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The southern Appalachians preserve evidence for three Paleozoic orogenies that contributed to construction of the composite southern Appalachian orogen, including the Taconic (480-440 Ma), Neoacadian (380-340 Ma), and Alleghanian (330-280 Ma) events. However, the complexity of thermal-metamorphic overprinting and polydeformation has impeded efforts to examine questions related to tectonic processes such as the crustal escape flow hypothesis in the southern Appalachians. To address this, new monazite and xenotime laser ablation split-stream U-Pb and hornblende 40Ar/39Ar dates have been produced for the Blue Ridge (BR) and Inner Piedmont (IP), and these data are being compiled with all previously available geo-thermochronological and quantitative P-T data to construct P-T-t histories for different parts of the orogen. Monazite U-Pb dates from prograde monazites in the North Carolina BR yield a clear Taconic (464-441 Ma) metamorphic signal for conditions up to granulite facies, which is interpreted to result from development of a Taconic accretion-subduction complex. Following the Taconic arcs collision, this part of the BR was cooled during Neoacadian and Alleghanian uplift and exhumation pulses, as indicated by thermochronologic dates spanning a wide range of closure temperatures (~550-220 °C). In the IP and Sauratown Mountains window, U-Pb dates of mostly prograde monazite growth yield a dominant Neoacadian signal (369-358 Ma), which corroborates previous age estimates for IP Barrovian metamorphism up to sillimanite II grade. In the IP, hornblende 40Ar/39Ar ages of 380-345 Ma generally indicate syn-Neoacadian cooling below ~500 °C. In the IP between the Brevard and Brindle Creek fault zones, Y-rich monazites yield younger dates (~330 Ma) that overlap with hornblende 40Ar/39Ar yield ages (335-324 Ma). Combined, these ages are interpreted to reflect post-Neoacadian reactivation and retrogression of the Brevard fault zone and potential folding(?) of the Brindle Creek fault zones during early Alleghanian deformation. This retrograde deformation persists until at least 297 Ma, as reflected by xenotime dates in the Brevard zone (311-297 Ma). Future work will address how the timing and extent of metamorphism, deformation, and exhumation may vary south of the present study area. 

The Appalachian Mountains expose one of the most complete deeply exhumed orogenic belts in the world. These rocks provide the opportunity to understand tectonic processes in the mid- to lower- crust that can be linked to upper crustal processes interpreted from less exhumed orogenic belts. However, 3 Paleozoic orogenies (Taconic, Neoacadian, Alleghanian) in the southern Appalachians produced a complicated thermal-metamorphic history that is poorly understood. Recently obtained monazite U-Pb ages in the western, central, and eastern Blue Ridge of Tennessee and the Carolinas range from 459 to 441 Ma, indicating that this part of the Blue Ridge preserves Taconic (Ordovician) metamorphic mineral assemblages and were not significantly reheated during Neoacadian (Devonian) or Alleghanian (Mississippian) orogenesis. Five published garnet Sm-Nd ages from the eastern Blue Ridge in Alabama and Georgia of 331 to 320 Ma indicate widespread Alleghanian metamorphism. The northwestern extent of these Alleghanian metamorphic rocks is constrained by a garnet Sm-Nd age of 357±3 Ma from NW of the transtensional Goodwater-Enitachopco fault. However, published metamorphic age constraints are lacking SE of and along strike to the NE of the Alleghanian rocks. We report new garnet Sm-Nd ages for northern Georgia that constrain the extent of the Alleghanian metamorphic rocks. Garnet-staurolite-hornblende gneiss in the Pumpkinvine Creek Formation yields an Alleghanian age of 323±3 Ma (MSWD=6.6, N=7). To the NE, garnet-muscovite-biotite gneiss from within the structural window at Brasstown Bald and migmatiticsillimanite- and spinel-bearing garnet-biotite neiss from outside the window at Blood Mountain have ages of 446±6 (MSWD=0.7, N=4) and 448±8 (MSWD=6, N=7) Ma, respectively. These 2 indistinguishable ages confirm the premetamorphic stacking of thrust sheets exposed in the structural window. Comparison of these new ages indicates post metamorphic displacement on the Allatoona fault between the Dahlonega terrane and the western Blue Ridge. Additional garnet ages spatially distributed across the Piedmont of east central Alabama and the Murphy belt of NE Georgia extent are currently in-progress. The full data set will be used to test tectonic models including possible out-of-sequence thrusting and crustal channel flow. 

