Paper No. 28-7
Presentation Time: 8:00 AM-12:00 PM
REGIONAL STRAIN IN THE EASTERN BLUE RIDGE OF ALABAMA: IMPLICATIONS FOR INTERPRETATION OF THE ALEXANDER CITY FAULT
The eastern Blue Ridge Ashland-Wedowee-Emuckfaw belt (AWEB) of Alabama consists of Cambrian to Ordovician metasedimentary lithologies intruded by Ordovician to Carboniferous orthogneiss bodies. The belt is bounded on its northwest by the Hollins Line thrust system and on its southeast by the Brevard fault zone, but also contains a number of internal faults, including the Goodwater-Enitachopco fault and Alexander City fault (ACF). Over the past 50 years, a number of interpretations for the timing and kinematics of these faults have been proposed, with different researchers arguing for normal, thrust, and strike-slip displacement under cataclastic to mylonitic conditions. Along the ACF, for example, workers have interpreted it as a Permian-Triassic normal fault, Alleghanian thrust fault, and Devonian to Carboniferous right-lateral, mylonitic shear zone. In the latter interpretation, mylonitic-phyllonitic fabrics in metapelite lithologies of the Wedowee and Emuckfaw Groups, as well as ductily deformed feldspar grains in Elkahatchee orthogneiss, were used to argue for dextral strike slip kinematics. Complicating this interpretation, however, are pervasive mylonitic-phyllonitic fabrics throughout the AWEB. In an attempt to determine whether these ductile shear fabrics formed due to strain partitioning along the ACF or from widespread ductile shearing under Carboniferous middle-upper amphibolite facies regional metamorphic conditions, we performed Rf-phi analysis on plastically-deformed feldspar crystals from lithologically similar orthogneiss bodies across the region. 19 samples along a >30 km, strike-perpendicular transect yielded bulk strain (Rs) values between 1 and 9. A closely spaced series of samples across the ACF (200 m transect) yielded Rs values between 1 and 2, similar to the predominant regional Rs values for the AWEB. Interestingly, the largest Rs values do not occur in proximity to any identified fault, but instead are associated with localized shearing along a significant mechanical contrast at the contact between an Emuckfaw Group metapelite and Ordovician megacrystic orthogneiss. Our data provide a cautionary tale in the use of mylonitic fabrics as kinematic indicators for fault zones in regionally metamorphosed terranes deformed under ductile conditions.