The Humber Zone of the western Newfoundland Appalachians represents a Cambrian-Ordovician passive continental margin which was deformed in Taconian (mid-Ordovician) and Acadian (Silurian-Devonian) orogenic events. A deformation front is imaged in seismic reflection data offshore of western Newfoundland. Structures associated with this deformation front are exposed on Port au Port peninsula, where Silurian rocks are strongly deformed but Mississippian strata are flat lying, indicating that latest thrusting was Acadian. A gravity low in the Gulf of St. Lawrence corresponds to a sediment-filled Acadian foreland basin. Previous models suggest that the onland shelf succession is autochthonous to parautochthonous. However, two Lithoprobe seismic reflection transects show subhorizontal reflections between 2 and 5 s two-way travel time, which extend up to 85 km east of the thrust front. These are interpreted as autochthonous platform and basement. In this model, shallower reflectors and outcropping units include both allochthonous platform and basement, comprising the Acadian Port au Port allochthon. The Taconian Humber Arm allochthon was carried westward as a high structural slice during thrusting of this allochthon. No major structural discontinuity exists between Grenville age crystalline rocks of the Long Range massif and platform rocks interpreted as allochthonous in the northern seismic line. A monocline at the southern extremity of the Long Range probably represents an oblique or lateral hanging wall ramp above the basal detachment. Within the Long Range thrust zone at the western margin of the massif the Long Range thrust shows only a few kilometers of displacement. However, the Parsons Pond thrust, which we interpret to run offshore at Green Point, juxtaposes contrasting successions with different structural and thermal histories; it probably carries a much larger amount of the total displacement. The basal d¿collement of the Port au Port allochthon is therefore interpreted to pass beneath the southern part of the Long Range massif.