A SYNTHESIS OF RECENT CONTRIBUTIONS IN SOUTHERN APPALACHIAN MAPPING, GEOCHEMISTRY AND GEOCHRONOLOGY OF LAURENTIAN MARGIN ROCKS: PART II – SILURIAN THROUGH PERMIAN

Following development of an Ordovician back-arc basin and Taconic accretionary orogenesis on the Laurentian plate, southern Appalachian orogenic activity enters an enigmatic phase prior to the Alleghanian formation of Pangea. An early Silurian foreland clastic wedge suggests uplifts to the northeast of the AL promontory, while Silurian plutonic rocks in the EBR (e.g., Kowaliga) and WIP (Farmville) indicate silicic magmatism affecting the previously tectonized Ordovician margin. Silurian mafic rocks have not yet been documented in the WBR, EBR, or WIP. The mixed provenance (Laurentian and Gondwanan) Cat Square basin of the Inner Piedmont formed in the Silurian and is proposed to have originated in the New England segment of the orogen, being translated to its current position via strike-slip tectonics. Alternatively, the basin could be associated with a continuation of southern Appalachian extensional accretionary orogenesis, minimizing the magnitude of strike-slip emplacement. Completion of ongoing efforts to assign unequivocal ages to EBR plutons will be needed to establish a potential for Silurian source rocks in the southern Appalachians. By latest Silurian-earliest Devonian an extensional successor basin formed atop the distal Laurentian shelf (WBR Lay Dam Formation). Younger WBR rocks (upper Talladega Group) suggest a period of tectonic quiescence on the distal shelf until latest Devonian. However, Late Devonian plutonic rocks (e.g., Rockford, Elkahatchee, Bluff Springs) in the EBR suggest crustal thickening and B-type subduction beneath the Laurentian plate outboard of the margin. Latest Devonian-Middle Mississippian structural emplacement of proximal Ordovician back-arc rocks (Hillabee Greenstone) atop WBR shelf strata and coeval silicic igneous activity in the EBR (e.g. Blakes Ferry, Almond plutons) bracket peak (~340 Ma) metamorphism in the EBR and may be associated with a slab breakoff event and/or the arrival of Gondwanan tectonic elements at the Laurentian plate margin at the onset of Alleghanian orogenesis. Late Mississippian-Permian telescoping was responsible for the development of terrane bounding and internal thrust faults (e.g. Hollins Line), while normal faults (e.g., Alexander City) may have developed as a result of post-orogenic collapse and/or Mesozoic rifting.