THE POLYPHASE DEFORMATION HISTORY OF THE EMPLACEMENT OF THE DADEVILLE COMPLEX: GROWTH OF THE LAURENTIAN MARGIN DURING THE ACADIAN OROGENY, SOUTHERN APPALACHIAN MOUNTAINS ALABAMA

This project completed detailed geologic mapping and structural analysis of the Dadeville Complex (DC), an island arc complex formed during the Taconic orogeny, to better constrain the role of terrane accretion during the Acadian orogeny. The Cusseta and Ponders quads are located in east-central Alabama with the Cusseta quad including the DC and the Opelika Complex and the Ponders quad including the DC and Brevard Zone. The Cusseta and Ponders quads include meta-volcanic, meta-plutonic, and meta-sedimentary rocks with the Ropes Creek Amphibolite, Waverly Gneiss, and the Agricola Schist, respectively. The strike of the foliations, defined by amphibole and/or plagioclase, is broadly NE-SW with dips mostly to the SE, however, many foliations dip to the NW. Lineations are best recorded in the amphibolite defined by plagioclase and/or hornblende, measured in the field and in the lab on oriented samples. The majority of the lineations plunge shallowly to the east, but a few plunge to the west and north or south. The mapping pattern in each quad has repeating units of amphibolite and granitic gneisses such that one orientation of folds cannot fully explain the pattern. There are several instances where the repeating pattern can be seen trending N-S and E-W, making the fold axes close to perpendicular. It is likely that the N-S trending fold axis was formed first and then the area was refolded with a fold axis trending E-W. Since both fold axes are paralleled by amphibolite grade mineral lineations, and there is little to no greenschist grade overprint, it is likely that both folding events occurred near peak metamorphic conditions during the Acadian Orogeny. One potential explanation could be an initial collision between the Carolina superterrane pinning the DC against Laurentia causing the first generation of folding with E-W shortening. The second generation is a little harder to constrain. Ma et al. (2018) have suggested that DC was formed outboard of Tennessee, and translated SW along orogen parallel shear zones. Merschat et al. (2005) provided evidence that the Inner Piedmont from NC to GA show evidence for mid to lower crustal flow to the SW during transpression. During transpression, the shortening direction could have shifted more N-S towards the southern edge of Laurentia approaching the Alabama-Oklahoma Transform.