New insight into the nature of the Acadian orogeny in eastern Maine has been gained by combining detailed field studies in six lithotectonic belts with geochemical data from the igneous rocks of the region. Revised stratigraphies and deformation histories of the tracts reveal their sedimentological and structural evolution from Ordovician through Early Devonian times, and variations in the isotope geochemistry of the igneous rocks permit delineation of the basement blocks beneath the supracrustal belts. Combined, these results yield a model for plate interactions that followed Taconian deformation and culminated in the Acadian orogeny. Large basins (e.g., Aroostook-Matapedia, Central Maine) formed immediately after the Taconic orogeny on the recently accreted eastern margin of ancestral North America. These filled with thick clastic sequences derived from post-Taconian highlands during Late Ordovician through at least Middle Silurian times and characteristically preserve complex facies patterns at their margins. At the same time, sedimentation continued in the Fredericton Trough, inferred to be the only remaining oceanic crust in the region. This ocean basin separated the composite North American terrane from an equally complex Avalonian continent. Closing of this basin resulted in the Acadian orogeny. The onset of the Acadian suturing of Avalon to North America is indicated by a change from local basin filling to a more homogeneous blanket of sandstones whose deposition appears to have begun in the east (Flume Ridge Formation) and migrated westward. Collision of basement blocks led first to westward thrusting of parts of the Avalonian continent over the Fredericton belt. Later Acadian thrusting caused by final collision between Avalon and ancestral North America transported supracrustal Miramichi belt strata eastward over the Fredericton belt and parts of the Fredericton belt eastward over the western edge of the Avalonian allochthon. Acadian thrusting has displaced the original boundaries between supracrustal belts in southeastern Maine so that they no longer coincide with boundaries between the basement blocks that originally lay beneath them.

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.

Abstract Middle Paleozoic belts are recognized across the Canadian Appalachian region from Quebec to Nova Scotia, and across the island of Newfoundland. Middle Paleozoic rocks are most extensive along the Quebec-Nova Scotia transect and from northwest to southeast they define the Gaspé, Fredericton, Mascarene, Arisaig, Cape Breton, and Annapolis belts. Middle Paleozoic rocks are less abundant in Newfoundland. From west to east they define the Clam Bank, Springdale, Cape Ray, Badger, Botwood, La Poile, and Fortune belts (Map 2). Rocks of the middle Paleozoic belts do not express the early Paleozoic zonation of the orogen, except in a few cases where middle Paleozoic rocks occur within the confines of a particular zone. The middle Paleozoic belts are less continuous than early Paleozoic zones and there have been few attempts to correlate middle Paleozoic rocks and interpret their significance for the entire Canadian Appalachian region. This is because (a) there are few complete sections with fossiliferous rocks so that the record is fragmentary, (b) occurrences in some places are small and of unknown age, thus precluding broad correlations and linkages across the orogen, (c) there are no middle Paleozoic ophiolite suites, few mélanges, and other rocks that can be related to píate boundaries, (d) contrasts among middle Paleozoic faunas are less pronounced than those among early Paleozoic faunas, and (e) many middle Paleozoic rocks are terrestrial redbeds and associated volcanic rocks that are post accretionary cover sequences, some of which overstep the early Paleozoic elements of the orogen. Stratigraphic sections of most

Abstract This chapter summarizes, and repeats without referencing, information presented in preceding chapters. Some of these chapters were written more than 5 years ago and there have been important subsequent changes. A few new ref- erences are therefore cited. This chapter was written also with an eye toward suitability for a Canadian overview volume. The summary follows the systematics introduced ear- lier, treating all rocks according to the four broad temporal divisions; lower Paleozoic and older rocks, middle Paleozoic rocks, upper Paleozoic rocks, and Mesozoic rocks. The rocks of each temporal division are subdivided into spatial divisions. Thus, the lower Paleozoic and older rocks are separated into the Humber, Dunnage, Gander, Avalon, and Meguma zones and subzones as depicted in Figure 11.1. The middle Paleozoic rocks are separated into belts: Gaspé, Fredericton, Mascarene, Arisaig, Cape Breton, and Annapolis for the mainland; and Clam Bank, Springdale, Cape Ray, Badger, La Poile, Botwood, and Fortune for Newfoundland (Fig. 11.2). The upper Paleozoic rocks define a number of basins, and Mesozoic rocks define graben (Fig. 11.3). A compilation and classification of volcanic rocks for the Canadian Appalachian region is provided for comparisons with other divisions (Fig. 11.4). The Canadian Appalachians provide an excellent example of an orogen that built up through accretion and eventual continental collision. In this model of a typical Wilson cycle, the Humber Zone is the Appalachian miogeocline or continental margin of Laurentia, and outboard zones are accreted parts of the orogen or suspect terranes. These zones are the fundamental divisions