14-5 REGIONAL EXTENT OF THE BREVARD ZONE LITHOSTRATIGRAPHY AND EMPLACEMENT OF THE OVERLYING WESTERN INNER PIEDMONT ALLOCHTHON, SOUTHERN APPALACHIAN OROGEN, ALABAMA AND GEORGIA, USA

Session: New Research in the Appalachian-Ouachita Orogen: Integrated studies from the Foreland to the Hinterland

Presenting Author:

Authors:

Abstract:

Along its linear ~N50°E trend the distinctive Brevard Zone (BZ) lithostratigraphy (Jacksons Gap Group-JGG) in Alabama (AL) and Georgia (GA) is bounded by the late Alleghanian Katy Creek fault (KCF) on the southeast below the higher grade western Inner Piedmont (WIP) Dadeville complex (DC) volcanic arc allochthon. However, the JGG and underlying rocks of the eastern Blue Ridge, both of which are intruded by Ordovician-Silurian (~459-430 Ma) granites, curve south beneath the Coastal Plain (CP) around the southwestern hinge of the Tallassee-Newnan-Tucker synform (TNTS) to reappear to the northeast as the northwest-dipping Loachapoka Formation structurally beneath the DC along the older Stonewall Line thrust (SLT). The BZ sequence, intrusive granitoids and overlying SLT can be traced continuously from the CP >160 km northeastward into the Snellville Formation in north central GA, and then around the northeast hinge of the TNTS directly into units correlative with the Chauga River Formation immediately southeast of the BZ. Farther northeast, the BZ-equivalent sequence continues beneath the WIP Alto allochthon klippe along the Toccoa Falls-Shorts Mill thrust and lies structurally below the WIP Six Mile thrust sheet to the east along the Seneca thrust. Thus, the JGG and correlative units form a continuous footwall flat covering >18,000 km² traceable around an ~500 km elongated oval that lies structurally beneath the WIP allochthon. The WIP bounding faults are all post-metamorphic (post-Neoacadian), placing higher grade on lower grade rocks, and are discordant to syn-metamorphic fabrics, compositional layering and migmatitic veining in the WIP, thus requiring palinspastic restoration of the WIP >66 km to the southeast. Unlike the KCF, the SLF is folded by three post-metamorphic map-scale fold phases and stitched by post-metamorphic early Alleghanian (~335 Ma) plutons on both TNTS limbs. This suggests the WIP allochthon was emplaced above the BZ and equivalent units to the southeast between ~345 and 337 Ma, and did not move relative to the BZ after that time except for minor oblique thrust displacement on the rotational late Alleghanian (post ~300 Ma) KCF northeast of Jacksons Gap, AL. Because the BZ and equivalent units to the southeast are a continuous part of the same thrust sheet completely encircling the WIP DC, Alleghanian strike slip motion between the BZ and WIP must be extremely limited in magnitude.

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