TWO- AND THREE-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

Two- and three-dimensional strain analysis of the Long Island Creek Gneiss (LICG) was conducted from natural exposures in 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, Georgia, 7.5-minute quadrangle. The LICG is typically a sphene (titanite)-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 Brevard Zone (BZ). The LICG is a recognizable marker unit that has been mapped for ~130km 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.

A total of 13 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. 3.7.3). 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 (R_f) and final orientation (ɸ) 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 R_f=3.3 near the ZIS to R_f=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 (R_f/km). The measured strain in the LICG generally decreases from R_f=3.3 in the ZIS to R_f=2.5 south of the ZIS near Roanoke, AL, with a dip-parallel strain gradient of 0.230 (R_f/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 that culminated around ~317 Ma (Crawford and Kath, 2015, Poole, per com).