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The Acadian orogeny in the North Atlantic region is assessed in this chapter in the light of mid-Paleozoic tectonics; throughout, plate tectonic nomenclature is used, and cycles are avoided. In North America nine regions bearing the imprint of the Acadian orogeny are recognized.

In Newfoundland, in the Maritime Provinces of Canada, and in Vermont and New Hampshire a continuous sequence of lithotectonic belts correlates along the orogen. The Bronson Hill belt, although a continuous structure in southern New England, is not recognized as such but splits into two structures northeast of the Maine-New Hampshire border: the Boundary Mountain anticlinorium and the Lobster Mountain anticlinorium. Other lithotectonic belts are partly continuous from Canada into the United States; they include: (1) North-Central Maine belt, (2) Aroostook-Matapedia belt, (3) Miramichi belt, (4) Fredericton-Central Maine belt, (5) Richmond belt, (6) Casco Bay belt, (7) Benner Hill belt, (8) St. Croix-Ellsworth belt, (9) Mascarene belt, and (10) Avalon belt. The decision as to whether each of these belts represents a separate terrane is at present reserved. In the coastal Maine zone the situation is particularly complex, and belts 6 through 10 can be recognized there.

In Massachusetts, we interpret the Merrimack Trough belt as in fault contact with both the Kearsarge-Central Maine and Bronson Hill belts to the northwest, and in Connecticut, with the Bronson Hill belt alone. Additionally, the Merrimack Trough belt is in fault contact with the Putnam-Nashoba belt to the southeast. The latter shows mainly a Taconian metamorphism and extensive intrusion of granites; clear evidence for Acadian orogenic effects in the Putnam-Nashoba belt is lacking.

In Newfoundland the main orogeny appears to be Silurian in age, and the same is true of New Brunswick, whereas in the Meguma of Nova Scotia the Devonian deformation and intrusive activity continue from the Devonian to the Carboniferous.

Correlations with the south-central Appalachians indicate a possibility of significant Acadian transpressional effects. The most recent evidence of a new microfossil find, however, implies that considerable Acadian deformation occurred in the Southern Appalachians, although it may have been directly continuous with earlier Taconian events.

The Acadian metamorphism in the Northern Appalachians is associated with numerous granites, in general ranging in age from the Silurian to the Carboniferous. The earlier Silurian granites may have originated along the Iapetus suture or may be associated with transcurrent faults.

The plate tectonic interpretation of the orogenic system is based on a model of successive blocks (terranes) approaching and colliding with North America and squeezing intervening sediments and volcanics. This took place over a fairly prolonged period of time.

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