Relationship between the Dadeville Complex and Opelika Group, Appalachian Inner Piedmont of Alabama and Georgia

The southern Appalachian Inner Piedmont in east central Alabama and west Georgia consists of two tectonically separated Ordovician lithotectonic units: the structurally upper Dadeville Complex and the underlying Opelika Group. The Dadeville Complex, 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 Opelika Group, composed predominantly of metasedimentary rocks, is part of an Ordovician back-arc basin. The Dadeville Complex and Opelika Group are separated by a bounding Alleghanian thrust, the Stonewall Line. These two tectonic entities have been mapped in the Lanett S. 7.5’ quadrangle, located along the Chattahoochee River on the Alabama/Georgia border. All units generally strike northeast and dip northwest. In this area, the Dadeville Complex consists of the Ordovician Ropes Creek Amphibolite, whose protolith was mostly tholeiitic basalt, but also includes thin interlayers of dacitic meta-tuff and metasedimentary rocks. The Opelika Group consists of the Loachapoka Schist, a kyanite/sillimanite schist with interlayered quartzite/metaconglomerate and thin amphibolite. The Auburn Gneiss occurs structurally below the Loachapoka Schist and consists of interlayered biotite gneiss and migmatitic muscovite schist. Both of the latter units were intruded by the Farmville Granite, but this unit does not cross the Stonewall Line. The Stonewall Line is ~5-10-meter-wide fault zone, with units of the Ropes Creek Amphibolite thrust above thin lenses of Farmville Granite and the Loachapoka Schist. In the study area, units in both hanging and footwall are semi-concordant, but regional mapping shows that to the northwest, the basal thrust cuts up section several kilometers in the hanging wall, while remaining concordant to footwall units. This geometry indicates that the Stonewall Line’s thrust trajectory was that of a large hanging wall ramp (Dadeville Complex) on a footwall flat (Opelika Group), as the arc complex was emplaced to the northwest upon its companion back-arc basin.