Southeastern Section - 67th Annual Meeting - 2018

Paper No. 25-11
Presentation Time: 11:40 AM


LANGILLE, Jackie1, CORRADINO, Julia1 and SEWARD, Gareth G.E.2, (1)Department of Environmental Science, UNC Asheville, Asheville, NC 28804, (2)Department of Earth Science, University of California, Santa Barbara, CA 93106

The Appalachian Orogen records three orogenic events attributed to the assembly of Pangaea; the Taconic, Acadian, and the Alleghenian orogenies. In the Blue Ridge of western North Carolina, the rocks of the Ashe Metamorphic Suite (AMS) are juxtaposed on ~ 1 Ga Grenville basement rocks, thought to have occurred during the Taconic. However, Acadian dextral strike-slip displacement has been traced for ~100 km along this contact from the Grandfather Mountain window south to Asheville, North Carolina along the Burnsville shear zone (BSZ) and is the only documented Acadian fault in the Blue Ridge. Previous studies suggest that the BSZ could have accommodated hundreds of km’s of dextral displacement, potentially juxtaposing the AMS onto Grenville basement rocks and that this shear zone and the Inner Piedmont define the boundaries of a major dextral transform margin. However, no geologic markers exist to evaluate the amount of displacement along the BSZ making correlation of units across the fault difficult. This study presents detailed field mapping and kinematic analysis of rocks collected along a transect through the 2 km-wide BSZ near Asheville to evaluate its role within this dextral transform margin. Vorticy (Wm) analyses and electron-backscattered diffraction (EBSD) of quartz [c] and <a> axes were combined to quantify Rxz and the amount of thinning and extension across the shear zone. Wm estimates were obtained from quartz oblique grain-shape fabrics and rigid rotated grains. Quartz fabrics yield Wm values ~0.90-1 suggesting dominantly simple shear. Rotated grains support simple shear. Rxz values range from ~5-150 and combined with Wm estimates suggest that shortening perpendicular to and extension parallel to the shear zone increased toward the interior of the shear zone. These data suggest that the BSZ accommodated ~10-20 km of displacement. While this should be considered a minimum as additional displacement could have been accommodated along narrow ultramylonite zones, this study suggests that dextral displacement was not likely enough to juxtapose the AMS and Grenville basement rocks. Strain due to oblique convergence during the Acadian that was partitioned into the Blue Ridge, was partitioned into the AMS along a preexisting weakness defined by the Taconic contact with the basement rocks.