SILURIAN EXTENSION IN CENTRAL PIEDMONT, SOUTHEASTERN UNITED STATES AND THE ORIGIN OF A MIDDLE PALEOZOIC BASIN IN CAROLINAS-GEORGIA SEGMENT OF THE SOUTHERN APPALACHIANS

A variety of lines of evidence suggest that Carolinia accreted to Laurentia in the Middle Ordo, and as well imply that this collision occurred in the central Appalachians. Following this collision and prior to “Acadian” orogenesis, translation of the composite terrane at least 500 km south, and emplacement of these rocks as a crystalline thrust sheet on Laurentia, a significant episode of lithospheric extension has been recognized. Aspects of this extension include deposition of a retroarc foreland basin containing detritus from peri-Laurentian and peri-Gondwanan terranes (Wenlock-Pridoli?), early crustal anatexis (Wenlock-Lochkovian?), and later alkalic- to subalkalic mantle-derived bimodal magmatism (Emsian). Upper amphibolite to granulite (charnockite-bearing) facies paragneisses of the Cat Square terrane contain detrital zircons as young as 430 Ma (Bream, 2002). To the west Cat Square gneisses overlie rocks of the Tugaloo terrane along a pre-thermal peak (i.e., > ca. 360 Ma) feature named the Brindle Creek fault. I interpret the Brindle Creek fault to be an unconformity. Petrography, U-Pb (monazite) TIMS and SHRIMP analyses suggest migration of hydrothermal fluids along the Brindle Creek unconformity multiple times 330-320 Ma, with significant retrogression. There is little or no evidence for shearing or strain concentration along this feature in South Carolina. Basin formation was accompanied by crustal anatexis (e.g., Clouds Creek, Newberry, Lake Murray, Anderson Mill). These plutons contain Late Ordo inherited zircons, have –3 < ε_Nd < 1, and ISR > 0.706. Lithospheric thinning accelerated at the Sil-Dev boundary and by Emsian time, a bimodal suite of alkalic gabbros + syenites were intruded, primarily on the Carolinia side of the suture. ε_Nd of these plutons ranges between +3 - +5, and ISRs are as low as 0.702. Collapse of this thinned lithosphere is recorded by the so-called “NeoAcadian” orogeny, and deposition of the Catskill clastic wedge. Restoration of the southern Appalachian crystalline Piedmont to its Devonian location extends the strike-length of the internal Salinic basins (Connecticut Valley- Gaspé Trough, Central Maine Trough, and northward extensions into Nfld) about 50%. This Salinic basin system and associated mantle-derived magmatism are remarkable for their length and the narrow time window during which this orogen-scale feature formed.