The leading edge of the Humber Arm allochthon in western Newfoundland coincides with the western limit of both the Taconian (Ordovician) and Acadian (Devonian) deformed zones. Taconian deformation is west-directed and confined to rocks of the allochthon. Acadian deformation extends to deeper crustal levels and affects underlying shelf carbonates and Grenville basement. Acadian structural style changes from westward-directed thrusts and folds forming an emergent thrust front along the northern and southern margins of the allochthon to eastward-directed thrusts and folds forming a buried front along the intervening central eastern margin. In this central region, cover and basement are delaminated by westward basement wedging and concomitant eastward backthrusting at a higher structural level.

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