Reprocessed Lithoprobe seismic reflection data across the Appalachian orogen in Newfoundland provide images of an Ordovician–Devonian collision zone that separates Laurentia from Ganderia, an accreted peri-Gondwanan microcontinent. Prominent reflectivity within Ganderian basement tapers westward and merges with reflections that project beneath the Moho, outlining a probable Ordovician to Devonian subduction zone. Reflectivity within Ganderian basement likely originates from transposed compositional layering within Cambrian–Neoproterozoic arc basement. Migmatites and other high-grade rocks of the Meelpaeg allochthon were likely extruded in the Devonian toward the northeast. The reflection Moho may have been established in the Devonian in parts of Newfoundland by partial melting of the lower crust. Reflection truncations outline a near-vertical Carboniferous strike-slip fault zone that cuts the entire crust.