INSIGHTS INTO PALEOZOIC OROGENESIS AND MESOZOIC RIFTING FROM DETAILED MAPPING IN THE GEORGIA INNER PIEDMONT

Detailed and reconnaissance geologic mapping in the central GA Inner Piedmont (IP) has recently been compiled at 1:100K scale. It incorporates new mapping with previous work done in the vicinity, and overlaps four lithotectonic terranes: the Tugaloo (W IP), Cat Square (E IP), Carolina superterrane, and the NE end of the Pine Mountain window. The terrane boundaries, delineated via detailed mapping and analysis of aeromagnetic data, have been confirmed with granitoid ages and detrital zircon geochronology acquired using SHRIMP. The Brindle Creek fault (BCf) separates the two IP terranes, and is a NE-striking, steeply-dipping, dextral strike-slip fault. NW of the fault (Tugaloo terrane), dominant lithologies consist of the 450 Ma Lithonia granitoid gneiss, along with an amphibolite-rich metagraywacke unit (lower Tallulah Falls Fm. ?); small bodies of Grenville basement are also exposed here. SE of the BCf, metasedimentary rocks consist of aluminous schist and amphibolite-poor metagraywacke, and at least three distinct granitoids. The High Falls granite is the oldest Cat Square granitoid (424-382 Ma). All other granitoids, in both of the IP terranes, are Alleghanian. The Cat Square granitoids mostly occur between the BCf and Towaliga fault (Tf) in a zone of intensely deformed crosscutting plutonism. The Tf frames the N edge of the PMW trending ~070, and changes to a more northerly ~030 strike through the Cat Square terrane. Fault-rock fabrics range from garnet-grade mylonite (Alleghanian) to lower-temperature ribbon quartz mylonite and isolated, map-scale rhombs and pods of siliceous cataclasite (Mesozoic). In general, more sinuous Alleghanian shearing can occur up to 300 m off of the Mesozoic fault trace, and in places is truncated by the later brittle features. Interestingly, shear sense of the Alleghanian mylonites is dextral, whereas the Mesozoic cataclasites formed in dilational step-overs in a sinistral strike-slip system. The Oxford fault, found NW of the Brindle Creek fault, shares a similar Mesozoic history. Both the Oxford fault and Tf cut ~200 Ma diabase dikes, providing a maximum age for the brittle faulting, although their emplacement was likely coeval.