THREE PALEOZOIC APPALACHIAN OROGENIES IN THE SOUTHERN APPALACHIAN CRYSTALLINE CORE: BOXES WITH BROKEN EDGES?

In the southern Appalachian crystalline core three Paleozoic orogenies are recognized: Taconic, Acadian-Neoacadian, and Alleghanian. These orogenies are defined based on ages of deformation, metamorphism, magmatism, and sedimentation. However, despite this robust approach many questions remain regarding the timing, duration, and geodynamic setting of these seemingly distinct orogenies. Fundamental impediments to unravelling the history of the polymetamorphic Appalachian orogen include a geologic record that is especially lacking in preservation of rocks that record the earliest or initial deformation stage of each orogeny, obfuscation or obliteration of evidence by subsequent deformation, and substantial terrane translation that complicates reconstructions and correlations.

The general orogenic framework includes: (1) The Taconic orogeny, often described as diachronous along the entire orogen, that shows agreement between ages of the clastic wedge, magmatism and metamorphism, the latter defining a protracted event (465–448 Ma) in the Blue Ridge. Taconian magmatism extends into the earliest Silurian in peri-Laurentian Piedmont terranes. (2) The Acadian-Neoacadian orogeny that had a long duration (~60 myr), with two periods of deformation and tectonothermal events at 395–385 Ma and 376–340 Ma. Restoration of dextral motion on the Brevard fault zone links the Acadian hinterland with the Devonian to Mississippian clastic wedge. (3) The Alleghanian orogeny extended from 335­–250 Ma and involved emplacement of the Blue Ridge-Piedmont thrust sheet (BRTS) ~300 km to the WNW of its root zone, followed by SSW translation of outboard terranes during the later stages.

Unlike the ~40 m.y. hiatus between the Taconic and Acadian orogenies, the gap between the Acadian-Neoacadian and Alleghanian orogenies continues to shrink as more data are collected. Recent muscovite and K-feldspar ⁴⁰Ar/³⁹Ar geochronologic data from the Catface and Stone Mountain faults, the leading edge of the BRTS in northeastern TN, and parts of NC and VA, constrain faulting to ~340 Ma. This suggests either kinematics changed from dextral transpression to NW-directed contraction during the Neoacadian, or the Alleghanian orogeny started by ~340 Ma.