26 Geologic Maps, Geophysical Maps, 3-D Geologic Models, Digital Mapping Techniques, Map Derivatives, and Digital Map preparation (Posters)
This poster session will highlight new geologic maps, mapping programs, and innovations in geological mapping techniques, including data management, web accessibility, 3-dimensional maps and cross sections, and applications in engineering geology and water/land management.
Tuesday, 10 March 2026
9:00 AM - 5:00 PM
RCC, Lower Level Hall
An updated map of the Brevard Zone showing Two-dimensional strain analysis of the Long Island Creek Gneiss with proximity to the zone of most intense shearing in the Brevard Zone in western Georgia and eastern Alabama
Paper number: 26-1
Program: Technical Sessions
Time: 9:00 AM – 9:00 AM
Authors
Abstract
An updated geologic map of the Brevard Zone (BZ) is presented showing two-dimensional strain analysis of the Long Island Creek Gneiss (LICG) from western Georgia and eastern Alabama. The LICG was first mapped and described by Higgins (1966, 1968) for exposures at and near Long Island Creek on the Sandy Springs, GA, 7.5-minute quadrangle. The LICG is typically a sphene-epidote-biotite-quartz-feldspar gneissic granite that is medium- to coarse-grained and moderately- to well- foliated; locally the unit contains muscovite and microcline. The outcrop belt of the LICG is one of the longest and most continuous exposures of gneissic granite in the region, and is the most continuous lithology found within the BZ. The LICG is a recognizable marker unit that has been mapped for ~148km along strike, from Sandy Springs, GA, to southwest of Roanoke, AL, and is crossed by the zone of most intense shearing (ZIS) within the BZ. The LICG and BZ are subparallel with a divergence/convergence angle of ~7 degrees.
Thirteen hand samples of the LICG were collected along the strike belt from Sandy Springs, GA to Roanoke, AL. Strain analysis was conducted on the hand samples by cutting three orthogonal faces and digitizing feldspar porphyroclasts using Ellipse Fit (ver. 3.8.2). The three orthogonal faces were oriented: 1) perpendicular to foliation and parallel to the long axis of the porphyroclasts; 2) perpendicular to foliation and perpendicular to the long axis of the porphyroclasts; and 3) parallel to foliation and parallel to the long axis of the porphyroclasts. The final ellipticity (Rf) and final orientation (ΙΈ) were calculated from the digitized feldspar porphyroclasts using Ellipse Fit and using the method of Chew (2003).
The measured strain in the LICG generally decreases from Rf=3.3 near the ZIS to Rf=1.8 near Sandy Springs, GA, north of the ZIS. The dip-parallel strain gradient is nearly linear along this transect with a gradient of 0.152 (Rf/km). The measured strain in the LICG generally decreases from Rf=3.3 in the ZIS to Rf=2.5 south of the ZIS near Roanoke, AL, with a dip-parallel strain gradient of 0.230 (Rf/km). Based on the nearly linear strain gradients with proximity to the ZIS, the shear foliation within the LICG appears to be due to BZ shearing.
Geological Society of America Abstracts with Programs. Vol. 58, No. 1
doi: 10.1130/abs/2026SE-411905
© Copyright 2026 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.