The southern Appalachians record three Paleozoic collisional events, including the Taconic (Ordovician), Neoacadian (Devonian-Mississippian), and Alleghanian (Carboniferous-Permian) orogenies. The complex nature of thermal and structural overprinting related to these events, coupled with a lack of widespread modern geo-, thermo-, and petrochronologic studies here has limited our ability to unravel the precise timing, spatial extent, and conditions of Paleozoic deformation and metamorphism. In the Blue Ridge (BR) and Inner Piedmont (IP) of Tennessee, North Carolina, and Georgia, which represents the orogenic core of the composite southern Appalachians, new monazite laser ablation split stream (LASS) analyses, amphibole 40Ar/39Ar dates, and metamorphic phase equilibria models are integrated with pre-existing geo- and thermochronology data to test holistic models of Paleozoic orogenesis. In the BR west of the Brevard fault zone (BFZ), monazite U-Pb dates are 459-441 Ma and are related to a pronounced Taconic metamorphic peak (to upper amphibolite facies) during development of an eastern Laurentian subduction-accretionary complex, followed by exhumation and cooling during later Neoacadian and Alleghanian thrust stacking, indicated by thermochronologic data. In the BFZ and the IP to the east, monazite U-Pb dates range from 373-356 Ma and delimit the timing of peak Neoacadian kyanite-sillimanite II metamorphism in the IP driven by accretion and partial subduction of Laurentian and mixed-affinity IP rocks beneath the overriding Carolina superterrane. The relatively clear separation of Taconic and Neoacadian monazite dates across the BFZ indicate that this shear zone acted as a Neoacadian thermal-rheologic transition zone that partitioned SW-directed crustal “escape” channel flow of melt-weakened material, as proposed by earlier studies. Late Paleozoic monazite U-Pb dates derived from within the BFZ (~335 Ma) and in the southeasternmost parts of the IP (~324 Ma) reflect Alleghanian reactivation of the BFZ and the northwesternmost extent of Alleghanian Barrovian metamorphism, respectively, but the majority of the BR and the IP in the study area reveal no evidence of post-Neoacadian metamorphic overprinting. 

Abstract The tectonometamorphic evolution of the southern Appalachians, which results from multiple Paleozoic orogenies (Taconic, Neoacadian, and Alleghanian), has lacked a consensus interpretation regarding its thermal‐metamorphic history. The Blue Ridge terranes have remained the focus of the debate, with the interpreted timing of regional Barrovian metamorphism and associated deformation ranging from early (Taconic) to late Paleozoic (Alleghanian). New monazite U‐Pb geochronology and thermobarometric data are integrated with previously reported geo‐ and thermochronology to delimit the Paleozoic thermal‐metamorphic evolution of these terranes. Monazite compositional, textural, and U‐Pb age systematics are remarkably consistent for all samples, yielding a single dominant age mode for each sample. The western, central, and eastern Blue Ridge terranes yield weighted mean monazite U‐Pb ages of 450–441, 459–457, and 458–453 Ma, respectively. Thermodynamic modeling using mineral assemblages yields peak conditions of 600°C–650°C and 5.8–8.9 kbar for staurolite and kyanite grade western Blue Ridge units, including the stratigraphically youngest unit in the Murphy syncline, which also yields a weighted mean monazite U‐Pb age of 441 Ma. The Taconic metamorphic core of the central Blue Ridge yields peak conditions of 775°C and ∼11.5 kbar. Combined, these ages indicate that the relatively intact Barrovian metamorphic progression mapped across the Blue Ridge of Tennessee, North Carolina, and northern Georgia is solely of Ordovician (Taconic) age. Synthesis of this new data with existing geo‐ and thermochronology support a model of Barrovian metamorphism resulting from construction of a Taconic accretionary wedge and subduction complex, followed by post‐Taconic unroofing during Neoacadian and Alleghanian thrusting.