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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.
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Detrital zircon geothermochronology reveals pre-Alleghanian exhumation of regional Mississippian sediment sources in the southern Appalachian Valley and Ridge Province
Abstract The Black Warrior foreland basin records sedimentation associated with the development of intersecting Ouachita and Alleghanian thrust belts along the southern margin of Laurentia. Mississippian–Pennsylvanian units in the Black Warrior basin are interpreted to be sourced from either the northern Appalachians and mid-continent or more regionally from the southern Appalachians or nearby Ouachita thrust belt. We present detrital zircon U-Pb ages and Th/U values from Paleozoic units that indicate zircon from the Mississippian Hartselle Sandstone are temporally and chemically compatible with being sourced from the southern Appalachians. Zircon mixing models suggest sediment was primarily recycled from Cambrian, Ordovician, and Devonian strata in the Appalachian Valley and Ridge, with minor influx from Piedmont units. A ca. 415 Ma zircon population requires additional input from the Maya Block of the Yucatan Peninsula or similar outboard terranes. We present zircon (U-Th)/He analysis and thermal history modeling of Paleozoic units, which detail pre-Alleghanian exhumation in the Appalachian Valley and Ridge. Both the Cambrian Chilhowee Group and Pennsylvanian Pottsville Formation exhibit (U-Th)/He dates ranging from 507 to 263 Ma with a Mississippian subset (353–329 Ma, n = 4), which indicates rapid cooling and inferred exhumation during Late Devonian–Early Mississippian Neoacadian tectonism. We propose a Mississippian drainage system that transported material along southern Appalachian structural fabrics to the juncture between Appalachian and Ouachita thrust belts followed by a sediment-routing rotation toward the Black Warrior foreland. This interpretation honors chemical-age zircon data, accounts for metamorphic grains in thin section petrography, and matches Mississippian–Pennsylvanian Black Warrior foreland lithostratigraphic relationships.
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- Award ID(s):
- 1735492
- PAR ID:
- 10340760
- Date Published:
- Journal Name:
- Geosphere
- Volume:
- 17
- Issue:
- 6
- ISSN:
- 1553-040X
- Page Range / eLocation ID:
- 1840 to 1860
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/GES02427.1
