed and clast boundaries in the undeformed rock were traced for subsequent Rf-Phi analysis.

Rs and anisotropy values were calculated using EllipseFit for the undeformed rock, and then calculated

for a progressively distorted digital image to mimic homogenous deformation in a natural setting.

Preliminary results suggest at least some of the observed relationship between grains size and Rs might

be a function of grain-size dependent anisotropy in conglomerates.

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

doi: 10.1130/abs/2023AM-395285

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