Paper No. 6
Presentation Time: 3:00 PM
OVERVIEW OF TACONIAN TECTONISN IN THE SOUTHERN BLUE RIDGE - STILL THE SAME, AFTER ALL THESE YEARS
As in the northern Appalachians, conventional interpretations of orogenesis in the southern Appalachian Blue Ridge Province involve Ordovician (Taconian) collision of an accretionary complex with the Laurentian passive margin. Recent provocative proposals for Devonian (Acadian) or Carboniferous (Alleghanian) arc accretion would upend the conventional view of Taconian orogenesis, which produced the main phase of metamorphism in the Western Blue Ridge (WBR) province. Regional Barrovian isograds, mapped within the Ocoee Supergroup from the GA-NC-TN area (including the Murphy Belt) extend NE to the Grandfather Mtn. Window, parallel the strike of the orogen and increase in grade to the SE, extending into the westernmost Eastern Blue Ridge (EBR) province (Tallulah Falls/Ashe suites). Sil+Kfs grade/granulite facies metamorphism is attained over a large area in the south, and eclogite facies is preserved in the Blue Ridge thrust sheets to the NE. High precision zircon U-Pb TIMS geochronology in granulite and eclogite facies rocks in the Eastern Blue Ridge, and monazite U-Pb TIMS and U-Th-Pb chemical age distributions in St to Ky-Sil grade WBR rocks, support the conventional interpretation of Ordovician metamorphism of an accretionary complex. Granulite and eclogite facies conditions were simultaneously attained at 458 Ma, albeit 150 km apart. The 458 Ma age (time of the thermal and baric peak, based on thermobarometry) corresponds to the Llandeilian-Caradocian boundary and the youngest time of deposition in the Sevier foreland basin in eastern TN. Taconian orogenesis also accounts for published Silurian Hbl (post-Taconian cooling), Devonian Ms (post-Taconian cooling or regional F3 folding and slip cleavage formation), and early Paleozoic slate/phyllite whole rock 40Ar-39Ar ages in the WBR. Acadian tectonism and magmatism are demonstrated in the southern and southeasternmost Blue Ridge, but generally not on the NW side of the Blue Ridge in TN, where a significant Acadian clastic wedge is absent. Alleghanian greenschist facies deformation and overprinting of Taconian assemblages, structures, and fabrics are pervasive throughout the southern Blue Ridge. As in the NE Appalachians, much work remains to sort out the details of the distribution and intensity of these post-Taconian orogenic pulses.