EVIDENCE FOR THE (NEO-) ACADIAN OROGENY IN THE SOUTHERN APPALACHIANS

Modern geochronologic and existing stratigraphic data indicate that a mid-Paleozoic (380-355 Ma) orogeny affected the southern Appalachians. Post-Silurian folding and faulting have long been documented in Central and Eastern TN beneath the pre-Chattanooga (Late Devonian-early Mississippian, LDeM) unconformity. SHRIMP zircon rim and TIMS monazite (Dennis and Wright) geochronology has revealed 360-350 Ma metamorphism, along with 400, 380, and 366 Ma plutons. Early Silurian detrital zircons, a unique lithologic assemblage, mixed Gondwanan and Laurentian affinities, and middle to late Devonian plutonism define the Cat Square terrane (CSt) in the eastern Inner Piedmont. CSt plutons appear to be 380 Ma and younger, and are predominantly anatectic. The 400-380 Ma plutonic suite in the Carolina superterrane (CS), a 360 Ma thermal overprint (⁴⁰Ar/³⁹Ar ages, Dallmeyer et al.) in the western CS, and 600 and 500 Ma peri-Gondwanan detrital zircons deposited with Silurian detrital zircons in the CSt, provide the earliest real connection between the CS and Laurentian terranes to the W. These data not only document the mid-Paleozoic Neoacadian orogeny, but preclude docking of CS earlier than LDeM time. A plate tectonic scenario would involve formation of the more westerly Tugaloo terrane (Tt) as a Neoproterozoic to Middle Ordovician deep-ocean depositional to arc/MORB volcanic system. Other more westerly terranes (Dahlonega gold belt, Cowrock, Cartoogechaye) were accreted to Laurentia during the Ordovician, while the Tt remained outboard in the Theic(?) ocean. CSt sediments were deposited in a remnant ocean that remained between the Tt and CS through the Silurian and much of the Devonian. Eastward subduction of the intervening CSt remnant ocean basin by ~400 Ma produced the Devonian plutonic suite in the CS. CSt sediments were buried deeply enough beneath CS by 380 Ma to initiate anatectic melting. The CSt began to detach and move onto the Tt shortly thereafter, subducting, melting, and deforming the Tt. By 366 Ma additional melting, migmatization, and an imbricate channel had formed, and wholesale SW-directed flow of the Inner Piedmont occurred, detaching on the Brevard fault zone. The western Tt was later obducted onto the more westerly terranes along the Chattachoochee-Holland Mountain fault as either a late Neoacadian or early Alleghanian suture.