Search for: All records

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.