STRAIN ANALYSIS ACROSS THE MARGINS OF THE ELKAHATCHEE AND COLEY CREEK PLUTONS, ALABAMA EASTERN BLUE RIDGE: IMPLICATIONS FOR THE ALEXANDER CITY FAULT

The Ashland-Wedowee-Emuckfaw belt of the eastern Blue Ridge consists of metamorphosed Neoproterozoic-Ordovician continental margin and Ordovician back-arc sedimentary/volcanic sequences intruded by Ordovician-Mississippian granitic plutons. Two of these plutons, the Elkahatchee Quartz Diorite and Coley Creek Orthogneiss exhibit zones of high strain, evidenced by mylonitic fabrics, ductile deformation of feldspar grains, grain size reduction, and changes in mica content at their margins. Geologic mapping in the vicinity of the Coley Creek pluton shows no evidence for a ductile shear zone beyond its margins and thus, is unlikely to be associated with a major fault. More likely, this high strain zone is the result of mechanical differences between schist of the adjacent Emuckfaw Group and quartzofeldspathic rocks of the Coley Creek Orthogneiss, in conjunction with pervasive chemical alteration during metamorphic dewatering of adjacent pelites. Similar high strain zones observed along the margin of the Elkahatchee batholith have been attributed to a major ductile shear zone associated with the Alexander City fault. This ductile shear zone, along the southeastern margin of the batholith where it borders Wedowee Group graphitic schist, is projected by some workers to the AL-GA state line, in which case it would have significant implications for the local and regional geology. Other workers argue, however, that the regional geology does not support this interpretation, and that the ductile shear zone cannot be mapped beyond the Elkahatchee batholith. Importantly, shear zones observed along the southeastern margin of the Elkahatchee batholith are similar in nature to the shear zone observed along the margins of the Coley Creek pluton, where a major fault is not present. We utilize Rf-Ф analysis, along with mineralogical and grain size analysis, on samples from regular intervals across the intrusive contacts of both plutons with their metasedimentary country rock towards the interiors of each pluton, to compare and contrast the mylonitic fabric observed along each margin. Our work suggests the sheared margins of both are similar in nature and provides an alternative explanation for the ductile shear zone mapped as the Alexander City fault along the margin of the Elkahatchee batholith.