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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. 

In the last two decades, crustal channel and escape flow, wherein long-wavelength ductile flow of lower crustal material transports mass and heat out of the collision zone, have remained among the most impactful ideas proposed to explain shortening accommodation in continental collisions. In the Inner Piedmont (IP), southern Appalachians, channel and escape flow have been previously proposed for the Devonian-Mississippian Neoacadian orogeny, and the deep exhumational level of the IP relative to other orogens in which channel flow has been proposed makes it ideal for testing the channel and escape flow models. In the IP channel flow model, the Brevard fault zone (BFZ) footwall is interpreted to buttress orogen-normal crustal flow of the hot IP in northwestern North Carolina and drive escape flow to the southwest. However, the polymetamorphic and deformational history of the southern Appalachians has made it difficult to isolate the spatial and temporal extent of thermal and deformational events driving flow of the interpreted channel. To address this, we use in situ laser ablation split stream monazite (Mz) U-Pb geochronology and geochemistry coupled with quantitative P-T data to define the extent and conditions of Paleozoic metamorphic events in the southern Appalachians of North Carolina. In this area, northwest of the BFZ, Mz dates indicate mostly Taconic (~462 Ma) and minor Neoacadian metamorphism (~368 Ma) whereas IP data show Neoacadian metamorphism (~363–330 Ma) with no Taconic ages. IP Mz also records a transition over time from HREE-poor to HREE-rich compositions, indicating Mz growth associated with both garnet growth and breakdown, respectively. This, along with diffuse chemical profiles and resorption textures in garnet, suggests that IP Mz records prograde to retrograde metamorphism. Furthermore, P-T estimates from the eastern Blue Ridge of northwestern NC are 5–9 kbar and 565–730 °C, whereas peak Neoacadian metamorphism in the IP core reached 5–8 kbar and 750–850 °C. We interpret this to indicate that the BFZ footwall acted as both a thermal and rheological boundary in northwestern NC during Neoacadian metamorphism, supporting earlier interpretations. Future work will assess the timing and conditions of metamorphism further south into the Blue Ridge and IP of South Carolina, Georgia, and Alabama. 

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.