The climactic, middle Paleozoic event that affected the Newfoundland Appalachians has been referred to traditionally as Acadian orogeny. The latest Field studies and isotopic ages indicate that it began in the Early Silurian and continued into the Devonian. It affected the Newfoundland Humber, Dunnage, and Gander zones and western parts of the Avalon Zone. Intensities of the effects of the orogeny decrease westward across the Humber Zone (Appalachian miogeocline) and eastward across the onland Avalon Zone. Offshore, the wide Avalon Zone is virtually unaffected by Paleozoic deformation.
The most intense regional metamorphism coincides mainly with the Gander Zone. Plutonism affected a wider area—from the eastern Humber Zone of White Bay to the western Avalon Zone of Placentia Bay. Deformation affected the widest area, from the Appalachian Structural front, which defines the western boundary of the Humber Zone, to the Avalon Peninsula of the western Avalon Zone.
A change from marine to terrestrial conditions preceded the Silurian-Devonian deformation. Uninterrupted shallow marine conditions prevailed in bordering regions outside the Acadian deformed zone.
The Acadian orogen spans the eastern portion of the Grenville lower crustal block that underlies the Humber Zone and western parts of the Dunnage Zone. It spans the Central lower crustal block that underlies eastern parts of the Dunnage Zone and the Gander Zone, and it spans the western part of the Avalon lower crustal block. Orogenic effects are most intense above the narrow Central lower crustal block and diminish outward across the margins of the opposing Grenville and Avalon lower crustal blocks. This spatial relationship between the surface orogen and lower crustal blocks implies that collisional interaction among lower crustal blocks controlled the tectonothermal effects of Acadian orogeny.