IMPLICATIONS REGARDING THE EVOLUTION OF THE INNER PIEDMONT FROM THE SOUTHWEST END OF THE CAT SQUARE TERRANE, CENTRAL GEORGIA

The southern Appalachian Inner Piedmont (IP) consists of two lithotectonic terranes distinguished by notable differences in lithology, sedimentary provenance, and a clear partitioning of granitoid ages. The Cat Square terrane (CSt, E IP) was initially separated from the Tugaloo terrane (W IP) on the basis of U-Pb SHRIMP geochronology in the NC IP, which revealed a distinct detrital zircon suite containing Grenville, peri-Gondwanan, and ~430 Ma (Laurentian?) ages. Aeromagnetic data indicate the Brindle Creek fault, which separates the IP terranes, continues into central GA where it is truncated by the Alleghanian Towaliga fault. Detailed geologic mapping and U-Pb SHRIMP geochronology confirm the SW-continuation of the CSt, and also reveal subtle differences between the CSt of NC and GA that may be a consequence of crucial tectonic elements necessary to understand the evolution of the southern Appalachian orogen. Existing tectonic models envision the CSt as a Siluro-Devonian remnant ocean basin that diachronously closed from NE to SW as the Carolina superterrane (CS) accreted to Laurentia near the NY promontory during the Devonian-Mississippian. An implication of this model is that deformation, metamorphism, and plutonism should progressively young to the SW; geochronologic data from central GA CSt granitic plutons contradict this hypothesis. The High Falls Granite (GA) yields ages 424-382 Ma, which precedes the oldest plutonism in the NC CSt by nearly 20 m.y. (Walker Top Granite, 407-357 Ma). The CSt detrital zircon suite also differs between NC and GA. Both contain Grenville and peri-Gondwanan zircons, although the prominent ~430 Ma suite in NC metasedimentary rocks is missing from GA samples. The age and catazonal nature of the High Falls Granite suggest subduction of this part of the CSt occurred prior to uplift and erosion of the ~430 Ma source material. A modified tectonic model involves the CSt as a doubly closing remnant ocean basin, with initial closure at the SW end resulting from the CS colliding with an irregular Laurentian margin. Alternatively, piecemeal accretion of the CS could explain these observations. Initial closure of the CSt at the SW end remains compatible with numerous structural data that suggest overall dextral accretion of the CS and subsequent SW-directed lateral extrusion of the IP.