A COMPARISON OF THE ACADIAN AND NEOACADIAN OROGENIES IN THE SOUTHERN AND NORTHERN APPALACHIANS

Defined in the Northern Appalachians (NA), the Acadian orogeny is described as the 423–380 Ma collision of the peri-Gondwanan Avalon terrane with Laurentia. The west-dipping collision subducted the Avalon plate beneath Laurentia, shed flysch cratonward into foredeeps, and produced a continental magmatic arc at 423–388 Ma. These elements and their respective basement were metamorphosed and transported westward as fold-thrust nappes recognized throughout New England. The Neoacadian orogeny, 366–350 Ma, involved collision of the Meguma terrane outboard of Avalon, and resulted in amphibolite to granulite-facies metamorphism and deformation, and granitic to tonalitic magmatism.

In the Southern Appalachians (SA) evidence of the Acadian and Neoacadian orogenies can be found in the Inner Piedmont (IP), parts of the Blue Ridge, the Talladega belt, and the peri-Gondwanan Carolina superterrane (Ct). In the IP, the Cat Square terrane contains Late Silurian to Devonian clastic sedimentary rocks and rare ultramafic and mafic rocks juxtaposed over peri-Laurentian terranes. Detrital zircon analyses of the Cat Square terrane recognizes a mixed Laurentian (1.0–1.4 Ga) and peri-Gondwanan (0.5, 0.6, 1.6, 1.7, 1.9 Ga) provenance. The maximum depostional age of the rocks is constrained to ~435 Ma; magmatic ages constrain a minimum age of deposition of ~415 Ma. Magmatism ranges from 417–355 Ma and is partitioned into the Cat Square and Ct. East-directed subduction of oceanic crust beneath the Ct resulted in intrusion of the 417–372 Ma Concord Plutonic Suite. Granitic magmatism, 415–355 Ma, in the Cat Square terrane is broadly coeval with garnet to sillimanite + K-feldspar zone metamorphism in the IP. U-Pb metamorphic zircon ages range from 396–340 Ma, with relative probability peaks at 396, 376, 363, and 346 Ma, related to the oblique collision of the Ct, which overthrust the IP and parts of the Blue Ridge.

Although temporally similar, west- and east-directed subduction in the NA and SA is problematic. Palinspastic restoration of the Brevard fault zone places the IP near the Pennsylvania embayment. The New York promontory may have acted as an inflection point causing subduction polarity to switch from west-directed in the NA to east-directed in the SA, and corresponded to the location of a transform fault.