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.
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DECIPHERING THE “FOOTPRINT” OF THE NEOACADIAN METAMORPHIC EVENT IN THE SOUTHERN APPALACHIANS: INSIGHTS FROM 40 AR/ 39 AR GEOCHRONOLOGY IN THE EASTERN BLUE RIDGE AND INNER PIEDMONT OF NORTH CAROLINA
The southern Appalachians preserve a composite thermal and deformational history that involves at least four orogenic pulses and spans >800 Myr from the middle Mesoproterozoic to the late Paleozoic. This complex history has made the discrete separation of metamorphic and deformational events persistently challenging, particularly for Paleozoic events. To address this, we report results from a new suite of amphibole 40Ar/39Ar data from the eastern Blue Ridge and Inner Piedmont (Tugaloo and Cat Square terranes) of western North Carolina to constrain the timing of cooling from thermal conditions greater than ~550 °C. Samples were collected along an orogen-normal transect from the Newton window, near the Central Piedmont suture, to the eastern Blue Ridge, west of the Grandfather Mountain window. Amphibole gneiss from the Spruce Pine area and Wilson Creek gneiss from the Grandfather Mountain window record dates of 380 and 496 Ma, respectively. Across the Brevard Fault zone, one sample from the western Inner Piedmont Tugaloo terrane yields a date of 377 Ma. Southeast of the Brindle Creek fault in the Cat Square terrane, three samples yield dates of 344-320 Ma while two samples from Tugaloo rocks of the Newton window yield dates of 335-330 Ma. Together, these dates suggest regional cooling of the Inner Piedmont progressed from northwest to southeast following the Neoacadian orogeny. The data also indicate either that high-grade (>550 °C) Alleghanian metamorphism may have only reset the amphibole40Ar/39Ar systematics in the most southeastern parts of the Inner Piedmont, near the Central Piedmont suture, or that these high-grade Neoacadian rocks were not exhumed from the mid-crust until the Alleghanian. Additionally, our new amphibole 40Ar/39Ar dates northwest of the Brevard Fault zone are not reset during Neoacadian orogenesis. Collectively, we interpret that high-grade metamorphism in this part of the southern Appalachians must have occurred prior to the Alleghanian, placing limits on the northwesternmost extent of mid- to high-grade Alleghanian metamorphism in this part of southern Appalachians. Additional data from the Sauratown Mountains window to the northeast and South Carolina and Georgia to the southwest may further constrain the spatial extent of the Neoacadian thermal event in the southern Appalachians.
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- Award ID(s):
- 1802730
- NSF-PAR ID:
- 10471369
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Abstracts Geological Society of America
- ISSN:
- 0435-3986
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1130/abs/2021SE-362173
