The Burnsville Fault: Evidence for the Timing and Kinematics of Acadian Dextral Transform Tectonics in the Southern Appalachians

There has been a conspicuous absence of documented Acadian structures in the Blue Ridge province of the southern Appalachians even though a growing body of evidence suggests that the middle Paleozoic Acadian orogeny affected the rocks of this region. New mapping along with structural, petrologic, and geochronologic data from western North Carolina show that the contact between the metasedimentary and metavolcanic rocks of the Ashe Metamorphic Suite and the Grenville basement rocks of the western Blue Ridge corresponds to a Devonian high-grade, dextral strike-slip fault zone, the Burnsville fault. Timing of motion on the Burnsville fault is constrained by field relationships, metamorphic fabrics and assemblages, and radiometric ages. A U-Pb zircon crystallization age for a pegmatite sheared by the Burnsville fault shows that the last movement on the fault must be younger than 377 Ma. Previously published ⁴⁰ Ar/ ³⁹ Ar cooling ages show that the Burnsville fault must be older than ca. 360 Ma. We have mapped the Burnsville fault for ∼100 km in northwestern North Carolina. The Gossan Lead fault of northwesternmost North Carolina and southwestern Virginia is the likely continuation of the Burnsville fault to the northeast. Southwest of Asheville, North Carolina, the Burnsville fault may connect to the Devonian Dahlonega shear zone, or may be cut by post-Devonian thrust faults associated with the Alleghanian orogeny. Diachroneity of Acadian clastic wedges, the presence of Silurian-Devonian pull-apart basins, the presence of Devonian high-grade, dextral shearing in the Inner Piedmont, and recent plate reconstructions for the middle Paleozoic, support the interpretation that the late phase of the Acadian orogeny in the southern Appalachians was primarily a dextral transpressional event. The Burnsville fault and the Inner Piedmont formed the boundaries of a dextral transform margin that may have extended the length of the Appalachian orogen. This interpretation requires that models for the Paleozoic tectonics of the Appalachians incorporate major pre-Alleghanian dextral displacement.