RELATIONSHIP BETWEEN THE DADEVILLE COMPLEX AND OPELIKA GROUP, APPALACHIAN INNER PIEDMONT OF ALABAMA AND GEORGIA

The southern Appalachian Inner Piedmont in E. central Alabama and W. Georgia consists of two tectonically separated Ordovician lithotectonic units: the structurally upper Dadeville complex (DC) and the underlying Opelika Group (OG). The DC, a large Taconic arc fragment, now occurs as a klippe composed of prominently metavolcanic and metaplutonic rocks that forms the core of the regional NW-plunging Tallassee synform. The structurally underlying OG, composed predominantly of metasedimentary rocks, is part of an Ordovician back-arc basin. The DC and OG are separated by a bounding Alleghanian thrust, the Stonewall line (SL). These two tectonic entities have been mapped in the Lanett S. 7.5’ quadrangle, located along the Chattahoochee River on the Alabama/Georgia boarder. All units generally strike northeast and dip northwest. In this area, the DC consists of the Ordovician Ropes Creek Amphibolite (RCA), whose protolith was mostly tholeiitic basalt, but also includes thin interlayers of dacitic meta-tuff and metasedimentary rocks. The OG consists of the Loachapoka Schist (LS), a kyanite/sillimanite schist with interlayered quartzite/metaconglomerate and thin amphibolite. The Auburn Gneiss occurs structurally below the LS and consists of interlayered biotite gneiss and migmatic muscovite schist. Both of the latter units were intruded by the Farmville Granite, but this unit does not cross the SL. The SL is an ~10-15 m wide fault zone, with units of the RCA thrust above thin lenses of Farmville Granite and the LS. In the study area, units in both hanging and foot wall are semi-concordant, but regional mapping shows that to the NW, the basal thrust cuts up section several kilometers in the hanging wall, while remaining concordant to foot wall units. This geometry indicates that the SL’s thrust trajectory was that of a large hanging wall ramp (DC) on a footwall flat (OG), as the arc complex was emplaced to the NW upon its companion back arc basin.